[From the U.S. Government Printing Office, www.gpo.gov]







                                                      Ref. No. [UMCEES]CBL 92-033


                              NA90AA-D-CZ795


                                  Task 21



            INFLUENCE OF RAFTED OYSTER AQUACULTURE

                           ON SEDIMENT PROCESSES



 
   
                                  DRAFT FINAL REPORT
    
                                             to:

                          Maryland Department of Natural Resources
                                  Tidewater Administration
                             Maryland Coastal Research Division
                               DNR Contract No. C224-91-004




  


                                        AUTHORS:


                                        Jon H. Tuttle
                              The University of Maryland System
                        Center for Environmental and Estuarine Studies
                               Chesapeake Biological Laboratory
                                 Solomons, MD 20688-0038

          QL                                and
          430.7
          .09                          Robert B. Jonas
          T88                       Department of Biology
          1992
                                  George Mason University
                                     Fairfax, VA 22030
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                                                  TABLE OF CONTENTS






              ABSTRACT                                                                                     i


              LIST OF TABLES                                                                              ii


              LIST OF FIGURES                                                                            iii


              PROJECT RATIONALE AND OBJECTIVES                                                            I


              BACKGROUND                                                                                  3


                      Ecosystem Rationale for Oyster Replenishment                                        3
                      Oyster Replenishment as a Bioremediation Tool                                       8
                      Replenishment Strategies: Oyster Reef vs. Oyster Aquaculture                        9
                      Field and Ecosystem Modeling Validations                                           I I
                      Potentially Negative Impacts                                                       13

              METHODS                                                                                    14


                      Study Site Description                                                             14
                      Coring Device Design and Sediment Collection                                       18
                      Hydrographic Measurements                                                          19
                      Pore Water Collection                                                              21
                      Chemical Determinations and Porosity                                               22
                      Biochemical Oxygen Demand                                                          23
                      Dark Assimilation of Carbon Dioxide                                                24
                      Sulfate Reduction                                                                  25


              RESULTS                                                                                    26


                      Weather and Hydrographic Data                                                      26
                      Sediment Characteristics                                                           29
                      Carbon: POC and PON                                                                29
                      Carbon: Biochemical Oxygen Demand                                                  34
                      Carbon: POC by Combustion                                                          39
                      Dark Assimilation of Carbon Dioxide                                                42
                      Sulfur: Sulfate Profiles                                                           52
                      Sulfur: Sulfate Reduction                                                          61
                      Sulfur: Sulfide Profiles                                                           69
                      Sulfur: Total Reduced Sulfur                                                       71


              DISCUSSION







          EXECUTIVE SUMMARY            US Department of Commrece
                Technical Findings     NOAA Coastal Services Center Library
                                       2234 South Hobson Avenue
                Recommendations        Charleston, SC  29405-2413

          ACKNOWLEDGEMENTS

          REFERENCES                                                          83
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                                                  LIST OF TABLES



             Table 1. Weather conditions and other observations made at the Pintail Point Farm oyster
             aquaculture facility on sediment sampling days.

             Table 2. Hydrographic and secchi disk data collected at the Pintail Point Farm oyster
             aquaculture facility on days when sediments were sampled. Oysters were rafted at stations 2
             and 3. Station 6 was located in the main channel of the adjacent Wye River. ND indicates
             no data collected.


             Table 3. Mean particulate organic carbon, particulate organic nitrogen, and molar carbon:
             nitrogen ratios in the surficial 2 cm of sediment at stations 1 through 3.

             Table 4. Comparison of sediment POC measured directly, calculated by regression and
             estimated from loss on combustion.

             Table 5. Loss of POC between the 0-2 cm and 4-6 cm segments of cores collected July 12 to
             September 15, 1991.









                                                    LIST OF FIGURES


              Figure 1. Current state of production (carbon flow) in the Chesapeake Bay ecosystem.
              Adapted from Draft 1989 State of the Bay report.

              Figure 2. Schematic diagram of the Deep Cove Creek rafted oyster aquaculture site.

              Figure 3. Porosity profiles of sediments at stations 1-5.

              Figure 4. POC, PON, and POC:PON molar ratios in surficial sediments.

              Figure 5. Five-day and twenty-day biochemical oxygen demand in surficial sediments.

              Figure 6. Comparisons of five-day and twenty-day biochemical oxygen demand values at
              stations 2 and 3 with values at the other stations.

              Figure 7. Intercomparison of five-day and twenty-day biochemical oxygen demand values
              among stations where rafts were not present.

              Figure 8. Regression of surficial sediment weight loss on combustion on POC determined by
              elemental analysis.

              Figure 9. Distributions of POC in sediments at stations 1-5.

              Figure 10. Distributions of total inorganic carbon (TIC) in sediments at stations 1-5.

              Figure 11. Rates of dark assimilation of CO, as a function of sediment depth at station 1.

              Figure 12.  Rates of dark assimilation of C02 as a function of sediment depth at station 2.

              Figure 13.  Rates of dark assimilation of CO, as a function of sediment depth at station 3.

              Figure 14.  Rates of dark assimilation of CO, as a function of sediment depth at station 4.

              Figure 15.  Rates of dark assimilation of CO, as a function of sediment depth at station 5.

              Figure 16.  Depth-integrated areal rates of dark assimilation of CO, over the study period.

              Figure 17.  Depth profiles of pore water sulfate concentration at station 1.

              Figure 18.  Depth profiles of pore water sulfate concentration at station 2.,

              Figure 19.  Depth profiles of pore water sulfate concentration at station 3.

              Figure 20.  Depth profiles of pore water sulfate concentration at station 4.

              Figure 21. Depth profiles of pore water sulfate concentration at station 5.







              Figure 22. Sulfate reduction rate as a function of sediment depth at station 1.

              Figure 23.   Sulfate reduction rate as a function of sediment depth at station 2.

              Figure 24.   Sulfate reduction rate as a function of sediment depth at station 3.

              Figure 25.   Sulfate reduction rate as a function of sediment depth at station 4.

              Figure 26.   Sulfate reduction rate as a function of sediment depth at station 5.

              Figure 27.   Depth-integrated areal sulfate reduction rates over the study period.

              Figure 28.   Depth profiles of pore water HS- concentration at station 1.

              Figure 29.   Depth profiles of pore water HS- concentration at station 2.

              Figure 30.   Depth profiles of pore water HS- concentration at station 3.

              Figure 31.   Depth profiles of pore water HS- concentration at station 4.

              Figure 32.   Depth profiles of pore water HS- concentration at station 5.

              Figure 33.   Distributions of total reduced sulfur in sediments at station 1.

              Figure 34.   Distributions of total reduced sulfur in sediments at station 2.

              Figure 35.   Distributions of total reduced sulfur in sediments at station 3.

              Figure 36.   Distributions of total reduced sulfur in sediments at station 4.

              Figure 37.   Distributions of total reduced sulfur in sediments at station 5.

              Figure 38.   Depth-integrated areal total reduced sulfur content of sediments over the study
              period.















                                                             iv









                                      PROJECT RATIONALE AND OBJECTIVES



                     Comparison of historical Chesapeake Bay oyster standing stocks with those existing

              now indicate that an important mechanism for removing phytoplankton biomass from Bay

              water has all but disappeared (Newell 1988; Newell et al. 1989). The Bay ecosystem appears

              to have changed such that a substantial portion of primary production is now dissipated by

              bacterial metabolism (Baird and Ulanowicz 1989), leading to increased oxygen consumption

              -n the water column and sulfide production in sediments. These microbial processes have

              been chiefly responsible for increasing the intensity, duration, and spatial extent of hypoxic

              and anoxic conditions in mesohaline Bay waters (Tuttle et al. 1987a). A simple, benthic

              suspension feeding model predicts that rafted oyster population densities sufficient to reduce

              current phytoplankton biomass by 40% are theoretically sustainable (Gerritsen et al. 1989).

              All these observations lead to the hypothesis that increasing Bay oyster stocks, by rafted

              oyster mariculture or by augmenting oyster reef densities, could substantially improve water

              quality in the Chesapeake Bay and/or its tributaries and, at the same time, revitalize the

              declining shellfish industry in Maryland.

                     Field studies conducted by our laboratory in 1989 and 1990 at the St. George Oyster

              Company oyster mariculture facility in St. Mary's County, Maryland, demonstrated that

              rafted oysters can remove significant quantities of phytoplankton carbon and biochemical

              oxygen demand from the estuarine water column (Jonas and Tuttle 1991; Tuttle and Jonas, in

              preparation). The field data are consistent with the trophic consequences of increasing oyster

              densities predicted by a quasi-equilibrium, mass action model of the exchanges transpiring in

              the mid-Bay ecosystem (Ulanowicz and Tuttle, in press). Despite mounting evidence of the



                                                             1







             efficacy of increasing oyster stocks as a useful bioremediation technique to augment the

             nutrient reduction strategy, it remains unclear whether increased loading of sediments with

             oyster feces and pseudofeces would significantly increase oxygen demand in the sediments.

             If so, many of the benefits gained from removing excess phytoplankton from the water

             column could be negated.

                     Based on information gained using a variety of techniques, many of which we have

             applied over the past eight years to examine the formation and maintenance of anoxia in the

             mesohaline Chesapeake Bay, we report here on the influences of oyster rafting mariculture on

             underlying estuarine sediments in comparison with similar, nearby sediments outside the raft

             area. Our goals were to establish whether oyster fecal loading substantially increases

             sediment oxygen deficits and to examine how this loading quantitatively and qualitatively

             affects key estuarine microbial sediment processes on a seasonal basis.

                     To attain these goals, our objectives were:


                     1. To compare the quantity and quality of organic carbon reaching the

                     sediments beneath and away from rafting areas;


                     2. To determine integrated microbial metabolism in these sediments;


                     3. To compare rates of sediment sulfate reduction, the key bacterial process

                     occurring in Chesapeake Bay sediments during the periods of rapid oyster

                     growth; and,


                     4. To assess sediment oxygen demand in sediments beneath and away from

                     rafting areas.




                                                           2









                                                   BACKGROUND



             Ecosystem Rationale for Oyste Replenishment


                    Recent studies on Chesapeake Bay water quality and assessments of the current state

             of the mid-Bay's ecosystem (Malone et al. 1986; Tuttle et al. 1985, 1987a, 1987b; Jonas

             1987; Jonas et al 1988a, 1988c; Baird and Ulanowicz 1989; Jonas and Tuttle 1990; Baird et

             al. 1991) all suggest that there has been a major shift in the trophic food web structure of the

             Bay's ecosystem. Especially during the warmer months of the year, the ecosystem now

             appears to be dominated by bacteria and their associated physiological processes rather than

             by grazer-based food webs. This trophic shift is particularly evident in the water column

             where increased phytoplankton production resulting from anthropogenic inputs of nitrogen

             and phosphorous is consumed to a major degree by free-living bacteria. In the mesohaline

             reaches of the Bay, for example, abundances of these microorganisms reach and remain at 10

             to 20 x 101 cells 1;'. Such sustained abundances, which have been observed over several

             years and locations throughout the mid-Bay region (Malone et al. 1986; Tuttle et al. 1987a;

             Ducklow et al. 1987; Jonas and Tuttle 1990; Malone et al. 1991), are high compared to other

             coastal environments (Williams 1984). These elevated bacterial abundances are responsible

             for high rates of pelagic oxygen consumption and represent one of the primary factors driving

             the development and maintenance of hypoxia and anoxia commonly observed in the mainstern

             and subestuaries of the Bay (Tuttle et al. 1987a).

                    High levels of microbially labile dissolved organic carbon (DOC) fueling the

             production and metabolism (and thus, oxygen consumption) of the pelagic, heterotrophic

             bacteria derive from: 1) direct release of DOC by living phytoplankton (Malone et al. 1991);

             2) indirect release of DOC through grazing and excretion by zooplankton (e.g. Caron et al.

                                                           3








               1988; Roman et al. 1988); and 3) DOC release from decaying phytoplankton cells (Tuttle et

               al. 1987a; Jonas and Tuttle 1990; Bell 1990). About I I % of mainstem mid-Bay total DOC

               (typically 3.5 mg C L-'; Malone et al. 1991) consists of highly labile organic carbon (0.38

               mg C L:' measured as dissolved, 5-day biochemical oxygen demand; Jonas and Tuttle 1990)

               which in turn is composed of such substances as carbohydrates, amino acids, and short chain

               organic acids (Bell et al. 1988; Jonas et al. 1988a; Bell 1990). Like bacterial abundances,

               rates of oxygen-consuming metabolism of this labile organic carbon are also unexpectedly

               high compared to other estuarine systems. In the mid-Bay mainstem, for example, dissolved

               monosaccharide concentrations average I to 2 jM (0.07 to 0.14 mg C L;'), dissolved free

               amino acid (DFAA) concentrations approach I IAM, and their metabolism can average 225 n

               mol L;' h-' (16.2 ILg C L;' h@') and 20 n mol 1;' h-', respectively (Bell 1990). Thus, dissolved

               monosaccharides, comprising up to 37% of labile DOC, represent a preferred source of

               carbon and energy for pelagic bacterial metabolism and production in the mid-Bay and exhibit

               a very rapid turnover ranging from 8.9 to 4.3 h.

                     Recent inter-ecosystem comparisons of the Chesapeake Bay with other coastal

               environments suggest that the Bay is stressed, giving rise to increased recycling of materials

               (i.e. fixed carbon and nutrients) via short intense loops (Baird et al. 1991). Thus, although

               there have been suggestions that secondary producer-based bacterial food webs may be

               -mportant links to higher trophic levels in aquatic ecosystems, this phytoplankton/bacterial

               food web ("microbial loop") is probably unproductive in terms of transferring carbon and

               energy to harvestable biological resources. Factors such as increased food web length (Baird

               and Ulanowicz 1989) and the apparent lack of efficient bactivors (small animals which

               consume bacteria) seem to result in Bay planktonic food webs in which the bacteria act as an


                                                             4







             organic carbon sink in which carbon and plant nutrients are rapidly recycled (Ducklow et al.

             1986; Baird and Ulanowicz 1989; Malone and Ducklow 1990; Fig. 1). The amount of

             phytoplankton net production "lost" through mid-Bay water column microbial metabolism

             alone has been estimated at 50, 30, 28 and 25% in summer, fall, winter, and spring,

             respectively (Baird and Ulanowicz 1989). Indeed, the 1989 State of the Bay Report

             concludes: "...only a small percentage of the plant production eventually reaches the

             economically important finfish and shellfish levels. The task of managers is to restore the

             Chesapeake to a balanced ecosystem in which as much of this energy as possible is funneled

             into important and useful biological yields -- oysters, striped bass, and waterfowl, among

             others. In the Bay's current state, much of the plant production does not reach these higher

             levels but ends up decomposing on the Bay bottom, robbing the water of much needed

             oxygen."

                    Recent evidence indicates that increased carbon flow through microbial loop

             communities is occurring in Bay sediments as well as in the water column. Sulfate reduction,

             a process confined chiefly to sediments and catalyzed by obligately anaerobic bacteria, has

             increased in mid-Bay sediments since colonial times (Cooper and Brush 1991). Directly

             measured rates of sediment sulfate reduction (sulfide production) in the mid- and lower Bay

             are among the highest found in marine environments (Tuttle et al. 1987a, 1987b; Roden and

             Tuttle in press, in revision) and rates in the upper Bay are substantial, even at limiting

             sediment pore water sulfate concentrations (Roden and Tuttle in review).

                    Although there are probably multiple causes for this shift in trophic structure of the

             Chesapeake Bay ecosystem, it is likely that a key factor has been the decline in abundance of

             benthic suspension feeders, such as oysters and clams, and consequent reduction in cropping


                                                           5











                                          The Bay Pyramid

                           Ft s



                                                                            Sunlicht

                           Senthos
                                                                                       (Hurnan
                                                                                      Ac
                                                                                   /    livitles)
                           Zooplankton                                         Nutrients




                           Primary Production





                           TO tal
                                                                       Decomposition
                           I                                             by Bacteria
                                  Annual Prodwdon of Elomass
                                 (tons cubw In mesottallne                 `5
                                                                   @IN


























              Figure 1. Current state of production (carbon flow) in the Chesapeake Bay ecosystem.
              Adapted from Draft 1989 State of the Bay report. (Thanks to Dr. J.A. Mihursky, CRQ.


                                                              6








              of phytoplankton. Baird and Ulanowicz (1989) noted that deposit-feeding organisms now

              dominate the benthic community of the Bay and that water clarity, once maintained by large

              populations of filter feeders, has decreased. Low dissolved oxygen concentrations, high rates

              of sediment deposition, overharvesting and parasitic diseases (e.g. MSX and Dermo) have all

              contributed to this decline of benthic suspension feeders. Newell (1988) estimates that

              Chesapeake Bay standing stocks of the American oyster, Crassostrea virginica, declined from

              188 x 10' kg dry weight (-355 million bushels) in the late 19th century to about 1.9 x 10' kg

              dry weight (-4 million bushels) currently. Oyster landings in the 1800's amounted to about

              20 million bushels annually from the northern, mesohaline portion of the Bay, whereas

              current harvests are of the order of 0.5 million bushels or less.


                     Calculations based on oyster abundances in the Bay and laboratory-determined oyster

              feeding rates (Newell 1988; Newell et al. 1989) suggest that oyster abundances in the late

              19th century were sufficient to filter the entire volume of the Maryland portion of the Bay in

              three to six days. Current oyster stocks filter the same volume in about 325 days. Thus, a

              major biological mechanism for removing phytoplankton and organic detritus has been nearly

              eliminated from large areas of the Bay.

                     Although deposit-feeding macrobenthic invertebrates may have replaced the oyster to

              some extent (Baird and Ulanowicz 1989), it is possible that benthic biomass is actually

              reduced due to the loss of oyster feces and pseudofeces (Haven and Morales-Alamo 1986;

              Holland et al. 1987; Jordan 1987). Large segments of the macrobenthos may depend either

              on the organic materials deposited by oysters or the large surface area provided by oyster

              shell. In either event, inhospitable bottom water and sediment chemical conditions probably

              limit annual recolonization success.



                                                             7







                      It is reasonable to conclude that oysters, key filter feeders, were but are no longer a

              quantitatively important component of the trophic structure of the Chesapeake Bay ecosystem.

              Based on calculations such as those discussed above and on the current trophic state of the

              Bay, we (Tuttle et al. 1987b) and others (Newell 1988; Newell et al. 1989; Gerritsen et al.

              1989) have proposed that replenishing oyster populations should have a major positive impact

              on Bay water quality as well as a revitalizing influence on the nearly extinct Chesapeake Bay

              shellfish industry.


              Qyste ftlenishment as -4 Bioremediation Tool


                      Given the likely condition that the magnitude of bacterial biomass, production, and

              metabolism in Chesapeake Bay is ultimately related to the amount of internal organic carbon

              production (autochthonous primary production) and external inputs (allochthonous carbon

              from terrestrial sources), it is theoretically feasible to control bacterioplankton communities

              by:


                      1. decreasing autochthonous production through nutrient reduction strategies

                      and allochthonous carbon inputs by effective waste treatment and land

                      management practices (bottom-up or supply side controls) and by


                      2. redirecting a portion of autochthonous production and allochthonous carbon

                      through ecosystem compartments at higher trophic levels than the microbial

                      loop (top-down or demand side control).


                      The first of these strategies, already implemented as the cornerstone of Bay cleanup



                                                              8







             efforts and targeted primarily at decreasing phytoplankton biomass and production, would be

             expected to decrease microbial loop biomass and production as well with a concurrent

             decrease in oxygen consumption and improved water quality (eg. higher dissolved oxygen

             levels in deep waters, increased water clarity). Unfortunately, it is becoming increasingly

             apparent that nutrient reduction goals (40% reduction of N and P inputs) are probably not

             attainable, particularly in the case of N where substantial inputs as NO. enter the Bay

             watershed from air pollution (see eg. Blankenship 1990, 1991). Furthermore, there is no

             reason to conclude & priori that the V=ortio of primary production metabolized by

             microbial communities would be altered, even if nutrient reduction goals were attained (i.e.

             even less production would be available for transfer to higher trophic levels, such that stocks

             of desirable species could be decreased).

                    The second strategy, a bioremediation procedure relying on higher trophic level

             consumers (oysters) whose population density could be directly managed, would permit a

             greater proportion of primary production to be captured by species more desirable than

             microorganisms. This top-down control scheme has, in fact, been proposed as a practical

             means to augment the nutrient reduction strategy (Gerritsen et al. 1989; Ulanowicz and

             Tuttle, in press).


             Replenishment Strategies: Oyste Reef versus Oy        AQuacul


                    Oyster standing stocks could be increased by replenishing historic oyster reefs,

             establishing new reefs, or by raft mariculture of oys ters. Although our arguments are based

             on delineating those practices which will optimize water quality improvement, we are well

             aware of political and sociological considerations which may ultimately drive the


                                                           9







             implementation of oyster replenishment strategies.

                    Gerritsen et al. (1989) used a simple, benthic: suspension feeding model to predict the

             ability of suspension feeding bivalves to remove excess phytoplankton production from Bay

             waters, thereby improving water quality and decreasing oxygen deficits caused by decaying

             phytoplankton. Their model predicts that benthic oyster populations increased to a density

             sufficient to remove 40% of the phytoplankton standing crop (requiring an oyster biomass

             about ten-fold greater than that which existed historically in the Maryland portion of

             Chesapeake Bay) would be unsustainable due to problems of crowding, food limitation, and

             periodic hypoxic and anoxic conditions. In contrast, rafted oyster mariculture population

             density sufficient to meet the 40% nutrient reduction (i.e., 40% phytoplankton biomass

             removal) target of Maryland's Chesapeake Bay Initiatives are theoretically sustainable. It

             should be pointed out that this model scenario assumes the use of oyster replenishment alone

             to meet established water quality goals. If on the other hand oyster replenishment were used

             to augmen nutrient reduction strategies (Ulanowicz and Tuttle, in press), oyster biomass

             supplementation could be reduced and oyster replenishment to the benthos would become an

             effective option.

                    There are other factors, however, which argue for oyster aquaculture. Viewed most

             simply, rafting of oysters within the mixed zone of the water column removes the oyster in

             the "Bay Pyramid" (Fig. 1) from its usual benthic position to the level between primary

             production and bacterial decomposition processes. In this new position, the oyster

             "competes" with bacterioplankton and, if successful, oysters could sequester a substantial

             amount of organic carbon from immediate bacterial metabolism. Redirecting the carbon flow

             by increasing oyster populations would be expected to reduce suspended particulate matter


                                                           10







              (including phytoplankton, bacteria, and detritus) and oxygen demand while improving water

              clarity and increasing a harvestable fisheries resource. Rafted aquaculture is also likely to

              alleviate two major problems faced by bottom dwelling oysters. First, bottom dwelling

              oysters are particularly subject to deleterious influences of hydrogen sulfide arising from

              sulfate reduction in the sediments or from inundation by advected anoxic water. Secondly,

              oysters in more saline regions (e.g., the mesohaline Bay) are threatened by the parasitic

              disease MSX and Dermo, chronic infections often fatal to the animals before they reach

              market size. By suspending oysters near the surface, away from sediments, the threat of low-

              dissolved oxygen concentrations is greatly reduced or eliminated, and the more rapid oyster

              growth rates attainable in rafted aquaculture should lessen the effects of parasitic diseases.


              Field and Ecosyste Modeling Validations


                     Although simple modeling exercises such as those discussed above suggest that

              increased oyster populations could improve Bay water quality and restore balance to the Bay's

              ecosystem, field evidence and ecosystem effects confirming the efficacy of this proposed

              bioremediation strategy has been lacking. Dining 1989 and 1990, however, our laboratory

              conducted at the St. George Oyster Company in St. Mary's County, MD, a small-scale

              demonstration study aimed at determining how rafted oyster mariculture influences

              phytoplankton and bacterioplankton biomass and production, water column nutrients, and

              organic carbon lability. Details of this work may be found elsewhere (Jonas and Tuttle 1991;

              Ulanowicz and Tuttle, in press) or are in preparation (Tuttle and Jonas, in prep), but several

              key findings are worthy of mention here. We compared parameters measured within a raft

              field with measurements made in adjacent, open water areas.


                                                            1







                    Over a late May - early October period, phytoplankton biomass was reduced in the

             raft field (60,000 oysters in a 300 m' area) an average of 50% (range 17% to 72%) with a

             concommitted reduction in primary production (average 44% reduction). Thus, suggestions

             that raft mariculture should decrease phytoplankton biomass but might leave primary

             production unaffected (Gerritsen et al. 1989) or even increased (Tenore et al. 1982) seem

             unfounded. Bacterioplankton abundances were reduced significantly but less dramatically

             (average 20%, range 10 to 40%) than phytoplankton. Bacterial production and metabolism

             (glucose turnover), however, were nearly identical in rafted and open water areas despite

             decreased abundances in the former, indicating increased cell-specific rates of both processes.

             This finding is consistent with observations that bacterioplankton in Bay waters respond

             primarily to labile DOC (Jonas and Tuttle 1990; Malone et al. 1991). Indeed, filterable

             (functionally dissolved) 5-day biochemical oxygen demand (BOD) was decreased an average

             of only 9 % (range 20 % decrease to 21 % increase) within the oyster raft field. Total BOD,

             however, was reduced an average of 30% within the rafts area and particulate BOD was

             decreased by an average of 47% (maximum of 64%), comparable to decreases in

             phytoplankton biomass and production.

                    A quasi-equilibrium, mass action model designed to assess the influence of increasing

             oyster densities on the mid-Bay ecosystem (Ulanowicz and Tuttle, in press) predicts

             qualitatively similar changes resulting from decreasing oyster stock exploitation by 23 %

             (oyster biomass increased by 150%). Although smaller in magnitude (the aquaculture oyster

             densities were much greater), the predicted changes (decreases of 11 %, 6 %, and 5 % for

             phytoplankton abundance, bacterioplankton abundance, and suspended POC-attached bacterial

             mass, respectively) were all in the same direction as parameters measured at the aquaculture


                                                          12







              facility. Interestingly, the ratio of phytoplankton abundance decrease:bacterioplankton

              abundance decrease (about 2:1) was stffdngly similar to that measured in the field. Other

              desirable changes predicted by the model included: decreased gelatinous zooplankton (-89%);

              increased benthic diatoms (+ 29 %), mesozooplankton (+ 5 %), carnivorous fishes (+ 18 %),

              filter feeding fishes (+5 %); and virtually unchanged DOC (+ I %). Thus, the evidence so far

              supports the contention that increasing oyster populations in the Bay and/or its tributaries will

              positively impact the Bay's ecosystem and yield improved water quality.


              Potentially Negative Impac


                     Despite proposed, predicted, and measured benefits of an oyster replenishment

              strategy, there are several possible negative impacts associated with increased oyster densities.

              One of these, namely increased phytoplankton production in the vicinity of the oysters, has

              been discounted (Jonas and Tuttle 1991; Ulanowicz and Tuttle, in press). Another, increased

              nutrient concentrations (particularly N) arising from oyster excretion or from increased

              sediment flux, has not been confirmed by our 1990 study of water quality and pelagic

              microbial processes at the St. George Oyster Company (Tuttle and Jonas, in prep). Indeed,

              levels of ammonia and nitrate + nitrite were not elevated in an oyster raft area compared to

              nearby open waters.

                     Perhaps of greatest concern is the possibility that increased oyster densities could,

              through increased deposition of highly labile organic material (oyster feces and pseudofeces)

              increase sediment microbial processes and, thereby, increase sediment oxygen demand and

              sediment nutrient regeneration. In one sense then, what had been a problem of excess

              oxygen consumption and nutrient regeneration in the water column could be relocated to the


                                                            13








              sediments. This shift appears to occur in mariculture raft areas of Spanish rias (Tenore et al.

              1982) where sediments beneath rafts exhibit higher rates of anaerobic microbial processes

              (e.g. sulfate reduction) than sediments in rias where aquaculture is not conducted. The

              Spanish ria environments, however, are not nearly so productive as the Chesapeake Bay.

              Interestingly, ammonia regeneration in sediments underlying rafted rias is less intense than in

              open rias, probably due to decreased bioturbation in the former (Tenore et al. 1982).

                     The potential for increased carbon loading to sediments by increased oyster densities

              has been pinpointed as the critical outstanding question which needs to be addressed before

              oyster augmentation as a water quality improvement strategy can be implemented (Gerritsen

              et al. 1989). The work reported here was undertaken to directly investigate this potential

              problem. We have hypothesized that sediments underlying rafted oysters will receive greater

              quantities of microbially labile organic matter than outlying sediments and that key, anaerobic

              processes, such as sulfate reduction, will be increased. Both aerobic decomposition of the

              newly deposited material and sulfide produced from sulfate reduction should increase oxygen

              demand beneath the rafts. Accordingly, we selected for measurement parameters to assess

              sediment carbon quantity (POC) and quality (PON and BOD), overall sediment microbial

              activity (dark assimilation of carbon dioxide), and sulfate reduction (rates and pool sizes of

              sulfate and sulfide).



                                                        METHODS



              S
              Auddy Site De%@doon


                     Our proposal identified the St. George Oyster Company oyster aquaculture facility,



                                                             14








              located along the Potomac River in St. Mary's County, MD and operated by the Stewart

              Petroleum Company, as our study area. We intended to examine sediments beneath "oyster

              bag" raft arrays set along the eastern shore of Piney Point Creek where we had conducted in

              1990 a study of the influence of rafted oyster aquaculture on water quality; phytoplankton

              biomass and production; bacterioplankton  biomass, production and metabolism; and water

              column carbon quality (Tuttle and Jonas, in prep). This site had the advantages that: we had

              already collected a substantial body of water column data to serve as a backdrop for our

              sediment study; the oyster raft experimental study area, consisting of adjacent rows of oyster

              bag rafts, bags without oysters, and surrounded by open water on all sides, remained virtually

              -unmanipulated" by aquaculture personnel through an agreement we had with the St. George

              Oyster Company; the site was within 1/2 hour of our laboratory in Solomons; and outside

              access to the site by land was controlled by Stewart Petroleum which owned the property.

              Unfortunately, the St. George Oyster Company was closed by Stewart Petroleum in February

              1991, just before our sediment study was to begin. We are unsure of the causes which

              forced the closure, but continued financial losses incurred by the ambitious St. George Oyster

              Company undertaking and possibly infection of the aquacultured oyster stock by the disease

              Dermo and/or MSX (caused by the parasitic protozoans, Perkinsus marinus and

              Haplospofidium nelsoni, respectively) may well have been key factors.

                     We are indebted to our grant officer, David Bleil of the Maryland Department of

              Natural Resources Tidewater Administration, for identifying and arranging for our access to

              an alternate aquaculture site located at Pintail Point Farm (near the Wye River Institute) on

              Maryland's Eastern Shore. Unfortunately, this site is more than two hours from our

              laboratory and had lower oyster densities and smaller oysters than the Piney Point Creek site.


                                                             15







              Perhaps more importantly, however, financial and research personnel constraints on the

              project inhibited us ftom making pelagic measurements of the sort we had done previously at

              the St. George facility, and it was inappropriate at such a late date for us to interfere with

              operations (e.g., moving oyster trays and aerating the water) critical to the success of the

              commercial enterprise. On the positive side, the sediments at Pintail Point Farm (Deep

              Cover Creek, Paynter and Burreson 1991) were softer and less shell-littered, both of which

              facilitated coring. Moreover, the cove environment was more protected than at Piney Point

              Creek, thus minimizing sediment resuspension events, and the oysters at this lower salinity

              site were not infected with MSX or Dermo (Paynter and Burreson 1991).

                     The oyster raft area of the Deep Cove Creek site is shown schematically in Fig. 2.

              Two oyster raft fields were located approximately 5 m from each other and 15 m out from

              the northerly shore. Water depth throughout the cove ranged from about 1 m (station 1) to 2

              m (station 4) at mean low tide. The raft stations 2 and 3 were located in the approximate

              center of each of the two raft fields. Oyster culture employees set out smaller oysters in the

              field surrounding station 3 for initial grow up, and then moved these oysters to the raft field

              surrounding station 2 for further growth. The larger oysters at station 2 were transported to

              high salinity sites in Virginia for final grow out before they were sold commercially. This

              scheme is employed to take advantage of the observations that the young oysters could be

              grown disease-free at the low salinity site but higher growth rates could be achieved at high

              salinity sites, so long as the oysters were not held at the latter for time periods long enough

              for Dermo infection to occur (Paynter and Burreson 1991).

                     The oysters were rafted in mesh trays (Paynter and Burreson 1991) similar in size and

              design to the "bread tray" rafts used by St. George Oyster Company in 1989 (Ulanowicz and


                                                              16





                                                     @@AIPOT FROM
                                                     ACqJACULTURE :3L-:-rTLj?jt pr



    NI                                          A




                                                 OYSTER
                                         C       R A r-TS

                                           4)
                                  Ob      0
                                     4:P QQ

                              C    0
     PIQTAIL
     Pci@T
     P7ARM


                                  Li        3











                              Sta. 4-














            Figure 2. Schematic diagram of the Deep Cove Creek rafted oyster aquaculture site (not to
             scale).

                                                            17








              Tuttle, in press). The maximum number of oysters rafted in the cove was 600,000

              individuals (K. Paynter, personal communication).

                     Intermittent terrestrial input from an aquaculture waste pond settling at the cove's head

              influenced our selection of control (away from rafts) stations. Station 1 was located about 50

              in down-cove from the raft field around station 2 in order to assess effects of this input.

              Station 4 was established about 35 m out from the cove mouth side of the raft field


              surrounding station 3. Thus, stations 1-4 formed a transect along the main axis of the cove

              which permitted us, within the bounds of original project resources, to assess oyster-related

              influences against the background of potential effects of non-oyster inputs. We also

              established a third control site (station 5) midway into a second cove whose mouth was

              adjacent to the mouth of the rafted cove. This cove received runoff from agricultural fields

              at its head and on its southern shore, but it was unaffected by aquaculture pond input and, to

              our knowledge, had not been used for oyster aquaculture.


              Coring Device Design and Sedime Collecfion


                     In order to sample soft sediments such as those of the oyster raft fields from a small

              skiff, we needed to design and fabricate light, easy to use hand-held coring devices. These

              were constructed chiefly of light, inexpensive PVC tubing. The core barrel consisted of a 15

              cm length of thin-walled, I in. ID PVC plumbing pipe fitted with a threaded male PVC

              connector. The core barrel was screwed into a 1" ID brass flapper valve with its plug

              removed. The flapper valve retained cores in the core barrel liner during retrieval. Core

              barrel liners were cut from 2.5 cm ID plexiglass stock (Read Plastics, Rockville, MD) to a

              length of 20-25 cm. One end was beveled to serve as a cutting edge. The core liner, which


                                                             18








             fit easily but snugly into the core barrel, was held in place by a strip of duct tape wrapped

             around the bottom of the core barrel and the protruding core liner. The core handle,

             consisting of a thick-walled, I" ID, 1.75 m length of PVC pipe, was connected to the top of

             the flapper valve by another threaded male PVC connector. The overall length of the

             assembled corer, about 2 m, was sufficient for most coring at the depths encountered at the

             site. For deeper samples, an additional 1.4 m length of pipe could be easily fitted to the

             handle by means of male and female connectors.

                    A synopsis of sampling visits is shown in Table 1. Full suites of measurements were

             made on June 10 (station 2 only) and on all subsequent visits (stations 1-4, occasionally 5).

             Cores were collected by lowering the core barrel to the sediment surface, followed by a

             rapid, even push of the corer into the sediment to a depth of about 15-20 cm. The cores

             were retrieved gently into the skiff and the bottom of the liner immediately plugged with a

             #51h rubber stopper. The liner was then carefully removed from the core barrel and the top,

             filled to overflowing with overlying water was carefully stoppered to exclude air. Collected

             cores were placed upright in core racks and transported to the laboratory in coolers containing

             ambient temperature cove water. Eleven to 12 cores collected at each station were required

             for a full suite of measurements.



             Hydrogmj2hic Measurements


                    Temperature, salinity, dissolved oxygen (DO), and Secchi disk depth were determined

             routinely at each station. Salinity was measured using a YSI Model 33 salinity meter, and

             temperature and DO with a YSI Model 57 digital dissolved oxygen meter.




                                                           19











                   Table I . leather conditions and other observations made at the Pintail Point Farm"
                             oyster aquaculture facility on sediment sampling days.

                     DATE       WEATHER CONDITIONS              OTHER OBSERVATIONS
                   May-29 ------- Fair-and-hot,-calm  ------------- Dissolved-oxygen-in-rafted-cove-lower ---------
                                                                 than in unrafted cove where the minimum %
                                                                 oxygen saturation ( 67% ) was found at the
                                                                 head of the cove.


                   June 10      Fair and warm, calm.


                   July 12      Fair and warm. Wind light       Air bubblers in operation between the two
                                 from west ( into rafted         oyster raft areas ( Stations 2 and 3
                                 cove ).

                   Aug I        Fair and warm. Wind light       Additional air bubblers installed along
                                and northerly   across           the main axis of the oyster raft areas.
                                 rafted cove                     Bubblers in operation. "Red tide" observed
                                                                 at Station 4 and offshore into mouth of cove.

                   Sep 6        Rainy and cool. Wind gusty      Oyster rafts removed from Station 2 ( largest
                                from the southwest. Water        oysters ) area. Rafts still in place at
                                surface of unrafted cove         Station 3. Bubblers not in operation.
                                most affected by wind.

                   Sep 27       Fair and cool. Wind moderate    Some rafts of larger oysters have been placed
                                 from north ( across both        back at Station 2. Only a few oyster rafts at
                                 coves                           Station 3. Bubblers not in operation.

                   Nov 15       Fair and warm, calm.            All rafts removed from Station 2 area. Still a
                                                                 few rafts at Station 3. Bubblers not in
                                                                 operation.



















                                                                     20









              Pore Water Collection



                     Collection of sediment pore water was required for analysis of free sulfide (HS7),

              sulfate, and total inorganic carbon (TIC). Upon returning to the laboratory, three cores from

              each station were selected for pore water collection. Overlying water was carefully removed

              with a syringe from the top of the core as the core surface was gassed with O,-free N, A #4

              rubber stopper was inverted into the bottom of the liner and a 1.5 cm thick-walled PVC tube

              was applied against the stopper as a core pusher. Two cm thick segments of sediment were

              extruded directly into a gassed-out 50 mf centrifuge tube which was then tightly capped to

              prevent air intrusion. Two additional segments from replicate cores were removed to the

              tube. A total of 6 tubes were filled with sediments from each station so that profiles over a

              12 cm depth at 2 cm intervals were obtained.

                     Capped tubes of pooled sediment were centriftiged at 3000 to 4000 rpm with an IEC

              swinging bucket rotor for 30 to 40 min. For pore water HS and sulfate analyses, the caps

              were removed, the pore water surface was gassed with 02-free N2, and 5 to 8 mL of pore

              water was taken up into a gassed-out syringe. The contents of the syringe were filtered

              through a N,-purged, in-line filter (0.22 Jim pore size membrane) into a pre-weighed

              scintillation vial containing 0.5 mL of 10% (w/v) zinc acetate to fix HS_ as ZnS. The vial

              was then weighed again so that a dilution factor to allow for the zinc acetate could be

              determined.

                     A portion of the zinc acetate-treated pore water was removed for immediate HS_
              analysis or held overnight at 40C for analysis the following day. The remainder was filtered

              (0.22 gm pore size membrane) to remove precipitated ZnS and the filtrate was stored at 40C



                                                            21







             in a plastic scintillation vial for subsequent sulfate analysis.

                    An additional portion of untreated pore water was removed to a small plastic vial

             fitted with a tight cap. The vials were stored frozen and analyzed later for TIC.


             Chemical Determinations and Porosijy


                    Pore water HS   and total reduced sulfur (TRS) in sediments were measured according

             to the colorimetric method of Cline (1969). Samples analyzed for TRS consisted of

             appropriately diluted material from zinc acetate traps used to collect reduced sulfur from

             chromium distillations (see below).

                    Sulfate was measured with a Dionex Model 2020i ion chromatograph operated in the

             30 u Siemens detector range. Ions were separated on an APS-4 column (Dionex) and eluted

             with filtered (0.45 jim pore size membrane) 2.8 mM NaHC03/2.25 MM Na,C03. The

             column was regenerated with 0.025 N H,SO,.

                    Porosity of sediments is defined as the water content (by weight) of I g of wet

             (whole) sediment. Two cm thick segments were cut from fresh cores into pre-weighed

             aluminum pans and the wet weight determined. The segments were then dried to constant
             weight at 75 " C.

                    POC and PON content of the surficial sediments (0-2 cm) was determined by

             elemental analysis of dried sediments, ground to a powder with a mortar and pestle. POC
             was also estimated from gravimetric loss from dry sediments after combustion at 450-C.

             TIC in pore water was estimated by acidifying samples and quantifying purged C02with a

             carbon analyzer fitted with an infrared detector (CBL Analytical Services).




                                                         22








             Biochemica OUgen Demand


                     Five-day and 20-day biochemical oxygen demand (BOD-5 and BOD-20, respectively)

             estimates were made using a newly-developed procedure for sediments. A 2 cm thick

             surficial sediment segment was cut from a freshly coflected core into a 150 mL beaker to

             which was added 90 mL of a nutrient-buffer solution prepared just prior to use from the two

             sterile stock solutions described below. Solution A had the composition: NaCl, 24 g;

             MgSO, - 7H20, 7 g; KCI, 0.7 g; MgC12 - 6H,O, 5.3 g; deionized distilled water, I L.

             Solution A was sterilized by autoclaving. Solution B was composed of: K2HPO, 21.75 g;

             KH2P041 8.5 g; NaHP04 - 7H20, 33.4 g; KN03, 3.2 g; deionized-distilled water, I L. The

             pH was adjusted to 7.2 and the solution was filter-sterilized (0.22 ktrn pore size membrane).

             Solution A was diluted for use to the salinity of sample site surface water (usually about 8

             ppt) with deionized-distilled water and solution B was added at the rate of 1 mL/L of diluted

             solution A to prepare the nutrient-buffer solution.

                     The diluted sediment was vigorously stirred and aerated for 30 min. to oxidize free

             sulfide which in the reduced sediments could contribute greatly to BOD. Two mL portions

             of well stirred sediments were transferred to each of 3-150 mL glass stoppered bottles,

             followed by an "inoculum" of I mL of surface water collected at the aquaculture site. The

             bottles were then filled with additional nutrient-buffer solution and DO was measured with a


             stirring DO probe. Two 4 mL portions of diluted sediments were filtered through pre-

             weighed GF/F glass fiber filters. The collected sediment was washed with two 5 mL

             portions of deionized water and dried to a constant weight. The dry weight data were used to

             normalize BOD data to unit weight of dry sediment.



                                                           23







                    BOD samples were incubated in the dark at 200C and their DO content was measured

             at 5 and 20 days. Controls consisted of "inoculated" nutrient-buffer solution lacking

             sediments. BOD values for nutrient-buffer solution were subtracted from values for diluted


             sediments. Replicate cores from each station were analyzed for BOD.


             D     Assimilation _o_f Carbon Dioxide



                    Dark CO, assimilation as a measure of integrated microbial metabolism was

             determined by measuring the rate of NaH`CO, assimilation in duplicate intact sediment cores

             collected at each station and incubated at in situ temperature in the dark. The methods were

             similar to those described by Radway et al. (1987), but with some important modifications.

                    Overlying water was removed to a depth of 2 cm above the sediment surface.

             Measurements were started by injecting with a Hamilton syringe 10 AL (10 AQ portions of

             NaH"C03 (ICN Radiochemicals) into the sediments through silicone-sealed ports in the side

             of the core liner. Injections began 1 cm beneath the sediment water interface and were

             repeated at 2 cm intervals to a depth of 11 cm. Following incubation for 12 h (optimal

             incubation time was determined from time course measurements up to 24 h with station 2

             sediments collected on June 10), the cores were frozen and held at -700C until alkaline

             extraction of radiolabeled organic material was done.

                    For analysis, frozen core segments were sliced directly into tared, 125 ml, wide

             mouth flasks which were then re-weighed to estimate sediment wet weight. Purging of

             thawed, acidified sediment with air to remove remaining   MC-labeled inorganic carbon and

             boiling alkali extraction of labeled organic material was done according to Radway et al.

             (1987). Radioactivity of extracts was quantified using a Packard 3340 Minaxi liquid


                                                           24








             scintillation counter (LSQ operated in the DPM mode. Water was added to 10 niL Instaget

             (Packard Instruments) to form a gel so that the samples did not phase during counting.

                    Dark CO, assimilation rates were calculated from turnover constants

             corrected for isotope discrimination (factor = 1.06). The turnover constants were also

             corrected for amount of radiolabel recovered when cores were frozen immediately after they

             had been injected (T = 0 correction). This correction factor, determined on a sediment dry

             weight basis, varied in magnitude with the turnover constant. For example, when extracted

             DPM was 0.075% of DPM injected into a segment, the T = 0 correction accounted for 65%

             of the turnover constant. When extracted DPM exceeded 1.2% of injected DPM, the T = 0

             correction accounted for only 4.6% of the turnover constant. Turnover constants, adjusted

             for the T = 0 correction, were corrected for porosity (to convert to a "whole" or wet

             sediment basis) and multiplied by the TIC pool size of the appropriate segment. Assimilation

             rates used to construct core profiles have the units:


                                          ng Carbon/g Wet Sediment x Day.


             Areal rates were calculated by multiplying the per gram rate of each individual segment by

             wet weight of that segment and then summing the segment values to obtain an integrated

             value for the entire 12 cm core. These integrated values are expressed as:


                                                mg Carbon/m' x Day.




             Sulfate Reductio



                    Sediment sulfate reduction was assessed by "S- radiotracer methods (Jorgensen 1978).


                                                          25







             Initial core preparation and injection procedures were identical to those for CO, assimilation

             measurements (see above), except that injections consisted of 10 to 15 FL portions (1.2 to 2.0

             ACi) of carrier-free Na2'SO, (ICN Radiochemicals). Incubations were for 4 h as determined

             from time course incubations (0, 1, 2, 4, and 6 h) conducted with station 2 sediments

             collected on June 10. Sulfate reduction was stopped by injecting 200 gL of 30% (w/v) zinc

             acetate into the sediments through the same injection ports through which radioactivity had

             been added. This procedure "fixed" reduced sulfur by retarding its reoxidation. Fixed cores
             were immediately frozen and held at -70 -C.

                    Total reduced "S was liberated by acidic chromium reduction (Zhabina and Volkov

             1978; Fossing and Jorgensen 1989; Roden and Tuttle, in press). Cores were removed from

             the freezer, gently warmed so that they could be extruded from the core liner, and 2 cm thick

             segments cut and placed into tared 125 mL three-neck boiling flasks. The flasks were rapidly

             Te-weighed, connected to distillation apparatus, and reduced "S was distilled into collection

             traps containing 100 mL of 10% (w/v) zinc acetate.

                    Radioactivity of Zn"S was quantified by LSC counting of LSC 2 ml, of trap contents

             in 7 mL of Instagel. The TRS content of the trap material was measured as described above.

             Calculations made to determine sulfate reduction rates were similar to those used for the dark


             CO, assimilation measurements (see above). T = 0 corrections averaged 2.8% of sulfate

             reduction turnover constants. Sulfate reduction rates for core profiles have been expressed

             as: n mol S/9 Wet Sediment x Day. Areal rates are in the units: in mol S/m@ x Day.


                                                      RESULTS



             Weather and Hydroanhic Data


                                                          26







                    Weather conditions and hydrographic data collected during each of the visits to the

             sample sites are recorded in Table 1 and 2. All samples were collected during mid-
             to late-morning. Highest water temperatures, sometimes exceeding 300C, occurred in May,

             although temperatures remained at 240C or above throughout the season until late September.

             Salinity during the entire period ranged from about 6.8 ppt to 10.0 ppt. Surface water

             dissolved oxygen, when unaffected by operation of air bubblers, followed a pattern in the

             rafted cove of lowest DO at the head to highest values at the mouth (stations 1-4, May 29).

             Lowest observed DO (Table 2) occurred near the bottom of the channel of the Wye River at

             a depth of about 3 meters, suggesting that oxygen consuming in situ processes were at least

             as intense in the river as in the aquaculture cove area. Highest summertime DO in surface

             water occurred at station 4 on August 1. This coincided with the observation of an apparent

             "Red Tide" extending from station 4 to the mouth of the rafted cove.

                 Although DO values were sometimes low, < 3.5 mg/l, they were not biologically

             critical, at least during daylight hours, even without mechanical aeration. Bottom water DO

             tended to be lower than at the surface and this was true of both rafted and unrafted stations.


             When mechanical aeration commenced on July 12, bottom and surface water DO appeared to

             increase within the rafted cove areas relative to station 4 (Table 1). Mechanical bubbling was

             discontinued on or before September 6 even though oysters were still being rafted at station

             3. Substantial DO sags were not noted in the period after June 10.

                   Secchi depths ranged from about 37 to 101 cm (Table 2). Highest Secchi depth values,

             indicating greatest water clarity, occurred in November likely as a result of natural seasonal

             variation in phytoplankton standing stocks. Usually there were no apparent significant

             differences in Secchi depth between rafted and unrafted areas. This suggests that oysters did


                                                          27










                     Table 2   Hydroqraphic and secchi disk data collected at the Pintail Point Farm
                               oyster aquaculture facility on days when sediments were sampled. Oysters
                               were rafted at Stations 2 and 3. Station 6 was located in the main channel
                               of the adjacent Wye River. ND indicates data not collected.

                                         SURFACE BOTTOM     SURFACE BOTTOM      SURFACE BOTTOM      SECCHI
                                           TEMP      TEMP  SALINITY SALINITY        DO       DO      DEPTH
                       DATE    STATION    deg C     deg C       ppt      ppt     mg/L     mq/L        cz

                     May 29         1        29.9        ND       7.0       ND      3.10        ND       >70
                                    2        30.1     28.9        6.9       6.9     3.51     3.42        69
                                    3        29.7     28.6        6.8       6.9     3.30     3.75        85
                                    4        30.5     28.1        6.9       7.0     6.35     5.22        68
                                    5        31.3     30.8        7.1       7.1     6.16     5.60        50
                                    6        29.6     27.3        7.1       7.5     8.44     2.89        ND


                     June 10        2        25.9     25.4        6.9       6.9     5.85     4.48        45


                     July 12        1        26.9     26.6        7.0       7.0     6.79     5.70        53
                                    2        27.4     27.3        7.0       7.1     6.79     5.65        52
                                    3        27.8     27.5        7.1       7.2     6.41     5.33        52
                                    4        28.2     27.5        7.1       7.1     7.49     6.13        57


                     Aug 1          1        26.8     26.4        8.3       8.3     5.65     4.95        53
                                    2        27.0     26.6        8.4       8.4     6.34     4.91        54
                                    3        27.1     26.8        8.8       8.8     6.35     6.09        56
                                    4        27.3     26.5        8.8       8.8     10.00    6.08        45


                     Sep 6          1        24.3     24.2        8.2       8.1     6.16     4.75        43
                                    2        24.9     25.0        8.2       8.2     5.91     5.61        42
                                    3        25.1     25.2        8.2       8.2     5.81     5.56        39
                                    4        25.0     25.2        8.4       8.5     6.10     5.32        47
                                    5        24.7        ND       8.2       MD      6.59        ND       37


                     Sep 27         1        20.3     20.3        7.8       7.8     5.09     5.10        55
                                    2        20.7     20.8        7.8       7.9     5.80     5.62        52
                                    3        20.5     20.6        8.0       8.0     6.07     5.85        52
                                    4        20.7     20.6        7.8       8.0     6.70     5.69        65
                                    5        20.2        ND       7.3       ND      6.43        ND       40


                     Nov 15         1         8.5        8.5      9.9       10.0    12.40    12.01       97
                                    2         8.3        8.3      9.8       9.9     13.24    13.16       98
                                    3         8.0        8.2      9.4       9.6     13.05    10.71       101
                                    4         8.9        8.3      9.3       9.5     12.27    11.80       101
                                    5         8.2        8.0      9.2       9.2     12.42    11.24       90











                                                                         28







              not detectably influence water clarity as determined by this relatively crude method.


              Sgdimen Chaja=jjs@ju


                     The sediments consisted of "clayey" sand material at all stations except station 4

              which was noticeably sandier. All stations, except station 4, appeared to be chemically

              reducing below 2 cm, as evidenced by a black appearance of the sampled cores. Surficial.

              sediments, even in the rafted areas, often contained macrofauna, usually polychaete worms.

              At station 4, however, bioturbation, as indicated by worm tracks, was often apparent to a

              depth of at least 4 cm. Undecomposed or only partially decomposed leaves were sometimes

              encountered in cores taken at stations 1 through 3. These leaves were found at various

              depths, even as deep as 12 cm.

                     Porosity of the sediments exhibited a high degree of uniformity from sampling date to

              sampling date at individual stations (Fig. 3). Surface porosities were in the range of 0.75 to

              0.80 at stations 1, 2, 3 and 5. Throughout the sediment profile, stations 1 and 2 were

              similar, as were stations 3 and 5. However, the profile for station 4 showed lower values

              than any of the others. The differences between station groups were most noticeable at the

              bottom of the sampled profile (10-12 cm). Mean porosity values were 0.67 to 0.68 at

              stations I and 2 (Figs. 3a and 3b), 0.60 to 0.62 at stations 3 and 5 (Figs. 3c and 3e), and

              0.55 at stafion 4 (Fig. 3d).



                     : POC and PON



                     POC and PON, measured by elemental analysis, were analyzed only in the 0-2 cm

              core segments. POC decreased from station I to 4 (Fig. 4a). The pattern for PON was


                                                            29











                  0
                                          A                          B                          c


                  2



                  4

               E  6
               LU


               U-
               q:

               F-
               Z10
               U,



               q:
               W
               < 12
                  0                       D                          E
               LU
               2
               0  2
               Uj


               0  4
               w                                                                 MEAN ï¿½STD
               'a
               :E 6

               LU
               C) 8


                  10



                  12           7- -7-
                   0.5   0.6   0.7  0.8   0.9  0.5  0.6   0 7   0.8   0.9


                           POROSITY (ml WATER/g WET SEDIMENT)












            Figure 3. Porosity profiles of sediments at (A): station 1, (B): station 2, (Q: station 3,
            (13): station 4, and (E): station 5.


                                                      30


















                                               4b..



                                               C)


                                                                                                                                    POC            PON (Molar Ratio)                                       PON (mg N/g DRY SEDIMENT)                                                      POC
                                               C)
                                               z

                                                                                                                           C3        ro 4 0) w                                        m         4@1       0                      N.) w .4 cn 0) 0




                                               0




                                                                                                                                                                                                                                            x x x x x x A 4 x
                                               z


                                                                                                  K)       c-                                              X.
                        LJ                                                                                 C
                                                                                                           >

                                                                                                                                                                                                                            -S
                                                                                                  L]                  C.0


                                                                                                  LIJ                 -4
                                                                                                                      0                                                                                                                               -A
                                                                                                                                                                      2.-1    All
                                                                                                  (n
                                                                                                                      CA)

                                                                                                                      W







                                               CD








             similar except that PON was highest at station 2 on two occasions (Fig. 4b). It is noteworthy

             that station 5, having sediment characteristics most resembling station 3, consistently had both

             lower POC and PON values than station 3 (Figs. 4a and 4b). The consistency of this pattern

             suggests an oyster signal in the sediments at station 3.

                   The surficial sediments at all station were carbon rich compared to the Redfield Ratio,

             C/N = 8 (Fig. 4c), although station I appeared to be the most enriched (Fig. 4a, Table 3).

             At station 2, POC was relatively constant until removal of oyster rafts on September 6, after

             which POC declined slightly. PON decreased simultaneously, although the decrease in PON

             was slightly greater than that of POC as evidenced by an increasing POC/PON ratio (Fig.

             4c). These data suggest that while oysters were present the surficial organic matter was of

             higher nutritional quality. Overall the POUPON ratio at station 2 was lower than at other

             locations (Table 3).

                    POC at station 3 was variable. The relatively high values of POC and PON on

             November 15 might reflect deposition of primary production from a fall phytoplankton

             bloom. At station 4 the trends in both POC and PON were negative after July 12, except for

             September 27 when both were elevated. The POC/PON ratio at station 4 showed the least

             variability. Trends in the data from station to station (Table 3) are suggestive of two effects.

             Decreasing POC from station 1 to 4 implies organic inputs from upland sources such as the

             aquaculture sedimentation pond located in the drainage basin of the cove. Superimposed on

             this gradient is an oyster effect as evidenced by the lower POC/PON ratio at station 2 and

             higher POC and PON at station 3 than at station 5.






                                                          32
















     Table 3. Mean particulate organic carbon, particulate organic nitrogen,
               and molar carbon : nitrogen ratios in the surficial 2cm of sediment
               at stations 1 through 5.



               POC (mg C/g Dry Sed) PON (mg N/g Dry Sed) C:N (Molar Ratio)
                 -----------------      -----------------    ----------------
      STATION      MEAN        STD         MEAN        STD     MEAN       STD



                       51.2       0.8          5.1       0.1      11.8      0.3
         2             44.6       2.1          5.0       0.4      10.4      0.4
         13            37.5       1.6          4.1       0.3      10.8      0.3
         4             29.7       3.2          3.2       0.4      10.7      0.1
         5             32.1       3.0          3.4       0.3      10.9      0.3







































                                            33








             Carbon: Biochemical OUgen Deman


                    Five-day BOD data are shown in Fig. 5a. Excluding the August I sampling date

             when values were skewed by an undetermined amount of BOD in the diluent (see Methods),

             BOD ranged from 0.75 to 2.75 ing C/g dry weight of sediment. Highest values were always

             found at station 2 except on November 15 when almost identical values were found at stations

             2 and 3. Five-day BOD at stations 1 and 3 were similar except on November 15 when it was

             substantially higher at station 3 than at station 1. Station 4 BOD-5 was always lower than at

             station 1 and was usually lowest, except on August 1 where the data may be in error. On the

             final three sampling dates (September 6 - November 15) BOD-5 was higher at station 5 than

             at station 4 but they varied in the same pattern on the different dates. There was a trend of

             decreasing BOD-5 at each station over time (note August 1 possible error) until November 15

             when it increasing at all stations. This observation supports our hypothesis of increasing rate

             of deposition of fresh phytoplankton derived organic carbon during this period. This effect

             could not be due to oysters since most had already been removed (Table 1).

                    The carbon in the surficial sediments at station 2 was clearly more labile than at other

             stations (Fig. 5a) even though POC was not the highest (Fig 4a and Table 3). This

             conclusion is supported by the POC/PON ratios at station 2 (Fig. 4c, Table 3). The BOD-5

             data suggest the same pattern as do the POC/PON data, that of decreasing organic matter

             lability (nutritional quality) in the down-cove direction but with an intermediate oyster signal

             of increased lability.

                    Our goal in estimating both 5 and 20 day BOD was to develop an index of BOD

             lability, i.e. high BOD-5 to BOD-20 ratios would indicate increased carbon lability. On



                                                          34









                                       3-


                                 E    2.5-

                                             <
                                                         >
                                       2-    <           >
                                             <           >
                                             <
                                                         >
                                 M    1.5-               >


                                 E
                                 U?   0.5-
                                 0
                                       0                    A
                                       6   149     161  1 9@'    3   249   270    319-
                                 CD      -
                                 E     5-

                                 Cn                 <
                                       4-           <
                                             >@     <
                                       3-    X      <

                                 E     2-

                                 C@
                                 0                  < A
                                 CO    0
                                      100    161    193    213     249    270 -319

                                      80-
                                                                   X
                                 C@                                >   IN
                                 0    60-
                                 0

                                      40-
                                 U?
                                 0
                                 0                                                >
                                      20                                          >


                                                                                  >
                                       0
                                             1@1    193    213     249    270     319

                                                          JULIAN DAY
                                                   1 2 2 7 3 71        4 El    5








             Figure 5. Five-day and twenty-day biochemical oxygen demand (BOD-5 amd BOD-20,
             respectively) in surficial (0-2 cm) sediments. Bar codes  represent different stations.

                                                            35







              several occasions, however, high amounts of BOD in diluent water resulted in very low or

              zero DO before the end of the 20-day incubation period. Obviously these 20 day values are

              not then useful. Reasonable data were accumulated from the July 12 (Julian Day 193) and

              September 27 (Julian Day 270) sampling trips. On these two dates, the trend in BOD-20 was

              the same as that for BOD-5 and the ratios of BOD-5/BOD-20 were not substantially different

              among the stations (Fig. 4c).

                     With the exception of one occurrence at station 3 (November 15), BOD-5 at station 2

              always exceeded, by as much as 3 times, that at all other stations (Fig. 6a). This increased

              sediment BOD clearly indicates an organic signal in surficial sediments associated with the

              presence of oysters in the overlying water. Similarly, BOD-20 at station 2 also exceeded that

              at other locations except on November 15 (Fig. 6c). At station 3, the other oyster-raft site,

              BOD was greater than at station 4, the down-cove, unrafted, control site. However, station 3

              BOD was variable compared to BOD at stations I and 5, the up-cove and out-cove control

              sites, respectively (Figs. 6b and 6d). On August I (Julian Day 213) BOD at station 3 was

              2-3 times greater than at station 1, but at other times they were about the same. The oyster

              signal in BOD was not nearly as clear at station 3 where smaller oysters were rafted. Even

              where a fairly strong oyster effect was observed it appeared to be gone by November 15 in

              both cases. The oysters effects on labile sediment carbon seem to be local in geographic

              extent and of short-term duration.


                     From an experimental design perspective, there is a question as to the efficacy of

              station I as an up-cove control site. Terrestrial carbon inputs seemed to elevate BOD at that

              site compared to station 4 and 5 (Figs. 7a and 7b). There is no reason to suspect that station

              5 has been influenced by oysters in the recent past. The data shown in Fig. 7c suggest no


                                                            36



                                                                                                                             mop



                           400                                                 400

                                                                             LO
                        0  300-                                                300-
                    U?  CO                                               LO  0
                        LO                                                   LO
                    0   If                                               0
                        c'! 200-                                               200 -
                                                                         Cf)
                    C\1
                                                                             F-
                    U)  CO
                           100-                                                  00
                        0
                                                                             0-0
                        0-0

                                            4-
                             0         1                1111                      0
                                 1 @9 161   193  213 249    270   319                 193     213    249     270     3@9
                           400                                                 400
                                                                    C                                                   D
                        C\j                                                  C14
                    0   0  300-                                          0     300-
                    C@  CO                                               C\1 0
                        LO
                                                                         0   LQ
                    0   @i
                        c6 200-                                          CO    200 -
                                                                         m
                    C\1
                                                                         <   <
                                                                         U)  U)
                        U) 100-                                              - 100-
                                                                             0
                        0
                                                                             0-0
                        1-0
                        01
                                   IL     -,'oL!l ------                          0
                              0    T                      T
                                  193    213     249     270     319                  193     213     249    270     319

                                                                        JULIAN DAY
                                                                  1 0  2 F@ 3 El   4  F]  5




               Figure 6. Comparisons of five-day and twenty-day biochemical oxygen demand values at
               stations 2 and 3 with values at the other stations. Panel (A): BOD-5 at station 2
               compared to BOD-5 at stations 1, 3, 4, and 5; (B): BOD-5 at station 3 compared to BOD-5 at
               stations 1, 4, and 5; (C): BOD-20 at station 2 compared to BOD-20 at stations 1, 3, 4, and
               5; and (D): BOD-20 at station 3 compared to BOD-20 at stations 1, 4, and 5. Bar codes
               represent different stations.






                          300 M1 E12 E23 [14           5       A
                          250

                    U?
                    0   0C 200
                        0
                    0   LO
                    CO  -
                          150


                          100-1
                    W   4-        11.                      1
                        0

                           50
                             0 L
                          300
                                 1  Z12 E3 74 D 5               B
                          250,


                    CM  0 200
                        CD
                    0   LO
                        -ti 150


                        W 100
                    (n
                        0

                        0
                            50-
                        -0


                             0
                          300 ii 5 day      20 day              C
                          250

                        0
                    0   m 200-11
                              1
                    03  to
                          150


                          100
                        I-RO
                            50


                             0
                               149 161   193 213 249 270 319

                                           JULIAN DAY






          Figure 7. Intercomparison of five-day and twenty-day biochemical oxygen demand values
          among stations where rafts were not present. Panel (A): BOD-5 at station 1 compared to
          BOD-5 at stations 4 and 5; (B): BOD-20 at station I compared to BOD-20 at stations 4 and
          5; and (Q: BOD at station 4 compared to BOD at station 5. Bar codes represent different
          stations in panels (A) and (B) and BOD- 5 and BOD-20 in panel (C).

                                                38








             significant differences in BOD between stations 4 and 5.


             Carbon: POC 4 Combustion


                    From a practical perspective determination of organic content of the sediments by

             weight loss after combustion is an inexpensive and rapid technique useful for processing

             multiple samples. Carbon contents of synoptic surficial sediment samples analyzed by

             elemental analysis and, separately, by combustion weight loss were compared (Fig. 8, Table

             4). Although there is some scatter in the data, particularly at low carbon concentrations,

             these two estimates yielded comparable results (r' = 0.83).

                    In order to validate conversion of down-core combustion loss data to POC values and


             to characterize the oxidation state of the sediment POC, we compared mean measured POC

             (elemental analysis), mean POC calculated from the regression shown in Fig. 8 and POC

             values calculated from combustion data assuming the carbon was at the oxidation state of

             carbohydrate (combustion loss * 12/32). The results (Table 4) indicate that, when data from

             all stations were used, the three methods yielded very similar results. Therefore, it is

             reasonable to conclude that the POC, on average, is at or near the oxidation state of

             carbohydrate. POC profiles were generated using this assumption about POC oxidation state.

                    When we compared the stations individually, calculated POC values differed from

             combustion loss POC values at four of the five stations (Table 4). Stations I and 2 evidenced

             higher regression based POC values than were suggested by the carbohydrate-based

             calculations, whereas, these ratios of the values were lower than expected at stations 4 and 5.

             Similar relationships held for ratios of measured POC against combustion loss POC only at

             stations I and 5. This suggests that at station 1, and possibly station 2, POC was more


                                                          39














                   130


                            Y = 1.874X + 24.637

               Z   120      2 =0.825
                            r
               W
                            n  22                                           0
                                                                                               0
               W                                                        C)
                   110
                                                                  0                00
               0
               0)
               0)  100
               E                                             0
               z                                                  0
               0
                    90                  0
               W

               CIO
               2
               0
                    80
               z
               0                            00 0
               (1)
               U)   70
               0
                                  0


                    60           -

                       20         25          30         35          40         45          50         55
                                                  TOC (mg C/g DRY SEDIMENT)
            Figure 8.  Regression of surficial   sediment weight loss on combustion on POC determined by
             elemental analysis.




















       Table 4. Comparison of sediment POC      0-2cm depth ) measured
                 directly ( by CHN analysis      calculated by regression
                 ( see Fig. 8 ) and estimated from loss on combustion
                 (assuming lost material had the composition : CHOH

                                    MEAN
                        MEAN      COMBUSTION   MEAN   CALCULATED MEASURED
                     CALCULATED LOSS POC MEASURED         POC:       POC:
                          POC     (AS CHOH)     POC   COMBUSTION COMBUSTION
                       mg C/g.     mg C/g     mg C/g   LOSS POC    LOSS POC
       STATION    n    DRY SED     DRY SED    DRY SED       %           %


           1      5        50.30      47.57     51.22       105.7       107.7
           2      5        46.96      45.06     44.76       104.2        99.3
           3      4        38,42      38,66     37,45        99*4        96,9
           4      5        26-30      29.57     29.74        88.9       100.6
           5      3        34.41      35.66     32.10        96.5        90.0

       -------------
       ALL DATA 22         39.76      39.66     39.76       100.2       100.2






























                                            41







              reduced than carbohydrate, and at station 5, and possibly station 4, it was more oxidized.

                     Station 1 was carbon rich throughout the sediment profile when compared to all other

              stations (Fig. 9a). Station 2 was enriched only at the surface, and showed the greatest

              decrease in carbon with depth (Fig.9b). Stations 3 and 5 had very sin-filar profiles (Figs. 9c

              and 9e), while station 4 sediments had the lowest carbon content throughout the 0 - 12 cm.

              horizon (Fig. 9d). Whereas there may be an oyster signal of increased carbon at stations 2

              and 3 (note variance in 0 - 4 cm. horizon at station 3), this effect appears to be small and is

              masked by the non-oyster related, down-cove gradient in POC.

                     Station 2 exhibited an unusually large decrease in carbon between the surface and the

              6 cm horizon (Fig  . 9b). We investigated this further by comparing decreases in POC from

              the surface to the 6 cm. horizon on each sampling date. The largest declines were seen at

              station 2 and sometimes at station 3 (Table 5). This effect could be explained by carbon

              enrichment in the surficial sediments (i.e. deposition by oysters) or by increased microbial

              metabolic activity at depth. On the average, the difference at station 2 was nearly twice that

              at any other station.


              Dark Assimilation Qf Carbon Dioxide


                     Total inorganic carbon (TIC) was measured to determine the size of the inorganic

              carbon pool which is necessary to convert "CO, uptake rates to actual carbon assimilation

              values. The TIC content of sediment pore waters represents the balance between

              assimilatory/dissimilatory microbial processes and diffusion (abiotic: transport within the

              sediments). Increases in TIC with depth indicate an imbalance between microbial organic

              carbon degradation and the sum of TIC assimilation and diffusive processes. Assimilatory


                                                             42






                  0   A                            B                 K3        C           W,    0
                  2



                  4

              E
              0
              -   6
              W
                                    >
              LL
              a:  8
              W                       >
              z

              q:
              W
                                    >                                                       >

                  12

              Z   0
              W       D                            E

              W   2
                                >                          0

                  4
              W
                                                             0

                  6
              W               0                              0

                  8



                  10
                       lit,

                  12     ............

                    15 20 25  30 35 40 45 50     15 20  25 30 35 40 45 50

                           COMBUSTION LOSS POC (mg C/g DRY SED)

                                    4   5    6   7    8 AVG
                                    13  >    0   0    10,









             Figure 9. Distributions of POC in sediments at (A) station 1, (B) station 2, (C) station 3,
             (D) station 4, and (E) station 5. Different symbols represent different sampling days whose
             number codes are as follows: July 12 (4); August 1 (5); September 6 (6); September 27 (7);
             and November 15 (8).
                          0@1 I
                                                           0



                                                             0



                              0                              0



                              0                            0
































                                                        43
























                           TABLE 5. Loss of POC ( determined from combustion of dry sediment ) between
                                    the 0-2cm and 4-6ca segments of cores collected July 12 to September
                                    15, 1991.



                                       POC LOSS (mg C/g Dry Sediment) on SAMPLE DATE
                                       -----------------------------------------------
                            STATION      07-12      09-01      09-06     09-27      11-15    K EAR       STD


                               1         3.75        2.68      3.56       5.32      1.27     3.32      1.33
                               2         6.81       10.39      6.29       8.58      6.05     7.62      1.64
                               3         6.31          HD      7.21       4.20      2.58     4.06      2.59
                               4         6.75        4.69      3.02       3.58      4.59     4.53      1.28
                               5            ND         ND      5.59       2.76      2.98     2.27      2.10
































                                                                        44







              process and diff-usion out of the sediments would tend to decrease TIC whereas dissimilatory

              processes would increase TIC.

                     TIC profiles for the five stations are shown in Fig. 10. In nearly every case, TIC

              increased with depth. Profiles for stations I and 4 showed the least scatter from cruise to

              cruise (Fig. 10a and 10d), but TIC increased more substantially with depth at station 1. The

              variability in TIC among sampling dates was particularly evident at stations 2 and 3 (Fig. 10b

              and 10c). The profiles for station 5 were similar to stations 2 and 3 on the same sampling

              dates, indicating a similar balance in carbon dynamics at these three stations (Figs. 10b, 10c

              and l0e). The trend at station 2 was for high TIC values, particularly in surficial sediments

              when temperatures were high and oysters were present (samplings 3 - 5, Fig. 10b). Aeration

              in the water column (see Table 1) tended to depress surficial TIC. Stations 2 and 3, affected

              by deposition from oysters tended to have similar TIC profiles, except on July 12 (sampling

              4) when the station 3 profile resembled that of station 4. Because smaller oysters were rafted

              at station 3, this relationship was probably due to insufficient oyster biomass to generate a

              detectable signal at this early date.

                     Dark CO, assimilation profiles are reported in Figures 11 - 15. Assimilation rates

              were generally high in the surficial sediments (100 - 300 ng C/gd) but declined rapidly with

              depth to less than 50 ng C/gd below the 6 cm horizon. A notable exception occurred in

              station 2 sediments on July 12 (Fig. 12b) where rates in the 10 - 12 cm segment of one of the

              replicates exceeded 1. 1 jug C/gd. This was probably the result of methanogenesis (bacterial

              autotrophic metabolism). This explanation is supported by sulfate depletion in sediments

              below 9 cm on this day at station 2 (see Fig. 18b). Dark CO, assimilation profiles at station

              4 (Fig. 14) differed from those at other stations in that maximum surficial rates (except on


                                                            45







                    0                      A-                           B                           C

                    2



                    4
              -E
              W     6
                                                                                                  (P

              U-
              a:    8
              W
              F-
              z

              W                                                1>

                    12

              Z     0
              W                            D                            E
                          >                                                             3

              W
                    2
              W
                                                                                        4

                    4
              W
                                                                                        5 >

              F-
              CL    6
              W                                                                         6

                    8
                                                                                        7 11@


                    10
                                                                                        8

                    12
                     0   50   100 150 200 250     0   50 100 150 200 250

                              TOTAL INORGANIC CARBON (ug C/ml)









            Figure 10. Distributions of total inorganic carbon (TIC) in sediments at (A) station 1, (13)
            station 2; (C) station 3; (D) station 4; and (E) station 5. Different symbols represent
            different sampling days whose number codes are as follows: June 10 (3); July 12 (4); August
            1 (5); September 6 (6); September 27 (7); and November 15 (8). Lines are drawn through
            symbols for July 12 and September 6 to highlight the extremes at station 3.


                                                          46







                                                        Station 1
               0-                      A                                                             C
                                                            El      0

               2-

                                                                                             El
            -E 4-      C
            0                        0            0        El

            LU 6-
            C)
            <                                    0
            LL
            CE 8-
            LU
            F-
            z
            a: 10
            LU
            <                      July 12                         Aug 1                          Sep 6
               12
                  0     100      280      300    0     100     280      300    0      100     200      300
            z
            Uj 0 -
                             0 DI        D                El       0    E
            LU 2 -
            Cn
            3:     0       El                       El             304.420
            0  4 -
            w
                  0                                   El     0

               6 -


            LU

               8-

                                                 D  7

               10                                -
                                    Sep 27       0                Nov15
               12                                -  I  .     I I
                  0     1@0'     280'     300    0     180      280'    300

                             C02 Assimilation, ng C/g day






            Figure 11. Rates of dark assimilation of C02 as a function of sediment depth at station 1.
            Different symbols represent values for individual cores and the line depicts the average. The
            number in panel (E) gives the magnitude of an officale rate.


                                                         47







                                                                  Station 2
                  0 -                            A                                  B                                 C
                                      El                         El          0                               0 0

                  2 -

                                                                                                    0 El

               -E             F-I
               0
               LU 6 -
               0            D
               <
               U-

               Uj
                             0
               'r-


               z
               cc lu -
               LU                                         0                1156.14 El
                                         June 10                July 12                                          Aug I
               < 12
                     0       100       200       300    0       100       200       300    0       100       200       300

               w  0
                                                 D                 0     0          E                   F-I     0      F
               LU 2
               U)
               3:                                               0 0                                       0
               0  4
               Ui
               C"

                  6
               CL
               LU

                  8



                  10

                                           Sep 6                             Sep 27                             Nov 15
                  12                       @ I I   i    I , , .  ' '-'           I, - I    I    I . I @ I I    I ;  '  . I
                     0       100       200       360    0       100       200       300    0       100       200       300
                                                      C02 Assimilation, ng C/gd





               Figure 12. Rates of dark assimilation of CO, as a function of sediment depth at station 2.
               Different symbols represent values for individual cores and the line depicts the average. The
               number in panel (B) gives the magnitude of an officale rate.


                                                                  48







                                                         Station 3
               0   A                               B                             C
                           El      0                           0 D                       0       0
               2                                                                   0        0
               4
            E
            0                                      1:1   0                       Ej
            1-
            UJ 6

                     0                                 0
            U-
            q: 8
            Ir-
            z       0

            Cr '0
            LU
            1--
            <                       July 12                         Aug 1                          Sep 6
              12
                  0      100      2@O     '300   0"1@0 . . . 280 . . . 300      0      180      280    'Ino
            z
            W  0
                    D                             E
                                                                            343.42
            LU 2

                        C
            0
            -J 4
            LU
                                                          0 El

            l--6
            LU                                      0
               8                                  0
              10

              12                      Sep 27                       Nov 15
                  0       1@0'    200      3 0   0      100      200      30'0
                               Cq Assimilation, ng C/g day






              Figure 13. Rates of dark assimilation of CO, as a function of sediment depth at station 3.
              Different symbols represent values for individual cores and the line depicts the average. The
              number adjacent to panel (E) gives the magnitude of an officale rate.
                                                                  Li







                     C




































                                                          49







                                                                 Station 4
                 0-      E 1)                 A                        1:1        B                                   C
                 2                                                                                  0
                 41
              E
              -C_

              LWLJ6
              0
              <     I                                        El 0                           D     0
              U-
              T- 8
              F-                                          El  0                            11
              z
              LL,


              irl 0
              w
              !;@                                          E]    0                         E
                                        July 12                               Aug 1                               Sep 6
                12                        1 1             . @ I  I  . I @ I I I @               ; I @ . I . I     . I
                    0       100      200       300     0       100       200       300     0       100       200       300
              z
              W  0
                                               D                                  E

              w
                 2 -

              0
                J4
              w
              CO
              'r
              F- 6

              w          0 01



                        013                               C 7

                10
                                         Sep 27             0               Nov 15
                12    , i , 1 @ 1 , ,                          I i . I . I I I I I
                    0       100       200       300    0        100       200      300
                                  C02 Assimilation, ng C/g day







                Figure 14. Rates of dark assimilationof C02 as a function of sediment depth at station 4.
                Different symbols represent values for individual cores and the line depicts the average.


                                                                  50







                                                     Station 5
               E   0 -
               0
                                                      A                                         B
               w                                                                         0

               u-  2
               Cr
               w
                                                                   0 0
               z
               Cc  4
               w
               <        0

                   6
               z
               w


               0
               w   8 -


               0
               -j
               w  10 -
               M

               CL                                Sep 6                                    Nov 15
               w   12
               0      0         100        200         300      0          1@0        2@O        300

                                             C02Assimilation, ng C/g day
















            Figure 15. Rates of dark assimilation of CO, as a function of sediment depth at station 5.
            Different symbols represent values for individual cores and the line depicts the average.


                                                        51







             August 1) never exceeded 100 ng C/gd and rates at depth were the same or higher than at

             other stations (note the station 2 exception, see above).

                    It appears that the dark CO, assimilation measurement, except for times when

             methanogenesis is occurring, estimates microbial respiratory activity which causes increasing

             TIC levels particularly at stations 1, 2, 3, and 5. The more uniform depth profiles at station

             4 indicate reduced respiratory activity in the upper 6 cm of the sediments (compare Fig. 14

             with Figs. 11 - 13 and 15). This relationship is more readily apparent when areal dark CO,

             assimilation rates are compared (Fig. 16).

                    There were no clear patterns among the stations in areal dark CO, assimilation rates

             integrated from 0 - 12 cm depth (Fig. 16a). In the upper 6 cm, however, the down-cove

             effect, decreasing rates from station 1 to station 4, could be seen from July 12 through

             September 27. Data for August 1 and September 27 suggest a possible oyster effect at station

             2 , but this difference was probably not significant (Fig. 16b). In general, microbial

             respiratory activity in the top 6 cm of sediment was higher in stations I - 3 than at stations 4

             and 5, but possible oyster effects at stations 2 and 3 were small in magnitude and/or masked

             by the down-cove gradient previously described- Interestingly, temperature seems not to have

             greatly affected darkC02 assimilation during sampling period.


             Sulfur: Sulfate Profiles



                    Under anoxic conditions in marine sediments such as those studied here, sulfate

             reduction is the major microbial metabolic process consuming organic matter (anaerobic

             respiration) (Roden and Tuttle, in press). Sulfate concentrations must be known in order

             to calculate sulfate reduction rates from the radiotracer experiments, but they are of


                                                           52










                             30 -
                                                                                         A
                           CY      0-12 cm
                           E 25 -

                           E 20 -

                           0
                             15

                           E
                             10 -

                              5                A
                           0
                              0


                             14 -                                                         B
                           N       0-6 cm
                           E
                             12 -

                           E 10

                              8 -

                              6 -
                           E
                                                         vp
                           cn
                              4
                           <                                        IR
                              2

                              0

                             100


                      0
                              80
                      S9
                      E
                           E
                              60
                      <    04
                      0    E
                           C.) 40


                      CD      20
                      CL

                               0
                                                                                        Rl
                                      1@1      193       213      249       270      319
                                                         Julian Day
                                E Stal       9, Sta 2      7 Sta3       El Sta 4         Sta 5






             Figure 16. Depth-integrated areal rates of dark assimilation of CO, over the study period.
             Panel (A): integrated over 12 cm depth; (B): integrated over the 0-6 cm depth interval; (Q:
             ratio of areal rate over 0-6 cm to that over 0-12 cm.
                                                                   V














































                                                           53







              importance in their own right in that their level is indicative of the balance between sulfate

              supported anaerobic respiration and sulfide oxidation. In general, in sediments with active

              sulfate reduction, sulfate concentrations in sediment pore water are expected to be high at the

              surface and to decrease with depth. On the other hand, if substantial sulfate depletion is

              evident in pore waters of surficial sediments, sulfate reduction exceeds sulfide oxidation

              and/or influx of new sulfate from overlying water.

                     Sulfate profiles for the five stations are shown in Figures 17 - 21. The overall pattern

              indicates depletion of sulfate with depth except occasionally at station 4. At station I the

              profiles from July 12 to September 27 were almost identical (Fig. 17) suggesting a

              quasi-equilibrium between reductive and oxidative plus diffusion processes. On November 25

              sulfate concentrations had increased, however, indicating that the balance shifted in favor

              of oxidative processes and/or diffusion. High salinity on this date (Table 2) supports

              increased diffusion of sulfate into sediments.


                     Sulfate profiles at station 2 (Fig. 18) were qualitatively similar to station I profiles

              (Fig. 17) but differed quantitatively. Specifically, throughout the profiles sulfate was more

              depleted than at station 1 indicating increased rates of sulfate reduction at this oyster raft site.

              Even in the 0 - 2 cm horizon in the presence of aeration, sulfate concentrations were

              substantially lower on July 12 and August I at station 2 (Figs. l8b and 18c) than at station 1

              (Figs. 17a and 17b). In the deeper horizons on July 12 (Fig. 18b) sulfate had nearly

              disappeared below 7 cm and on August I (Fig. 18c) below 5 cm. After removal of oysters

              from station 2 (September 6, Table 1) sulfate concentrations in the pore waters above 6 cm

              depth increased dramatically (Fig. 18d) such that the concentrations were even higher than at

              station I (Fig. 17c). This strongly implicates oysters in the overlying water as a source of


                                                            54







                                                                                           Station 1


                                                                                   ...........
                                                                  1 ->vx                                                         A
                                                                     -.1-11.1 .. ... ......... . .............................
                                                                  3 -Y\
                                                                  5-

                                                                  9 M@
                                                                                                 July 12



                                                                                             .. ........... I..-, . . . . . . ................ . .
                                                      E                                                   77j                    B
                                                      0
                                                                  3
                                                      W           5-

                                                                  7
                                                                     _70
                                                                    -7Z
                                                      Ir          9
                                                      Ui
                                                      F_                                          Aug 1
                                                      z

                                                      Cr
                                                      Ui
                                                                                                                                 C
                                                                  3-

                                                                  5
                                                      z           7
                                                      LU          9
                                                      Z>
                                                      Fn       11 -V                              Sep 6
                                                      LU
                                                      U)

                                                                                                                                 D
                                                      0

                                                      LU
                                                      m           5 T@,@@_
                                                                  7-=
                                                                  9 -
                                                      Ui
                                                                                                 Sep 27



                                                                                                     . ...... . ..               E

                                                                                    ............ .. ....................
                                                                  3
                                                                  5                              ...... . .....

                                                                                  . . .... . .............. .
                                                                             ................... .......................... . ..
                                                                  7         in-'..........."..-I.-I., . ........
                                                                                   ................ ....... . .
                                                                                  ...... . ..... ............... .--.-
                                                                                  ... ............. . .. ...................... .

                                                                              ..........
                                                                              ...........
                                                                              ...........
                                                                             .............
                                                                  9
                                                                11                               Nov 15
                                                                    0                                            16        1@        14
                                                                          So 2,CONCENTRATION (mM)
                                                                                4


                     Figure 17. Depth profiles of pore water sulfate concentration at station 1.



                                                                                            55







                                                                       Station 2


                                                              .............. .
                                                                                                   A
                                                 3-


                                                 7
                                                 9
                                                                          June 10




                                                                                                   B
                                                 3
                                        E        5
                                                 7
                                        W        9
                                        C)
                                        <                                  July 12
                                        LL
                                        CE
                                        W
                                                                                                   C
                                        z        3
                                        Cc       5 -5"
                                        W        7
                                                 9V
                                                                            Aug 1


                                        z
                                                          .......................... .. .
                                        W
                                                                                                   D
                                                 3-
                                                 5-
                                        W
                                                 7  -__P


                                                                            Sep 6
                                        0
                                          i
                                        W
                                        CO
                                                                                                   E
                                                 3
                                                                     ...... ....I
                                                 5-
                                        W        7-
                                                 9
                                                                           Sep 27


                                                                     ..........                    F
                                                 3
                                                                     . . . . . . . . . . . . . . .
                                                                    . ..........
                                                 5               . ...
                                                 7

                                                    - - - - - - - - - - - -
                                                                           Nov 15

                                                    0     2      4      6       8     10     12      14
                                                        So 2-CONCENTRATION (MM)
                                                            4



                        Figure 18. Depth profiles of pore water sulfate concentration at station 2.


                                                                     56








              organic matter fueling increased sulfate reduction in July and August. Sulfate continued to

              increase in pore waters at station 2 throughout the remainder of the study (Figs 18d - 18f).

              In fact, by November 15 the profiles at stations 1, 2, 3 and 5 were very similar (Figs. l7e,

              18f, l9e and 21b).

                     At station 3, where the smaller oysters were rafted and which had less total biomass,

              sulfate concentrations were high relative to station 2 and were reasonably constant to a depth

              of 9 cin (Figs. 18b and 19a). However, by August 1 the profile at station 3 (Fig. 19b) was

              almost identical to that at station 2 on the same date (Fig. 18c). On September 6, when

              oysters were still present at station 3, sulfate concentrations were lower than at station 2,

              particularly in the upper 7 cm. This supports the contention that oyster deposition is

              responsible for increased sulfate reduction and, therefore, sulfate depletion. Oysters were

              kept at station 3 until almost the end of the study (Table 1). In this respect the persistent

              sulfate depletion in sediments at this station (Figs. 19d and l9e) compared to stations 1 (Fig.

              17d and l7e) and 2 (Figs. l8e and 18f) strongly illustrate that oysters increase the rate of

              anaerobic sediment microbial processes.

                      Comparison of sulfate profiles at stations 3 and 5 on September 6 (Figs. 19c and 21a)

              and November 15 (Figs. l9e and 21b) indicates that, although they were qualitatively similar,

              sulfate was more depleted at station 3. Because these sediments were so similar in character,

              this depletion further supports the conclusion of an oyster effect.

                      Sulfate profiles at station 4 differed from those at an the other stations (Fig. 20).

              Apart from July 12 (Fig. 20a) there was little or no sulfate depletion throughout the entire

              depth profile. This is probably due to the sandy nature of these sediments, lack of oyster

              influences and reduced input of allochthonous organic material in this down-cove area.


                                                              57






                                                        Station 3


                                                      ...................
                                                                               A
                                         3

                                         5-
                                                          ....... . .....
                                         7-
                                                            ......... .
                                         9-
                                       11                  July 12


                                         1 -1ZZ11A),
                                 E       3
                                 Q
                                         5 V
                                 Ui
                                         71
                                 <
                                 LL      9V
                                 CE   I 10                  Aug 1
                                 U.j
                                 1--
                                 z
                                 CI:     1
                                 LU      3
                                 <       5 fl
                                         7
                                         94
                                                            Sep 6

                                 LU

                                 3:      3 - \\w"\7@V
                                 0       5-
                                 LLJ     7
                                 m
                                 -r      9
                                         i                 Sep 27
                                 a-
                                 LLJ

                                                  ... . . ..........           E
                                                  . ...............
                                         3        ..... ....... . . . . . . . . ....
                                                 ...........
                                         5
                                              . ............
                                                .............
                                         7

                                         9. . . . .... ..
                                                           Nov 15
                                         0                            10,   12    14

                                             So ZCONCENTRATION (mM)
                                                4




                    Figure 19. Depth profiles of pore water sulfate concentration at station 3.


                                                           58









                                                                                                      69


                                              --p uopvjs It, uo!ILAua:)uoo ;)ILjlns mium wod jo s;)Igoid tpd;D(I -OZ ain2ij



                                                                     (INW) NOliVHiN30NOaz;"OS

                                                            t, L      z 1       0            8        9          V        z           0

                                                                                                                    ..............
                                                                                                                        ..........
                                                                 SL AON
                                                                                                              . ...... ......................... .... .. .... -6
                                                                                       . . ... ... ...... . ... . ...... ..... ..
                                                                                                     .......................... .............. .............. . . . ......... .... ...... .

                                                                                                                                 . .... . .....
                                                                                                                    ........... . ... .... ........
                                                                                                 . .. . . ..... . .. .. ... ...................... .... . .........
                                                                                                                  ........................
                                                                            . . ............
                                                                                                                         ......... ....... .
                                                                                                    ......... . . . ..........  ...............
                                                                                                ..... . .... .
                                                                                                      ...                  . . . . . . . New


                                                                                                                                 ......... ..
                                                                 3                   ........... ........... ........ . .. .....................................


                                                                 LZ des


                                                                                                                   ... .. ... ... ... -- ...........
                                                                                                                                        L        m
                                                                                                                        ................ . . ......... . ...0

                                                                                                                                                 (n
                                                                 cl                                                                              m


                                                                 9 des                                                                           M



                                                                                                                                                 >
                                                                                                                                                    i
                                                                                                                                                 m
                                                                                              E.... . . . . . . .. . .. . ...... .
                                                                                              .42


                                                                 LBnV
                                                                                                                     ... . ......
                                                                                                                                                 -n
                                                                                                                                                 >






                                                                                              . .. .............. ..... .......




                                                                 z I Ainr


                                                                                                                                        L


                                                                                                                     ........... . ....  -.4


                                                                                                    . ..... ...........
                                                                 v

                                                                                           t, U01juls







                                                                            Station 5



                                                                     .... ........ %@
                                                               .. .. ... .............                                       A



                                          ... . .... . .... . .. .. .. ..... ..... .... . .............. ...
                                  3

                          E
                          0       5
                          W
                          L)
                          <       7
                          U-

                                          AO@
                          W

                          z

                          W                                                                                           Sep 6


                          z
                          W
                                                                     ................ . ... ...
                                                                          .... ......... ..................... . ............... .............................
                                                                ................... .......... .. . ... ...............
                                                                                        ...... ........
                                                                                                                              B

                          W
                          U)              . ....... . ...... ... ..... ... .. ... ........................ .... ..... .. ....... .................. . ..........
                                                                                     . ... ........ ..........................
                                                             ........... ........ . ...........




                                          ....... ............                  .................... .
                                          .................... -.- . ... . . . ............................ ... ... ... ... .. ... ... ... ... ...........
                                          -.....................
                                          ................. ............................... . .... . ..
                                                                                ..................
                                          ..........
                          W                     . ...... .. . .. . .. .......  ..............
                          CO      5


                                          . . .. ....... ........ .... .. . . ...................... . .
                                            ........ . . ............ ......
                                                               .............. . .........
                                                                     .............
                          EL
                                                     ...........     .............. . ................. .
                                          I.. ... .. . .................... ...... ... ...............
                                          . . . . . ...........   .........
                          W                     ...........   .. ... . .... .... .......
                                                  ........................-
                                                                     . . . . ...................


                                            .. ......
                                          . .. ............. ...
                                                      .. ... .......
                                            ..... ...r. . ........
                                          ggi,
                                                                                                                    Nov 15


                                       0           2             4           6            8           10           12           14

                                                              So 2-CONCENTRATION (mM)
                                                                     4







                           Figure 2 1. Depth profiles of pore water sulfate concentration at station 5.


                                                                            60








             Sulfur: Sulfa Reduction



                    Profiles of measured sulfate reduction rates are shown in Figs. 22 - 26. On the final

             two sampling dates sulfate reduction rate profiles were similar at all stations examined.

             Sulfate reduction rates were low, with mean values not exceeding 50 nmol S/gd throughout

             the sediment profiles. Maximum rates of sulfate reduction occurred during summer, warm

             weather conditions, probably indicating effects of temperature, redox condition and

             the availability of suitable substrates supporting sulfate reduction.

                    On June 10 the sulfate reduction profile at station 2 was bimodal, having peaks near

             the sediment water interface and at 8 - 10 cm. (Fig 23a). On July 12 sulfate reduction was

             bimodal at station I (Fig. 22a) but at station 2 (Fig. 23b) sulfate reduction was highest near

             the surface and declined with depth. The replicate values were highly variable in the top 8

             cin of the cores from station 2. The mean profile is probably more representative of sulfate

             reduction rates than is the core which yielded the lower rate estimates since sulfate at station

             2 was highly depleted with depth on that date (Fig. l8b). The station 3 profile (Fig. 24a)

             was markedly different from stations I and 2. Sulfate reduction occurred mainly below 8

             cm, in agreement with sulfate profiles which indicated depletion, particularly in the deeper

             sediments (Fig. 19a). Despite high rates of sulfate reduction at stations 1, 2 and 3, rates at

             station 4 on July 12 were low throughout the sediment column, never exceeding about 40

             nmol S/gd (Fig. 25a).

                    On August I rates at the sediment surface at station I had decreased from July 12, but

             the minimum in the 4 - 6 cin horizon remained evident and rates below 6 cin increased (Fig.

             22b). Stations 2 and 3 (Figs. 23c and 24b) exhibited very similar sulfate reduction profiles



                                                           61







                                   Station 1
                 0                  A                    B
                         0     El             F1
                 2

                                           El 0

                 4
              E
              C.)
              Lu 6
              C)
              <     0                    El 0
              LL
              CC
              w  8

              z           0          El    El   0
              Cc 10
              LIJ
              <                 July 12               Aug 1
                12
              Z  0
              LU                    C                    D


              w  2 j


              0  4


                 6

              CL
              w


                      0                   El

                10
                12              Sep 27        T   r  Nov'15,
                           1 '0     200 0       100      200
                   0        0
                        SO,2-HEDUCTION RATE (nmol S/gd)






         Figure 22. Sulfate reduction rate as a function of sediment depth at station 1. Different
         symbols represent values for individual cores and the line depicts the average.
                                             0'






                                       62









                                                           Station 2
               0
                                         A                               B                              C
                                   0                     El        308.56
               2

                                                   El         317.74

               4-11
           E       !j -
                                                                  0

               6

           <                                                     0
           U-
               8

                                 0
           z
              10

           <                        June 10                         July 12                          Aug 1
           LL,
           R
           z   0
           LU                             D                              E
           U0  2

           0   4
           -j

           ca
           7-  6


           0   8


              10 -
              12                      Sep 27                         Nov 15
                   0          100          200    0          100          200
                         S04  2REDUCTION RATE (nmol S/g day)






              Figure 23. Sulfate reduction rate as a function of sediment depth at station 2. Different
              symbols represent values for individual cores and the line depicts the average. The numbers
              in panel (B) give values for officale rates.


                                                           63








                                                                      Station 3


                                 F-1                                                             0      227.52



                                                                         209.10 1:1

                    4-
             E
                                                                                                  0
             w      6
             <
             U-
             cr
             w      8
             z          0                                  11   0                              0
             Er   10 -                                     -
             w
                        0            307.30                DD                                  0 El
             <                     July 12                               Aug 1                             Sep 6
                  12    -                - - - - - - -7-

             z      0-
             w                                    D                                   E
                         C

                    2
             w

             0      4-
             w
             M
                    6
             (L         0
             w
                    8

                         0 0
                  10-

                                   Sep 27                             Nov 15
                  12   1 .                    I .          I I I . I . i                  i
                       0            100            200     0            100            200

                            S04 REDUCTION RATE (nmol S/9 day)







                Figure 24. Sulfate reduction rate as a function of sediment depth at station 3. Different
                symbols represent values for individual cores and the line depicts the average. The numbers
                in panels (A), (B), and (C) give values for officale rates.


                                                                    64







                                                           Station 4
                 0-                        A                              B                               C
                                                    1:1 0                          E)

                 2-


                 4-
           E

           Lu    6  -
           <        ED
           U-
                 8  -

           z
           I--

           Cr  10
           LU
           1@       -I                 July 12                         Aug I                          Sep b
               12

           z     0
           w                                D                              E
           Z>
           F5    2
           Uj

           0     4
           LLj
           03
                 6


           Lij
                 8


               10

                                       Sep 27                         Nov 15
               12                       , , i             I . I I i L
                    0          100          200    0          100          200
                         S04  2REDUCTION RATE (nmol S/g day)







             Figure 25. Sulfate reduction rate as a function of sediment depth at station 4. Different
             symbols represent values for individual cores and the line depicts the average.


                                                           65







                                                    Station 5
                   0
                                                     A
               E         07

               w
               0   2
               <
               U-
               Cr.
               LU
               z   4
               w
               <       ED 0

               z   6
               w
                           El 0

               w
                   8
               3:
               0

               w

                   10
               a_
               w                                                C :1
                                                 Sep 6                                   Nov 15
                   12
                      0              100              200       0              100              200
                                       Sq2-REDUCTION RATE (nmol S/g day)















            Figure 26. Sulfate reduction rate as a function of sediment depth at station 5. Different
            symbols represent values for individual cores and the line depicts the average.


                                                       66








              with maxima within the 2 - 4 cm horizon. Decreased surficial rates at these two stations may

              have been due to oxygen input at the sediment water interface due to mechanical bubbling of

              the overlying water (Table 1). Despite relatively high rates at station 2 and 3 sulfate

              reduction at station 4 remained low (Fig. 25b).

                     Unfortunately, data for stations I and 2 on the September 6 were lost due to improper

              processing of cores. At station 3 (Fig. 24c), however, sulfate reduction rates remained high

              in contrast to the companion station 5 (Fig. 26a). Station 4 continued to evidence low sulfate

              reduction rates (Fig. 25c). Similarities in the magnitude and trend in the sulfate reduction

              rate profiles at stations 2 and 3 compared to stations 1, 4 and 5 during the summer indicate a

              marked oyster effect.

                     By September 27 sulfate reduction rate profiles at stations 1, 2 and 3 appeared to be

              qualitatively similar (Figs. 22c, 23d and 24d). However, mean rates in the deepest 6 cm at

              station 3 were noticeably higher than at the other two stations. This may represent an effect

              of oysters having been removed later from station 3 than from station 2 (Table 1).

                     Relationships among sulfate reduction rates in time and space are most easily seen by

              comparing depth integrated data (Fig. 27). At station 1 sulfate reduction rates decreased

              consistently over the period July 12 to November 15 (Julian Day 193 to 319) (Fig. 27a).

              Data for station 2 followed a similar pattern of change with a peak occurring on July 12. In

              contrast, sulfate reduction rates at station 3 peaked on August 1, whereas, integrated sulfate

              reduction at station 4 remained low throughout the study. Although sulfate reduction was

              measured at station 5 on only two occasions, the sulfate reduction rate patterns were not

              inconsistent with those at stations I and 3.


                     Oyster rafting had a substantial, although transient, effect on sediment sulfate


                                                            67










                             14-
                                  0-12 cm Depth
                     Cts     12-
                     m
                     C
                     0   -COV 10
                      :          -
                     t5  _E
                         U)   8  -
                     CD  0    6  -
                     Cr  E                                K
                                                          K
                     42) E    4 -                         K
                                                          K
                     Cz                                   K
                                      >1
                              2 L     >@                  K
                                      K
                                                          K
                                      K                                              Ell i
                     CO



                             14
                                  0-6 cm Depth
                             12-1
                     Cr
                     C   P   10
                     0
                         E
                              8
                                                KI
                                                ><
                                                K
                         0    6
                     CI: E                      J
                                                K         K
                              4                 K         K
                     Cz                                   K
                                                x         >d A
                     75       2                 4
                              0  L              >'<       ><

                     C       100
                     0
                     4-          -
                     Q
                             80
                         E                                <
                                                K         <
                             60                           </                 <
                         Cj                               </                 <
                     Ca                         K         <                  <
                                                KK
                             40       <         K         ><                 <
                     U)               <                                      <
                         (0
                     CD      20       <
                     C.)      0
                     CL
                                      161       193       213      249       270       319
                                                          Julian Day
                                 M Sta 1        @2 Sta 2     7 Sta 3       F-I Sta 4     ZSI Sta 5





              Figure 27. Depth-integrated areal sulfate reduction rates over the study period. Panel (A):
              integrated over 12 cm depth; (B): integrated over the 0-6 cm depth interval; (Q: ratio of
              areal rate over 0-6 cm to that over 0-12 cm.
                                                                            a2i





























                                                            68







              reduction. When oysters were present at station 2 (Table 1) sulfate reduction rates there were

              substantially higher than at either of the control stations I and 4. This effect was most

              noticeable in the upper 6 cm of the sediments (Fig. 27b). After oyster removal from the raft

              area at station 2, sulfate reduction rates in the sediments there were not significantly different

              from rates at any of the control stations.

                    Our conclusions are reinforced by comparing sulfate reduction rates at station 3 with

              those at stations 1, 4 and 5. Presumably the oysters at station 3 were smallest on July 12 and

              increased in size (biomass) as time progressed. It is interesting in this respect that the

              integrated rate of sulfate reduction at station 3 on July 12 was about the same as at station I

              and its distribution in the sediments was also similar to that at station 1 (Figs. 27a - 27c).

              Only 3 weeks later, however, the sulfate reduction rate at station 3 was substantially greater

              than at station 1, and this effect was most noticeable in the upper portions of the sediment.

              This is consistent with increased deposition of labile organic material to the sediment surface.

              Oysters were removed from station 3 at a date later than when they were removed from

              station 2. It is significant that from August 1 through September 27 sulfate reduction rates at

              station 3 were higher than at any of the other stations. However, by November 15 the rates

              were very similar across the rafted cove transect with a possible trend of decreasing sulfate

              reduction from the head of the cove to its mouth.



              Sulfur: Sulfide Profiles


                      Distribution of HS- in sediment pore water supports the sulfate reduction rate data and

              sulfate distribution profiles (Figs. 28 - 32). At station 1, HS- tended to accumulate at depth

              rather than at the sediment surface (Fig. 28). When oysters were present at stations 2 and 3


                                                             69





                                                                       Station 1

                                                                                                           A
                                                   3-0
                                                   5
                                                   7-1
                                                   9                                              July 12

                                         E                                                                 B
                                         0
                                                   3
                                         W
                                         0         5 -zzzzzzZZA-)
                                         U_        7
                                         cc:                   ..., ..........
                                         W         9-
                                                                                                    Aug 1
                                         z        11

                                         W         1                                                       C
                                                   3-
                                                   5-
                                                                                ........... . ..
                                         z         7-
                                         W         9-1
                                                             .. . ..........                        Sep 6
                                         W
                                                                                                           D7@
                                                               .............. ....................
                                         0         3-
                                         W         5           ........ . ........
                                                                                  ......... .....
                                                   7-
                                                                                  ... ........ ......
                                                   9-
                                         W                                                        Sep 27

                                                                                                           E
                                                   3
                                                   5
                                                                  ...... . ...
                                                   7
                                                                            . . . . . .. . . . .. .
                                                                            .............
                                                                            . . .... ... .
                                                                           ...........
                                                   9
                                                  11                                              N  ov 1  5
                                                      0        0.5                     1'5         2         2.5

                                                            HS-CONCENTRATION (mM)



                           Figure 28. Depth profiles of pore water HS- concentration at station 1.


                                                                              70







             (Figs. 29 and 30) HS- tended to accumulate near the surficial sediments, and, were it not for

             mechanical aeration, it might well have contributed to anoxic conditions in the water column.

             Also in concert with sulfate reduction rate data, when oysters were removed from stations 2

             and 3 (before September 6 and 27, respectively), HS rapidly disappeared from surficial

             sediments and the profiles had the same patterns as at station I (Fig. 28). In this respect,

             comparison of the HS profile on September 6 at station 3 (Fig. 30c) with station 5 (Fig.

             32a) also demonstrates an effect of the oysters.

                    Although station 4 was only about 35 meters from the oyster rafts, HS concentrations

             were very low throughout the sediment profiles during the entire sampling period (Fig. 3 1).

             The only case in which a substantially higher than background value was detected was on

             July 12 in the 10 - 12 cin segment (Fig. 3 1 a).


             Sulfur: Total Reduce Sulfur



                    The general distribution pattern for TRS was an increase with depth in the sediment

             profile Figs. 33 - 37). Despite differences in sulfate reduction rates and HS- concentrations,

             TRS in the sediments was reasonably similar at most of the stations (Figs. 33 - 38). A

             possible exception was at station 4 where TRS appeared to be lower overall (Fig. 38d).

             Nevertheless, there appeared to be an increase in TRS at stations 2 and 3 (Figs. 38b and 38c)

             at the same time that areal sulfate reduction rates peaked (Figs. 27a and 27b).

                    The fact that TRS levels in the sediments were relatively similar at all the stations may

             be related to inputs of metals which can react with sulfide, forming insoluble metal sulfides

             such as FeS whose oxidation is very slow. Thus, at station 4 where sulfate reduction rates

             were low most of the sulfide produced would accumulate as metal sulfide. The failure of


                                                           71





                                         Station 2

                             i iii-mw                        A
                             3 M@@
                             5
                             7
                             9                         June 10

                             1                               B
                                                  ...... . . . ...... .. ............. ...... ... ...
                             3-
                     E       5
                             7 -
                     W       9 _m5NxXXXXAXP
                                                        July 12
                     L.L
                     X
                     W                             ........ .... .
                             1 _K11171171111117              C
                     z       3-1V//Z//////Z/ZZI.)
                     cc      5 q2zzzz///@
                     W       7 -@77=77V
                                                          Augl


                     W                                       D
                             3
                             5
                     W         . . .............
                     U)      71
                             9
                     3:                                  Sep 6
                     0

                     W
                     m                                       E
                     m       3
                     (L      5
                     W
                     0       7 \\\\7715  . .. ....... ......... .
                             9
                                                        Sep 27


                                                             F
                             3
                             5
                             7
                             9                           Nov15
                           11 t=m    '
                               0    0.5     1     1.5    2     2.5

                                  HS -CONCENTRATION (mM)



               Figure 29. Depth profiles of pore water HS- concentration at station 2.


                                            72





                                          Station 3

                                                                A
                            3
                            5 4
                            7-0

                                                           July 12



                                ..............-
                                                                B
                                                ... . .... . ..... .... . .... . .
                       E
                            3-
                            5-


                       U_
                       CC   9-
                       W    11                              Aug 1
                              _P      I      T      I
                       z

                       CC
                       W    1                                   C
                       <    3
                            5 -
                            7-9
                            9 -
                                                            Sep 6
                       F5
                       W
                       U)
                                                                D
                       0
                        1   3
                       W
                            5
                            7-0
                                .......... . .. ....
                       (L   9-
                                                           Sep 27  1


                                                                E
                            34
                            5
                            7-0

                                                           Nov 15  i

                                                                  2 5
                              0     05             1.5


                                    HS -CONCENTRATION (mM)


                Figure 30. Depth profiles of pore water HS- concentration at station 3.



                                              73





                                           Station 4

                                                                  A
                              3
                              5
                              7 4"
                              9-1
                                                            July 12


                              1 4.                                B
                              3-1
                        W     5 41
                              7-1
                        U-
                        Cc    9-0
                        LU                                   Aug 1

                        z

                        cc:
                        W                                         C
                        <     3-1
                              5
                        z     7
                        Ui    9-3
                        2
                                                              Sep 6
                        LU             T


                        0
                              3-0
                        m     5
                        m     7
                        a_    9
                        W
                                                            Sep 27


                                                                  E
                              34
                              5
                              7
                              9
                                                            Nov 15

                                0     05      1      1.5     2      2.5


                                     HS-CONCENTRATION (mM)

                 Figure 31. Depth profiles of pore water HS- concentration at station 4.
                                 1-1
                                4*.'@










































                                               74





                                                  Station 5


               E                                                                        A
               U
                .*    3 -
              Lij

              <       5 -
              I I
              rr
              W
              z       9 -
              CC
              LU
                                                                                 Sep 6


              F-
              z
              LU


              LU      3 -
              U)
              3:      5
              0
              -j
              W       7
                      9-0
              CL
              W                                                                Nov 15


                          0           0.5           1            1.5           2            2.5


                                    HS- CONCENTRATION (mM)



                   Figure 32. Depth profiles of pore water HS- concentration at station 5.


                                                       75







                                                Station 1
                      0 -
                                                  A             0 7                  B
                      2-                    July 12                              Aug 1  1


                 E    4 -
                 Q
                 LU                                              0        El
                      6 -

                 LL

                 LU   8 -
                 F-
                 z                        0    Ej
                 a:   10 -
                 LU
                 F-
                 <
                      12

                 Z    0-
                 W                                C                                  D

                 Q
                 Lj   2-                     Sep 27                             Nov 15
                 U)
                 3:   4-
                 0
                 -i
                 uj         11      0                                 0 0
                 C13



                 CL
                 LU   8-
                 Q
                                  0                                     0    0

                      10-



                      12
                        0        60       120       180    0        6@       1 @O      180
                              TOTAL REDUCED S (mmol/kg WET SEDIMENT)

                                           CORE 1    CORE 2 MEAN

                                              11        0








                   Figure 33. Distributions of total reduced sulfur (TRS) in sediments at station 1.



                                                      76








                                                                Station 2
                     0
                                                 A                               B                              C


                     2

                                                                        1:3
               E     4
               0
                                                                    0 0
               W
                     6
               LL                                                                                     0 0
               cr_
               W     8
               z
               cr   10 -
               W
                                          June 10        July 12                          Aug 1            0 0
                    12
               z     0 -
               W                                 D                               E


               W     2 -
               (D
               3:                                                                            CORE 1       o
               0     4 -
               -j                                                                            CORE 2       o
               M
               -r    6                                                                       CORE 3
               LU


               F-                      0                             ED 0                    CORE 4
               a_
               W
                     8 -                                                                     CORE 5        A,

                                                                   0     0                    MEAN
                    10 -       27                        Nov 15
                    12
                        o       60       120      180 0         60       1@0      180
                        TOTAL REDUCED S (mmol/kg WET SEDIMENT)









               Figure 34. Distributions of total reduced sulfur (TRS) in sediments at station 2. Note in
               panel (A) that 5 replicate cores were examined as part of a time course sulfate reduction
                            I @d







               experiment.



                                                                77








                                                           Station 3
                   0
                                             A                           B                            C


                   2

                                                                 D
              E    4
              0

              w
                   6
              <
              U-
              CC                                   J
              w    8

              z                 0     D                         El 0

              CE: 10
              w
                                                                                           0 11
                  12  July 12                       Aug 1                        Sep 6

              z    0
              w                              D                            E

              C)   2                               7:
              w
                                                           E-] 0
                                                                                     CORE 1     o
              0    4
              -j                                                                     CORE 2     o
              w               0 10
              M                                                                      MEAN
                   6 7
              -r
                              0     0
              W    8

                                   0  E],                       0

                  10

                      Sep 27                        Nov 15
                  12
                     0       60      120     180   0      6@      1 @O    180

                      TOTAL REDUCED S (mmol/kg WET SEDIMENT)











                    Figure 35. Distributions of total reduced sulfur (TRS) in sediments at station 3.
                                     C






























                                                           78






               0                              Station 4

                     El                    El   0                  11 0
               2 -              July 12                 Aug 1                  Sep 6
                                                                   ET
           E   4
           w        0
           Q   6

           U-
           cc
           w   8
           z                                                            0 El
           cc: 10 -
           w
                                                   Cl 0

               12

           Z   0
           w                        D                      E

           w   2 -              Sep 27                  Nov 15


           0
               4                                                    CORE 1   0
           -j                                                       CORE 2   o
           w             C                    0 El
           co  6 -                                                   MEAN

           (L            0
           w   8 -

                                                07

               10 -



               12
                 0     60     120   180 0       60   120    180
                 TOTAL REDUCED S (mmol/kg WET SEDIMENT)











                Figure 36. Distributions of total reduced sulfur (TRS) in sediments at station 4.
                                                                       00



                                                                        0



























































                                                79







                                                    Station 5
                   0                                  A
               E                                                      11 0
               0

               w
                   2-

               U-
               Ir                                                             Ej
               w
               z   4-
               a:
               w
                                El                                         1:1      .(D

                   6-
               z
               w
               2
               0                         D 0                                      E 0
               w
                   8 -

               w                          E]     0
               M
                  10
               a_
               w
                  12   Sep 6              11       0              Nov 15
                      0         60         120        180       0          60        120        180

                                  TOTAL REDUCED S (mmol/kg WET SEDIMENT)

                                               CORE 1     CORE2 MEAN

                                                  0         0

















                   Figure 37. Distributions of total reduced sulfur (TRS) in sediments at station 5.


                                                       80











                   20-
                         A                           Station 1         B                            Station 2


                   15-



                   10-



                    5-


                                 /T
                CE  0
                0  20-
                E        C                           Station 3         D                            Station 4

                   15-
                U_
                _j
                (0 10-

                W

                Z)  5-

                W
                a:
                    0
                                                                       161    193   213   249    270   319
                0  20-
                         E                           Station 5                    JULIAN DAY
                   15-                                                0-4cm      0-6cm E 6-12cm El 0-12cm

                   10



                    5-



                    0
                         161   1@3   2@3    249    270  319

                                    JULIAN DAY
















              Figure 38. Depth-integrated areal total reduced sulfur (TRS) content of sediments over the
              study period. Bar codes indicate the depth intervals over which values for individual
              segments were integrated.
                                                                           8  LA
                                                                          A

















                                                             81







              metal sulfides to continue accumulating at the other stations where the sulfate reduction rates

              were much higher probably means that the supply of metals available to react with sulfide is

              depleted and, thus, HS_ accumulates. This contention is supported by the data for station 2

              (Figs. 34a, 34b and 38b) where TRS increased about two-fold between June 10 and July 12

              and remained relatively constant thereafter.






































                                                           82









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                                                       86







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