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<doc callnum="QK930.D38 R47 1980">
<metadata>
	<titleStmt>
		<mainTitle nfc="0"><title>Responses of submersed vascular plant communities to environmental change</title>:<titleExt>performed for Eastern Energy and Land Use Team, National Water Resources Analysis Group, Office of Biological Services, Fish and Wildlife Service, U.S. Department of the Interior</titleExt>/<respStmt>by Graham J. Davis and Mark M. Brinson ; project officer, Charles Segelquist.</respStmt></mainTitle>
	</titleStmt>
	<authorStmt>
		<persAuthor mainEntry="y"><name type="surname">Davis, Graham J.</name></persAuthor>
		<persAuthor><name type="surname">Brinson, Mark M.</name>,<authMisc code="e">joint author.</authMisc></persAuthor>
		<persAuthor><name type="surname">Segelquist, Charles.</name></persAuthor>
		<corpAuthor><name>Eastern Energy and Land Use Team.</name></corpAuthor>
		<corpAuthor><name>Biological Services Program (U.S.)</name></corpAuthor>
	</authorStmt>
	<imprint><pubPlace>Washington, D.C.</pubPlace>:<pubName>Fish and Wildlife Service, U.S. Dept. of the Interior</pubName>,<pubDate>1980.</pubDate></imprint>
	<classStmt>
		<locClass>
			<subject cat="top">Aquatic plants</subject>
			<subject cat="gen">Ecology.</subject>
		</locClass>
		<locClass>
			<subject cat="top">Plant communities.</subject>
		</locClass>
		<locClass>
			<subject cat="top">Plants, Effect of light on.</subject>
		</locClass>
		<locClass>
			<subject cat="top">Aquatic ecology.</subject>
		</locClass>
		<locClass>
			<subject cat="top">Plants, Effect of stress on.</subject>
		</locClass>
		<locClass>
			<subject cat="top">Lake plants</subject>
			<subject cat="geo">North America.</subject>
		</locClass>
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<text xml:space="preserve">
<pb n="1" />

         Biological Services Program

         FWS/OBS-79/33
         August 1980

                                  T
                                  T
                                             @A

         Responses            of
         Submersed Vascular
         Plant Communities, to
         Environmental Change

                         N

                                                                      Y

                 i rr od,-' Wildlife Service

      QK930
      .D38       Department of the             Interior
      R47
      1980
<pb n="2" />

                                        The Biological Services Program was established within the U.S. Fish and
                                  Wildlife Service to supply scientific information and methodologies on key
                                  environmental issues that impact fish and wildlife resources and their supporting
                                  ecosystems.
                                        Projects have been initiated in the following areas: coal extraction and
                                  conversion; power plants; mineral development; water resource analysis, including
                                  stream alterations and western water allocation; coastal ecosystems and Outer
                                  Continental Shelf development; environmental contaminants; National Wetland
                                  Inventory; habitat classification and evaluation; inventory and data management
                                  systems; and information management.
                                        The Biologica[Services Program consists of the Off ice of Biological Services in
                                  Washington, D.C., which is responsible for overall planning and management;
                                  National Teams, which provide the Program's central scientific and technical
                                  expertise and arrange for development of information and technology by contracting
                                  with States, universities, consulting firms, and others; Regional Teams, which
                                  provide local expertise and are an important link between the National Teams and
                                  the problems at the operating level; and staff at certain Fish and Wildlife Service
                                  research facilities, who conduct inhouse research studies.

                                             For sale by the Superintendent of Documents. U.S. Government Printing Office
                                                                   Washington, D.C. 20402
<pb n="3" />

                                                                       FWS/OBS-79/33
                                                                       August 1980

                   RESPONSES OF SUBMERSED VASCULAR PLANT COMMUNITIES
                                TO ENVIRONMENTAL CHANGE

                                           by

                         Graham 0. Davis and Mark M. Brinson
                                Department of Biology
                               East Carolina University
                           Greenville, North Carolina 27834

                            Contract No. 14-16-0009-78-032

                                   Project Officer

                                  Charles Segelquist
                          Eastern Energy and Land Use Team
                                    Route 3, Box 44
                         Kearneysville, West Virginia 25430

                                property of CSC Librar,'Y

                                  Performed for:
                          Eastern Energy and Land Use Team
                       National Water Resources Analysis Group
                           Office of Biological Services
                             Fish and Wildlife Service
                          U.S. DEPARTMENT OF THE INTERIOR

         &lt;=&gt;                   US Department of Commerce
                               NOAA Coastal Services Center Library
                               2234 South Hobson Avenue
         &lt;C                    Charleston, SC 29405-2413
<pb n="4" />

                            Library of Congress Catalog Card Number: 80-607178

                                 wc2d"I
<pb n="5" />

                                              PREFACE

                 Many construction activities in or near streams and lakes cause changes
            in the aquatic environment that affect submersed vascular aquatic plants.
            This report was prepared to give biologists, engineers, and planners an over-
            view of how submersed plants respond to changing environmental conditions
            including light transmission, fluctuating water levels, currents and waves,
            and other physical and biotic factors.

                 A companion document, FWS/OBS-80/42, is a summary of this technical
            report. Inquiries concerning the availability of either report should be
            directed to:

                                       Information Transfer Specialist
                                       U. S. Fish and Wildlife Service
                                       Eastern Energy and Land Use Team
                                       Route 3, Box 44
                                       Kearneysville, West Virginia 25430
<pb n="6" />

                                                EXECUTIVE SUMMARY

                     Submersed vascular plants are native to many aquatic ecosystems where
                they influence a number of ecosystem processes and provide food and shelter
                for fish and wildlife. This report examines factors that 3ffect the light
                environment of submersed macrophytes and eval 'uates the responses of submersed
                plant communities to changing light conditions. Other requirements and stres-
                ses important to submersed macrophyte communities are discussed also.

                     The amount of light available for photosynthesis and growth of submersed
                plants depends on the combination of the turbidity of the water and the depth
                at which they grow. Turbidity due to suspended sediments may vary greatly
                depending on the energy of currents and waves that keep them in suspension or
                on human activities that may c-eate new sources of particulate matter. Depo-
                sition of sediments directly on @i.eaif surfaces may reduce light available to
                plants. Increases in plankton density, matting filamentous algae, and dense
                epiphytic growth, all common consequences of eutrophication, can reduce the
                amount of light available to submersed macrophytes. Other factors affecting
                the survival and growth of aquatic macrophytes are grazing and feeding acti-
                vities of fish and waterfowl, fluctuating water levels, hydrostatic pressure5
                and sediment type. All of these factors are reviewed briefly and examples
                given from pertinent studies.

                     Data were assembled on depth distribution records of submersed angio-
                sperms and the Secchi transparencies of water for mostly North American lakes.
                These were lakes in which the maximum depth of submersed macrophyte establish-
                ment was limited by light availability rather than shallow depth. Although
                the data base lacks rigorous precision, several patterns reveal that species
                respond quite differently to reduced light levels with increasing depth. Lab-
                oratory and field studies generally show that most shade tolerant species have
                a rapid photosynthetic response to increasing light in the low range of inten-
                sities. In clear shallow waters, the competitive advantage is shifted toward
                species in which photosynthesis is saturated only by extremely high light in-
                tensities.

                     Further analysis of depth distribution patterns of species and water
                transparency allowed identification of turbidity tolerant and non-tolerant
                species. This was expressed as a turbidity tolerance index, which in turbid
                waters (Secchi transparency &lt; 2.5 m), is the ratio of the depth maxima of
                species to the Secchi transp-arency depth. The index ranked ten species
                according to their resistance to adverse effects of turbid systems.

                                                        iv
<pb n="7" />

                 Submersed macrophyte communities may respond differently to short term
            and long term perturbations.    Short term changes may vary from a few weeks
            to a few years and usually have no lasting detrimental effects on the aquatic
            macrophytes. Long term changes associated with factors such as eutrophi-
            cation result in elimination of species sensitive to light reduction and often
            in the appearance of exotic species.

                 Based on long term studies of northern lakes, a survival index for sub-
            mersed species was developed. This index and the turbidity tolerance index
            were used, along with other informati6n,@, to identify five groups of species
            that have varying degrees of resistance to ecosystem alteration.

                 Finally the possible effects of human activities on'alterations in
            aquatic ecosystems and an array of impacts on submersed plant communities are
            considered. Since a wide range of ecosystem changes can be associated with a
            single type of perturbation and since responses of submersed macrophytes to
            system changes cannot be predicted with confidence, these impact evaluations
            are tentative.

                                                   v
<pb n="8" />

                                                        TABLE OF CONTENTS

                                                                                                             Page

                   PREFACE  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                  iii
                   EXECUTIVE SUMMARY      . . . . . . . . . . . . . . . . . . . . . . . . . . .                iv
                   LIST OF FIGURES        . . . . . . . . . . . . . . . . . . . . . . . . . . .              viii
                   LIST OF TABLES      . . . . ... . .. . . . . . . . . . . . . . . . . . . . .                ix
                   ACKNOWLEDGMENTS     . . . . . . . . . . . . . . . . . . . . . . . . . . . .

                   INTRODUCTION      . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
                   PHYSICAL, CHEMICAL, AND BIOLOGICAL          FACTORS  AFFECTING   SUBMERSED
                     PLANT GROWTH      . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    3
                     Fluctuating Water Levels           . . . . . . . . . . . . . . . . . . . . . .               3
                     Currents and Waves        . .  *   I' *  * '  * '  * * '          * . . . . . . . .          4
                     Suspended Sediments and Their Effects           on Submer@ed*  P*Iants   . . . . . .         5
                     Growing Season and Dormancy         . . . . . . . . . . . . . . . . . . . . .                9
                     Rutri 'ent Availability and Uptake         . . . . . . . . . . . . . . . . . .               9
                     Biological Factors        . . . . . . . . . . . . . . . . . . . . . . . . .               10
                     Hydrostatic Pressure        . . . . . . . . . . . . . . . . . . . . . . . .               12
                   RESPONSES OF SUBMERSED      PLANTS TO LIGHT AND      TURBIDITY      . . . . . . . . .       14
                    .Light Attenuation      . . . . . . . . . . . . . . . . . . . . . . . . . .                14
                     Relationship of Actual Light Transmission and Secchi              Disc
                        Transparency Estimates to the Euphotic          Zone   . . . . . . . . . . . .         15
                     -Depth Zonation and Turbidity Tolerance of Submersed Species                              17
                        Relationship between Maximum Depth Distribution and Secchi
                          Transparency       . . . . . . . . . . . . . . . . . . . . . . . . . .               26
                        Turbidity Tolerance Index        . . . . . . . . . . . . . . . . . . . . .             29
                    ,,Photosynthesis and Growth in       Response to    Light     . . . . . . . . . . .        30
                        Laboratory Experiments          . . . . . . . . . . . . . . . . . . . . . .            31
                        Field Studies        . . . . . . . . . . . . . . . . . . . . . .           *  .  . .   34
                  -RESPONSES OF SUBMERSED PLANTS        TO ECOSYSTEM    CHANGE    . . . . . . . . . . .        35
                     Community Responses to the Duration of Perturbations                . . . . . . . .       35
                        Short Term Perturbations         . . . . . . . . . . . . . . . . . . . . .             35
                        Long Term Perturbations         . . . . . . . . . . . . . . . . . . . . . .            37
                     Categories of Species Based        on Resistance to     Ecosystem    Alteration     . .   42
                        Rosulate Species       . . . . . . . . . . . . . . . . . . . . . .            I. . .   43
                        Northern Potamogetons           . . . . . . . . . . . . . . . . . . . . . .            43
                        Tolerant Species Normally -with Low        Biomass   in Disturbed Systems              44
                        Tolerant Species Nonnally Dominant         or Subdominant in Disturbed
                          Systems    . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                 44
                        Adventive Species         . . . . . . . . . . . . . . . . . . . . . . . .              45

                                                               vi
<pb n="9" />

           EFFECTS OF ENVIRONMENTAL ALTERATIONS ON SUBMERSED AQUATIC PLANTS       . . .   46

           REFERENCES    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      52
           APPENDICES
             A.  Regions of North America that Correspond to the
                 Distribution Codes of Appendix B     : * * ' ' * * ' I ' * ' * ' * *     62
             B.  Species, Family, Common Name, and Distribution
                 of North American Aquatic Macrophytes Mentioned
                 in Text   . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      63

                                                 vii
<pb n="10" />

                                                LIST OF FIGURES

                Figure                                                                         Page

                          Comparison of settling velocities described by Stokes law
                          and.the impact law . . . . . . . . . . . . . . . . . . . . . . .        7

                    2     The relationship between current velocity and particle size
                          which determines whether particles will be eroded,
                          transported, or deposited   . . . . . . . . . . . . . . . . . . .       7

                    3     Schematic diagram of factors that contribute towards a
                          tendency for nutrient absorption by roots or shoots     . . . . . .    11

                    4     Hypothetical changes in relative primary productivity of submersed,
                          emergent, epiphytic, and planktonic communities with
                          increasing nutrient enrichment  . . . . . . . . . . . . . . . . .      12

                    5     Depth maxima for aquatic vascular plants and Nitella spp.
                          as related to the 1 percent light transmission depth     for
                          some British lakes . . . . . . . . . . . . . . . . . . . . . . .       13

                    6     Absorption spectra of water from Lake George, Wisconsin,
                          compared with pure water  . . . . . . . . . . . . . . . . . . . .      16
                    7     Depth distributions for submersed macrophytes    . . . . . . . . .     18

                    8     Maximum depth distribution of selected species    from Figure
                          7 plotted against Secchi disc transparency of the waters
                          where the distributions were observed    . . . . . . . . . . . . .     27

                    9     Conceptual model illustrating the effects of environmental
                          forces on submersed aquatic macrophyte communities    . . . . . . .    47

                                                      viii
<pb n="11" />

                                             LIST OF TABLES

            Table                                                                        'Page

                    Turbidity tolerance index for selected species of Figure 7
                    expressed as the ratio of depth maxima to Secchi depth where
                    Secchi depth is 2.5 m or less . . . . . . . . . . . . . . . . . .     30

               2    Changes in submersed macrophyte populations in Reelfoot Lake,
                    Tennessee . . . . . . . . . . . . . . . . . . . . . . . . . . . .     36

               3    Long term changes in macrophyte populations in lakes and bays
                    in northern United States . . . . . . . . . . . . . . . . . . . .     38

               4    Evaluation of the effects of various types of environmental
                    alteration on submersed plant communities  . . . . . . . . . . . .    49

                                                  ix

            328-607 0 - 80 - 2
<pb n="12" />

                                              ACKNOWLEDGMENTS

                    The following persons contributed to the preparation of this manuscript:
                Randy Creech prepared the illustrations, Carol Lunney photographically repro-
                duced the figures, Sue Williams and Margaret Schiller typed preliminary drafts,
                and Cindy Stack of the Institute for Coastal and Marine Resources typed the
                final copy.

                    The cover diagrams were drawn by Ray Moore. An attempt was made to
                illustrate the effects of phytoplankton turbidity on submersed plant community
                structure in two otherwise similar areas of eutrophic Little John Lake and
                oligotrophic Silver Lake in Wisconsin (after Wilson, 1935).

                                                      x
<pb n="13" />

                                                  INTRODUCTION

                   There are many examples of waters that at one time supported luxuriant
              growths of macrophytes but are now largely devoid of these plants.          The causes
              for the reduced abundance vary, but most aye associated with increases in
              water turbidity.    However, the absence of submersed macrophytes does not al-
              ways imply that adequate light is lacking.       Submersed macrophytes generally
              colonize high energy and rather unstable zones of lakes, reservoirs, rivers,
              and estuaries and are subjected to a number of stresses that may exclude
              them.  Some of these stresses are discussed to put in proper perspective the
              importance of light availability.      However, the main subject area of this re-
              view is the effects that reduced light availability and increased turbidity
              have on submersed plants in inland waters.

                   The probable effects that increased water turbidity might have on sub-
              mersed angiosperms; will be evaluated. A number of human activities such as
              dredging, waste disposal, boat traffic, road construction, land use, etc. may
              add suspended solids to lakes and rivers at levels above background.          How-
              ever, background or "natural" turbidity can vary greatly in rivers- depending
              on current velocity following heavy rains or snow melt and in lakes depending
              on mixing by wind, bank erosion, and inflows from turbid rivers and streams.
              The changes in turbidity and thus the light environment of submersed macro-
              phytes corresponding with these events need to be evaluated relative to other
              factors that limit macrophyte distribution.       The importance of these "resi-
              duals" is often veyy difficult to quantify.

                   The importance of submersed macrophytes in aquatic ecosystems is widely
              recognized by biologists, but public perception of their value focuses pri-
              marily on their utilization by migratory waterfowl as food. Many aquatic
              vertebrates and invertebrates utilize aquatic macrophytes for food and cover.
              In fact, the metabolism of many flowing and limnetic ecosystems is dominated
              by photosynthesis and respiration of aquatic macrophytes which are at the
              base of the detritus -food web. Exchange of nutrients among the compartments
              of the sediment-plant-water system and calcium carbonate precipitation in
              plant beds can have striking effects on the water chemistry of aquatic eco-
              systems.

                   Most of the data and examples reviewed are from North America. Examples
              from north-central and northeastern United States and contiguous Canadian
              areas dominate the review.     This is due partly to the abundance of lakes in
              those districts and partly to the large number of limnological studies con-
              ducted there.    Data are presented for areas other than North America, but a
<pb n="14" />

               serious review of the world literature was not made. Some of the most rele-
               vant studies on turbidity-macrophyte relationships have been conducted in
               shallow estuaries rather than lakes.

                    While the studies cited date back to the early decades of this century,,
               many of the relevant data are of recent origin.  Problems associated with
               overabundance of certain macrophytes (e.g., Myriophyllum spicatum, Hydrilla
               verticillata)have provided the impetus for some of these studies. A few
               investigators in the field have recently focused much of their effort toward
               resolving the relationship between distribution of aquatic macrophytes and
               water transparency. We hope that this review will provide additional in-
               sight, identify some critical gaps in our knowledge, and suggest important
               areas of research.

                    North American species mentioned in this report are listed alphabeti-
               cally in the table in the Appendices with additional information on the
               family, common name, and distribution. Distribution is given according to
               the regions developed in Shelter and Skog (1978).
<pb n="15" />

                              PHYSICAL, CHEMICAL, AND BIOLOGICAL FACTORS
                                   AFFECTING SUBMERSED PLANT GROWTH

                 Among the submersed vascular plants, there is great variation in life
            cycle, morphology, physiology, and reproduction which somewhat reflects the
            diversity of their terrestrial ancestors (Arber, 1920).    This diversity is
            illustrated by the wide range of physical and chemical conditions to which
            various species are adapted.   The species composition and abundance in a
            submersed plant community will depend upon the totality of these factors to
            which an area is subjected. The purpose of this section is to provide an
            overview of the importance of factors other than light which may limit the
            abundance, or even the occurrence, of aquatic vascular plants.    While the
            discussion focuses on the submersed life forms, many of the factors mentioned
            also relate to emergent and floating leaved species.

            FLUCTUATING 14ATER LEVELS

                 Fluctuating water levels are common features in many shallow aquatic
            ecosystems. The distribution of wetland plants in response to a drop and
            subsequent rise in water level in a prairie pothole marsh is illustrated in
            a study by van der Valk and Davis (1976).   The submerged zone was completely
            exposed during a summer drought which almost eliminated Ceratophyllum demer-
            sum when reflooding occurred the following year.    Another submersed species,
            Potamogeton sp. aff. pusillus, was little affected but moved 4 or 5 m closer
            to shore after reflooTi@ngThere was a tendency during the drought for emer-
            gent species to germinate and invade the submersed zone such that biomass and
            species richness of the zone increased after reflooding.    This illustrates
            that although some submersed species are severely affected by extreme water
            level fluctuation (e.g., Ceratophyllum, which is not rooted), others adapt by
            a shift in zonation. However, considering the community as a whole, irres-
            pective of life form, there was little overall change in community production
            and diversity due to water level fluctuation.

                 Where drawdown can be artificially controlled it is commonly used in con-
            trol procedures for aquatic macrophytes.   Peltier and Welch (1970) suggested
            that drawdown, along with low rainfall, res'ulted in greatly increased cove-
            rage by Najas spp. in an Alabama reservoir. Similarly, Jackson and Starrett
            (1959) n5-t-ed that Potamogeton pectinatus grew best when water levels remained
            low in the shallow, floodplain of Lake Chlataqua, Illi'nois. In.the Chippewa
            Flowage, Wisconsin, which has received repeated winter drawdowns for 50

                                                    3
<pb n="16" />

                years, Nichols (1975) identified five submersed species that either recover-
                ed, or increased in coverage after repeated water fluctuation.       On the other
                h-and, if drawdowns are not properly conducted, problems resulting from exces-
                sive macrophyte growth may result especially in areas with long growing seasons,
                as predicted by Hestand et al. (.1971) for Lake Ocklawaha, F.lorida.

                      Where drawdowns persist for several years or are frequent during a sin-
                gle growing season, as in some reservoirs, submersed vascular plants will
                not survive.   Reservoirs typically have highly turbid waters and few shallow
                areas which further reduces chances for establishment of submersed vegetation.
                Controlled drawdowns have been used in the TVA reservoir system for management
                of Mriophyllum spicatum, a nuisance plant (Leon Bates, personal communica-
                tion).

                      Submersed species may be limited in their length of growth by shallow
                water, but it is uncertain whether this would affect rates of production.
                For example, Lind and Cottam (1969) found that Myriophyllum exalbescens, in
                Lake Mendota, Wisconsin, wasabout half as dense at Z.0 to Z.5 m depth as at
                1 m., but the average weight per plant wasabout twice as great in the deeper
                water. Whether Myriophyll6m would respond to increasing water depths during
                a single growing season by elongation and reduction in density is uncertain.
                Alternatively, reduced depths may result in greater fragmentation and uproot-
                ing from turbulence as more of the plant floats near the surface.

                      Martin and Uhler (1939) stated that at least a few inches of water must
                be retained for truly aquatic plants to remain established.       Of the species
                that they suggested for propogation in periodically exposed wetlands, none
                are submersed aquatics.    None of the seeds of aquatic plants mentioned by
                Sculthorpe (1967) that require drying before germination were from submersed
                species.

                CURRENTS AND WAVES

                      In medium to large lakes the eroding forces of waves may prevent the
                establishment or result in the fragmentation of submersed aquatic plants in
                the shallowest zone. Their absence is probably more commonly due to the
                abrasive action of waves rather than instability of the substrates.        In ma-
                ture basins, the amount of erodable inorganic material in this high energy
                zone is usually small, having long since been removed (Hutchinson, 1975).
                There is a tendency for submersed vascular plants that grow in these situa-
                tions to be small and their occurrence probably depends on resistance to
                fragmentation.

                      Fragmentation due to wave action was determined by Jupp and Spence
                (1977b) for Potamogeton filiformis in Loch Leven, Scotland. By comparing
                peak biomass--oT plots protected from wave action and waterfowl grazi
                (243 g/m2) with plots protected only from waterfowl grazing (125 g/m2?, a
                loss of 118 g/m2 was attributable to wave action. Since plant densities did
                not differ significantly between treatments, all of this loss was due to wave
                pruning of shoots. It is uncertain, however, to what extent fragmentation

                                                          4
<pb n="17" />

            calculated by this procedure actually occurred and to what degree wave action
            may have merely inhibited potential growth.

                 In running waters only rooted growth forms become established and thus
            they are restricted to areas of sediment deposition, with the notable excep-
            tion of members of the mostly tropical Podostemaceae which are attached to
            rocks in fast flowing water (Arber, 1920). Hynes (1970a) stated that no
            rooted plants show any special adaptation to running water, and the species
            that occur in streams and rivers have tough, flexible stems or leaves, a
            creeping growth habit, frequent adventitious roots, and strictly vegetative
            reproduction.

                 According to Westlake (personal communication) the relationship between
            plant distribution, depth, light and compensation point are complex, parti-
            cularly in the relatively shallow depths colonized in fairly turbid river
            waters. In these conditions many plants are capable of growing from the
            bottom and creating a leaf array containing most of their biomass near the
            illuminated surface. Such stands are ultimately limited in biomass partly
            by turbidity, but mainly by self-shading. Their depth limits, as defined by
            the deepest water in which they are rooted, are probably fixed by their
            capacity to grow from reserves, out of the dark bottom waters and into the
            light before other plants shade them. Size of storage organs is therefore
            important. These principles apply also to still water conditions.

                 In many flowing water situations, the occurrence of spates or freshlets
            following heavy rains severely limits the abundance of submersed species due
            to strong currents. For example, Bilby (1977) quantified the effects of
            macrophyte distribution in a stream pool before and after a spate resulting
            from a large rainstorm in New York. The two dominant species, Elodea canadensis
            and Potamogeton crispus underwent a pattern of displacement toward lower
            current speeds following the spate. Most of the reduction in macrophyte
            cover was where current speeds were highest.

                 Where current movement is slow as in irrigation canals, submersed plants
            may become exceedingly abundant, as in south Florida (Blackburn et al., 1968).
            Likewise, the steady flow and transparent waters of Florida spring runs often
            support high biomass of submersed species, whose year-round productivity is
            limited by light (Odum, 1956). In these latter two examples water current
            serves as an important auxiliary energy source by increasing nutrient availa-
            bility and exporting waste products. Within a range of slow currents for
            which flow is laminar (0.02 - 0.5 cm/sec), Westlake (1967) demonstrated that
            photosynthesis of submersed plants in the laboratory increased with increasing
            current velocity. However, the high velocities that occur during flooding
            of most streams and rivers would represent stressful and often quite damaging
            conditiens for macrophytes (Haslam, 1978).

            SUSPENDED SEDIMENTS AND THEIR EFFECTS ON SUBMERSED PLANTS

                 Suspended sediments have effects on submersed macrophytes in addition to
            those directly related to a reduction in available light. For example, the

                                                 5
<pb n="18" />

                composition of bottom materials in which plants are rooted depends on the
                balance between the rate of sediment supply and the rate at which sediments
                are carried away by currents. Current velocity, particle density, and par-
                ticle size are mainly responsible for this balance. These variables will be
                treated briefly prior to discussing the relationship between macrODhytes and
                sediment types.

                     The settling velocity of particles is classically described by Stokes
                law which is formalized as

                                              v                 gr2
                                                   9       n

                where v is the velocity of the particle (cm/sec), r is its radius (cm), g is
                acceleration of gravity, n is the viscosity of the f    Iuid (poises), and dl and
                d2 are the densities of the particle and fluid (g/cm ), respectively. If all
                other conditions are constant, then the settling velocity is directly propor-
                tional to the square of the particle radius, and the equation simplifies to
                v = Cjr2, where Cl represents the various constants.      This law does not hold
                for large particles because above about 0.1 mm in diameter the settling velo-
                cities are proportional to the square root of the radius according to the
                impact law.   The simplified form of the impact law is v = C2\l-r1 where various
                constants are included in C2.    Viscous forces operable in Stokes     law become
                negligible. Thus the settling velocity will follow the experimental curve
                shown in Figure 1.

                     In streams and lakes which have turbulent flow, particles are kept in
                suspension by kinetic energy that overcomes the gravitational and cohesive
                forces.  The relationship between velocity, particle size and the fate of the
                particles is shown by the HjDstrom scheme in Figure 2.       This graph incorpor-
                ates the critical erosion velocity in addition to the settling velocity which
                brackets the regions in which particles will be eroded, transported, or de-
                posited.  This conceptual model is based on a number of assumptions, few of
                which have much applicability to field situations where the flow velocity is
                stochastic and particles are seldom spherical, of homogeneous size, or of
                similar density.

                     Nevertheless, Figure 2 correctly conveys the concept of segregation of
                particle size with respect to flow velocity. In considering dredging acti-
                vities, fine particles with their slow settling velocities will remain in
                suspension longer and will tend to be transported greater distances than
                larger particles. Particles from sediments largely composed of organic mat-
                ter have lower densities and will have an even greater tendency to remain in
                suspension.   It becomes obvious that the duration of shading and the extinc-
                tion coefficient of the water will depend greatly on the composition of the
                material brought into suspension, whether by natural (floods) or by human
                activity (dredging or other disturbances).

                     Submersed macrophytes and other structural features may act as sediment
                traps because of their effectiveness in reducing flow velocity.      Growth of

                                                         6
<pb n="19" />

                                    20                        1
                                                    Stokes Low
                                  Z

                                  E  15

                                  0                Impact ow
                                     10-

                                                        Experimental
                                                              Curve

                                     5-

                                      0--
                                       0          0.5         1.0        1.5
                                                   Diameter (mm)

            Fi gure 1.  Comparison of settling velocities described by Stoke's law and the
            impact law. The experimental curve is also shown. From STRATIGRAPHY AND
            SEDIMENTATION, Second Edition, by W.C. Krumbein and L.L. Sloss. W.H. Freeman
            and Company.    Copyright 1963.

                               1000

                                                      Erosion

                                100

                            E

                                 10

                            CD
                                          Transportation
                            &gt;                                     Deposition
                                1.0

                                0.1-
                                  .001    .01      0.1        1.0      10       100
                                                     Diameter (mm)

            Figure 2.    The relationship between current velocity and particle size which
            determines whether particles will be eroded, transported, or deposited (after
            Hju'ilstrom, 1935).
                                                         L

                                                       7

             328-607 0 - 80 - 3
<pb n="20" />

                 rhizomes and roots in the sediment further stabilizes the substratum.     The res -
                 triction of vascular plant beds to the relatively low energy sectors of
                 streams contributes to the extremely patchy distribution that is often ob-
                 served.

                      In highly organic bottoms, such as in open water areas of peat bogs, the
                 soft ooze severely restricts the establishment of aquatic macrophytes.       Plant
                 beds are often restricted to species that produce dense and persistent net-
                 works of rhizomes, as in the case of members of the Nymphaeceae. Although
                 this family is characterized by floating leaves, some members have submersed
                 11water leaves" which may persist year round in the southeastern United States
                 (Brinson and Davis, 1976).   The importance of these leaves to the carbon
                 balance of the plant has never been established.

                      Not only must sediments be stable for successful colonization of macro-
                 phytes, but the particle size distribution' also influences the species that
                 occur.  Spence (1964) showed that in Scottish lochs submersed broad leaf forms
                 predominated in water greater than 150 cm deep only when the sediments were
                 composed of fine muds.   However, other factors such as light and turbulence
                 all change with depth, so it is not possible to single out substrate type as
                 the most important variable except perhaps by controlled experiments (Pond,
                 1903; Brown, 1913; Bourn, 1932; Misra, 1938).

                      Pearsall's (1920) work on the English lakes during the early part of this
                 century singled out the physicochemical. nature of the sediment as the main
                 factor in determining composition of the vegetation, although the original
                 interpretation of these results is somewhat questionable (Spence, 1967).
                 Isoetes was restricted to stony areas with thin silt.     This genus apparently
                 cannot colonize areas of sediment deposition because it cannot alter its root
                 level. Potamo2@@ perfoliatus grows in areas with a high clay fraction,
                 which ma@ -also be related to nutrient availability, rather than texture alone.
                 However, life forms with a stoloniferous habit are probably able to adjust to
                 changes in sediment depth except in the most extreme cases of accumulation.

                      Another aspect of siltation is the accumulation of material on leaf sur-
                 faces which reduces light transmission to photosynthetic surfaces and possi-
                 bly alters gas and nutrient exchange.    Sculthorpe (1967) suggested that the
                 linear leaves of Potamogeton pectinatus remain free of settling particles and
                 thus the species may colonize areas unsuited for submersed plants with leaf
                 forms more amenable to silt accumulation.     Schiemer and Prosser (1976) con-
                 finned that the sediment coating on Myriophyllum spicatum, which has finely
                 ,divided, feathery leaves, is markedly greater than for F_ pectinatus in
                 sheltered bays of Neusiedlersee, Austria.     In addition, they suggested that
                 silt deposition is enhanced by the presence of epiphytic algae on heavily
                 infested macrophytes.   Increased plant weight due to silt deposition was also
                 noted as having an inhibitory effect on macrophyte growth.      These factors in
                 addition to wave action appear to be largely responsible for the distribution
                 of M. spicatum in Neusiedlersee.

                                                         8
<pb n="21" />

              GROWING SEASON AND DORMANCY

                   Submersed macrophytes may resist the effects of freezing by colonizing
              depths below the zone of surface ice formation as compared with emergent or
              floating leaved species that are exposed to freezing temperatures.     In spite
              of this, many temperate submersed species undergo a period of dormancy during
              the winter and a few species are anatomically and physiologically adapted to
              overwintering.  However, a number of submersed perennials merely subsist with
              reduced or negligible growth rates and reduced biomass until more favorable
              light and temperature conditions at the onset of the growing season.      Except
              in cases of a limited number of annuals where viable seed development and
              favorable conditions for germination must occur, most submersed species are
              perennial and overwinter by means of vegetative structures.

                   Weber and Nooden (1976a, b) described the role of turions in the over-
              wintering of Myriophyllum verticillatum. In this species turions are special-
              ized compact buds that develop from nodes late in the growing season as a
              response to photoperiod and possibly temperature.    These reproductive struc-
              tures sink to the bottom after detachment from the parent plant.     Dormancy is
              broken by cold temperatures (0    40 C) which compares well with observed
              turion germination before ice breakup.

                   Apart from highly modified organs such as turions, many other less spe-
              cialized organs appear equally capable of overwintering.     These include dor-
              mant apices and offsets, root tubers, stolons, and rhizomes.     Dormant apices
              and offsets as well as turions   can be important in plant dispersal.     These
              structures in submersed aquatic plants appear to substitute for seed disper-
              sal more commonly found in emergent or floating leaved species.

                   In addition to lower water temperatures and reduced day length during
              the nongrowing season, the presence of an ice and snow cover in northern lati-
              tudes poses severe restrictions on light penetration.    Species that may have
              only reduced biomass and growth during the winter season if open water per-
              sists will be much reduced in more northern waters that become completely iced
              in.  This may result in regional differences in standing crops of submersed
              plants at the beginning of the growing season.

                   The presence of ice may also result in physical disruption of macrophyte
              communities'  Martin and Uhler (1939) described the scouring action of ice
              masses during spring breakup in flowing waters and shallow lentic habitats
              that may cause severe damage to beds of submersed plants.

              NUTRIENT AVAILABILITY AND UPTAKE

                   There has been considerable controversy concerning the importance of
              roots in nutrient uptake from sediments.    It is clear from a number of studies
              that roots do accumulate nutrients from the sediments and these may be trans-
              located to the shoots.   However, in many aquatic plants, significant ion
              absorption occurs by leaves and there appears to be a great    diversity in
              the relative importance of roots and shoots in mineral nutrition.
<pb n="22" />

                     Figure 3 illustrates the spectrum of all possible cases for aquatic
               macrophytes in which the x-axis represents a gradient in life form, root-
               shoot ratio, or anatomical complexity (Denny, 1972). Emergent species will
               obtain most of their mineral nutrition from the sediments, while those that
               have floating leaves are intermediate between emergent and submersed plants.

                     The actual amount of nutrient absorption in field situations may be re-
               lated also to the relative supply in the water and the sediments.          For exam-
               ple, Bole and Allan (1978) demonstrated that Myriophyllum spicatum and
               Hydrilla verticillata utilized phosphorus from the sedimenT @until concentra-
               tions in the water reach a threshold value which differs for the two species.
               Above these water concentrations uptake from the water column increases.
               Nutrient availability is not based solely on concentration since flowing
               waters of low concentration may actually be a better source of nutrients         than
               higher concentrations in quiescent waters.       Dense gyniths of submersed commu-
               nities often require nitrogen and phosphorus in excess of the amount present
               in the water at any one time. Sediment texture and cation exchange capacity
               may also be important in the nutrient supply to roots.        Since it has been
               demonstrated that some aquatic macrophytes translocate phosphorus (Twilley
               et al., 1977) and nitrogen (Nichols and Keeney, 1976) both from roots to
               shoots and from shoots to roots, it is unlikely that the sediments or the
               water alone are the singular source of nutrients.

                     Although nitrogen and phosphorus are generally believed to be the most
               important limiting nutrients in fresh waters, there are no clear cut cases
               where submersed macrophytes are excluded by the paucity of either.          Rather it
               would seem that the rate of    productivity may be limited by the supply of these
               nutrients.   However, in the   case of soft and hard waters (low and high CaCOI
               concentration, respectively) there appears to be an important dichot        omy in
               species distribution.      This has been reviewed extensively by Hutchinson (1975)
               who also noted that pH may play an important role in species distribution in
               soft waters.

               BIOLOGICAL FACTORS

                     The absence of submersed aquatic plants in fertile lakes and ponds has
               often been attributed to shading by dense populations of phytoplankton.          Jupp
               and Spence (1977a) reported an inverse correlation between biomass of P6tamo-
               geton filiformis, and open water chlorophyll a concentrations at certai6_tF`mes
               during a three-year study of Loch Leven, Scotland.        Scums of blue-green algae
               accumulating near the shore during Anabaena fl'os-aguae blooms intensified
               shading in macrophyte beds.     Jupp and Spence suggested that these algal blooms,
               apparently enhanced by high levels of phosphorus from cultural eutrophication,
               retard macrophyte growth by shading and possibly by producing elevated pH con-
               ditions.

                                                         10
<pb n="23" />

                                 100%,

                                                                    Shoot

                              4)

                              .0

                              0

                                                                       ,Root

                              Z       01              life form           -
                                       Emergent     root/shoot ratio      0 Submersed
                                       High                               io Low
                                       Complex       anatomy              o Simple

               Figure 3.    Schematic diagram of factors that contribute towards a tendency
               for nutrient absorption by roots or shoots.         Axes are arbitrary.      Modified
               from Denny (1972).

                    However, Phillips et al. (1978) set forth a convincing argument for the
               role of epiphytes and filamentous algae in suppressing submersed macrophyte
               growth due to shading.      They suggest that dense phytoplankton develops sub-
               sequently to the macrophyte decline rather than being its cause.             Although
               the effects of shading will be treated more fully in later sections, the
               scheme of Wetzel and Hough (1973) in the succession of littoral communities
               with increasing fertility (Figure 4) is of interest here.           According to this,
               nutrients are initially limiting to macrophyte productivity and growth is
               proportional to nutrient availability.        At high concentrations of nutrients,
               submersed macrophytes will be excluded due to shading by phytoplankton, epi-
               hytes, and filamentous algae. The model probably applies somewhat to flowing
               water situations although phytoplankton is expected to be less important and
               physical factors more important than in lakes.

                     The importance of grazing on submersed plants has never received a com.-
               prehensive review.      Repeated mowings during a single growing season may in
               some ways simulate high grazing intensities (Davis, in preparation). However,
               when one considers the total effect of consumer activity, the disruptive
               activity of feeding, whether on macrophytes or other food sources, may be
               quite substantial. For example, the feeding attivities of carp in Lake
               Mattamuskeet, North Carolina, increased turbidity so greatly that submersed
               waterfowl food plants did not become established until the fish were removed
               (Cahoon, 1953).
<pb n="24" />

                                  Shading of Submersed Macrophytes
                       Nutrient               by Epiphytes and
                    &gt;.. Limited                                           Phytoplankton
                                            Filament us Algae             Dominance

                    0
                    L_
                    CL                                                         Emergent
                                                                               *,Macrophytes
                    E
                    0-                                                     .,.--Phytoplankton
                    (D                                                        Epiphytes and
                    &gt;
                                                                                Filamentous
                    7@                                                           Algae
                    ir
                                                                              Submersed
                                                                                 Macrophytes
                       Ofigotrophic   meso-to Eutrophic          Hypereutrop
                                        Increasing Fertility

               Figure 4. Hypothetical changes in relative primary productivity of submersed,
               emergent, epiphytic, and planktonic communities withlincreasing nutrient en-
               richment (after Hough and Wetzel, 1973, with modifications.according to
               Phillins et al., 1978).

                     Many migratory waterfowl species are primary consumers and may have tem-
               porarily devastating effects on wetlands, oarticularly marshes that receive
               overgrazing by geese (Lynch, et al. , 1947) .    Muskrat "eat outs" have also been
               observed, but again it is the conspicuous emergent species studied that are
               reported to have heavy damage.     Anderson and Low (1976) studied grazing rates
               on Potamogeton pectinatus by ducks in the open water region of a Mantioba
               prairie marsh.   By comparing biomass in enclosures and in areas not excluding
               birds, they estimated that 40 percent of the peak standing crop of foliage and
               18 percent of the peak standing crop of tubers were removed.        Some of this re-
               duced biomass was not consumed but was lost by activities associated with
               feeding.   By comparison, Jupp and Spence (1977b) calculated that 30 percent of
               th.e peak standing crop of P. filiformis was removed by waterfowl grazing in
               Loch Leven.   In this case, only shoot biomass showed significant grazing los-
               ses, perhaps due to difficulty of uprooting tubers in the fine clay substrate.

               HYDROSTATIC PRESSURE

                     In exceptionally clear lakes of great depth, it would appear that there
               is adequate light for macrophyte growth at depths beyond the observed plant

                                                        12
<pb n="25" />

                           12-

                           10-                                ---&amp;-Nitella spp.
                                                              --o-Nascular Plants

                         (D
                           8-

                           7-
                         0
                         h_
                         (D 6-                       __0__0 - - - - - - -0

                           5-              0
                         -2            /0
                           4-         0-
                         E 3-         0

                             0    5      10    15    20      25   30
                                  I % Light Transmission Depth  (m)

           Figure 5. Depth maxima for aquatic vascular plants  and Nitella spp. as re-
           lated to the 1 percent light transmission depth for some-9-ri-ti-s-h lakes (data
           from Spence, 1976).

           distribution. At lower altitudes one atmosphere of excess pressure is equal
           to the pressure in about 10 m of water. Hence, depth maxima of species.tole-
           rant to low light levels suggest that hydrostatic pressure is a factor in
           depth limitation of plant-presence. As reviewed by Hutchinson (1975), charo-
           phytes, mosses, and the lower vascular plants tend to grow at greater depths
           than submersed angiosperms. R. G. Wetzel (quoted in Hutchinson, 1975) found
           that 0.5 atmosphere of excess pressure reduced photosynthesis in Najas flexi-
           lis by 50 percent. Spence (1976) compared depth maxima of vascular plants
           T-i"ncluding lower vascular plants ) with various nonvascular plants in 23
           freshwater lakes. Depth maxima summarized by Spence for submersed vascular
           plants and the non-vascular charophyte, Nitella spp., in some British lakes
           are plotted against the depth of 1 percent light transmission for each lake
           (Figure 5). Only the data collected by divers (as compared with from a boat)
           are used. The depth maxima for the vascular plants plateau at around 5.5 to
           6 m regardless of water transparency. This suggests that factors other than
           available light limited the depth maxima for vascular plants and a case for
           hydrostatic pressure effects is strongly suggested. Depth maxima for Nitella
           spp., on the other hand, increased throughout the range with increasing light
           penetration, suggesting light limitation as being of primary importance.

                Adaptations which result in resistance to hydrostatic pressure are un-
           clear. In laboratory experiments physiological and/or anatomical and growth
           changes in submersed angiosperms become apparent when hydrostatic pressures
           of 0.5 to 1 atmosphere excess are applied (Gessner, 1952; Ferling, 1957).
                                                       0

           One response is a decrease in the size of intercellular air spaces. Of
           course, environmentalfactors such as sediment characteristics, nutrient
           distribution, dissolved oxygen, temperature, and the quality of light reach-
           ing the bottom may play a part in restricting depth penetration by plants.

                                              13
<pb n="26" />

                            RESPONSES OF SUBMERSED PLANTS TO LIGHT AND TURBIDITY

                LIGHT ATTENUATIOii

                     The depth to which submersed aquatic vascular plants are distributed
                depends on the availability of light, if no other factors such as hydrostatic
                pressure, nutrient supply, substrate composition, an-d turbulence limit growth.
                Three factors affect light attenuation:   absorption by water itsel.f, absorp-
                tion by suspended particles, and absorption by dissolved substances.

                     Monochromatic light passing through chemically pure water is absorbed
                exponentially and thus decreases at a constant rate with increasing incre-
                ments of depth. This relationship can be expressed as the extinction coeffi-
                cient, ri, which decreases from the red to the blue end of the visible spec-
                trum. The extinction coefficient, q, is a function of the light intensity at
                the surface (10) and the intensity at depth z in meters

                                                  in I0 - in Iz

                                               T1=
                                                        z

                The extinction coefficients of  natural waters deviate greatly from those of
                pure water due to the presence  of dissolved and particulate substances which
                absorb and scatter light.   Th espectral quality of light is particularly af-
                fected by dissolved substances, since light scattering by particulate matter
                is relatively nonspecific in optical effects.

                     The extinction coefficient of natural waters is separated into three
                components such that

                                              nt = qw + np + nc

                where nt is the total extinction coefficient, and the remaining terms are
                due to water, suspended particulates, and dissolved color, respectively.
                Thus the expression can be rearranged so that the intensity of light, Iz, at
                a depth of one meter below I0 is

                                                      14
<pb n="27" />

                                                   -nw     _11p   _11c
                                          I z  i0e     x e     x e

                   Figure 6, adapted from the data of James and Birge (1938) for Lake
              George in Wisconsin, clearly shows the effects of absorption by each of these
              components.   (The data are graphed as percenti'le absorption, expressed by the
              formula, 100 (1    - jz)I0, where I    is at 1 m depth.) Short wavelengths (vio@
              let and blue) aroe most strongly a6sorbed by the dissolved material which most-
              ly consists of dissolved organic compounds, absorption of long wavelengths
              (red and infrared) is due mostly to water, while the particulate matter is
              quite nonspecific in its absorption properties, at least when in low concen-
              tration. However, in using sediment concentrations between 50 and 5,000 ppm,
              Otto and Enger (1960) found that the red wavelengths penetrated somewhat fur-
              ther than blue.    The spectral discrimination was greater for a commercial
              sodium base montmorillonite type bentonite than for a sediment obtained from
              a reservoir.   This indicates that although suspended sediments may have a
              negligible effect on spectral quality at low concentrations (&lt; 50 ppm),
              higher concentrations may cause significant shifts in the rel-itive penetra-
              tion of various wavelengths.     Selective absorption by algal pigments may
              occur in the blue (400-500 nm) and red (640-680 nm) when phytoplankton are
              dense, but the amount is minor when compared with the attenuation by particu-
              late matter throughout the visible spectrum (Westlake, 1966).

                   Vertical extinction coefficients then represent a composite of all wave-
              lengths and vary considerably among natural waters depending on the contribu-
              tion by suspended particulate and dissolved components.         Values of nt range
              from about 0.2 for exceptionally clear lakes such as Lake Tahoe, California,
              to values in excess of 10 where turbidity is extremely high such as for re-
              servoirs receiving inputs from flooding rivers (Wetzel, 1975; Westlake, 1966).
              Ice cover reduces light transmission to variable degrees depending on whether
              the ice is clear, contains bubbles, or is stained. Theoretically, clear ice
              transmits light better than natural water because the dissolved substances
              have been reduced.    However, snow cover reduces transmission considerably.
              Self shading by submerged macrophytes may be large depending on biomass, and
              as little as 0.1 percent of the surface light may reach the bottom of a river
              weed bed, mostly in the green wavebands (Westlake, 1966).

              RELATIONSHIP OF ACTUAL LIGHT TRANSMISSION AND
              SECCHI DISC TRANSPARENCY ESTIMATES TO THE EUPHOTIC ZONE

                   The euphotic zone is the region from the surface to the depth at which
              99 percent of the incident surface light has disappeared.         Work based on
              response of phytoplankton suggests that the      'intensity of light at this level,
              i.e., 1 percent of the surface light, represents the compensation light in-
              tensity at which photosynthesis and plant respiration are in balance.           For
              many of the studies we cite, only Secchi disc transparency depths are availa-
              ble.  It would be valuable to be able to relate the Secchi depth to the more

                                                       15

               328-607 0 - 80 - 4
<pb n="28" />

                 100
                                              Lake George                        otal Absorption
                                                                                Pure H20
                  80

                  60-
               n

               0

               CL
               U_ 40-
               0

                  20-
               a)                                                            Dissolved organic Color
               C)
                                                                          Suspended Particulates
                   400            500            600           700            800
                                       Wavelength (nm)

                Figure 6. Total absorption spectrum of water from Lake George, Wisconsin,
                as compared with spectra for pure water, dissolved   .'organic color, and sus-
                pended particulates. Modified from James and Birge (1938).
                theoretically sound measuremenis of extinction coefficient or percent light
                penetration at which the Secchi disc disappears.     Attempts have been made to
                do this (Poole and Atkins, 1929; Verduin, 1956; Cole and Barry, 1973).       As
                Hutchinson (1957) pointed out, the Secchi depth measurement actually is based
                on a comparison of the brightness of the disc and the water surrounding it.
                Thus light reflected from the bottom in shallow water or scattered upward by
                silt-laden waters can introduce considerable error.     Nevertheless, factors
                of 2.7 to 3.0 times the Secchi depth have been found to approximate the 1
                percent level in many cases (Cole, 1975).     Based on empirical evidence for
                coastal waters, Holmes (1970) suggested that a factor of 3.5 might be most
                appropriate in water with a Secchi depth of less than 5 m and a factor of
                2.0 for water with a Secchi depth between 5 and 12 m.     However, as will be
                discussed later, lower factors appear to be more appropriate for relatively
                clear waters.

                     Within a single lake, s  eston would be expected to have a greater corre-
                lation with Secchi depth transparency than dissolved colored compounds be-
                cause of greater seasonal change in suspended material.     However, between
                lakes, the confounding effects of varying color would weaken the relationship
                between Secchi depths and extinction coefficients.

                                                      16
<pb n="29" />

               DEPTH ZONATION AND TURBIDITY
               TOLERANCE OF SUBMERSED SPECIES

                     Species that become established and grow in the deeperregi       'ons.of aqua-
               tic ecosystems where only a small fraction of the surface irradiation remains
               are'better adapted,to survival at low levels of light than those-restricted
               to shallower and better illuminated zones. In shallow aquatic ecosystems
               where light is rapidly attenuated by high concentrations of suspended sedi-
               ments, the same species tolerant to low levels of light might be expected to
               compete more effectively than those requiring high light intensities. To
               test this hypothesis, the depth distributions for a number of species from
               diverse systems are presented graphically with their Secchi depth estimates
               ,in Figure 7. Available North American depth distribution records of submers-
               ed angiosperms have been tabulated for aquatic systems where water depths
               were such that the maximum depth of submersed plants would likely be limited
               by irradiance rather than the shallowness of the system. Some data are given
               for areas other than North America but an extensive search of world litera-
               ture was not made.

                     The variety of methods of collecting and reporting the data summarized
               in Figure 7 and the complexity and variations within and between the ecosys-
               tems put the comparative analyses to be made later within the realm of approx-
               imations.   Normally for each species the shallowest and greatest depths re-
               ported are shown along the depth axis and the area of greatest frequency,
               density, cover, or standing crop is indicated by cross-hatching.         No absolute
               values are given; only the relative occurrence of a species along the depth
               gradient is graphed.    These data were often given in the literature just as
               reported here, but in some cases we have given our best estimate.         For exam-
               ple, depth distribution may have been reported within range classes such as
               percent frequency at 0-1, 1-3 and 3-8 m (Rickett, 1922, 1924; Wilson, 1935,
               1941).   In this specific case the minimum depth would be graphed as 0.5 m if
               the species occurred at 0-1 m while the maximum depth would be graphed as
               5.5 m if the species occurred at 3-8 m.

                     Secchi disc depths as given in the    literature or as estimated from sub-
               marine photometer readings aregiven when available.         There are many problems
               associated with attempting to relate Secchi readings to the light environment
               of the plant. These range from the visual acuity of the observer to the
               necessity of using Secchi data taken at times other than the growing season
               for the plants in question.     One possible source of error in Secchi readings
               which doesn't appear to be serious is the lack of s-tandardization in disc
               size and contrast (white as compared with black and white) common in earlier
               studies.   Baker and Magnuson (1976) found no significant differences in trans-
               parencies in Crystal Lake, Wisconsin, as measured with a 20 cm black and
               white disc and a 10 cmwhite disc.

                     To convert occasional clear water light transmission data taken with a
               submarine photometer to Secchi depth transparency estimates, the depth (m) at
               the 1 percent light level was divided by 1.7.       This factor is consistent with
               observations by Wile (in preparation) for fresh waters in southern Canada and

                                                        17
<pb n="30" />

                            Figure 7.            Depth distributions for submersed macrophytes.

                                                      PLANT DEPTH RANGE                                                                SHALLO11 LIMIT KNOW, DEEP LIMIT
                                                      DEPTH RANGE OF HIGHEST ABUNDANCE (DENSITY,                                           UNKNOWN
                                                        FREQUENCY, BIOMASS, OR COVER)                                                  DEEP LIMIT KNOWN SHALLOW LIMIT
                                        - ------ I    ABUNDANCE ALONG DEPTH GRADIENT UNKNOWN                                               UNKNOWN
                                                      ESTIMATES BASED ON DEPTH INTERVALS OR                                        I   SECCHI DEPTH TRANSPARENCY
                                                        GENERAL STATEMENT
                                                                             Depth 0W
                                       Specles              0 1 2 3          4 5 6 7 8 9 10                           Locality                              Source
                             ALsmA GRAMINEUM VAR. SEYERI                                                  SALINE LAKES, ND                METCALF, 1931

                             APMORACIA AQUATICA                                                            GREEN LAKE, WI                  RICKETT, 1924
                             BIDENS BECKII                                                                 LAKE GEORGE, NY                 SHELDON A BOYLER, 1977
                                                                                                           TROUT LAKE, WI                  WILSON, 1941

                                                                                                           SOUTHERN ONTARIO LAKES          1. FILE, IN PREPARATION

                             CERATOPHYLLUM DEMERSUM                                                        WAKULLA SPRINGS, FL             MARTIN &amp; UHLER, 1939
                                                                           T4RO4M0MM/00704M0M                    GREEN LAKE, VI                  RICKETT, 1924

                                                                                                           LAKE BUNYONYI, UGANDA           PENNY, 1973

                                                               SOUTHERN ONTARIO LAKES          1. WILE, IN PREPARATION

                                                                                                           LAKE FURES6, DENMARK            SEIDELIN RAUNKIAER 9 BOVE PETERSON, 1917

                                                                                                           LAKE WEST OKOOJI, IA           CRUM A RACHMANN, 1973

                                                                                                           LAKE OPINICON, ONTARIO          CROWDER ET AL,,1977
                                                                                                           TROUT LAKE, WI                  WILSON, 1941

                                                                                                           LAKE FURE &amp; DENMARK             CHRISTENSEN 9 ANDERSON, 1958

                                                                                                           LAKE MENDOTA, WI                DENNISTON, 1921
                                                                                                           BIG SPIRIT LAKE,  ]A            CRUM &amp; BACHMANN, 1973
                                                                                                           LAWRENCE LAKE, MI               RICH ET AL. 1971
                             ELATINE MINIMA                                                                WEBER LAKE, WI                  POTZGER It VAN ENGEL, 1942
                                                                                                           MIRROR LAKE, NH                 MOELLER, 1975

                                                                                                           LAKE GEORGE, NY                 SHELDON &amp; BOYLEN, 1977

                             ELEOCHARIS ACICULAR IS                                                         MIRROR LAKE, RH                 MOELLER, 1975
                                                                                                           SOUTHERN ONTARIO LAKES          1. WILE, IN PREPARATION

                                                                                                           SILVER LAKE, WI                 WILSON, 1935

                                                                                                           TROUT LAKE, WI                  WILSON, 1941

                                                                                                           LAKE MUSKELLUNGE, WI            WILSON, 1935
                                                                                                           PAR POND, SC                    GRACE &amp; TILLY, 197F

                                                                                                           LITTLE JOHN LAKE, WI            WILSON, 1935
                             ELEOCHARIS PARVULA                                                            BACK BAY, VAJ CURRITUCK, NC     SINCOCK, 1965
                             ELODEA CANADENSIS                        711 Wr I 7m- LAKE GEORGE, My        SHELDON &amp; BOYLEN, 1977
                                                                                                           SOUTHERN ONTARIO LAKES          1. WILE, IN PREPARATION

                                                                                                           LONG LAKE, MN                   SCHMID, 1965
                                                                                                           LAKE FURES6, DENMARK            SEIDELIN RAUNKIAER 9 BoYE PETERSON. 191@

                                                                                                           LAKE WEST OKOBOJI, IA           CRUM 9 BACHMANN, 1973
                                                                            07/40747M04M,                       GREEN LAKE, WI                  RiCKETT, 1924
                                                                                                           TROUT LAKE, WI                  WILSON, 1941
                                                                                                           LAKE FURESO, DENMARK            CHRISTENSEN &amp; ANDERSON, 1958
                                                                                                           LITTLE JOHN LAKE, WI            WILSON, 1935

                                                                                                  Continued

                                                                                                           18
<pb n="31" />

                            Figure 7 (Continued)
                                                                                          Depth (m)
                                                 Species                 0 1 2 3          4 5 6 7 8             9 10                  Locality                                 Source
                                                                         0 413     1   1    . ,      I   , I -r--1
                                             CANADENS (CONT.)         -                                             LAKE EAST OKOBOJI, IA             CRUM   BACHMANN, 1973
                                                                                                                         LAKE CHAUTAUQUA, NY               NICHOLSON P, AROYO, 1975
                                                                            1 q3                                          BACK BAY, VAi CURRITUCK, 11C      SINCOCK, 1965
                                                                          ED       1                                     LAKE MENDOTA, W1                  DENNISTON, 192.1
                                                                         I.-qJq]                                           UPPER CAR LAKE, IA                CRUM 9 PACHMANN, 1973
                                                                         [::3                                            LAKE MINNEWASHTA, [A              CRUM 9 PACHMANN, 1973
                                     ERIOCAULON SEPTANGULARE                                                             SOUTHERN ONTARIO LAKES            1. WILE, IN PREPARATION

                                                                                                                         MIRROR LAKE, NH                   MOELLER, 1975

                                                                                                                         LAKE GEORGE, NY                   SHELDON 9 BOYLEN, 1977
                                                                                                                         WEBER LAKE, Wl                    POTZGER &amp; VAN ENGEL, 1942

                                     GRATIOLA AURE                      								MIRROR LAKE, NH                   MOELLER, 1975
                                                                                                                         SILVER LAKE, W1                   WILSON, 1935

                                                                                                                         TROUT LAKE, Wl                    WILSON, 1941

                                     GRATIOLA LUTEA F. PUSILLA                                                           WEBER LAKE, Wl                    POTZGER 9 VAN ENGEL, 1942

                                     HE TERANTHERA DUBIA                                                                  LONG LAKE, MN                     SCHMID, 1965

                                                                                                                         LAKE VEST OKOBOJI, [A             CRUM 9 BACHMANN, 1973

                                                                                                                         SOUTHERN ONTARIO LAKES            1, WILE, IN PREPARATION
                                                                           5 qMR/qm/qm/,                                   GREEN LAKE, Wl                    RiCKETT, 1924

                                                                                                                         LAKE GEORGE, NY                   SHELDON 9BOYLEN, 1977
                                                                             AE                                          LAKE EAST OKOBOJI, IA             CRUM 9 BACHMANN, 1973
                                                                           qiqr_-_----c q:q3                                 BIG SPIRIT LAKE, IA               CRum &amp; BACHMANN, 1973
                                                                                                                         LAKE CHAUTAUQUA, NY               NICHOLSON It ARoyo, 1975
                                     HYDRILLA VERTICILLATA                                                               SOUTH FLORIDA CANALS              BLACKBURN ET AL., 1969
                                                                                                                         CONWAY LAKE (SOUTH POOL), FL      NALL 9 SCHARDT, 1278 &amp; PERS, COMM.
                                                                            :     Z                                    LAKE BuNyomys, UGANDA             DENNY, 1973
                                                                                                                         CONWAY LAKE (WEST POOL), FL       NALL 9 SCHARDT, 1978 P, PERS. COMM,
                                                                                                                         CONWAY LAKE (EAST POOL% FL        NALL &amp; SCHARDT, 1978 &amp; PEns. Comm.
                                     JUNCUS PELOCARPUS                                                                   SOUTHERN ONTARIO LAKES            1, WILE, IN PREPARATION
                                                                                                                         SILVER LAKE, Wl                   WILSON, 1935
                                                                                                                         MIRROR LAKE, NR                   MOELLER, 1975

                                                                                                                         LAKE MUSKELLUNGE, Wl              WILSON, 1935
                                                                                                                         TROUT LAKE, W1                    WILSON, 1941
                                     JUNCUS PELOCARPUS F. SUBMERSUS                                                      WEBER LAKE, W                    POTZGER &amp; VAN ENGEL, 1942
                                     LITTORELLA AMERICANA                                                                LAKE FIOLEN, SWEDEN               TmuNMARK, 1931
                                                                                                                         LAKE MUSKELLUNGE, Wl              WILSON, 1935
                                                                                                                         TROUT LAKE, Wl                    WILSON, 1941
                                     LoBELIA DORTMANNA                                                                   WEBER LAKE, Wl                    POTZ61EA I VAN ENGEL, 1942
                                                                         =3                                              LAKE FJOLEN, SWEDEN               TWUNMARK, 1931

                                                                                                                         MIRROR LAKE, NH                   MOELLER, 1975
                                                                                                                         LAKE GEORGE, MY                   SHELDON &amp; BoYLEN, 1977

                                                                                                                         SILVER LAKE, WI                   WILSON, 1935
                                     MYRIqOPqHYLLUM ALTERNIFLORUM                                                          SOUTHERN ONTARIO LAKES            1. WILE, IN PREPARATION
                                                                                                                         LAKE GEORGE, NY                   SHELDON 9 BoYLEN, 1977

                                                                                                                         LAKE MUSKELLUNGE, Wl              WILSON, 1935
                                                                                                                         TROUT LAKE, Wl                    WILSON, 1941

                                                                                                       Continued

                                                                                                                19
<pb n="32" />

                            Figure 7 (Continued)

                                                                               Depth 0qW
                                       Species                0 1 2.q3 4 5 6 7 8 9 10                                      Locality                                Source
                            MYRIOPHYLLUM EXALBESCENS                                                           LONG LAKE, PIN                   SCHMID, 1965
                                                                                                               SOUTHERN ONTARIO LAKES           1. WILE, IN PREPARATION

                                                                                                               LAKE WEST OKOBOJI, 1A            CRUM &amp; BACHMANN, 1973
                                                                  17---  8qX --I                                BIG SPIRIT LAKE, JA              CRUM &amp; BACHMANN, 1973
                            MYRIOPHYLLUM HETEROPHYLLUM                                                        SOUTHERN ONTARIO LAKES           1. WILE, IN PREPARATION
                                                                                                               LAWRENCE LAKE, MI                RICH ET AL., 1971
                            MYRIOPHYLLUM SPICATUM                                                    1-11.5    LAKE TOWDA, JAPAN                JIMBqA ET AL- 1955
                                                                                                     1-14      LAKE VARANA, YUGOSLAVIA          GOLUBIC, 1963
                                                                                                               LAKE FURESO, DENMARK             SEIDELIN RAUNKIAER &amp; BOYE PETERSON, 1917

                                                                                                               SOUTHERN ONTARIO LAKES           1. WILE, IN PREPARATION
                                                                       FA                                      LAKE FUREs6, DENMARK             CHRISTENSEN &amp; ANDERSON, 1953
                                                                         F/qmqm/qm/                              PAR POND, SC                     GRACE &amp; TILLY, 1976

                                                                                                               CHESAPEAKE BAY OSYTER BEDS       STEENIS ET AL- 1962

                                                                                                               CURRITUCK SOUND, NC              DAVIS, IN PREPARATION

                            MYRIOPHYLLUM TENELLUM                                                              SOUTHERN ONTARIO LAKES           1. WILE, IN PREPARATION

                                                                                                               TROUT LAKE, W1                   WILSON, 1941

                                                                                                               WEBER LAKE, W1                   POTZGER &amp; VAN ENGEL, 1942

                                                                                                               LAKE GEORGE,-NY                  SHELDON &amp; BoYLEN, 1977

                                                                                                               LAKE MUSKELLUNGE, W1             WILSON, 1935

                            MYRIOPHYLLUM VERTICILLATUM                                                         GREEN LAKE, W1                   RiCKETT, 1924

                                                                                                               LAKES, U.S. &amp; CANADA             MARTIN &amp; UHLER, 1939
                                                                         q3 qC                                   LAKE MENDOTA, W1                 DENNISTON, 1921
                                                                                                               TROUT LAKE, W1                   WILSON, 1941

                            NAJAS FLEXILUS                                                                     LAKE GEORGE, NY                  SHELDON &amp; BOYLEN, 1977
                                                                                                               LAKE WEST OKOBOJI, 1A            CRUM &amp; BACHMANN, 1973

                                                                                                               WAKULLA SPRINGS, FL              MARTIN &amp; UHLER, 1939
                                                                                                               GREEN LAKE, W1                   RiCKETT, 1624

                                                                                                               SILVER LAKE, W1                  WILSON, 1935

                                                                                                               LAKE MUSKELLUNGE, W1             WILSON, 1935

                                                                                                               LONG LAKE, MN                    SCHMID, 1965

                                                                                                               TROUT LAKE, W1                   WILSON, 1941

                                                                                                               LAKE MENDOTA, W1                 DENNISTON, 1921
                                                                                                               SOUTHERN ONTARIO   LAKES         1. WILE, IN PREPARATION

                                                                                                               LAWRENCE LAKE, MI                RICH ET AL., 1971

                                                                                                               LAKE OPINICON, ONTARIO           CROWDER ET AL., 1977

                                                                                                               BIG SPIRIT LAKE, 1A              CRUM 9 BACHMANN, 1973

                                                                                                               LITTLE JOHN LAKE, W1             WILSON, 1935
                                                                  q3qE                                           LAKE CHAUTAUQUA, NY              NICHOLSON &amp; AROYO, 1975
                                                                                                               LAKE EAST OKOBOJI, 1A            CRUM 9 BACHMANN, 1973

                             NAJAS GUADALUPENSIS                                                               WAKULLA SPRINGS, FL              MARTIN &amp; UHLER, 1939

                                                                                                               LAKE OPINICON, ONTARIO           CROWDER ET AL., 1977
                                                                                                               BACK BAY, VA; CURRITUCK, NC      SINCOCKq, 1965
                                                                                                               PAR POND, SC                     GRACE &amp; TILLY, 1976

                                                                                                    Continued

                                                                                                            8qz8qo
<pb n="33" />

                          Figure 7              (Continued)

                                                                                       Depth (m)
                                             Species                 0      1   2  3   4 5 6 7 8 91o                                Locality                                   Source
                                                                            I   I   I   I    I   I    I    I   1 -1
                                    MAJAS GUADALUPENSIS (CONT.)                                                           PICKWICK RESERVOIR, AL              PEIIER &amp; WELCH, 1969

                                                                                                                          PAMLICO RIVER, NC                   DAVIS &amp; BRINSON, 1976
                                                                       WE                                                 BACK BAY, VAj CURRITUCK, NC         BOURN, 1932
                                                                          0qn                                               CONWAY LAKE (@OUTH POOL), FL        NALL &amp; SCHARDT, 1978
                                    NAJAS MINOR                                                                           PICKWICK RESERVOIR, AL              PELTIER 9 WELCH, 1969
                                    POTAMOGETON ALPINUS                          a=                                       LAKE.MENDOTA, Wl                    DENNISTON, 1921
                                    POTAMOGETON AMPLIFOLIUS                                                               SOUTHERN ONTARIO LAKES              1. WILE, IN PREPARATION
                                                                                                  LAKE GEORGE, NY                     SHELDON 9 POYLEN, 1977

                                                                                                                          GREEN LAKE, W1                      RICKETT, 1924
                                                                                                                          LONG LAKE, MN                       SCHMID, 1965
                                                                                                                          LAKE WEST OKOBOJI, IA               CRUM &amp; BACHMANN, 1973
                                                                         qm                                   LAKE MUSKELLUNGE, W1                WILSON, 1935
                                                                                                                          TROUT LAKE, W1                      WILSON, 1941

                                                                                                                          LAWRENCE LAKE, Ml                   RICH ET AL., 1971
                                                                                                                          LAKE MENDOTA, W1                    DENNISTON, 1921
                                                                                                                          SILVER LAKE, WI                     WILSON, 1935
                                                                                                                          LITTLE JOHN LAKE, W1                WILSON, 1935

                                    POTAMOGETON BERCHTOLDII                 I                                             MIRROR LAKE, NH                     MDELLER,1975
                                                                                                                          BACK BAY, VAj CURRITUCK, NC         SINCOCK, 1965

                                    POTAMOGETON CRISPUS                                                                   SOUTHERN ONTARIO LAKES              1. WILE, IN PREPARATION
                                                                                                                          LAKE WEST DKOROJI, IA               CRUM 9 BACHMANN, 1973
                                                                                                                          LAKE GEORGE, NY                     SHELDON 9 BOYLEN, 1977

                                                                                                                          LAKE CHAUTAUQUA, NY                 NICHOLSON 9 Am, 1975
                                    POTAMOGETON EPIHYDRUS                                                       SOUTHERN ONTARIO LAKES              1. WILE, IN PREPARATION
                                                                                                                          MIRROR LAKE, NH                     MOELLER, 1975
                                                                                                                       LAKE MUSKELLUNGE, W1                WILSON, 1935
                                                                                                                          TROUT LAKE, WI                      WILSON, 1941
                                    POTAMOGETON FOIOSUS                    49                                            GREEN LAKE, W1                      RICKETT, 1924
                                                                                                                          LAWRENCE LAKE,   MI                 WILSON, 1935
                                    POTAMOGETON GRAMINEUS                                                                 SOUTHERN ONTARIO LAKES              1. WILE, IN PREPARATION
                                                                                                                          LAKE GEORGE, NY                     SHELDON &amp; BOYLEN, 1977

                                                                                                mk                        SILVER LAKE, W1                     POTZGER &amp; VAN ENGEL, 1942

                                                                                                                          LAKE MUSKELLUNGE, W1                WILSON, 1935

                                                                                                                          TROUT LAKE, W1                      WILSON, 1941
                                                                        17-71                                             LAWRENCE LAKE, W1                   RICH ET At., 1971
                                                                                                                          LITTLE JOHN LAKE,    W1             WILSON, 1935
                                    POTAMOGETON ILLINOENSIS                                                               MONTEZUMA WELL, AZ                  COLE It BATCHELDER, 1969
                                                                                                                          LAKE GEORGE, NY                     SHELDON &amp; BOYLEN, 1977

                                                                                                                          SOUTHERN ONTARIO LAKES              1. WILE, IN PREPARATION
                                                                                                                          LAWRENCE LAKE, W1                   RICH ET AL., 1971

                                                                                                                          CONWAY LAKE (WEST POOL), FL         NALL &amp; SCHARDT, PqERSq. COMM.
                                                                                                                          LAKE WEST qOqKOBOJqI, IA               CRUM q&amp; BACHMANN, 1973

                                                                                                                          CONWAY LAKE (MIDDLE PqOOL)q,FL        NALL &amp; SCHARqDT, PERqSq. COMM

                                                                                                   Continued

                                                                                                            21
<pb n="34" />

                             Figure 7            (Continued)
                                                                             Depth (m)
                           Species            0 1 2 3 4 5 6 7 8 9 10            Locality                               Source

Potamogeton Illinoensis (cont.)                                             Conway Lake (South Pool),   FL Mall &amp; Schardt, Pers. Comm.
                                                                            Conway Lake (East Pool)     FL Mall &amp; Schardt, Pers. Comm.
Potamogeton Lucens									    Lake Fureso, Denmark        Seidelin Raunkiaer &amp; boye Peterson, 1917
												    Lake Mendota, WI	   	  Denniston, 1921
												    Lake Fureso, Denmark	  Christensen &amp; Anderson, 1958
Potamogeton Natans									    Green Lake, WI		  Rickett, 1924
												    Lake West Okoboji, IA	  Crum &amp; Bachman, 1973
												    Trout Lake, WI		  Wilson, 1941
												    Lake Muskellunge, WI	  Wilson, 1935
Potamogeton Obtusifolius						11.5            Trout Lake, WI		  Wilson, 1941
Potamogeton Pectinatus									    Lake Towda, Japan		  Jimbo et al., 1955
												    Lake Fureso, Denmark	  Seidelin Raunkiaer &amp; Boye Peterson, 1917
                                                            1-14		    Lake Varana, Yugoslavia     Golubic, 1963
												    Lake West Okoboji, IA	  Crum &amp; Bachmann, 1973
                                                                            Lake Sibayi, South Africa	  Boltt et al., 1969
                                                                            Southern Ontario Lakes      I. Wile, in preparation
                                                                            Long Lake, MN			  Schmid, 1965
                                                                            Green Lake, WI              Rickett, 1924
                                                                            Lake Bunyonyi, Uganda       Denny, 1973
												    Lake Fureso, Denmark	  Christensen &amp; Anderson, 1958
                                                                            Lake Mendota, WI            Denniston, 1921
                                                                            Big Spirit Lake, IA         Crum &amp; Bachmann, 1973
                                                                            Lake George, NY             Sheldon &amp; Boylen, 1977
												    Back Bay, VA; Currituck, NC Sincock, 1965
                                                                            Trout Lake, WI              Wilson, 1935
                                                                            Back Bay, VA; Currituck, NC Bourn, 1932
                                                                            Lake East Okoboji, IA       Crum &amp; Bachmann, 1973
												    Lake Chautaqua, IL		  Jackson &amp; Starrett, 1959
                                                                            Lawrence Lake, MI           Rich et al., 1971
                                                                            Upper Gar Lake, IA          Crum &amp; Bachmann, 1973
                                                                            Lake Minnewashta, IA        Crum &amp; Bachmann, 1973
                                                                            Lower Gar Lake, IA          Crum &amp; Bachmann, 1973
Potamogeton subsection                                                      Lake George, NY             Sheldon &amp; Boylen, 1977
Perfoliati (includes P.                                                     Lake Fureso, Denmark        Seidelin Raunkiaer &amp; Boye Peterson, 1971
Perfoliatus, P. Perfoliatus                                                 Southern Ontario Lakes      I Wile, in preparation
var. Bupleuroides and P.                                                    Lake Windermere, England    Pearsall, 1921
Richardson II)                                                              Green Lake, WI              Rickett, 1924
                                                                            Trout Lake, WI              Wilson, 1941
												    Lake Ullswater, England	  Pearsall, 1921
                                                                            Lake Fureso, Denmark        Christensen &amp; Anderson, 1953
                                                                            Lake Mendota, WI            Denniston, 1921
                                                                            Back Bay, VA; Currituck, NC Sincock, 1965
                                                                            Lake Derwentwater, England  Pearsall, 1921

                                    Continued

                                       22
<pb n="35" />

Figure 7  (Continued)

                                   Depth (m)

	Species                  0 1 2 3 4 5 6 7 8 9 10         	 Locality						Source

Potamogeton Subsection							Little John Lake, WI				Wilson, 1935
Perfoliato (includes P.							Lake Chautauqua, NY				Nicholson &amp; Aroyo, 1975
Perfoliatus, P. Perfoliatus						Lake East Okoboji, IA				Crum &amp; Bachmann, 1973
var. Bupleuroides and P.
RichardsonII) (cont.)							Back Bay, VA; Currituck, NC			Bourn, 1932
										Pamlico River, NC					Davis &amp; Brinson, 1976
										Upper Gar Lake, IA				Crum &amp; Bachmann, 1973
                     							Lake Minnewashta, IA				Crum &amp; Bachmann, 1973
        									Lake George, NY					Sheldon &amp; Boylen, 1977
      									Southern Ontario Lakes				I Wile, in preparation
                                                            Lake Windermere, England			Pearsall, 1921
     										Lake Ullswater, England				Pearsall, 1921
										Long Lake, MN					Schmid, 1965
										Lake Derwentwater, England			Pearsall, 1921
										Lake West Okoboji, IA				Crum &amp; Bachmann, 1973
                  							Trout Lake, WI					Wilson, 1935
										Big Spirit Lake, IA				Crum &amp; Bachmann, 1973
										Lawrence Lake, MI					Rich et al., 1971
										Lake Muskellunge, WI				Wilson, 1935
Potamogeton Pusillus							Lake George, NY					Sheldon &amp; Boylen, 1977
										Lake Ullswater, England				Pearsall, 1921
										Lake West Okoboji, IA				Crum &amp; Bachmann, 1973
										Lake Windemere, England				Pearsall, 1921
										Trout Lake, WI					Wilson, 1941
  										Lake Muskellunge, WI				Wilson, 1935
										Silver Lake, WI					Wilson, 1935
										Big Spirit Lake, IA				Crum &amp; Bachmann, 1973
										Little John Lake, WI				Wilson, 1935
										Wakulla Springs, FL				Martin &amp; Umler, 1939
										Lake East Okoboji, IA				Crum &amp; Bachmann, 1973
  										Upper Gar Lake, IA				Crum &amp; Bachmann, 1973
										Lake Minnewashta, IA				Crum &amp; Bachmann, 1973
Potamogeton Robbinsii                                       Lake George, NY					Sheldon &amp; Boylen, 1977
										Little Sissabogama Lake, WI			Steenis, Pers.-Comm.
										Southern Ontairo Lakes				I Mile, in preparation
										Trout Lake, WI					Wilson, 1941
										Lake Muskellunge, WI				Wilson, 1935
										Mirror Lake, NH					Moeller, 1975
										Lake George, NY					Sheldon &amp; Boylen, 1977
										Mirror Lake, NH					Moeller, 1975
										Lake Muskellunge, WI				Wilson, 1935
										Silver Lake, WI					Wilson, 1935
										Trout Lake, WI					Wilson, 1941
Potamogeton Strictifloius						Southern Ontario Lakes				I Mile in preparation

                                              Continued

                                                 23
<pb n="36" />

Figure 7  (Continued)

					Depth (m)
	Species			0 1 2 3 4 5 6 7 8 9 10	Locality				Source

Potamogeton zosteriformis					Long Lake, MN			Schmid, 1965
									Southern Ontario Lakes		I Mile, in preparation
									Lake West Okoboji, IA		Crum &amp; Bachmann, 1973
									Lake Mendota, WI			Denniston, 1921
									Green Lake, WI			Rickett, 1924
									Lake George, NY			Sheldon &amp; Boylen, 1977
									Big Spirit Lake, IA		Crum &amp; Bachmann, 1973
Ranunculus Aquatilis						Green Lake, WI			Rickett, 1924
									Southern Ontario Lakes		I Wile, in preparation
									Trout Lake, WI			Wilson, 1941
									Lake Bunyonyi, Uganda		Denny, 1973
									Lake George, NY			Sheldon &amp; Boylen, 1977
Ramunculus Longibostris						Lake West Okoboji, IA		Crum &amp; Bachmann, 1973
									Lake George, NY			Sheldon &amp; Boylen, 1977
Ruppia Maritima							Big Spirit Lake, IA		Crum &amp; Bachmann, 1973
									Back Bay, VA; Currituck, NC	Sincock, 1965
									Saline Lakes, ND			Metcalf, 1931
									Pamlico River, NC			Davis &amp; Brinson, 1976
									Back Bay, VA; Currituck, NC	Bourn, 1932
Sagittaria cuneata						Trout Lake, WI			Wilson, 1941
Sagittaria gramikea						Trout Lake, WI			Wilson, 1941
									Lake George, NY			Sheldon &amp; Boylen, 1977
									Mirror Lake, NH			Moeller, 1975
Sagittaria subulata						Back Bay, VA; Currituck, NC	Sincock, 1965
Scirpus acutus							Trout Lake, WI			Wilson, 1941
Scirpus subterminalis						Lawrence Lake, MI			Rich et al., 1971
Sparganium Angustifolium					Weber Lake, WI			Potzger &amp; Van Engel, 1942
									Mirror Lake, NH			Moeller, 1975
									Trout Lake, WI			Wilson, 1941
Subularia aquatica						Lake George, NY			Sheldon &amp; Boylen, 1977
									Lake Fiolen, Sweden		Thunmark, 1931
Utricularia cornuta						Lawrence Lake, MI			Rich et at., 1971
Utricularia gibba 						Lawrence Lake, MI			Rich et al., 1971
Utricularia intermedia						Lake Opinicon, Ontario		Craig, 1976
Utricularia minor							Lake Opinicon, Ontario		Craig, 1975
Utricularia purpurea						Mirror Lake, NH			Moeller, 1975
Utricularia resupinata						Lake George, NY			Sheldon &amp; Boylen, 1977
Utricularia vulgaris						Lake opinicon, Ontario		Craig, 1976
Vallisneria americana						Wakulla Springs, FL		Martin &amp; Uhler, 1939
									Lake George, NY			Sheldon &amp; Boylen, 1977
									Southern Ontario Lakes		I Wile, in preparation
									Lake West Okoboji, IA		Crum &amp; Bachmann, 1973
									Silver Lake, WI			Wilson, 1935

							Continued

							    24
<pb n="37" />

                         Figure 7 (Concluded)
                                                                            Depth W
                                                           0 1 2 3 4 5 6 7 8 9 10                                     Locality                                 Source
                                    Species
                         VALLISNERIA AMERICANA (CONT.)                                                      LAKE OPINICON, ONTARIO            CROWDER ET AL., 1977
                                                                                                            CONWAY LAKE (WEST POOL), F1       flALL P SCHARDT, PERS. CGM.

                                                                                                            DETROIT RIVER, M1                 HUNT, 1963
                                                                                                            BACK BAY, VA; CURRITUCK, ?K       SINCOCK, 1%5
                                                                                                            BIG SPIRIT LAKE, [A               CRUM &amp; BACHMANN, 1973
                                                                                                            CONWAY LAKE (EAST POOL), FL       NALL &amp; SCHARDT, PERS. COMM.

                                                                                                            GREEN LAKE, Wl                    PICKETT, 1924

                                                                                                            LAKE MUSKELLUNGE, W1              WILSON, 1935

                                                                                                            PAMLICO RIVER, K                  DAVIS &amp; BRINSON, 1976

                                                                                                            LAKE CHAUTAUOUA, NY               NICHOLSON Z AROYO, 1975
                                                                                                            LAKE EAST OKOBOJI, [A             CRUM 9 BACHMANN, 1973

                                                                                                            BACK BAY, VA; CURRITUCK, IC       BOURN. 1932
                                                             C:1                                            UPPER GAR LAKE, IA                CRUM &amp; BACHMANN, 1973
                          ZANN[CHELLIA PALUSTRIS                                                            fOWA GREAT LAKES, Ift.            CRUM &amp; BACHMANN, 1973

                         Wetzel (1975; Figures 5-9 and 5-16) for                                  Lawrence Lake, Michigan.                        Based on
                         studies of turbid waters in Back Bay, Virginia (Sincock, 1965), a conversion
                         factor of 2.5 was used for the data of Bourn (1932).

                                 Perhaps the maximum depths of plant growth recorded in Figure 7 are mom
                         reliable than other information given.                                   However, as discussed by Hutchinson
                         (1975) and Spence (1976), there can be problems in establishing the maximum
                         depth in a water body where a species is rooted and growing.                                                      Accuracy in-
                         cludes judgement as to whether a small number of plants (or the plant at a
                         depth are living and are truly rooted.                                   Ceratophyllum demersum is a special
                         problem because it does not form roots although a portion of the shoot often
                         becomes embedded in the sediment and thus "rooted" (Arber, 1920).                                                          Othe r
                         species such as Myriophyllum spicatum at times produce rooted floating shoot
                         fragments which normally sink with time.                                     Hence shoots of C. demersum and
                         other species may be carried to the deeper areas and sink -fo
                         where they may soon become moribund in the more stressful environment.                                                                Even
                         though some shoots were found as deep as 9.2 m, Spence (1976) set the macro-
                         phyte limit for Loch of Lowes at 3.9 m which was close to the 1 percent cover
                         line of 3.6 m.              The increasing use of SCUBA divers in studies of submersed
                         macrophytes should lead to more accurate data.

                                 Though the ecological importance of straggling plants surviving in the
                         lower depths is probably minimal , depth records suggest that there are
                         physiological limits due to hydrostatic pressure as discussed previously.
                         Hutchinson (1975), in summarizing depth records for a number of aquatic
                         macrophytes, rejected some of the published depths of colonization due to
                         problems mentioned above. He suggested that the record depth for submersed

                                                                                            25
<pb n="38" />

               freshwater angiosperms was for Potamogeton strictus Philippi, which is ap-
                                                 FT
               parently not found in North Ameri'ca (Shetler and Skog, 1978).     Tutin (1940)
               found this species at slightly deeper than 11 m in the high mountain Lake
               Titicaca, Peru-Bolivia. Recent data, especially those of Moel.ler (1975),
               Sheldon and Boylen (1977), and Wile (in preparation), all from the same geo-
               graphical area of northeastern United States and southeastern Canada, extend
               the depth ranges for a number of species native to North America.       Perhaps
               the most notable depth record is for Elodea canadensis which was found at one
               of the 12 m sampling stations in Lake George, New York (Sheldon and Boylen,
               1977).

               Relationship between Maximum Depth
               Distribution and Secchi Transparency

                    Figure 8 is a summary of depth records for 10 species in Figure 7 plot-
               ted as a function of Secchi depth.   Some trends are discernable and factors
               which may affect depth distribution will be discussed.     However, conclusions
               drawn from these graphs are necessarily tentative owing to the paucity of
               data points as well as problems already mentioned in interpretation of the
               original data.   The resistance of these species to environmental changes will
               be discussed further on page 44.

                    Since Eleocharis acicularis is a sedge, one might judge a priori that it
               is a shallow water species.    This is confirmed by depth records of 2 m or
               less for five of the seven data points (Figure 8a). The scattering of the
               points suggests that E. acicularis does not respond to water clarity in a
               predictable manner as do some other species that have increasing maximum
               depth distribution with increasing Secchi depth.     In fact, Wilson (1935)
               placed it in an ecological group of species (mainly rosulate) which becomes
               totally submerged only in response to changing lake conditions.      Other species
               placed in this group were Lobelia dortmanna, Juncus peloca*rpus,, and'GPatiola
               allrea. These relatively clear water species with generally shallow maximum
               deptF records (Figure 7) are probably limited in depth maxima by factors
               other than reduced irradiance. An affinity for sandy sediments which are
               normally characteristic of shallow areas subjected to fetch and water turbu-
               lence is one possible explanation for the depth distribution patterns.       Lab-
               oratory and field experiments have shown that certain species grow best when
               rooted in specific types of aquasoils (Pond, 1903; Brown, 1913; Bourn, 1932;
               Misra, 1938).

                    The relationship between depth maxima and Secchi depths for Potamo@eton
               praelongus (Figure 8b) can be considered representative of several species
               of this genus with a North American distribution, primarily in clear fresh
               waters in Canada and northern United States.     In addition to P. orciolongus
               these species i ncl ude P. , robbi ns i i $- P. zos teri f ormi s , P. amp I i fo I i us , and
               P
                  gramineus.   Since dep   maxima for this group are normally high, one might

               expect that these species would tend to survive under reduced light penetra-
               tion due to suspended particles.    However, some studies of long term changes
               in lakes where turbidity has increased show that these northern species tend
               to disappear or decrease in biomass (Volker and Smith, 1965; Lind and Cottam,

                                                       26
<pb n="39" />

                      15-                   15-                  15-
                                                     b                    C
                       10,                  10-                  10-

                                            5                    5-

                                                                                  1'5
                              5     0     15      5     10     15            0
                       15-                  15                   15-
                                d                                         f
                       10-                  10-                  10-

                                            5-                   5-

                     CL
                     0        5     10     1'5     5    10     15      5     10   15
                        15-                 15-                  15-
                     E                                h
                                9
                     E
                       10-                  10-                  10-
                        5-                   5                    5@
                               5    10     15      5     10    15       5    10    15
                                            15-

                                            10-

                                             5-

                                                   5     10     15

                                         Secchi Transparency (m)

              Figure 8. Maximum depth distribution of selected.species from Figure 7 plot-
              ted against Secchi disc transparencies of the waters where the distributions
              were observed: (a) Eleodharis acicUlaris, (b) PotaMogeton pra6 on_gus, (c) Najas
              flexilis, (d) Potamogeton pectinatus, (e) Myriophyllum spicatum, (f) *VaTlisne-
              ri a'arh6ti can a, ) Na'J'as U-6d-Alupensi (h             deme      (i) Tota-
                            _Fg         u          S,   ) 'Ceratoph,@' I um' Psum
              mogeton subsection Perfoliati, (j) Elodea CanaFensis.

                                                   27
<pb n="40" />

               1969; Stuckey, 1971; Nichols and Mori, 1971; Crum and Bachmann, 1973; Baumann
               et al., 1974; Bumby, 1977). Especially striking was the virtual elimination
               of the group with increasing human activities over 70 years in the vicinity
               of Put-In-Bay Harbor, Lake Erie, Ohio (Stuckey, 1971).       Though these species
               do well at the low light intensities of deeper waters, they appear to be re-
               stricted to rather narrow conditions which do not include highly turbid waters.

                     A number of physical and biological changes are likely to be associated
               with increasing turbidities from increasing suspended sediments.        These in-
               clude increasing inorganic nutrient levels and changes in biological compon-
               ents of the ecosystem.    Of 19 lakes and ponds of Southern Ontario studied by
               McCombie and Wile (1971), Potamogeton amplifolius was present only in the
               most oligotrophic impoundment.     Disappearance or reduced importance of northern
               species has also been associated with increased importance of other species,
               especially Myriophyllum spicatum (Lind and Cottam, 1969; Nichols and Mori,
               1971; Steenis, 1970) and perhaps P. crispus (Fassett, 1957; Stuckey, 1971;
               McCombie and Wile, 1971; McIntosh-et-al., 1978).       Both of these species have
               been naturalized from Europe.

                     Najas flexilis (Figure 8c) is found under a wider range of Secchi depths
               than the northern species. of Potamogeton.     This species principally has a
               northern distribution and does not range southward sufficiently to be consi-
               dered cosmopolitan.    There does not appear to be much tendency for an increase
               in maximum depth with increasing water transparency for the data points avail-
               able.

                     Compared with the three species just discussed, the remaining species all
               show some degree of linearity between Secchi transparency and maximum depth
               distribution (Figures 8d-8j).     All except Najas guadalupensis, which tends
               to have southern affinities, are strongly cosmopolitan. Potamogeton Vectina_
               tus grows well under a wide variety of conditions. This is consistent with
               the wide range of Secchi transparencies and depth records for the plant.         In
               their study of Canadian ponds and lakes, McCombie and Wile (1971) found P.
               pectinatus growing in waters of wide specific conductance range and spanning
               the complete range of Secchi transparencies from 0.9 to around 5.7 m.          This
               species may be found in waters high in suspended sediment and organic pollu-
               tion and is often rooted in silty sediments (Butcher, 1933; Hynes, 1970;
               Haslam, 1978; Ozimek, 1978). The essentially linear leaves have been obser-
               ved to be relatively free of the silt blanket which tends to cover submersed
               macrophytes in waters high in suspended sediments as discussed previously
               (McCombie and Wile, 1971; Schiemer and Prosser, 1976).

                     The data points for Ceratophyllum demersum (Figure 8h) indicate a dis-
               tribution at somewhat greater depths than other species for water bodies
               with Secchi depths less than 5 m.     This suggests a degree of shade tolerance
               for this species which will be discussed in more detail in a later section.
               For Elodea canadensis, the li-nearity between maximum depth and Secchi trans-
               M
                arency is remarkable.    The depth record of 12 m in Lake George, New York
                  eldon and Boylen, 1977) is the greatest depth reported for submersed
               angiosperms. Summer water temperature to 12 m ranged from 22-250C in Lake

                                                        28
<pb n="41" />

               George.  Light transmission to 12 m during the summer was about 10 percent
               of incident and the water column and sediments were aerobic at least to 12 m
               water depth. Thus, these factors probably were not limiting the maximum
               depth of growth.   Rather, hydrostatic pressure is important in limiting the
               depth of growth of deep water species in Lake George.     Elodea canadensis was
               somewhat more resistant to excess pressure than two other species studied by
               Ferling (1957).   A number of depth records in Figure 7 were for Lake George.

                    The linear pattern between maximum depth and Secchi transparency for
               Elodea canadensis differs from a tendency for some of the other species,
               such as   riop y      spicatum (Figure 8e), Vallisneria americana (Figure 8f),
               Najas  uadalupens.is. Figure TO, and the Potamogeton subsection Perfoliati
               group Nigure 8i), to reach a plateau at 6 to 7 m depth.       This plateau indi-
               cates that hydrostatic pressure rather than light availability controls maxi-
               mum depth distribution.

               Turbidity Tolerance Index

                    When the maximum depths for submersed angiosperms of Figure 7 are plot-
               ted against their Secchi depths, a linear relationship is apparent at shallow
               depths for most species (Figure 8). This suggests that, in the range of
               around 2.5 m Secchi depth or less, turbidity is an important factor affecting
               maximum depths of growth.   It follows that if a species is found in the deeper
               areas in water bodies with 2.5 m Secchi depths or less, the species would have
               a degree of turbidity tolerance.    More specifically, the higher the depth
               maxima to Secchi depth ratio in the turbidity-stressed systems, the higher
               the turbidity tolerance of the species. This ratio, along with related infor-
               mation, is given in Table 1 for species of Figure 8. Species with higher tur-
               bidity tolerance indices are better adapted for survival under conditions of
               low light transmission.

                    The absence of Potamogeton praelongus in the systems with turbidity stress
               typifies potamogetons that are mainly restricted to northern areas as discuss-
               ed previously.   The low ratio for Eleocharis acicularis is not surprising; the
               depth distribution of this species does not correlate with Secchi transparency.
               Elodea canadensis is apparently sensitive to turbidity, even though it may
               grow at great depths where turbidity is low.     Perhaps the effects of suspended
               particles on light quality reaching the plants is especially important for
               this species.  Of the remaining species Ceratophyllum demersum, Vallisneria
               americana, Najas guadalupensis, and P. pectinatus   'have th@7h_ighest turbidity
               tolerances. The mean depth of Ceratophyllum demersum is greater than that of
               other species (Figure 8).

                    Thus, it appears that not all species tolerant to low levels of light
               and growing at great depths in clear lakes will be successful in colonizing
               aquatic ecosystems of high turbidity.    These exceptions may be species that
               are sensitive to factors such as eutrophication, substrate type, siltation
               of leaves, or light quality, rather than just the quantity of light.* Never-
               theless, the turbidity tolerance index provides an approximation of the re-
               lative resilience of several species to turbidity stress and their rank is

                                                     29
<pb n="42" />

                  Table 1. Turbidity Tolerance Index for Selected Species of Figure 7
                         Expressed as the Ratio of Depth Maxima to Secchi Depth
                                  Where Secchi Depth is 2.5 m or Less

                                             No. Systems:     Total No.       Turbidity
                       Species               Secchi &lt; 2.5 m    Systems     Tolerance Index

              Potamogeton praelonqus                 0           11                 0

              Eleocharis acicularis                  2             7             0.5

              Elodea canadensis                      7           14              1.1

              Potamogeton s.s. Perfoliati            9           17              1.5

              Najas flexilis                         3           11              1.7
              Myriophyllum spicatuma                11           40              1.7

              Potamogeton pectinatus                 9           22              2-.0

              Naias guadalupensis                    5             6             2.2

              Vallisneria americana                  6           17              2.4

              Ceratophyllum demersum                 2           11              2.8

              aFrom data for lakes of southern Ontario, Canada.    Twe n ty - th re esampling
              stations were in Georgian Bay of Lake Huron with the rest from various
              other lakes (I. Wile, in preparation).

              supported by observations on distribution in nature.   Additional data are
              needed to firmly establish the relationship between submersed species and
              turbidity.

              PHOTOSYNTHESIS AND GROWTH IN RESPONSE TO LIGHT

                   It is'apparent from the foregoing discussion that some macrophyte spe-
              cies have an affini,ty for deeper or more turbid waters while others tend to
              be restricted to conditions of higher light intensities. It would be impossi-
              ble from the data given above to classify all species as being either shade
              tolerant or high light requiring although the approach used in Table 1 is prom-
              ising. Rather we will examine the experimental evidence available for a few
              species to see if it provides further insight to the possible light control of
              macrophyte zonation and turbidity tolerance.

                                                30
<pb n="43" />

                    Many of the experiments reported in the literature on apparent photosyn-
              thetic rates of submersed macrophytes relate to the light intensity at which
              the photosynthetic system is saturated. The lowest irradiance necessary to
              achieve the maximum rate of photosynthesis (saturation) provides a valuable
              point of reference for comparing species. Since the subject of this review
              is concerned more with the response of macrophytes to reduced levels of light,
              these experiments might appear irrelevant. However, where light is only oc-
              casionally limiting due to shading by high turbidity waters, the competitive
              advantage of species with a high photosynthetic efficiency may help to ex-
              plain their occurrence. Such conditions might occur in shallow zones of lakes
              or normally clear rivers with pulses in turbidity due to storms, dredging,
              high runoff, etc. Moreover, it might be proposed that species with high light
              saturation correspondingly have lower photosynthetic rates at low light levels
              and higher compensation points than shade adapted species.

                    The compensation point of light, i.e., where photosynthesis and respira-
              tion are in balance, should limit the lower depth distribution of a species.
              In laboratory and field experiments where compensation points are measured,
              they can only approximate depth distributions in nature. This is partly be-
              cause experiments are normally of short duration, while under natural condi-
              tions plants respond to a seasonal range of light conditions (turbidity, day
              length, solar angle, etc.). Furthermore, experiments are normally with active
              apical portions of plants and do not reflect respiratory utilization of photo-
              synthate by older stem portions and underground parts. There is another pro-
              blem with extrapolating low irradiance experiments in the laboratory to deep
              water conditions. In deep clear lakes, selective light absorption by water
              (red region) and by organic compounds (blue region) may be as great a factor
              in photosynthesis -as reduction in total irradiance (Figure 6). Research ap-
              pears to be lacking on this problem for aquatic macrophytes.
                    A further problem with interpretation of the experiments discussed below
              is the lack of consistency among experiments. For example, some workers re-
              port .light values as ill Iuminescence lux or foot candlesA while others more
              appropriately use irradiance (cal/cXhr or lleinsteins/m6-sec).          The two ex-
              pressions are not interconvertable because illuminescence does not take into
              account the variation in energy distribution of different spectral regions.
              Moreover, there are problems with differences in temperature, 1-ight source,
              enclosed biomass, and inorganic carbon availability both within and among
              experiments. Finally problems are associated with accumulation and utiliza-
              tion of oxygen in intercellular spaces (lacun@e) of submersed plants (Hartman
              and Brown, 1967). However, to the extent possible, these experiments will be
              discussed as they may relate to macrophyte depth zonation and turbidity toler-
              ance.

              Laboratory Experiments
                    In a series of laboratory experiments, Gessner (1938) determined light
              saturation for photosynthesis in six submersed species. Four of the species,
              Ceratophyllum demersum, Cabomba aquatica, Hottonia palustris, and Ranunculus
              aquatilis, appeared to saturate at approxi@_ately 10,000 to 40,000 T`ux. This
              low light saturation of C. demersum is in agreement with its high turbidity
              tolerance index.(depth maximum to Secchi depth ratio) reported in Table 1.

                                                     31
<pb n="44" />

               The other two species, Elodea crispa, and Potamogeton 'p6rfoliatus, did not
               saturate within the range oTlight i tensities used.       It would appear that
               particularly for P. perfoliatus light saturation was somewhat above 80,000
               lux. This concurs somewhat with the low turbidity tolerance index of Potamo-
               peton subsection Perfoliati of Table 1.     The photosynthetic curves for-C-.-
               demersum and C. aquatica appear to increase quite rapidly at the lower Tight
               intensities relative to the other species.

                    Boyd (1975) reported changes in net photosynthesis of 1 g samples with
               increasing light intensity.    Photosynthesis in three of the species reached
               light saturation at about 10,000 lux (Eleocharis acicularis, Elodea'dons-a,
               Naias flexilis), while light saturation occurred at 15,OOU-Iux for*Potamogeton
               sp. and 20,000 lux for Ceratophyllum demersum.      None of the species required
               more than-9,000 lux for@_5_0 percent of maximum photosynthesis.     These results
               are in general agreement with those of Gessner (1938) except for the two spe-
               cies that he reported which did not reach maximal photsynthesis.

                    Comparative photosynthetic rates for four species of submersed plants
               reported by Van et al. (1976) did n2t differ greatly in irradiance required
               for saturation (600-700 peinstein/m -sec), but light compensation points dif-
               fered substantiall,@.  The compensation point for Hydrilla Vortidflla@a was
               lowest, at 15 @E/m -sec, Cambomba caroliniana was highest at 55 jjE/          ec,
               while Myriophyllum spicat-u-m--an-c-FC-eratophyllum demersum were intermediate at
               35 pE/e*sec.    Even though H. verticillata and C. demersum had high to medium
               tolerances, respectively, @_o low light levels, _fheir maximal photosynthetic
               rates were higher than the other two species per unit of chlorophyll content.
               It would appear then that some species are adapted to a wide range of light
               conditions, being able to tolerate quite low levels of light and have high
               photosynthetic capacities at high light levels.      Since H. y6ftitillata exhi-
               bits th.is capability, this would explain its rapid biomass production under
               favorable light conditions as well as its large standing crops where self
               shading is high (Nall and Schardt, 1978). Carr (1969a, b) also characterized
               C. demersum as a shade plant, being adapted to low levels and saturating at
               'Ybout 40,000 lux in flasks and about 15,000 lux under artifical stream condi-
               tions.  She observed maximum photosynthesis in plants collected from 5 m
               depth in Lake Ohakuri, New Zealand, where light intensity was about 2 percent
               of surface.  However, Meyer and Heritage (1941) found this species to have
               maximum photosynthesis at the surface of Lake Erie.      As Carr (1969b) pointed
               out, this may have been a result of using only plants collected from the sur-
               face rather than intub-ating them at light intensities at the depth from
               which they were collected.    Further evidence for shade adaptation of Cerato-
               phyllum dermrsum relative to other species was reported by Guilizzon T_19_7_7T
               for @ake Wingra, Wisconsin.    Satration of photosynthesis occurred at 250
               jjE/m -sec compared with 800 -pE/m -sec for Myribphyllum
                    It is uncertain to what extent these short term (usually :S I hour) lab-
               oratory experiments, in which dissolved oxygen production rates are measured,
               are comparable to longer term studies in which growth is measured by increases
               in l.ength or biomass. For example, Blackburn et al. (1961) reported that
               Elddea d6nsa (-Planch) Caspary has a low light requirement and that long-term

                                                       37
<pb n="45" />

               growth (12 weeks under fluorescent lamps) was optimum at about 100 foot can-
               dles.  Above 125 foot candles rapid ch.lorosis and death occurred.        In com-
               parison,, 'H6tetarithere! dubia had a high light requirement with optimum inten-
               sity at 590 foot candles.    Long-term studies would appear to take into account
               differing abilities of species to adapt to light intensities.

                    A fundamental difference in response to light saturation by sun leaves
               (acclimated under high light intensity) and shade leaves was reported by
               Gessner (1938) for several species of submersed macrophytes. Proserpinaca
               palustris   Elodea crispa, and Potamooeton d6nsus all showed ligh.t saturation
               at about 40,000 lux or less, while the sun leaves continued to show a near
               linear photosynthetic response to increasing light abnve that -level.- Simi-
               larly, the emergent leaf form of the heterophyllous Proserpinaca palustris
               saturated at higher light levels than the submersed T-eaf form.        Th-is suggests
               that not only are there inherent differences in light saturation levels among
               species, but that there is considerable plasticity within a species depending
               on exposure to light conditions prior to experimentation.

                    This problem was addressed by Spence and Chrystal (1970a,b) who showed
               th at s o meIPotamogeton species had a greater reduction in photosynthesis with
               reduced irradiance than others.     There was a tendency for these species to
               be restricted to the more s4allow zones of lakes, and thus be less shade tole-
               rant, than the ones that underwent less reduction in photosynthesis.           The
               mechanism for the adaptation of shade species appeared to be related to lower
               leaf respiration and reduced thickness under low light conditions.         These fea-
               tures allow net photosynthesis to continue under low irradiance. In contrast,
               the thicker leaves of sun species were more efficient at higher irradiances.
                    Otto and Enger (1960) maintained submersed plants (Potamogeton pectina7
               tus, P. nodosus, Elodea canadensis) in tanks with varying concentrations of
               suspended sediment.    Growth reductions relative to controls were 20 to 40 per-
               cent for sediment concentrations of 50 ppm.       The amount of growth reduction
               was approximately linear with increasing sediment concentrations up to about
               1,250 ppm. Abnormalities in growth at higher concentrations over the 4-week
               growth period included elongation of stem internodes and chlorosis of stems
               and basal submersed leaves.     Potamogeton pectinatus appeared to be less tole-
               rant to suspended sediments than the other two species and the authors attri-
               bute this  to its lower leaf area. This does not agree with field observations
               (Table 1,  Figure 7) which show it to be reasonably turbidity tolerant.
                    This  research is of interest because it is the only known study in which
               suspended  sediment concentrations have been manipulated to observe growth
               responses  of submersed vascular plants.     However, the authors conclude that
               "Sediment  concentrations greater than 1,250 ppm would be necessary to cause
               plant growth reductions that might be considered critical to the plants'
               ability to survive." This statement should not be taken out of the context
               of the experiments conducted and applied to field conditions.         First, the
               plants were growing in only about 60 cm of water and the results may apply
               only to shallow water conditions. Secondly, and probably more importantly,
               many natural stands of submersed vascular plants may be living near their

                                                        33
<pb n="46" />

              limit of tolerance owing to physical, chemical, and biological factors discus-
              sed previously.   Additional light reduction due to turbidity, even at the low-
              est levels of suspended sediment used in these experiments (50 ppm), may ex-
              ceed the threshold of tolerance for plant communities already subjected to
              other stress factors.

              Field Studies

                   Few field studies have been conducted to determine photosynthetic and
              growth responses to light.    Because of the problems discussed above as well
              as changing light conditions with.in th-e course of a day or season, such experi-
              ments are difficult to interpret and extrapolate to field observations of
              plant distribution. However, they may have some value in a relative sense
              when several species are compared. For example, Meyer et al. (.1943) determined
              the compensation depth for shoot tips of several submersed species in Lake
              E ri e. Compensation light intensity was approximately 1.percent of the inten-
              sity measured at 5 cm depth for Elodea canadensis, 'POtamogeton ri.thaedtohii ,
                                                                  he value for'N6jas fl'
              Vallisneria aTfioricana and Het.6rahthera                                  exi Tis
              was somewhat higher at 2.6 percent.

                   Photosynthesis and respiration in    submersed species were studied in de-
              tail by Ikusima (1965, 1966, 1967).     He showed that photosynthesis decreased
              progressively from the upper part to the basal parts of community of'Potamo-
              geton crispus and Vallisn6,@ia 6siatica Miki, but that respirati.on was fairly
              constant throughout (Ikusima--l9--6-5T.-Pliotosynthesis in Val 1 ignetia 'deriseser-
              rulata Makino beds varied considerably from day to day depending o-n w-Fe-t-Fe-r
              tT-eweather was clear, cloudy, or rainy (.Ikusima, 1966). As expected, the
              compensation depth may vary from hour to hour during the day even though the
              light compensation point of the plants may remain constant. Monthly di ffe-
              rences occurred as well (Ikusima, 1967). Interception of light by apical
              portions of shoots, which reduces considerably the rate of photosynthesis of
              lower portions, results in a large respiratory demand where biomass is general-
              ly largest.   Communities during overcast or-rainy days may even have a negative
              net organic matter budget.

                   The results of the laboratory experiments and field studies cited above
              demonstrate a number of problems in interpreting instantaneous measurements of
              photosynthesis.   Species can be ranked according to their capacity for photo-
              synthesis at light saturation intensities, whicli-m&amp; be indicative of their
              success in competition if light saturation intensitie       , rsist under field
              conditions. However, distribution and abundance of aquati macrophytes in
              nature reflects the totality of the forcing functions that affect macrophyte
              growth, of which light is just one.

                                                       34
<pb n="47" />

                            RESPONSES OF SUBMERSED PLANTS TO ECOSYSTEM CHANGE

             COMMUNITY RESPONSES TO
             THE DURATION OF PERTURBATION

                  Factors affecting the growth and distribution of submersed plants h.ave
             been discussed.   In this section one example of short term and one of long
             term fluctuations in macrophyte community structure as related to natural and
             anthropogenic stresses will be considered.    Short term changes described in
             the literature are often related to a single perturbation with observations
             before and after the event.   The observation period may vary from weeks as for
             the study by Bilby (1977) of changes in cover and areal distribution of plants
             fo-11-bwing a stream spate to several years as for changes in macrophyte commu-
             nities in the Currituck Sound of North Carolina (Davis, in preparation).     On
             the other hand, some lakes in the northern U.S.A. have been studied sporadical-
             ly for over a half century as for the Iowa Great Lakes (Crum and Bachmann,
             1973)

                  Compared with responses of submersed plants to short term perturbations
             which are usually associated with increased suspended sediment loads in the
             water, long term changes in plant responses generally are harder to relate to
             specific environmental changes. Although changes in parameters with time,
             such as water transparencies, have been found during long term studies, short
             term changes associated with meteorological conditions and human activities
             such as dredging may be generally more important in effecting changes in sub-
             mersed plant populations.

             Short Term Perturbations

                  The observations of Steenis (1947) of the decimation of submersed macro-
             phytes in Reelfoot Lake, Tennessee, following heavy rains in June 1945 repre-
             sent changes which may occur following a single perturbation.     The water level
             rose and extensive siltation resulted from erosion of hills around the lake.
             Steenis' observations are summarized in Table 2.    Cer6toohyllum demersum was
             the dominant submersed plant before the rains, but it appeared to succumb to
             increased turbidity and wind action that resulted from increased fetch as the
             water level rose.   Lind and Cottam (1969) reported that C. demersum in Univer-
             sity Bay of Lake Mendota, Wisco'nsin, "anchors poorly" in thi@ -sediment and was
             restricted to areas of low turbulence.    Potamogeton pectinatus remained fairly
             abundant in places through 1945, but was limited in part by feeding activities
             of carp and competition with filamentous algae. In 1946 growth of P. pectinatus

                                                    35
<pb n="48" />

               Tabl e 2.  Changes in Submersed Macrophyte Populations in Reelfoot Lake, Tennessee (Steenis, 1947)

                 Species                  Before 1945                      Summer 1945                          1946

               Ceratophyllum        Dominant, many pure            Fragmentary, but some exten-        Showing definite signs
               demersum             stands                         sive beds in sheltered areas        of again dominating

               Najas                Limited and suppressed'        Scattered, sparce in much of        Extensive cover in some
               guadalupensis        growth in C. demersum          lake                                areas

               Potamogeton          Pyomirent in some areas        Scattered, sparce in much of        Extensive cover in some
               pusillus             but dominated by C.            lake                                areas; more aggressive
                                    demersum later in season                                           than N. guadalupensis

               Zannichellia         Suppressed fragmentary         Scattered, sparce in much of        Extensive growth; expan-
               palustris            growth in limited areas        lake                                sive and pioneer

               Potamogeton          Scattered small beds           Scattered, sparce in much of        Spots of existing growth
               nodosus.                                            lake; set back and slightly         increased about 10-fold
                                                                   redistributed

               Potamogeton          Scattered small beds;          Scattered, sparce in much of        Luxuriant beds in much
               pectinatus           greater range than P.          lake; fairly abundant in one        of the lake; largest
                                    americanus                     area but set back by carp           stands ever observed by
                                                                   action and overcovering with        Steenis for lake
                                                                   algae

               Heteranthera         Limited; two areas                                                 Plants that had survived
               dubia                                                                                   were slowly recovering

               Cabomba              Limited;  one area             Set back                            New growth in an addi-
               carolinana                                                                              tional area

               Utricularia          Mainly in shallow   'Nuphar                                        Continued without change
               vulgaris             advena dominated areas
<pb n="49" />

              was more luxuriant and widespread than Steenis had ever observed before.      Ac -
              celerated growth of Najas guadalupensis and P. pusillus also occurred in some
              regions of the lake Tn-1946.   Pdtam6geton pusillus was described as the more
              aggressive of the two.   Corat6phyllum .derheMum came back strongly in the year
              following the rains while there was a dramatic growth and spread of Zamnichel-
              lia palustris.  Here then, virtual elimination of the dominant species lgd to
              cf5-nging niches with rapid recovery and increase in macrophyte diversity.

                   Other short term changes in species composition and/or biomass have been
              reported for a shallow pond (Stuckey, 1971), a shallow flood plain (Jackson
              and Starrett, 1959), a lake (Oglesby et al., 1976), and a resevoir (Peltier
              and Welch, 1970). In all of these cases an increase in suspended sediment
              turbidity resulted from changing environmental conditions.

              Long Term Perturbations

                   Perhaps the most significant study of long term changes in submersed
              macrophyte populations was that by Crum and Bachmann (1973) of lakes of the
              Iowa Great Lakes region. They took advantage of an excellent opportunity to
              compare the submersed macrophytes in six essentially contiguous lakes which
              now have varying trophic states. The time element was added to their analy-
              sis through reference to three other studies, the first of which was in 1894.
              Depth distributions of submersed macrophytes given by Crum and Bachmann are
              included in Figure 7. These data were especially valuable in establishing
              relationships at the low end of,the Secchi depth scale for Several individual
              species (Figure 8).

                   At the time of the study in 1972, some of the northern potamogetons re-
              mained in Lake West Okoboji (4.2 m Secchi depth) and Big Spirit Lake (2.8 m
              Secchi depth) while none were in the more turbid lakes (0.7 to 1.6 m Secchi
              depth range) (Table 3). Generally, the species remaining in the more turbid
              lakes were those which have been established as turbidity tolerant (Table 1);
              however, these same species also were found in the clearer lakes.     These
              observations suggest that the northern potamogetons are sensitive to turbidity
              and associated changes while plants that are turbidity tolerant may thrive
              under a variety of conditions.

                   The initial invasion of the clearest lake by Potamogeton crispus is sur-
              prising since it often grows in disturbed or eutrophic systems.     Migration to
              the other lakes might be expected in the future.     The presence of Ruppia mari-
              tima in only one lake is also of interest.    Since this species is normal y
              To-und in saline environments (cf. Metcalf, 1931), a comparison of salinities
              of these lakes would be of interest.    Indeed, further analyses of physical and
              biological parameters that might affect the distribution of submersed macro-
              phytes seems warranted.

                   Based on a study in which macrophytes of Lake East Okoboji were compared
              with a 1915 survey, Volker and Smith (1965) suggested a number of factors res-
              ponsible for the observed changes.    These include nutrient enrichment from
              agricultural runoff and from sewage leading to algal blooms, siltation from

                                                     37
<pb n="50" />

Table 3.  Long Term Changes in Macrophyte Populations in Lakes and Bays in Northern United States.
__________________________________________________________________________________________________
                                                Iowa Great Lakes (1894-1972)
__________________________________________________________________________________________________
		Lake West Okoboji				Big Spirit Lake			Lake East Okoboji			Gar (three lakes)

Long term	Ceratophyllum denersum			Ceratophyllum Denersum		Ceratopyllum denersum		Ceratophyllum denersum
presence:	Elodea canadensis				Myriophyllum exalbescens	Elodea canadensis			Elodea canadensis
		Heteranthera dubia			Najas flexilis			Heteranthera dubis		Potamogeton pectinatus
		Myriophyllum exalbescens		Potamogeton amplifolus		Najan flexilis			P. richardsonii
		Najas flexilis				P. fflinoiensis			Potamogeton pectinatus		Vallisneria canadensis
		Potamogeton amplilolfus			P. nodosus				potamogeton sub sec
		P. natans					P. pectinatus			Pusilli
		P. nodosus					P. praelongus			P. richardsonii
		P. pectinatus				Potanogeton sub. sec		Vallisneria americana
		P. praelongus				  Pusilli				Aannichellia palustria
		Potanageton sub. sec.			P. richardsonii
		  Pusilli					P. zosteriformis
		P. richardsonii				Ruppia Maritina
		P. zosteriformis				Vallisneria americana
		Ranunculus longiristris			Zannichellia palustris
		Vallisneria Americana
		Zannichellia palustris

Disappeared:Megalodonta (Bidens) beckii		Elodea canadensis			Potamogeton amplifolius		Heteranthera dubia
		Potamogeton diversifolius		Megalodonta (Bidens) beckii	P. diversifolius			Hippuris vulgaris
		P. epihydrus				Potamogeton diversifolius	P. epihydrus			Megalodonta (bidens) beckii
		P. gramineus				P. epihydrus			P. gramineus			Myriophyllum exalbescens
								P. natans				P. natans				Najas Flexilis
													P. nodosus				Potamogeton amplifolius
													P. praelongus			P. natans
													P. zosteriformis			P. nodosus
													Megalodonta (Bidens) beckii	P. praelongus
													Myriophyllum exalbescens
													Ranunculus longirostris
Recent	Potamogeton crispus		Heteranthera dubia								Potamogeton sub. sec.
Adventive:	P. illinoiensus									 					  Pusilli
																		Zannichellia palustria
<pb n="51" />

0

                           Table 3 (Concluded)

                                          Put-in-Bay Harbor, Ohio                University Bay, Wisconsin               Green Lake, Wisconsin                Lake Wingra, Wisconsin
                       Condition               (1898-1967)                               (1912-1965)                        (1921-1971, 74)				(1929-1968, 69)

                       Long term          Butomus umbellatus                     Ceratophyllum demersumb                 Ceratophyl Ium demersum a            Certophyllum demersum
                        presence:            f. vallisnerifolius                 Elodea canadensis                       Chara sp.                            Elodea canadensis
                                          Ceratophyllum demersum                 Heteranthera dubia                      Elodea canadensis                    Heteranthera dubia
                                          Heteranthera dubia a                   Najas flexilis                          Heteranthera dubia		        Myriophyllum spicatum
                                          Myriophyllum exalbescens               Myriophyllum exalbescens b              Najas flexilis                       Najas flexilis
                                          Potamogeto berchtoldii                 Potamogeton crispus                     Potamogeton amplifolius              Potamogeton crispus
                                          P. crispus                             P. pectinatus                           P. gramineus                         P. foliosus
                                          P. pectinatus a                        P. richardsonii                         P. natans				        P. natans
                                                                                                                                                              P. pectinatus
                                          P. richardsonii a                      P. zosteriformis                        P. pectinatus                        P. richardsonif
                                                                                                                         P. richardsonii                      P. zosteriformis
                                          Vallisneria americana b                Vallisneria americana ab
                                          Zannichel1ia palustris                 Zannichellia palustris
                                                                                                                         P. zosteriformis                     Ranunculus longirostris
                                                                                                                         Ranunculus sp.                       Utricularia vulgaris
                                                                                                                         Zallisneria americana

                       Disappeared::      Elodea canadensis                      Potamogeton amplifolius a               Myriophyllum verticillatum           Potamogeton amplifolius
                                          Megalodonta (Bidens) beckii            P. illinoiensis                           var. pectinatum (perhaps) a        P. friesii
                                          Najas guadalupensis a                  P. natans                               Potamogeton foliosus		        P. illinoiensis
                                          N. flexilis a                          P. nodosus										        P. parelongus
                                          Potamogeton amplifolius                P. praelongus                                                                Vallisneria americana a
                                          P. filiformis
                                          P. foliosus
                                          P. friesii
                                          P. gramineus
                                          P. natans
                                          P. nodosus
                                          P. perfoliatus var.
                                            bupleuroides
                                          P. praelongus

                                          P. pusillus
                                          P. zosteriformis a
                       Recent             Elodea nuttallii                       Potamogeton foliosus                    Myriophyllum spicatum b               Myriophyllum spicatum
                       Adventive                                                                                         Potamogeton friesii                      (perhaps)
                                                                                                                                                               Potamogeton crispus
                                                                                                                                                                   (perhaps)

                       a Dominant or subdominant in an earlier survey.
                       b Dominant or subdominant at last survey.
<pb n="52" />

               agriculture, the use of algicides, depletion of dissolved oxygen, and fluctua-
               tions in water level. Crum and Bachmann (1973) suggested that stabilization
               of water levels in some of the lakes may have led to the demise of certain
               submersed macrophytes. The possible effects of herbicides in areas of intense
               agriculture might also be considered (Correll et al., 1978).

                    Long term changes in submersed plants for other systems are summarized in
               Table 3.  Environmental degradation associated with shoreline development and
               intense use of waterways in and around Put-in-Bay Harbor on an island in Lake
               Erie, Ohio, led to a drastic decrease in macrophyte presence and density over
               70 years (Stuckey, 1971).     Changes in the s:ubmersed macrophyte community over
               the period described by Stuckey did not begin with a pristine environment
               which gradually deteriorated due to human activities.        Poor agricultural prac-
               tices on the island apparently contributed to a heavy suspended sediment load
               in the study area from the beginning of the studies (Thorndale, 1898 as cited
               in Stuckey, 1971). Stuckey cites Pieters (1901) as reporting that in 1898 it
               was impossible to see plants in one area deeper than around 0.6-0.9 m due to
               turbi di ty. Stuckey suggested that the demise of the plants over time was re-
               lated to a combination of warming of the water, increased turbidity and de-
               creased dissolved oxygen.     Increased turbulence due to extensive bulkheading
               along the shore could also have been a factor.        In the Pamlico River, North
               Carolina, submersed macrophytes were usually limited to a narrow band in the
               deeper part of the littoral where bulkheading was extensive (Davis and
               Brinson, 1976).

                    Lind and Cottam (1969) studied the submersed macrophytes of University
               Bay of Lake Mendota, Wisconsin in 1966 and compared their results with those
               of surveys made by Denniston (19Z1), Rickett (192-2), and others.          Compared
               with Rickett's data, they found that the biomass of"M ribohyllurn bxAlbescens
               (or perhaps M. spicatum) had increased drastically              that of Vallisneria
               americana decreased and Ceratophyllum demersum increased (Table 3).           Lind
               and Cottam (1969) suggested that eutrophication caused the changes observed
               in University Bay.

                    Bumby (1977) compared the submersed macrophyte biomass and distribution
               in Green Lake, Wisconsin, in 1971-1974 with that in 1921 (Rickett, 1924). As
               for University Bay (Lind and Cottam, 1969), Myrioehyllum (M. spicatum) had
               become dominant, but relative biomass (46 percent) was not as great as for
               M. exalbescens in University Bay. Secchi depths were not given, but Bumby
               suggested thaf- there had been no change measured in light transmission since
               1942.  However, decreased light due to suspended sediments and seston in the
               littoral and the abundance of the filamentous alga, Cladophora,sp., were sug-
               gested as possible factors affecting the observed changes.

                    Lake Wingra is yet another Wisconsin lake in which Myriophyllum has be-
               come dominant (Nichols and Mori, 1971).       The relative frequency of M. Spica
               tum was 68 percent and no other submersed species had a relative frequency
               of over 10 percent.    The authors suggested that the lake was once dominated
               by potamogetons and Vallisneria americana. The species found in 1968-1969
               were much the same as described for the other Wisconsin lakes (Table 3).

                                                         40
<pb n="53" />

              Vallisneria amoritana and northern potamogetons had disappeared by 1929, as
              ,determined-b7a -herbarium survey.   Carp were introduced in the late 1800's
              and practically eliminated submersed macrophytes from the lake from the 1920's
              through 1955 (Baumann et al., 1974).    The demise of V. arnOPicana was associated
              with the carp infestation, apparently due to increas-ed turbidity.

                  All the long-terTned changes described thus far are for northern lakes
              which apparently had no rosulate populations typical of sandy, shallow areas
              of some northern oligotrophic lakes.    Flora of such lakes has been reported
              for northern Wisconsin (i.e. Steenis, 1932; Wilson, 1935; Potzger and Van
              Engel, 1942), New Hampshire (Moeller, 1975), Scottish Lochs (Spence, 1967)
              and Engl ish Lakes (i.e.,  Pearsall, 1920) as well as other northern European
              c.ourttries (Hutchinson, 1975).

                  Hutchinson (1975) suggested that the oligotrophic Weber Lake in northern
              Wisconsin had the simplest known macrophyte community for northern lakes of
              moderate altitudes and latitudes.   Potzger and Van Engel (1942) found that
              all species growing in Weber Lake were basically rooted in sandy sediments
              and suggested that growth of smaller plants (eight of nine species found) in
              deeper waters was restricted by a cover of organic sediments.      Even in the
              observed depth ranges, Myriophyllum tenellum and Isoetes macrospora Dur. were
              frequently etiolated up to near the tips du@_ to sediment cover.     Other low
              growing species included Elatine minima, Eriocaulon septangulare, Juncus pel.-
              ocarpus f. submersus, Gratiol-a-Titea f. pusil.La a    LoBeli   dortmanna.

                  In comparing three northern Wisconsin lakes of     varying trophic states,
              Wilson (1935) found that most of the rosulate forms    were not present in the
              eutrophic lake.  In the other lakes they were present in only trace amounts.
              Sow of the northern potamogetons were present at very low relative biomasses
              where Najas flexilis was dominant With Potamogeton richard8onii a weak sub-
              domi n Of.

                  From 1821 through 1894, several rosulate forms disappeared from Loch
              Leven, now eutrophic (Jupp et al., 1974).    Several of the species described
              here as northern potamogetons were present, but had disappeared by 1910.       By
              1966 Ceratophyllum demersum and perhaps M-riophyllum spicatum had disappeared.
              Potamogeton pectinatus anU-Zannichellia palustris were adventive by 1959 and
              P. crispus was first reported in 1966.

                  To summarize the long-term changes in submersed macrophyte communities
              described above, a survival index has been developed for plants of the north-
              ern lakes of Table 3 (Lake West Okoboji and Big Spirit Lake were omitted since
              there was comparatively little change in submersed species). This is simply
              a ratio of the number of lakes in which a species was reported in earlier
              surveys to the number of lakes in which the species was present when last
              studied. The survival index was calculated for species which were originally
              found in three or more of the lakes.    As for the turbidity tolerance index,
              other factors in addition to an increase in turbidity surely played a -part
              in changes in plant populations observed. Survival indices calculated and
              turbidity tolerance indices from Table 2 are:

                                                      41
<pb n="54" />

                                                                                   Turbid i ty
                                                            Survival Index     Tblerance Index

               Ceratophyllum demersum                             1.0                 2.8

               Potamogeton pectinatus                             1.0                 2.0

               P. richardsonii (s. s. Perfoliati)                 1.0                 1.5

               Zannichellia palustris                             1.0

               Elodea canadensis                                  0.8                 1.1

               Heteranthera dubia                                 0.8

               Vallisneria americana                              0.8                 2.4

               Najas flexilis                                     0.7                 1.7

               Potamogeton zosteriformis                          0.6

               P. foliosus                                        0.3

               P. gramineus                                       0.3

               P. natans                                          0.3

               ?_. amplifolius                                    0.2

               P. nodosus                                         0.0

                 praelongus                                       0.0                 0.0

                    The tabulation of the survival indices merely emphasizes what is apparent
               in Table 3.  The more cosmopolitan species tend to remain with ecosystem
               change while the northern potamogetons tend to disappear.    As would be expect-
               ed, there is a tendency toward a positive correlation between the survival
               index and the turbidity tolerance index.

               CATEGORIES OF SPECIES BASED
               ON RESISTANCE TO ECOSYSTEM ALTERATION

                   An arbitrary grouping of submersed macrophytes based on their tendency
               to decrease in biomass or disappear with increasing alteration of ecosystems
               has been developed. Ecosystem alteration can be caused by a number of fac-
               tors, but the net result is usually an increase in water turbidity.    Five
               categories ranked roughly in order of increasing resistance to change are sug-
               gested as follows:

                   1.   Rosulate species found mainly in northern lakes.

                                                     42
<pb n="55" />

                  2.  Northern potamogetons.

                  3.  Tolerant species normally with low biomass in disturbed systems.
                      These species may have relatively high biomass in pristine systems.

                  4.  Tolerant species normally dominant or subdominant in disturbed sys-
                      tems.

                  5.  Adventive species that appear in disturbed systems and may be domi-
                      nant to rare.

            Rosulate  Species

                  Significant mixed rosulate populations appear to be restricted to oligo-
            trophic northern  lakes. Their absence elsewhem may be associated with fac-
            tors related to increasing conductivity of the waters (Moyle, 1945) or, as
            Potzger an.d Van Engel (1942) suggested, increasing accumulations of finer
            sediments.   Wilson (1935) said that several rosulate species should be con-
            sidered terrestrial plants which "go aquatic" with submergence due to rising
            water level.   These are normally small plants found in shallow waters and
            maximum depth distribution correlates poorly with Secchi depth (Figures 7 and
            8). Myriophyllum tenellum, when present, is often included in rosulate popu-
            lations since it is 6suall   found with the rosulate species.    Some of the
            rosulate species are:

                  Elatine minima                                       Isoetes macrocarpa

                  Eleocharis acicularis                                Littorella americana

                  E. palustris                                         Lobolia dortmanna

                  Eriocaulon septangulare                              Myriophyllum tenellum

                  Gratiola aurea f. pusilla

            Northern Potamogetons

                  There is a preponderance of evidence that the potamogetons which are re-
            stricted to or are most widely distributed in northern areas do not survive
            the long-term changes that lead to eutrophica'tion.    Based primarily on the
            studies of the northern lakes, the potamogetons most sensitive to increasing
            eutrophication are:

                  Potamogeton amglifolius                              P. natans

                  P. foliosus                                          P. nodosus

                  P.  gramineus                                        P. peaelonous

                  P. illinoiensis                                      P. zostoriforMis

                                                    43
<pb n="56" />

                Of these species probably only P. foliosus can be considered truly cosmopoli-
                tan in distribution.

                     Potamogeton gramineus is one of the most sensitive of the northern pota-
                mogetons to ecosystem change.    Stuckey (1971) cited Pieters (1901) as finding
                this species in 1898 in Put-in-Bay only on a bar where presumably the sedi-
                ments were sandy and the water was shallow.      In comparing three northern
                Wisconsin lakes, Wilson (1935) concluded that the comparatively low biomass
                of P. gramineus found in the most eutrophic.lake was related to the greater
                accumulation of organic soils there.     Wilson suggested that this species is
                a colonizer of primitive soils and disappears as the system develops. Bumby
                (1977) found P. gramineus outside her transects, in Green Lake, Wisconsin         in
                1971, but was unable to find it again through 1973.       Of the northern potamo-
                getons, Potamogeton zosteriformis survives best.      Just as the disappearance
                of P. gramineus is an indicator of early ecosystem changes, the continued
                presence of P. zosteriformis as other northern potamogetons disappear is an
                indication oT further ecosystem change.

                Tolerant Species Normally with
                Low Biomass in Disturbed Systems

                     Any species may be dominant in a part of a system or throughout the sys-
                tem depending on the conditions.     However, for the systems reviewed here,
                there are some species which are resistant to ecosystem changes relating to
                decreased water transparency but which are commonly minor components of the
                systems. The species are:

                     Elodea canadensis                                      Pot6mogeton pusillus

                     Heteranthera dubia                                     P. richardsonii

                     Najas flexilis                                         ZanniChellia palUstris

                Except for Najas flexilis which tends to have a northern distribution and Pota-
                mogeton ricFa_rdson_i-i_`w`Fi__ch belongs to a subsection (Perfoliati) which extends
                southward, the species in this group are widely distributed.

                Tolerant Species Normally Dominant
                or Subdominant in Disturbed Systems

                     A few native species tend to maintain relatively high biomass in dis-
                turbe d sy stems. These include:

                     Ceratophyllum demersum                                 Potamogeton. perfol 1 atus
                                                                              var. bupleuroides

                     Naias guadalupensis                                    Vallisnoria americana
                Ceratophyllum demersum and Potamogeton pectinatus. are widespread, Vallisneria
                americana exteTds from south to north with ratHer spotty distribution, P.

                                                        44
<pb n="57" />

              perfoliatus var.  buple'uroldes. fo-m-s the southern extension of the subsection
              PerfoM_tiand Na'jas'gUAdalUpensis is the southern counterpart of N. flekilis.

                   Potamogeton pectinatus is probably the most widespread and abundant of
              all North American submersed species. In the 1930's this species was the
              most important of the potamogetons as a food source of game ducks. Potamogetons
              as a group were the most important waterfowl food in six of the eight North
              American regions described by Martin and Uhler (1939). Only in two areas, the
              lower Mississippi and Gulf Coast regions, were other species of greater impor-
              tance. The most used species there were Cyperus esculentus L. and Ruppia
              maritima, respectively. The resistance of P. pectinatus to suspended sediment
              loads aFd short and long-term ecosystem cha-nges for the most part is consis-
              tent with the widespread abundance and importance of this species.

              Adventive Species

                   As is now. apparent, Myriophyllum spicatum has spread in the past quarter
              century to many areas in the United States and Canada where infestations often
              cause *problems in human uses of lakes, streams, and reservoirs.     As reviewed
              by Davis and Steenis (1973) intertwining mats of M. spicatum may adversely
              affect swimming, boating and various types of fisfi-ing.   Quiescent waters in
              beds of M. spicatum may be conducive to breeding of mosquitos and decaying
              plants in windrows along the shore often cause obnoxious odors.     There is con-
              fusion in distinguishing between M. spicatum and M. exalbescens but M. @.@ca-
              tum is likely the species involveU in irruptions.

                   Observations in the Chesapeake Bay region (Steenis, 1970; Steenis et al.,
              1971) and Currituck Sound-Back Bay (Davis, in preparation) indicate that
              Myriophyllum spicatum is susceptible at least to short-term perturbations of
              the ecosystem.   Reestablishment of the species as the dominant is highly v.ar-
              iable and may not occur for many years.

                   Potambgeton crispus is another widespread adventive, especially in the
              northern region.   This species is well established as an invader of eutrophic
              or disturbed waters, but in the Chesapeake Bay area it fluctuates widely with
              changing conditions (Steenis etal., 1971).     Potamogeton crispus became a
              strong dominant in an Indiana lake, and measures were taken to control it
              (McIntosh et al., 1978).

                                                     45
<pb n="58" />

                               EFFECTS OF ENVIRONMENTAL ALTERATIONS ON
                                       SUBMERSED AQUATIC PLANTS

                   This section is intended to provide a better understanding of why reduc-
              tions in submersed macrophytes occur and how specific environmental altera-
              tions may affect submersed plants.   It should be emphasized at the outset
              that the capacity to predict changes in macrophyte communities due to natural
              or human induced environmental alterations is quite low. However, the exam-
              ples of long term and short term changes in macrophyte communities just des-
              cribed show that some generalizations can be made that may allow some predic-
              tive capacity.  Where natural episodes are responsible for changes, efforts
              toward conservation and management of submersed plant communities are not us-
              ually applicable. Thus management efforts must be directed toward human in-
              duced environmental changes.   The condition of "overabundance" of aquatic
              macrophytes that may result from environmental manipulation or from invasion
              of adventive species will not be considered.

                   Since reduction in abundance of submersed plants can result from an
              array of factors, it would be instructive to examine how the forcing functions
              that cause these changes affect macrophyte communities.   The diagram in Figure
              9 is a conceptual model that separates these forces into three categories:
              light attenuation, toxicity, and biomass removal.   Some of these may result in
              long term and others in short term changes of submersed plant communities.
              Light attenuation due to suspended sediments and eutrophication acts by    re -
              ducing the energy available for photosynthesis.   For suspended sediments, this
              may be either short term, as in the case of storms or floods, or long term,
              where the source of suspended sediments persists.   Eutrophication is generally
              a long term effect since nutrient levels in aquatic ecosystems can persist
              long after the source of these nutrients are eliminated. Sediments of aquatic
              ecosystems often have a high capacity for storing elemants critical to plank-
              ton growth and may continue to supply these nutrients to the water column af-
              te'r inputs to the system cease.

                   Toxicity due to herbicides, heavy metals, and other toxic substances acts
              by altering the metabolism of plants (Figure 9). The affinity of herbicides
              for small particles may result in the accumulation of these substances in the
              sediments as previously discussed, and effects may persist for long periods
              depending on the stability of the compounds in the environment and whether
              degradation products are also toxic.

                                                    46
<pb n="59" />

                                Sunlight                          LIGHT
                                                   Suspended       ATTENUATION
                                                   Sediments

                                       Reduced light
                                       penetration
                                                          Eutroph! -
                                       Shading by           cation

                                       phytoplankton
                                       and periphyton

                                                    Reproduction

                                                     Submersed
                                                     Macrophyte
                                                     Community

                                                 Altered
                                       Herbi-   metabolism
                                        ci des

                        TOXICITY                                Storm

                                                                 Floods
                                                         Waves,
                                                                      y
                                                         urrents

                                                                     Waterfowl

                                                                        Fish
                                                              Grazing

                                               BIOMASS
                                               REMOVAL                       Burial
                                                                     edimen-
                                                                     tation

              Figure 9. Conceptual model illustrating the effects of environmental forces
              on submersed aquatic macrophyte communities. These forces (circles) are se-
              parated into three categories of stress:     light attenuation, toxicity, and
              biomass removal. Lines represent pathways of energy flow and bold arrows may
              either reduce flow (indicated by "-" sign) or accelerate flow.      Thus all bold
              arrows represent stress on the macrophyte community, except for the one indi-
              cating a positive feedback of macrophyte. reproduction. Symbols after Odum
              (1971).

                                                       47
<pb n="60" />

                   Environmental factors resulting in biomass removal (Figure 9) are gene-
             rally short term so long as the reproduction of aquatic plants is not impair-.
             ed. Of these, only burial by sedimentaiton would normally be induced by
             human activities, such as dredging and instream mining, while the others are
             not amenable to control. Damage to macrophyte communities by the simultan-
             eous occurrence of more than one stress should also be considered as a possi-
             bility.

                   With this model as a basis for understanding stresses on submersed plants,
             the separation of factors due to human and natural forces is facilitated. The
             management of submersed plants in relation to anthropogenic influences is con-
             founded by a spectrum of problems associated with many variables such as the
             soil types which are disturbed (including aquasoils), the nature of the aqua-
             tic system (lotic vs. lentic, etc.) and the nature, time, and duration of the
             activity generating the pollution.     The resiliency of the submersed macrophyte
             system under stress will be related    to the ecological tolerance of the spec-
             ies present.   For example,, northern  potamogetons are very sensitive to stress-
             es associated with increased suspended sediments.      As discussed in other sec-
             tions, long term stresses may result in a greatly altered communities where
             exotic species often dominate.    Under extreme stress by one factor or a multi-
             plicity of several stresses, macrophyte communities may cease to exist.

                   Examples of human activities which may adversely affect natural systems
             and their possible effects on submersed plant populations are given in Table
             4 along with possible plant community responses.      These pertubations are a
             function of the stress factors discussed for Figure 9.      Literature reviews
             relating to the effects of suspended sediments and sedimentation on aquatic
             organisms include Cordone and Kelley (1961), Hynes (1970), Baxter (1977),--
             Morton (1977), Sorensen et al  .  (1977), and Stern and Stickle (1978).     Esti
             mates of the magnitude of pollution of aquatic ecosystems given in Table 4
             were compiled mainly from information in these reviews.      However, little in-
             formation on the impact of changes in aquatic systems affecting submersed
             plants was presented.    This paucity of research relating to clearcut examples
             of the effects of human activities on submersed plant communities is evident
             in this review as in others dealing with submersed plants (i.e., Spence, 1967;
             Westlake, 1968; Westlake, 1973; Wetzel and Hough, 1973).      Evidence that long
             term environmental degradation associated with agricultural and urban pollu-
             tion contributed to loss of many submersed species was given in a previous
             section.   Some other examples of community response to stress are cited in
             Table 4.

                                                     48
<pb n="61" />

              Table 4. Evaluation of the Effects of Various Types of Environmental Alteration on Submersed Plant
                         Communities

                                                     Possible Effects
               Environmental      Suspended
                Alteration        Sediments     Eutrophication     Toxicity     Sedimentation       Community Resiliency

              Instream mining     Varies de- Varies depending         Low      Plants buried by     Limited reestablishment
                                  pending on on bed sediments                  coarse sediments     after burial may be
                                  bed sedi-                                    in a downstream      possible for forms like
              Silviculture        ments                                        gradient             Podostemum.ceratophyllum
                 Selective           Low             Low              Low             Low           Low level continuous sil-
                 harvest                                                                            tation may be conducive
                                                                                                    to macrophyte establish-
                                                                                                    ment
                 Clear            High on      Low on long term    Low if      Variable             Impact ameliorated with
         ko      cutting          short term                       herbicides                       reestablishment of ground
                                                                   not used                         cover
                 Logging          High               Low              Low            High           Erosion of ditches and
                 roads                                                                              roadbed contributes
                                                                                                    sediments

              Urban
                 Construction      High        High if soils are      Low            High
                                               nutrient rich
                                               (i.3., phosphate)                                    A few species such as
                                                                                                    Potamogeton pectinatus
                 Waste waters        Low            High              Low             Low           may survive high levels
                  (secondary                                            to                          of urban pollution.a,b,c
                    treatment)                                        High                          Recovery of diverse com-
                                                                                                    munity possible unless
                                                                                                    sediments are toxis
                 Storm waters      High             High  (continued)  High        Medium
<pb n="62" />

              Table 4. (Continued)

                                                      Possible Effects
               Envi ronmental     T. ,-p e-n d _e T -
                Alteration        Sediments    Eutrophication       Toxicity     Sedimentation        Community Resiliency

              Agriculture            High            High              High          High             Adverse effects would be
                                                                                                      minimized with best av-
                                                                                                      ailable management tech-
                                                                                                      niques; substantial rec-
                                                                                                      overy of aquatic system
                                                                                                      would be expected from
                                                                                                      their application
              Road construction      High      High if eroding      Variable         High             Medium term pulse   of pol-
                                               soils are nutri-                                       lution with community re-
                                               ent rich                                               covery expected except in
                                                                                                      areas of extreme silta-
                                                                                                      tion. Aquatic habitat
         Ln
                                                                                                      may change due to "dam"
                                                                                                      effects of roadbed.
              Stream                 High            High           Variable         High             Decreased shading condu-
                channelization                                                                        cive to increased plant
                                                                                                      growth but spate stress
                                                                                                      increased; gradual recov-
                                                                                                      ery of nautral system
                                                                                                      possible wit   out channel
                                                                                                      maintenance.
              Dredging of         High (short        High           Varies with    Varies depending   Subsequent growth of
                navigation        term and                          sediment       on spoil dis-      submersed plants not
                channels          localized)                        content        posal techni-      buried may be enhanced
                                                                                   ques               by nutrient enrichment.e
              Nearshore           Variable     Variable             Variable       Variable           Effects vary according
                mining                                                                                to mining method and
                                                                                                      control procedures.

                                                     (continued)
<pb n="63" />

                Table 4. (Concluded)

                                                     Possible Effects
                Environmental    Suspended
                 Alteration      Sediments    Eutrophication      Toxicity    Sedimentation       Community Resiliency

                Dams and
                 impoundments
                 Upstream (lake) High             Variable          Low       High at mouth       Establishment of submer-
                                                                              of rivers           sed plants depends on
                                                                                                  width of littoral as well
                                                                                                  as extent and periodicity
                                                                                                  of drawdown.
                 Downstream        High       Varies; some          Low           Low             Depends on discharge
                                              nutrient deple-                                     procedure. Extensive
                                              tion in lake                                        plant beds may develop-  f
                                              likely
                Bulkhead, jetties, Low             Low              Low       Variable            Higher wave energy from
                 groins                                                                           bukl heads may reduce re-
                                                                                                  establishment;g lee
                                                                                                  side of jetties and
                                                                                                  groins conducive for
                                                                                                  growth

                aButcher (1933)
                bHaslam (1978)
                cOzimek (1978)
                do'Rear (1975)
                eOdum (1963)
                fHynes (1970)
                gDavis and Brinson (1976)
<pb n="64" />

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                                                          58
<pb n="71" />

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                                                          59
<pb n="72" />

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                                                            60
<pb n="73" />

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              Hydrobiol. 20:31-40.

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              phytes in lakes. An assessment. Pol. Arch. Hydrobiol. 20:9-19.

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              Wisconsin. Part I. The medium hard water lakes. Ecol. Monogr. 5:207-247.

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              County, Wisconsin. Trans. Wisc. Acad. Sci. Lett. 33:135-146.

                                                61
<pb n="74" />

               CANADA
               M Maritime
               N North
               W West
               P Plains
               E East                                                              CO
               GREENLAND G

               UNITED STATES
                   Northeast
               2   Southeast
               3   North Central             8
               4   North Plains
               5   Central Plains                7
               6   South Plains
               7   Southwest
               8   Intermountain
               9   Northwest
               0   California
               A   Alaska

               Appendix A. Regions of North America that correspond to the distribution
               codes of the Appendix B.

                                                  62
<pb n="75" />

                          Appendix B. Species, Family, Common Name, and Distribution of North American Aquatic
                                     Macrophytes Mentioned in Text.        The Distribution Code Corresponds
                                           with Regions in Appendix A Adapted from Shelter and Skog
                                                 (1978) Except as Indicated: (*) Fassett (1957)
                                                              and (**) Fernald (1950)

                              Species                             Family                 Common Name                 Distribution

                 Alisma gramineum var. geyeri J.             A.1 ismataceae            Narrowleaf Water           1,3,4,5,8,9,0
                 G. Gmel.                                                              Plantain

                 Armoracia aquatica (Eat.) Wieg.             Brassi caceae             Lake Cress                 1,2,3,6,E

                 Bi dens. becki i J. Torrey                  Asteraceae                Water Marigold             1,3,9,W,P,E,M

                 Butomus umbellatus f. vallisneri-           Butomaceae                Flowering Rush             1,3,9,P,E,M
                 foTTus L.

                 Cambomba caroliniana A. Gray                Nymphaeceae               Fanwort                    1,2,3,6

                 Ceratophyllum demersum L.                   Ceratophyllaceae          Coontai 1 , Hornwort       1,2,3,4,5,6,7,8,9
                                                                                                                  0,A,H,C,N,P,E,M

                 Cyperus esculentus L.                       Cyperaceae                Nut Grass, Ground          1,2,3,4,5,6,7,8,9
                                                                                       Almond                     0,A,E,M
                 Elatine minima (Nutt.) F. E. L.             Elatinaceae               Waterwort                  1,3,E,M
                 Fischer and C. . Meyer

                 Eleocharis acicularis (L.) J. J.            Cyperaceae                Slender Spikerush          1,2,3,4,5,6,7,8,9
                 J. Roem. and Schu'lt.                                                                            0,A,N,W,P,E,M,G
                 Eleocha   .s  arv la (J. J. Roem.           Cyperaceae                Dwarf  Spikerush           1,2,3,4,5,6,7,8,9,0
                 and Schult.)- Link                                                                               W,E,M
                                                                             Continued
<pb n="76" />

              Appendix B (Continued)

                          Species                         Family               Common Name             Distribution

              Elodea canadensis Michx.               Hydrocharitaceae,       Waterweed               1,2,3,4,5,6,7,8,9
                                                                                                     0,W,P,E,M

              Elodea nuttallii (Planch.) H.          Hydrocharitaceae        Waterweed               1,3,4,5,8,9,0,E,m
              St. John

              Eriocau-lon septangulare With.         Eriocaulaceae           Pipewort                1,2,3,6,E,M
              Gratiola aurea Pursch                  Scrophulariaceae        Hedge Hyssop,           1,2,3,4,5,E,M
                                                                             Goldenpert

              Gratiola lutea f. pUsilla              Scrophulariaceae        Dwarf Hyssop            1,3,E,M*
              ( F as s e t7t 7-enn e 11 .

              Heteranthera dubia (Jacq.)             Pontederiaceae          Water Stargrass         1,2,3,4,5,6,7,8,9
              MacMill.                                                                               O,E

              Hydrilla verticillata Royle            Hydrocharitaceae        Hydrilla                2
              Isoetes. macrospora Durieu             Isoetaceae              Quillwort               1,3,E,M
              Juncus pelocarpus E. Meyer             Juncaceae               Bog Rush                1,2,3,E,M
              Littorella americana Fern.             Plantaginaceae                                  1,2,3,E,M
              Lobelia dortmanna L.                   Campanulaceae           Water Lobelia           1,3,4,9,W,P,E,M
              Myriophyllum alterniflorum DC.         Haloragaceae            Li ttl e Watermi I -    1,A,N,P,E,M,G
                                                                             foil

                                                             Continued
<pb n="77" />

                  Appendix B (Continued)

                               Species                             Fami ly                 Common Name                 Distribution

                  Myriophyllum exalbescens Fern.              Haloragaceae               Northern Watermil-          1,2,3,4,5,6,8,9,0
                                                                                         foil                        A,N W,P,E,M,G*

                  Myriophyllum heterophyllum.Michx.           Haloragaceae               Variable Watermil-          1,2,3,4,5,6,E
                                                                                         foil

                  Myriophyllum spicatum L.                    Haloragaceae               Eurasian Watermil-          1,2,3,4,5,6,8,9,0
                                                                                         foil                        A,N,W,P,E,M,G

                  Myriophyllum tenellum Bigel.                Haloragaceae               Leafless Watermil-          1,3,E,M
                                                                                         foil

                  Myriophyllum verticillatum L.               Hal oragaceae              Whorled Watermilfoil        1,2,3,4,5,8,9,0,A

                  Najas flexilis (Willd.) Rostk.              Najadaceae                 Northern Naiad,             1,2,3,4,5,7,8,9,W
                  and ScF-midt                                                           Bushy Pondweed              P,E,M

                  Najas guadalupensis (Spreng.)               Najadaceae                 Southern Naiad,             1,2,3,4,5,6,7,8,9
                  Magnus                                                                 Bushy Pondweed              O,E

                  Najas minor All.                            Najadaceae                 Bushy Pondweed,             1,2,3
                                                                                         Naiad

                  Potamogeton amplifolius Tuckerm.            Potamogetonaceae           Largeleaf Pondweed          1,2j3q4y5p7s8s9,0
                                                                                                                     W,P,E,M

                  Potamogeton berchtoldii Fieber              Potamoge.tonaceae          Slender Pondweed            1,2,3,4,5,6,8,9,0
                                                                                                                     A,N,W,P,E,M,G**

                                                                        Continued
<pb n="78" />

               Appendix B (Continued)

                          Species                         Fami ly              Common Name              Distribution

               Potamogeton crispus L.                 Potamogetonaceae       Muck Pondweed,           1,2,3,4,5,6,8,9
                                                                             Curly Pondweed           0,E,M

               Potamogeton diversifolius Raf.         Potamogetonaceae       Snai Iseed Pondweed      1,2,3,4,5,6,7,8,9,
                                                                                                      0

               Potamogeton epihydrus Raf.             Potamogetonaceae       Ribbonleaf Pondweed      1,2,3,4,5,8,9,0
                                                                                                      A,W,P,E,M

               Potamogeton filifonnis Pers.           Potamogetonaceae       Threadleaf Pondweed      1,3,4,5,8,9,0,A

               Potamogeton foliosus Raf.              Potamogetonaceae       Leafy Pondweed           1,2,3,4,5,6,7,8,9,
                                                                                                      0,A,N,W,P,E,M

               Potamogeton friesii Rupr.              Potamogetonaceae       Fries Pondweed           1,3,4,8,9,A,N,W,P,
                                                                                                      E,M

               Potamogeton gramineus L.               Potamogetonaceae       Variableleaf Pond-       1,3,4,5,7,8,9,0
                                                                             weed                     A,N,W,P,E,M,G

               Potamogeton illinoensis Morong.        Potamogetonaceae       Variableleaf Pond-       1,2,3,4,5,6,7,8,9
                                                                             weed                     0,A,W,P,E,M

               Potamogeton lucens L.                  Potamogetonaceae       Variableleaf Pond-       1,2,3,4,5,6,7,8,9
                                                                             weed                     0,A,W,P,E,M

               Potamogeton natans L.                  Potamogetonaceae       Floatingleaf Pond-       1,2,3,4,5,6,7,8,9
                                                                             weed                     0,A,W,P,E,M,G

                                                               Continued
<pb n="79" />

                 Appendex B (Continued)

                             Species                          Fami ly               Common Name               Distribution

                 Potamogeton nodosus Poir.               Potamogetonaceae         Longleaf Pondweed         1,2,3,4,5,6,7,8,9
                                                                                                            0,W,E,M

                 Potamogeton obtusifolius   F. C.        Potamogetonaceae         Bluntleaf Pondweed        1,3,4,5,W,P,E,M
                 Mert. and W. C. J. Koch
                 Potamogeton pectinatus L.               Potamogetonaceae         Sago Pondweed             1,2,3,4,5,6,7,8,9
                                                                                                            0,A,N,W,P,E,M

                 Potamoqeton subsection Perfoliati       Potamogetonaceae         Redhead Grass,            1,2,3,4,5,A,N,E,M
                 (Includes P. perfoli tus L.,     P.                              Claspingleaf Pondweed     G
                 perfoliatus var. bupleuroides
                 (Fern-T-Fayw., ana P. richardsonii
                 (Ar. Benn.) Rybd. -    -
                 Potamogeton praelongus F. Wulf.         Potamogetonaceae         Whitestream Pond-         1,3,4,5,7,8,9,0
                                                                                  weed                      A,N W, P, E M, G

                 Potamogeton pusillus L.                 Potamogetonaceae         Slender Pondweed          1,2,3,4,5,6,7,8,9
                                                                                                            0,A,N,W,P,E,M,G

                 Potamogeton robbinsii Oakes             Potamogetonaceae         Fernleaf Pondweed         1,3,4,8,9,0,A,W,P
                                                                                                            E,M

                 Potamogeton spirillus Tuckerm.          Potamogetonaceae         Snailseed Pondweed        1,3,4,E,M
                 Potamogeton strictifolius Benn.         Potamogetonaceae         Pondweed                  1,3,4,5,8,9
                 Potamogeton zosteriformis Fern.         Potamogetonaceae         Flatstem Pondweed         1,3,4,5,8,9,0,A,M
                                                                                                            W,P,E,M

                                                                  Continued
<pb n="80" />

                Appendix B (Continuted)

                           Species                        Family                Common Name              Distribution

                Proserpinaca palustris.L.             Haloragaceae           Mermaid Weed              1,2,3,6,0,E,M

                Ranunculus aguatilis L.               Ranunculaceae          White Water Butter-       1,3,4,5,7,8,9,0qAs
                                                                             cup, Growfoot             N,W,P,E,M,G

                Ranunculus longirostris  Godron.      Ranunculaceae          White Water Butter-       1,3,5,6,7,E*
                                                                             cup

                Ruppia maritima L.                    Potamogentonaceae      Widgeon Grass             1,2,3,4,5,6,8,9,)
                                                                                                       A,W,P,E,M

                Sagittaria cuneata Sheld.             Alismataceae           Northern Arrowhead,       1,3,4,5,7,8,9,0,A
         CO                                                                  Duckpotato                N,W,P,E,M

                Sagittaria graminea Michx.            Alismataceae           Slender Arrowhead         1,2,3,4,5,6,7,E,M

                Sagittaria subulata (L.) Buckl.       Alismataceae           Water Arrowhead           1,2,9

                Scirpgs acutus Muhl.                  Cuperaceae             Hardstem Bulrush          1,2,3,4,5,6,7,8,9
                                                                                                       0

                Scirpus subterminalis J. Torr.        Cyperaceae             Water Bulrush             1,2,3,4,9,A,W,E,M

                Sparganium anqustifolium Michx.       Sparganiaceae          Burreed                   1,3,4,7,8,9,0,A,N,
                                                                                                       W,P,E,M,G

                Subularia aguatica L.                 Brassicaceae           Awlwort                   1,3,4,8,9,0,A,N,W
                                                                                                       P,E,M,G

                                                                (conti.nued)
<pb n="81" />

               Appendix B (Concluded)

                          Species                        Family              Common Name               Distribution

               Utricularia cornuta Michx.           Lentibulariaceae       Horned Bladderwort        1,2,3,6,0,P,E,M
               Utricularia gibba L.                 Lentibulariaceae       Eastern Bladderwort       1,2,3,5,6,0,C,E,M
               Utricularia intermedia Hayne         Lentibulariaceae       Flatleaf Bladderwort      1,3,4,8,9,0,A,N,W
                                                                                                     P,E,M,G

               Utricularia minor L.                 Lentibulariaceae       Purple Bladderwort        1,2,3,E,M

               Utricularia resupinata B. D.         Lentibulariaceae       Lavender Bladderwort      1,2,3,E,M
               Greene

               Utricularia vulgaris L.              Lentibulariaceae       Common Bladderwort        1,2,3,4,5,6,7,8,9
                                                                                                     0,A,N,W,P,E,M

               Vallisneria americana Michx.         Hydrocharitaceae       Wildcelery                1,2,4
               Zannichellia palustris L.            Zannichelliaceae-      Horned Pondweed           1,2,3,4,5,6,7,8,9
                                                                                                     0,A,N,W,P,E,M

               Zostera marina L.                    Potamogetonaceae       Eelgrass                  1,2,9,0,A,N,W,P,E
                                                                                                     M,G
<pb n="82" />

                                 50272 -101
                                 REPORT DOCUMENTATiO7. REPORT No.                                                            2.                          3. Recipient's Accession No.
                                              PAGE                             FWS/OBS-79/33                                1                          1
                                 4.   Title ;nd Subtitle                                                                                                 5. Report Date
                                      Re ponses of Submersed                   Vascular Plant Communities                       to Environ-                         August 1980
                                      mental Change                                                                                                      6.

                                 7.   Author(s)                                                                                                          &amp; Performing Organization Rept. No.
                                      Davis, Graham          J., and Brinson, Mark M.
                                 4.   Performing Organization Name and Address                                                                           10. Project/Task/Work Unit No.
                                      Department of Biology
                                      East Carolina University                                                                                           11. Contract(C) or Grant(G) No.
                                      Greenville, NC 27834                                                                                               (c)14-16-0009-78-032

                                                                                                                                                         (G)

                                 12 Sponsoring Organization Name and Address                                                                             13. Type of Report &amp; Period Covered
                                      tastern Energy and Land Use Team, Office of Biological Services,
                                      Fish and Wildlife Service, U. S. Department of the Interior                                                              Final report
                                      Route 3 Box 44, Kearneysville, West Virginia 25430                                                                 14.

                                 15. Supplementary Notes

                                 16. Abstract (Limit: 200 words)

                                      This report examines the response of submersed vascular plants to changing environmental
                                      conditions, primarily those that affect light transmission. Among the physical factors
                                      considered are fluctuating water levels, currents and waves, and suspended sediments.
                                      Chemical and biological factors affecting plant responses to light conditions are
                                      grwoing season and dormancy, nutrient availability, and plant-animal interactions. Depth
                                      distribution data for many North American species of submersed vascular miacrophytes are
                                      recorded as an indices of resistance to turbidity for some of the most tolerant of the
                                      species. Survival indices were also calculated for several species typically found in
                                      clearwater lakes. Turbidity tolerance and survival indices were then used to identify
                                      groups of species with varying degrees of resistance to ecosystem alterations. The
                                      report concludes with a brief discussion of the potential effects of human activities
                                      on submersed plant communities.

                                      There is a companion document which contains a summary of this literature review. It is
                                      entitled Responses of Submersed Vascular Plant Communities to Environmental Changes:
                                      Summary, FWS/OBS-80/42, August 1980.

                                 17. Document Analysis a. Descriptors
                                      Aquatic Plants, Aquatic Environments, Light Penetration, Light Intensity, Water Level
                                      Fluctuations, Suspended Solids, Waves, Currents

                                      b. 8d;ntr1fi      -Endeq Terms
                                          e ;e;60IVn
                                      Su               ascular palnts, Aquatic Macrophytes, Light Transmissivity, Turbidity,
                                      Suspended Sediment

                                      c. COSATI Field/Group       43F, 48G, 57H
                                 18. Availability Statement                                                                    19. Security Class (This Report)            21. No. of Pages
                                      Release unlimited                                                                       -Unclassified                                    79
                                                                                                                               20. Security Class (This Page)              22. Price
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                                      U.S. GOVERNMENT PRINTING OFFICE : 1980 0 - 328-607
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                                                                                                     tAT Op

                                                                                              1k

              As the Nation's principal conservation agency, the Department of                                   0
              the Interior has responsibility for most of our nationally owned
              public lands and natural resources. This includes fostering the
              wisest use of our land and water resources. protecting our fish and
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<pb n="84" />

      U. S. DEPARTMENT OF THE INTERIOR
             FISH AND WILDLIFE SERVICE                                                                       POSTAGE AND FEES PAID
           OFFICE OF BIOLOGICAL SERVICES                                                                 U 8 DEPARTMENT OF THE INTERIOR
        EASTERN ENERGY AND LAND USE TEAM                                                                            INT-423
                   ROUTE 3, BOX 44
              KEARNEYSVILLE, WV 25430

                 OFFICIAL BUSINESS

             PENALTY FOR PRIVATE USE. S300

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