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

                                                  Chesapeake Bay:
                                                   Introduction to
                                                       an Ecosystem

                                        U. S. DEPARTMENT OF COMMERCE NOAA
                                        COASTAL SERVICES CENTER
                                        2234 SOUTH HOBSON AVENUE
                                        CHARLESTON, SC 29405-2413

                Table of Contents
                 i Foreword
                 3 Introduction
                 6 The Geology of
                   the Chesapeake
                mo The Water & Sediments
                a8 Representative Biological
                   Communities
                z6 Food Production &
                   Consumption
                 33 The Future
                                         fill, ~~l

                                                    Property of CSC Library
    ii

    The Chesapeake Bay                    P,
    and its 'Eributaries                   P

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    Introduetion

              own through the years, residents and visitors alike have
     DF   found the Chesapeake imposing yet hospitable. The Al-
              gonquin Indians called it 'Chesepiooc," which loosely
    translates as the "Great Shellfish Bay' The Spanish explorers named it
    'Bahia de Santa Maria" and considered it the 'best and largest port in
    the world." Captain John Smith, who first mapped the Bay in
    preparation for English colonization, extolled the Chesapeake as, �...     �.�._
    a faire bay encompassed but for the mouth with fruitful and
    delightsome land.' All were impressed with its size, navigability and
    abundance of food.

    The Bay as an Important Resource
    Today, the Chesapeake is still one of this country's most valuable
    natural treasures. Even after centuries of intensive use, the Bay remains
    a highly productive natural resource. It provides millions of pounds of
    seafood, functions as a major hub for shipping and commerce,
    supplies a huge natural habitat for wildlife, and offers a wide variety of
    recreational opportunities for residents and visitors.
       Chesapeake Bay oysters and blue crabs are widely known del-
    icacies. The average annual oyster catch over the last fifty years has
    been approximately 27 million pounds of meats per year. Blue crab
    production totals about 55 million pounds annually, which makes the
    Chesapeake the largest producer in the world. More than half the total
    U.S. catch of both soft-shelled clams and blue crabs comes from the
    Chesapeake, along with more than a quarter of the nation's total yearly
    oyster catch. A thriving fin-fish industry, primarily based on menhaden
    and rockfish, rounds out the Chesapeake's major commercial seafood
    production. The value of the Bay's fishing catch exceeds $ioo million
    annually
        Baltimore's sage, H. L. Mencken once called the Bay, 'a great big
    outdoor protein factory" A recent study by the National Marine
    Fisheries Service ranks the Chesapeake as third in the nation in overall
    fishery catch. The Bay's production is exceeded only by the Atlantic
    and Pacific oceans. That's an impressive ranking, since the Bay covers a
    much smaller geographic area than the other major U.S. fishing
    centers.
       The Chesapeake is also a key commercial waterway, boasting two
    of this country's five major North Atlantic ports. The Hampton Roads
    Complex, which includes Portsmouth, Norfolk, Hampton and New-
    port News, dominates the mouth of the Bay At the northern end, the
    Port of Baltimore handles nearly z4 percent of the export commerce
    leaving the U. S. North Atlantic ports, making it one of the top three
    commercial shipping centers on the East Coast. More than 90 million
    tons of cargo, with a value of nearly $z4 billion, were shipped via the
    Chesapeake during i979.

                                                                                                                             3

                                                         Both Baltimore and Hampton Roads are situated near the
    Shipping Projections:                            coal-producing regions of Appalachia, making them essential to
    Chesapeake Bay                                   promoting the use of U.S. coal abroad. The Hampton Roads
                                                     Complex already leads the nation in exporting coal and lignite.
                          '~ ~  ":      ""              Nearly so,ooo commercial vessel trips are recorded annually from
                             ? ~ /aiT~pton Roa&      Chesapeake ports. Shipbuilding and other related industries also
                                                     depend on the Bay Industries and power companies use large volumes
                                                     of water from the Bay for industrial processes and cooling. The
                                                     estuary also assimilates wastes from some of these industries as well as
                                                     municipal wastes generated by the 13 million people who live within
            44  ~'                 .... '            the Chesapeake watershed.
                                                         Perhaps one of the Chesapeake's most valuable functions, yet one
                                                A    that is difficult to put a price tag on, is its role as a major wildlife
                                                     habitat. The Bay and surrounding wetlands provide a home for a
                                                     myriad of plants and animals.
                                                         Migratory birds and waterfowl use the Bay as a major stop along
                                                     the Atlantic Flyway Here they find food and shelter in the numerous
                                                     coves and marshes. The Chesapeake is also the winter home for
                                ~4'4Y~               approximately half a million Canadian geese and more than 40,000
                                                     whistling swans. It is a nesting area for the endangered bald eagle and
                                       ' -           the osprey, whose largest U.S. population is found in the Bay region.
                                                         The Chesapeake's tributaries provide spawning and nursery sites
                                                     for several important species of salt-water fish, such as the white perch,
                                                  striped bass and shad. During the warmer months, several marine
                                                     species including bluefish, weakfish, croaker, menhaden and spot enter
                          ;:�1         ~<:           the Bay to feed on its rich food supply
                                                         The hospitable climate, lush vegetation and natural beauty of the
           t980     I990 2ooo                       Chesapeake have made it an increasingly popular recreation area.
                                                     Boating, fishing, swimming, hunting and camping are the major
    Bi-state conference on Chesapeake Bay, i7.       attractions. Both power and sail boating have grown dramatically In
                                                     1979, more than n2,ooo pleasure craft were registered in the State of
                                                     Maryland alone.
                                                         Sportfishing is another major recreation activity in the Chesapeake.
                                                     A i979 survey by the National Marine Fisheries Service estimated the
                                                     annual sportfish catch at 28 million, which accentuates the value of the
                                                     Bay as a breeding ground for desirable species of sportfish.

                                                     Defining the Chesapeake
                                                     As well as being a national resource, the Chesapeake Bay is the largest
                                                     estuary in the contiguous United States. The Bay itself is only part of
                                                     an interconnected system which includes the mouths of many rivers
                                                     draining parts of New York, Pennsylvania, West Virginia, Maryland,
                                                     Delaware and Virginia. The Bay and all of its tidal tributaries comprise
                                                     the Chesapeake Bay ecosystem. We are just now beginning to see the
                                                     effects of human activities on the Bay's ecological structure. To assure
                                                     the Chesapeake's continued productivity, we must develop comprehen-
                                                     sive solutions to the often conflicting demands on the Bay's resources.
                                                     Growing commercial, industrial, recreational and urban activities in
                                                     the Bay area are putting substantial pressures on the Chesapeake's
                                                     regenerative powers.
                                                         Some potential problems are becoming apparent. For example, the
                                                     bay grasses, which perform so many crucial functions within the Bay
                                                     ecosystem, are declining in many areas. The oyster catch is diminish-
                                                     ing and kepone and other chemicals have shown up in the biota of the
                                                     Bay In addition, algal blooms have become more prevalent in the Bay
                                                     and its tributaries.

       To determine the causes of, and potential remedies for, those
    problems it is necessary to see the Bay from a new perspective. All too
    often we think of ourselves as external to our environment and ignore
    the many relationships between humans, other living creatures and
    their surroundings. If such relationships are ignored in determining
    solutions to environmental problems, more and greater problems may
    result. For example, after World War II, DDT was widely applied in
    the Bay area to control mosquitoes. An unexpected consequence was
    that the DDT interfered with egg shell development in several species
    of birds. Since the ban on widespread use of DDT some species such
    as the osprey have begun to increase in numbers. Solutions to the
    environmental problems are far more effective when they take into
    account the complex relationships involving all of the components of
    the ecosystem.
       When environmental problems are approached from an ecosystem
    perspective, proposed solutions to specific problems are evaluated in
    light of their effect on all other elements within the system. A truly
    effective solution not only corrects the problem, but avoids damaging
    other elements or relationships within the ecosystem. This approach
    makes problem-solving a great deal more challenging, but leads to
    more effective environmental management.
        In order to adequately define the Chesapeake ecosystem, we must
    go far beyond the actual shores of the Bay itself. The make-up and
    problems of the entire drainage basin significantly impact the func-
    tions and inter-relationships of the Bay proper. The weather, air, land,
    water, plants and animals all form a complex web of interdependencies    Population Proections:
                                                                                  Popultion rojetos
    which together make up the Chesapeake ecosystem. Lest we forget,         Chesapeake Bay Drainage Basin
    humans are also an important and very dependent part of this overall
    system.
       The purpose of this publication is to provide a glimpse of this       millions of people
    complex system along with enough background information to allow         I6
    the reader to understand the general processes involved. In order to
    simplify the presentation, we have divided the discussion into four
    major areas of interest:
    * Geological Make-Up -This section traces the geologic history of
       the Bay, describes the overall physical structure of the Bay proper
       and covers important aspects of the entire watershed.
    * Water & Sediments -This chapter reviews the estuarine proc-
       esses, describes the physical characteristics and chemical properties
       of the Bay waters, and examines the composition and distribution
       of sediments.
    * Key Biological Communities -Here we discuss major plant and
       animal populations living within the Bay itself, the communities in
       which they reside; and the ways in which they interact.
    ' Food Production and Consumption--This chapter explains the                                        g
       production of carbon by plants and how organisms use carbon and
       other nutrients to make food. In addition, it explains how carbon
       used by plants is distributed through the various trophic levels.
        Together, these chapters provide the reader with an appreciation
    for the interactions between the land, water and living creatures of the
    Bay.

                                                                                     1980         1990         2000

                                                                                                    ~~~~~~~~~~

                     The Geology of the Chesapeake

                               he Chesapeake Bay as we know it today is the result of
                               thousands of years of continuous change. Since its creation
                               following the end of the last ice age, the Chesapeake
                     ecosystem has been subjected to an unending modification process.
                     Nature, like a dissatisfied artist, is constantly reworking the details.
                     Some modifications enhance the Bay Others seem to detract from it;
                     but all affect the ecosystem and its interdependent parts. Some changes
                     are abrupt, while others take place over such a long period of time that
                     we can only recognize them as modifications by looking back into
      ,,~....,_~~geologic history.
    .5. AR,15TAONG       Humans are becoming more involved in the reshaping process,
                     often inadvertently initiating chains of events which reverberate
                     through the Bay's ecosystem. Because our actions can have a
                     potentially devastating effect on the entire system, it is essential that
                     we develop an adequate understanding of the Bay's geological
                     underpinnings.

                     Geologic History
                     In geological terms, the Chesapeake is very young. If the entire
                     geological calendar from the earliest fossil formations were equated to
                     one year, the Bay would be less than a minute old.
                         During the latter part of the Pleistocene epoch (which began a
                     million years ago), the region encompassing what is now the
                     Chesapeake was alternately exposed and submerged as massive glaciers
                     advanced and retreated up and down the North American continent,
                     causing sea levels to fall and rise in concert with glacial expansion and
                     contraction. The region still experiences small-scale changes in sea
                     level, easily observed over the duration of a century.
                         The most recent retreat of the glaciers, which began about io,ooo
                     years ago, marked the end of the Pleistocene epoch and brought about
                     the birth of the Chesapeake Bay The melting glacial ice resulted in a
                     corresponding increase in sea level that submerged coastal areas
                     including the Susquehanna River Valley along with many of the river's
                     tributaries. This complex of drowned stream beds now forms the basin
                     of the Chesapeake Bay and its tidal tributaries.

                     The Chesapeake Basin
                     The Bay proper is approximately zoo miles long and ranges in width
                     from about four miles near Annapolis, Maryland to 30 miles at its
                     widest point near the mouth of the Potomac. The water surface of the
                     Bay proper encompasses more than z,2oo square miles. When its
                     tributaries are included that figure nearly doubles. On average, the
                     Chesapeake holds about i8 trillion gallons of water. If the entire tidal
                     Bay system were drained, it would take more than a year to refill with
                     water from rivers, streams and runoff.
                         Fifty major tributaries pour water into the Chesapeake every day.
                     Almost 9o percent of the freshwater entering the Bay comes from the

    northern and western sides. The remaining mo to i5 percent is
    contributed by the eastern shore.
       Although the Bay's length and width assume dramatic pro-
    portions, the depth is another matter. The average depth is less than 30
    feet. In general, the Bay is shaped like a very shallow tray except for a
    few deep troughs which are believed to be remnants of the ancient
    Susquehanna River Valley Fortunately, these troughs provide a rather
    deep channel which runs along much of the length of the Bay Because
    it is so shallow, the Chesapeake is far more sensitive to temperature
    fluctuations and wind than is the open ocean.
       Even though the Bay proper lies totally within the Atlantic Coastal
    Plain, it draws water from an enormous 64.,ooo square-mile drainage
    basin which also includes part of the Piedmont Plateau, and the
    Allegheny Plateau. More than 5o tributaries contribute water to the
    Chesapeake, providing a mixture of waters with a broad geochemical
    range. The Bay is influenced by both the Atlantic Coastal Plain and
    the Piedmont Plateau, two radically different geological structures,
    each contributing a characteristic mixture of minerals, nutrients and
    sediments.
       The Atlantic Coastal Plain, whose waters drain directly into the
    Bay, is a relatively flat, low land area with a maximum elevation of
    about 300 feet above sea level. It extends from the edge of the
    continental shelf on the east, to a fall line that ranges from is to 90

                     Baltimore

            Washington DC

                                                                              The deepest parts of the Bay lie
                                                                              down its center and in some of
                                      '  ~~     .~the larger tributaries. These
                                                                              areas are believed to be
                                                                              remnants of the ancient
                                                                              Susquehanna River Valley,
                30'60'                                                      and are potential areas for
                6o'+                                                        accumulation of sediments
                                                                              which may contain toxic
                                                                              materials.

                                                                                                                                7

                               miles west of the Bay. This fall line forms the boundary between the
                               Piedmont Plateau and the coastal plain. Waterfalls and rapids dearly
                               mark this line, where the elevation sharply increases to approximately
                               ,oo feet, due to the erosion of the soft sediments of the coastal plain.
                                   Cities such as Fredericksburg, Richmond, Baltimore and Washing-
                               ton, D.C. have developed along this fall line, taking advantage of the
                               water power potentials of falls and rapids.
                                   The Atlantic Coastal Plain is supported by a bed of crystalline
                               rock, covered with southeasterly-dipping wedge-shaped layers of
                               relatively unconsolidated sand, clay and gravel. Water passing through
                               this loosely compacted mixture easily leaches out the mineral content.
                               The most soluble elements are iron, calcium and magnesium. Waters
                               of the Coastal Plain average moderately soft to moderately hard,
                               although extreme local levels of hardness can and do occur here.
                                   The Piedmont Plateau ranges from the fall line in the east to the
                               Appalachian Mountains in the west. This area is divided into two
                               geologically distinct regions by Parrs Ridge, which traverses Carroll,
                               Howard and Montgomery Counties in Maryland and adjacent coun-
                               ties in Pennsylvania. The eastern side is composed of several types of
                               dense crystalline rock, including slates, schists, marble, and granite.
                               This results in a very diverse topography Rocks of the Piedmont tend
                               to be relatively impermeable, and waters from the eastern side are
                               usually soft and flow directly into the Bay.
                                   The western side consists of sandstones, shales and siltstones,
                               underlain by limestone. This limestone bedrock contributes calcium
                               and magnesium to its water, making it hard. Waters from the western
                               side of Parrs Ridge flow into the Potomac River, one of the Bay's larger
                               tributaries.
                                   Clearly, the waters that flow into the Chesapeake Bay have a
                               chemical identity that significantly depends on the geology of their
                               place of origin. In turn, the nature of the Bay itself depends on the
                               characteristics and relative volumes of these contributing waters.

                               Erosion & Sedimentation
                               Since its creation, the Chesapeake's shoreline has undergone constant
                               modification by erosion, transport and deposition of sediments. In this
                               process, areas of strong relief, like peninsulas and headlands, are
                               eroded and smoothed by currents and tides, and the materials are
                               deposited in other areas of the Bay. Sediments carried by the river
                               currents are left at the margins of the Bay and major tributaries,
                               resulting in broad, flat deposits of mud and silt. Colonization by
                               marsh grasses stabilizes the sediments, and marshes develop. Land
                               build-up in the marshes continues until the area becomes part of the
                      .       .. ' " *-shoreline.
                                   The speed at which these modifying processes progress is deter-
                               mined by a multitude of factors, including weather, currents, composi-
                               tion of the affected land, tides, wind and human activities.
                                   The story of Sharp's Island, off the eastern shore of the
           �      .     "     Chesapeake, provides a telling example of the power and swiftness of
                               erosion in the Bay area. In colonial times, the island was a rich
        :"' '                 " plantation of six hundred acres. Today, it is completely submerged, a
                               victim of erosion. Sharp's Island disappeared so quickly that some
                               longtime residents of the eastern shore can still remember seeing the
                               white frame hotel that was situated on it.
    �'!'~":"' ':', ..            '"' :[i ""':  The forces of erosion and sedimentation are continually reshaping
        ";':'.    s           the details of the Bay.

                                                 The Chesapeake Bay Drainage Area
                             to A.d ~, ~~Approximately 64,000  square miles of territory in five states
                             iS ~~ ~z~                         ~ ~drain into the Chesapeake. The  rivers which empty into the
                       )     _                              Bay flow through a very diverse geology, and carry the
                   (      '                                many different substances which help to create the fertile
                          /'5_,4\~ f~~~i$                 chemical mixture  of the Bay.

        -T
    '' t.-,
                  , I-Finger Lakes

                   X~~~~~~~~ -i-
       IN

        miles                                                           Pa\
                                                                                               9

                                                       The Water & Sediments

                                                                  makes up approximately 80 percent of our total body
                                                                  weight. Without it, we cannot survive. Perhaps, because its
                                                       presence is so pervasive in our lives, we tend to think of water as being
                                                       homogenous rather than as a substance with extremely diverse
                                                       characteristics and properties.
                                                           Pure water is composed of two atoms of hydrogen bound to one
                                                       atom of oxygen. However, in nature water is never pure. It tends to
                                                       hold other substances in solution and easily enters into various
                                                       chemical reactions. It is this proclivity for "impurity" that makes water
                                                       an excellent environmental medium. Water normally contains dis-
                                                       solved gases such as oxygen, as well as a wide variety of organic
                                                       (containing carbon) and inorganic materials. The concentration and
                                                       distribution of these "impurities" can vary widely within a single body
                                                       of water. Add differences in temperature and circulation which can
                                                       enhance or retard certain chemical reactions, and the variety of
                                                       possible water environments vastly increases.
                                                           Of all bodies of water, estuarine systems offer the greatest diversity
                                                       in water composition. An estuary, according to oceanographer Donald
                                                       W. Pritchard, is a "semi-enclosed body of water which has free
                                                       connection with the open sea and within which sea water is
      'llypical Vertical Temperature measurably diluted by freshwater from land drainage." Within an
      'rpical Vertical Tem peiatur           e        gestuary, freshwater mixes with salt water, each contributing its own
      (ArfilMeanySpring                                variety of chemical and physical characteristics. The mixing of fresh
                                                       and salt waters creates unique chemical and physical environments,
      depth                                            each of which supports different communities of organisms particu-
      o, s                            _      _         larly suited to that type of water. The greater the number of different
                                                       environments available within a body of water, the greater the variety
                                                       of life that is likely to be sustained therein.

                   ..... iWater: Temperature, Salinity & Circulation
                    .......-- ~i::~cir:iiia .  ... The distribution and stability of Bay environments depends on three
                                                       very important physical characteristics of the water -temperature,
                                                       salinity and circulation. Each affects and is affected by the others, and
                           ........ i: ::::::::E~    ;together they determine the physical characteristics of the water at any
         i ii: ::':���   'RR :M!i::          given point in the Bay
         /i"        ,/...                                  Temperature dramatically affects the rates of chemical and
                                                       biochemical reaction within the water. A io-degree Celsius (i8-degrees
                                                       Fahrenheit) increase in temperature can double the speed of many
                                                       reactions. Because the Bay is so shallow, its capacity to store heat over
                                                       time is relatively small. As a result, water temperature fluctuates
                                                       considerably, ranging from o-degrees C to 29-degrees C (32-degrees F
                                                       to 84-degrees F) over the annual cycle. Fluctuations such as this are
                                                       significant because water temperature in turn affects other processes
                                       ....-    -       --------such as spawning which is partially regulated by water temperature.
                                                          The Bay's vertical temperature profile is fairly predictable. During
                                                       the spring and summer months, the surface waters are warmer than

    IO

      Spring Salinity                                            Autumn Salinity
      in parts per thousand                                      in parts per thousand
               0-5                                                        0-5
               5-10                                                       5-10

              15'-20                I                                     J'-2:

                                                                            25-30
            25-30                                                        53

                                                                                                    With the heavy influx of
                                                                                                    freshwater in the spring, the
    the deeper waters, due to the warmth of the sun. The water surface of the Bay-declines in
    temperature gradually decreases in relation to depth until a point is slnt.A rswtrfo
                                                                                                    declines in the autumn, salinity
    reached where the temperature drops abruptli. That point is known as from seawater increases.
    the thermocline. Below the thermocline, the temperature again
    resumes its gradual drop, until the coldest depths are reached at the
    bottom. Often by june, the turbulence of the waters helps to break
    down this layering and to cause the thermocline to lose definition.
       in the fall, the warming radiation of the sun begins to dimim'sh. As
    the surface water cools, it increases in dens'~ becoming heavier. Once
                                               It,

    'the surface water becomes colder and denser than the water toward
    the bottom, it begins to sink and vertical mi~xing occurs. Wimd may
    speed up the process. This mixing action can bring nutrients, materials
    essential to the growth of organisms, up ftom the bottom and into
    higher water levels. The wateover makes the nutrients available to
    phytoplankton and other organisms inhabiting the     u   pper water levels.
       During the wine.e     the ther         temperature becomes relatively
    constant from surface to bottom until March, when th e process of
    surfac                                    e warming begins    again.
       Saln ty is the second key factor a ffec ti g the physical make-up of
    the Bal it is the concentration of dissolved salts in the water, usually
    expressed in parts omes of and per 4ooo parts of water (ppt). Freshwater
    contains few salts (drinking water usually has a salinity of less then 0.5
    ppt), while seawater averages 3t ppt.

      Two-Layered Circulation Pattern with
      Zone of Maximum Turbidity

          fresh
         water  .  .

                                                             Since seawater enters the Bay through its mouth (located at the
                                                         southeastern edge), the salinity is highest at that point and gradually
                                                         diminishes as one moves toward the northern end. The salinity levels
                                                         within the Chesapeake vary, depending on the volume of freshwater
                                                         that flows into the Bay. Salinity declines in the spring when rainfall,
                                                         groundwater and melting snow cause large increases in freshwater
                                                         inflows. For example, the volume of the Susquehanna River, which
                                                         contributes about so percent of the Bay's freshwater, can vary is-fold
                                                         on a seasonal basis. In the fall, when freshwater inflows are greatly
                                                         reduced, high levels of salinity extend farther up the Bay.
                                                             Salinity can be indicated on a map or chart by lines called
                                                         isohalines. These lines connect points in the water which have the
                                                         same salinity. Because the greatest volume of freshwater enters the Bay
                                                         from the northern and western tributaries, the isohalines tend to show
                                                         a marked southwest to northeast tilt. This means higher salinity levels
                                                         extend farther up the Bay on its easter side. To a lesser extent, the
                                                         effect of the earth's rotation, called the Coriolis force, also influences
                                                         this tendency toward higher salinity levels along the eastern shore.
                                                             Salinity levels are graduated on a horizontal plain from one end of
                                                         the Bay to the other. They are also graduated vertically from top to
                                                         bottom. Since the presence of salts increases density, the lighter
                                                         freshwater tends to remain at the surface, while salinity increases with
                                                         depth. However, the relationship between depth and salinity is not
                                                         constant. From the surfac        e bottom, salinity increases gradually,
                                                         but there may be an intercent of the ays feshwatere the increase in salinity is
                                                         abrupt. This layer is known as the halocline. It normally extend farther up the Bay
                                                         less saline surface layer from the high saline bottom layer. As such, it
                                                         also disting uishes two different types of environments.
                                                             Perhaps the most important aspect of the Chisohalines graduated
                                                         salinity levels isup their effect on its ea distern side. To a lesser extentof the
                                                         various biological populations living in the  Bay
                                                             Estuarine circulation is also extremely important. The movement
                                                         of the waters transports plankton, eggs of fishes, shellfish larvae,
                                                         sediments, dissolved oxygen, minerals, nutrients and other chemicals.

    I2

       Freshwater from rivers, streams and run-off affects salinity. This
    effect is the primary factor driving circulation of the Bay and tidal
    tributaries. In the upper Bay and upper reaches of tidal tributaries,
    freshwater flows seaward over a layer of saltier, denser seawater
    flowing inward. The opposing movement of these two flows forms
    saltwater fronts, or intrusions, which move up and downstream
    primarily in response to freshwater inflow.
       These fronts are characterized by intensive mixing. The most
    vigorous mixing occurs near the head of the Bay and its major tribu-
    taries and causes suspension of bottom muds. Sediment particles, in-
    cluding inorganic, dead organic and living plankton, move seaward in
    the surface flow, then sink into the upstream-flowing saline water. The
    particles mix with the suspended muds and create a zone known as the
    turbidity maximum. Nutrients are mixed in the turbidity maximum.
    making these zones highly productive.
       Meteorological or tidal forces can alter this circulation pattern. The
    two-layered flow (fresh above, saline below) or stratification is
    strongest in the spring when freshwater inflow is greatest. The flow of
    freshwater slows down in the fall when there may be no distinct layers
    at all. Stratification may also vary within any season depending on
    rainfall or catastrophic events such as hurricanes. In addition, the tides
    are sometimes strong enough to mix the two layers.
       For the lower Bay, the two-layered circulation breaks down into a
    more complex pattern. The location of the major rivers, as well as the
    earth's rotation (Coriolis force), cause freshwater to accumulate along
    the western shore. Salinities on this side are thus lower than they are
    to the east. The two layers are tilted so that the freshwater stays mainly
    on the western shore. The result of this process is a more fully mixed
    water column in the lower Bay Here ocean water enters the Bay
    through the northeastern side of the Bay's mouth, and freshwater
    escapes primarily on the southwestern side of the mouth.
       Weather influences the circulation of the Bay system by disrupting
    the typical two-layered flow. Wind plays a role in the mixing of the
    Bay's waters. Another significant factor is barometric pressure, which
    depresses or raises the level of surface waters. Wind and barometric
    pressure can change the nature of the classic two-layered circulation,
    occasionally reversing flow direction for short periods of time. For
    example, strong, northwest winds associated with high pressure areas
    push water away from the coast, creating exceptionally low tides.
    Exceptionally high tides, on the other hand, result when strong
    northeast winds associate with low pressure areas.
       There are some other notable exceptions to this basic circulation
    pattern. Circulation of the smaller tributaries does not always follow
    the two-layered pattern outlined above. These very small tributaries,
    such as the Bush and Gunpowder, are flushed by tidal exchange, with
    little chance for stratification.
        The intermediate and larger tributaries of the upper Bay, such as
    the Patapsco (Baltimore Harbor), Chester, and Patuxent may have
    three distinct layers of flowing water at their mouths. This
    pattern may occur during the spring, when the Susquehanna River
    pours large volumes of freshwater into the mouths of these tributaries
    at the surface. This forces the surface freshwater to flow back
    upstream. The heavy, saline water from the deep channels of the Bay's
    main stem flows up the tributaries along the bottom. In the meantime,
    a layer is created in the middle at the mouth of the tributary. This layer
    contains a mixture of the top and bottom layers and flows outward
    with the water coming from the tributary itself.

                                                                                                                              '3

                                                    Chemical Factors
                                                    This section examines the chemistry of the Bay's waters and provides a
                                                    background for understanding food production in the Chesapeake.
                                                    These chemical processes play a major role in defining
                                                    physiological limits to the relative abundance and distribution of
                                                    plants and animals within the Bay In this context, it is important to
                                                    remember that many constituents contribute to the dynamic balance
                                                    among organisms, water and sediments.
                                                        The waters of the Chesapeake are a complex chemical mixture,
                                                    containing dissolved organic and inorganic materials, including dis-
                                                    solved gases, nutrients and a variety of other chemicals. The more
                                                    saline waters of the Bay are chemically similar to ocean water. Seawater
                                                    contains six major natural chemical components -chlorine, sodium,
                                                    magnesium, calcium, sulfate, potassium, with sodium chloride'(salt)
                                                    dominating. Ocean water also contains many minor elements, includ-
                                                    ing cobalt, manganese, iron, molybdenum and silica. These elements
                                                    are important in many biological reactions. For example, in minute
                                                    quantities, cobalt is required for living organisms to make vitamin B,.
                                                    Heavy metals such as mercury, lead and cadmium usually occur
                                                    naturally in very low concentrations. In some areas, though, human
                                                    activities add heavy metals to the water in quantities large enough to
                                                    create serious pollution problems.

    Dissolved Inorganic Compounds in Seawater

                                                                1acu-000

                          pure water

                                                           l               i                         -i
                                                           catrbon  -28

       Some of the components of sea water are "conservative" -they do
    not react easily with other chemicals and are not taken up by plants or
    animals. Their concentration is relatively constant. Sodium chloride is
    conservative, while the nutrients, nitrogen, and phosphorus, change
    seasonally These conservative elements provide a good measure of the
    extent of mixing between freshwater and seawater. In the Bay, salinity
    of i7.5 ppt indicates seawater has been diluted 50 percent by freshwater.
        Concentration of the major constituents of seawater is relatively
    constant from place to place. Freshwater, by contrast, contains a
    variable composition of salts that depends on the soils and rocks the
    water has come in contact with on its way to the Bay. Sodium chloride
    is negligible in freshwater. Silicon, however, which is important in the
    cell wall structure of diatoms (one-celled plants), occurs at higher
    concentrations in Susquehanna River water than in ocean water.
        Among the important chemical constituents found in waters of the
    Chesapeake are: organic and inorganic materials, dissolved oxygen,
    carbon dioxide, nitrogen and phosphorus. Both fresh and salt water
    contain a myriad of natural dissolved organic materials. These natural
    compounds are only now being identified, and are believed to be the
    breakdown products of proteins, carbohydrates and fats from plankton
    and higher organisms such as fish. These breakdown products come
    from several sources. Microorganisms decompose dead organisms and
    release dissolved organic compounds to Bay waters. Organisms are
    also "leaky" and excrete many organic compounds directly into the
    water. In addition, dissolved organic material flows into the Bay via its
    tributaries.
        Many of these organic compounds are required (at extremely low
    concentrations) for the growth of phytoplankton, microscopic plants
    found in open waters. For example, vitamin B. may occur at one
    nanogram per milliliter (or approximately I part in a billion) and still
    meet the growth requirements of many organisms. Some natural
    organic compounds also serve as chelators. This means that they are
    attracted to, and bind with, metals like iron. This keeps the metals in
    solution and biologically available.
        Synthetic (man-made) organic materials, some of which are toxic,
    may also be added to the waters of the Bay system. Many chemical
    wastes of industries and sewage treatment plants, as well as most
    herbicides and pesticides, are organic chemicals.
        Inorganic dissolved salts (such as sodium chloride) are impor-
    tant to the adaptive processes of many organisms. Some fish spawn in
    fresh or slightly brackish water and must move to more saline waters
    as they mature. These species have internal mechanisms which enable
    them to cope with the changes in salinity. In addition to being
    biologically important, natural dissolved materials also affect the
    physical properties of water such as lowering its freezing point or
    increasing its density
        Dissolved oxygen is essential for all plants and animals inhabiting
    the Bay. However, it is a particularly sensitive constituent because
    other chemicals present in the water, biological processes and tempera-
    ture exert a major influence on its availability during the year. The
    maximum amount of oxygen which can be dissolved in a given unit of
    water increases as the water becomes colder and decreases as the water
    becomes more saline. For example, at Is degrees Celsius, one liter of
    seawater has a saturation level of 5.8 milliliters of dissolved oxygen,
    while one liter of freshwater can hold 71 milliliters.
        Oxygen is transferred from the atmosphere into the surface waters
    by the aerating action of the wind. It is also added at or near the

                                                                                                                              Is

    surface as a by-product of plant photosynthesis. As a result, floating
    and rooted aquatic plants increase dissolved oxygen levels. Since the
    existence of plants also depends on the availability of light, the
    oxygen-producing processes occur only near the surface or in shallow
    waters.
       Surface water is at or near oxygen saturation all year long, while
    deep bottom waters range from saturation to nearly anoxic (no oxygen
    present). During the winter, respiration levels are relatively low.
    Vertical mixing is good and there is very little salinity stratification. As
    a result, dissolved oxygen is plentiful throughout the water column.
    During the spring and summer, increased levels of animal and
    microbial respiration, greater salinity stratification and reduced vertical
    mixing result in low levels of dissolved oxygen in deeper water. In fact,
    the deeper areas of some tributaries, such as the Patuxent and the
    Potomac, can become anoxic in the summer. Later, when the surface
    waters cool in the fall, vertical mixing occurs and the deep waters are
    reoxygenated to their winter condition.
       Carbon dioxide, another dissolved gas, is important to the
    well-being of the Bay's aquatic environment, acting as a buffer against
    rapid shifts in acidity. Such shifts can be detrimental to both plant and
    animal life in the Bay Carbon dioxide is also essential to plant life in
    the Bay because it provides the carbon with which plants produce new
    tissue during photosynthesis. Like oxygen, carbon dioxide is highly
    soluble in water. Its availability is also affected by temperature and
    salinity in much the same fashion as oxygen.
       Nitrogen is one of the major constituents in the production of
    plant and animal tissue. Its primary role is in the synthesis and
    maintenance of protein. The nitrogen enters the ecosystem in several
    chemical forms, including ammonia, nitrate and nitrite, although the
    preferred nutrient form for the growth of Bay phytoplankton is
    generally ammonia. Nitrogen also occurs in other dissolved organic
    and particulate forms, such as living and dead organisms.
       Some bacteria and blue-green algae can extract nitrogen gas from
    the atmosphere and transform it into organic nitrogen. This process,
    called nitrogen fixation, is an important pathway in the cycling of
    nitrogen between organic (living) and inorganic components.
       Phosphorus is another key nutrient in the Bay's food system. It is
    found in the water as dissolved organic and inorganic phosphorus as
    well as in particulate form. This nutrient is essential to cellular growth
    and reproduction. Phytoplankton and bacteria assimilate and use
    phosphorus in their growth cycles. Phosphates are the preferred form,
    but other simpler forms of organic phosphorus are also used when
    phosphates are unavailable.
       When phosphates are highly concentrated in waters which contain
    oxygen, they combine with iron and suspended particles and eventu-
    ally settle to the Bay bottom, becoming unavailable to phytoplankton
    and temporarily excluded from the cycling process. Phosphates
    sometimes become long-term constituents of the bottom sediments.
    Phosphorus compounds in Bay waters generally occur in greater
    concentrations in less saline areas, such as the upper portion of Bay
    tributaries. Overall, phosphorus concentrations range more widely in
    the summer than winter.
        Just as fertilizer aids the growth of agricultural crops, both
    nitrogen and phosphorus are vital to the growth of plants within the
    Bay These two elements are supplied in significant quantities by
    sewage treatment plants, food processing industries and urban,
    agricultural and forestry run-off. They are generally needed in a ratio

    of i6 parts nitrogen to one part phosphorus. If the availability of either
    drops below this general ratio, it becomes the limiting factor in the
    growth of plant life.
       Too many nutrients, on the other hand, can lead to an overabun-
    dance of phytoplankton, creating dense populations, or blooms, of
    plant cells. Blooms of green or blue-green phytoplankton can become
    a nuisance in the upper tidal freshwaters of some tributaries. As the
    blooms decay, oxygen is used up in decomposition. This can lead to
    anoxic (and odorous) conditions.                                                  Joppatown
       This situation can be improved. For example, in the upper tidal
    freshwater Potomac in recent years, minimum levels of dissolved
    oxygen have not been as low as they were during the 1960'S and early
    1970's, and algal blooms have decreased. This is at least in part
    attributable to the installation of new sewage treatment facilities in the
    mid I970'S. These installations significantly reduced the amounts of
    phosphorus entering the Potomac estuary, perhaps restoring some
    limits on the algal growth.

    Sediments: Composition & Effects
    In addition to chemicals, nutrients, and other dissolved materials, the
    waters of the Chesapeake and its tributaries also transport huge
    quantities of particulate matter known as sediments, which are
    composed of organic and inorganic materials. Sediments are distrib-
    uted by the Bay's circulation system.
       Researchers are particularly interested in the sediments and how
    they travel through the Bay because sediments can contain high
    concentrations of certain toxic materials. Individual sediment particles
    have a large surface area, and many molecules easily adsorb, or attach,
    to them. As a result, sediments can act as chemical sweeps by
    adsorbing metals, nutrients, oil, pesticides and other potentially toxic
    organics. Thus, areas of high sediment deposition may well have high
    concentrations of persistent (long-lasting) chemicals.
       While essential to the habitats of many Bay organisms, accumula-
    tion of sediments is, in many ways, undesirable. Accumulation of                    present coastline
    sediments on the bottom fills in waterways and ultimately leads to the              colonial coastline
    filling of the Bay. This sedimentation process has already caused Port
    Tobacco and Joppatown, Maryland, seaports during colonial times, to
    become landlocked. As they settle to the bottom of the Bay, the
    sediments can smother the benthos (bottom dwelling plants and
    animals). In addition, suspended sediments contribute to the turbidity
    of the water and thus decrease the light available for plant growth.
        The upper and lower sections of the Chesapeake have different
    sedimentation problems. In the upper Bay, the sediments discharged
    by rivers are primarily fine-grained silts and clays which are relatively
    light and can be carried long distances. Due to the two-directional
    water circulation pattern which is predominant in the upper Bay, these
    sediments are discharged into the fresh upper layer of water. As they
    move into the Bay, these particles slowly descend into the denser saline
    layer, where they reverse direction and flow back up into the tidal
    tributaries along with the lower layer of water. As the upstream flow
    terminates, the sediments settle to the bottom, mainly in the areas of
    maximum turbidity, thus turning these estuarine reaches into very
    effective sediment traps.
        In the lower Bay, by contrast, the sediments are somewhat sandier,
    and heavier. These particles result primarily from shore erosion. They
    drop fairly rapidly to the bottom, remain near their original source
    and are less likely to be resuspended than are finer silts.

                                                                                                                             17

                                                        Representative Biological
                                                        Communities

                                                                    ore than 2,700 species of plants and animals inhabit the
                                                                    Chesapeake and its shoreline. All depend on the Bay and
                                                        Hi alg, Vddtheir fellow inhabitants for food and shelter. Each, in turn,
                                                        contributes to the continued life of the entire Chesapeake ecosystem.
                                              /.. .� ..'  : �Each type of organism (species) has a set of physical and chemical
                                                .... s .requirements that must be satisfied in order for it to live. Different
                                                        species have different requirements for temperature, water, salinity,
                                                        nutrients, substrate, light, oxygen and shelter. These physical and
                                                        chemical variables largely determine which species will be found in a
                                                        particular habitat.
                                                            Within every habitat, communities of organisms are found which
                                                        exist in a close relationship with each other. Some provide shelter.
                                                        Others serve as prey or act as predators. The functions within a given
                                                        community are almost endless, and the Chesapeake provides countless
                                                        communities both large and small.
                                                            The composition of each community, or group of species,
                                                        undergoes change in population density due to environmental factors
                                                        and interactions among species. Germination of new plants, birth of
                                                        new animals, growth, changes in life stages (i.e., larval fish to
                                                        juvenile), local movement, migrations and stress due to changes in
                                          ---   1/   /E/ swater quality, habitat, over-fishing or other human activities are some
                                                        of the factors responsible for those fluctuations. Such change is
                                                        characteristic of most ecological communities and is true of the
                                                        Chesapeake Bay. Each species within a given community is subject to
                                                        major or minor population fluctuations with varying frequencies.
                                                            Some variations, such as seasonal changes in abundance, or the size
                                                        of populations, follow a predictable pattern. Others apparently follow
                                                        longer-term patterns or fluctuate randomly. Experience and studies
                                                        have shown that for a given season one can expect a characteristic
                                                        representation of species, although a few species may be missing at any
                                                        specific instant.
                                                            Some Bay communities are prone to rapid fluctuations in numbers
                                                        of one or more member species. This is particularly true for plankton
                                                        whose growth rate is correlated with their small size. Rapid changes in
                                                        diversity and abundance may occur hourly or daily- a complex result
                                                        of interacting biological, physical and chemical factors.
                                                            Many species show a long-term pattern in population abundance
                                                        and distribution. For example, striped bass were relatively rare in the
                                                        Bay during the 193o's and early I940's, while the croaker was abundant.
                                                        The bass increased until the early i97o's, when the population began to
                                                        decline. Its reduced numbers were followed by an increase in bluefish,
                                                        another predator. Such population cycles illustrate why it is so difficult
                                                        to separate natural patterns from those induced by human activity.
                                                            Bay communities can be as small as an oyster bar or as large as the
                                                        entire Bay. But whatever the size, these communities overlap and
                                                        intertwine with each other.

    i8

       Five major Bay communities that interact closely are the marsh
    dwellers, bay grass inhabitors, plankton, bottom residents and swim-
    mers. Each community represents a particular habitat within the Bay,
    and these habitats exhibit a wide range of characteristics. The marshes
    are relatively stable areas along the shoreline. The bay grass commu-
    nity extends from about mean low tide to a depth of about ten feet or
    when light becomes limiting. The plankton community is composed
    of minute creatures that float and drift with the movement of the
    water. The benthic environment includes the bottom of the Bay and
    its residents. Finally, the nekton are the fish and other swimmers who
    move freely throughout the Bay
       Each of these five communities--marsh, bay grass, plankton,
    bottom and swimming- along with the roles they play and how they
    interact, are described in the following sections.

    Marsh Dwellers
    Marshes fringe the Chesapeake and its tributaries, encompassing about
    425,000 acres, with 212,ooo acres in Maryland and 2z3,000 acres in
    Virginia. Forming a natural boundary between land and water, these
    spongy areas are dampened by rain, groundwater seepage, adjacent
    streams and by the Bay's tides. They are transitional areas that may
    slowly be converted to solid shoreline or may disappear due to
    erosion.
       Marshes help to reshape the Bay by removing sediments from the
    waters. Their emergent plants trap the sediments that they receive
    from streams and from tidal flooding. Without a rising sea level the
    growth of the marsh would be slow or nonexistent. Human activities
    'N\%NM      that affect sediment transport and deposition patterns,
                 such as levees and channels, may affect the development
                   of a marsh in unanticipated ways.

                                                                               Community
                                                                               The marsh habitat harbors a
                                                                  -*�   ~      seasonally abundant assemblage
                                                                               of plants and animals. The
                                                                               mid-salinity marsh shown here
                                                                               has a lower number of plant
                                                                               species than tidal fresh marshes,
                                                                                but ecological processes are
                                                                                representative. For example,
                                                                                intense     feeding by fish and
                                                                                birds occurs and nutrients,
                                                                                sediments and detritus are
                                                                               exchanged with surrounding
                                                                                estuarine environments.

                                                                                                                               x9

                                                             Salinity and frequency of tidal flooding are the most important
                                                         factors in determining the types of plant and animal populations that
                                                         inhabit a particular marsh. Some marshes along the eastern shore and
                                                         mouth of the Bay are subjected to high levels' of salinity. Others are fed
                                                         with brackish water, while those located in the northern reaches of the
                                                         Bay and its tributaries are, for the most part, freshwater marshes. Each
                                                         type offers different amenities and attracts different plant and animal
                                                         populations.
                                                             In general, freshwater marshes have larger and more diversified
                                                         plant populations. For instance, the overall production of plant
                                                         material (biomass) in freshwater marshes is estimated to be two to
                                                         three times greater than that of salt water marshes.
                                                             Poised between land and water, marshes absorb the erosive energy
                                                         of Bay waves and may also act as nutrient buffers, regulating the flow
                                                         of nutrients into the Bay. Nutrients flow into the marshes from land
                                                         sources, and from the Bay as a result of tidal action. Once there, the
                                                         nutrients are trapped and used by marsh grasses. They are gradually
                                                         released into the Bay during decomposition of the grasses.
                                                             Because of their bountiful supply of nutrients, marshes are
                                                         extremely productive. In fact, they are comparable to fertile agricul-
                                                         tural lands. This leads to a large quantity of plant biomass. There is a
                                                         huge amount of visible plant material in marshes. However, this just
                                                         constitutes above-ground biomass. The below-ground biomass com-
                                                         posed of root and rhizome material is often more than double the
                                                         above-ground biomass. This adds up to a tremendous reservoir of
                                                         nutrients and chemicals bound up in plant tissue and sediments.
                                                             Plant life varies according to salinity. Freshwater marshes include
                                                         cattails, reeds, arrow-arum, big cordgrass, wild rice, three-square,
                                                         tearthumb and pickeral weed. The coastal salt marshes of the mid and
                                                         lower Bay are dominated by salt meadow cordgrass, saltgrass and
                                                         saltmarsh cordgrass. Finally, the irregularly flooded salt marshes
                                                         possess the fewest species of plants and are dominated by needlerush.
                                                             The abundance of food and shelter provided by the marsh grasses
                                                         ensure a very favorable habitat for the other members of this
                                                         community. A host of intertebrates feed on decomposed plant material
                                                         and, in turn, provide food for numerous species of higher animals.
                                                         Another source of food is the dense layer of microscopic animals,
                                                         bacteria and algae that coats the stems of marsh plants. However,
                                                         decomposing plants and, to a lesser extent, dead animals provide the
                                                         major food for the marsh dwellers. Therefore, the primary food web
                                                         in the marsh environment is based on detritus (dead organic matter).

                                                        Bay Grass Communities
                                                        Approximately ten species of bay grasses occur in the Chesapeake.
                                                        They are often referred to as Submerged Aquatic Vegetation or SAV.
                                                        Most of the grasses cannot withstand extensive drying and live with
                                                        their leaves at or below the surface of the water. Light penetration
                                                        determines the depths to which bay grasses live. The water surround-
                                                        ing the grasses can contain substances that block or scatter the
                                                        transmission of light so that, in relatively dear waters, bay grasses can
                                                        exist in water depths up to about io feet.
                                                            Like the marsh grasses, the various species of submerged grasses
                                   .* l ,_              are distributed according to salinity. The substrate to which the roots
                                                        of bay grasses attach also affects their distribution. They thrive best in
                                                        relatively fine sediments, but can tolerate some organic matter or fine
                                    ARMTRoNG            sand. Turbulence caused by wave action can also restrict their
                                                        distribution. Although submerged grasses can act as 'baffles" to reduce

    20

                                                                                Bay Grass Community
                                                                                This community typically con-
                                                                                 tains a seasonally abundant
                                               sun                              group of plants and animals.
                                                                                 Shelter, food and nutrient
                                                                                 processing (such as reminerali-
                                                                                 zation of organically-bound
                                                                                 nutrients) are important charac-
                                                                ~    ~' - -     teristics. There is a dynamic ex-
                                                                                 change of materials between
                                                                                 the bay grass community and
                                                                           ;    the surrounding estuarine
                                                                                 environment.

         nutrients

    wave energy and slow water velocities, highly exposed locations create
    so much water movement that the grasses cannot survive.
        The chemistry of their surrounding water is critical to the survival
    of submerged grasses. Toxic materials such as herbicides or heavy
    metals can so disrupt the physiology of the plants that they die. On
    the other hand, enough needed materials like nutrients must be
    present in the sediment pore water for survival. Excessive nutrient
    enrichment may compromise the existence of submerged grasses by
    favoring the growth of phytoplankton and epiphytes (fouling plant
    growth) which in turn block out light available to grasses.
       Submerged grasses are an important link in the food chain of the
    Bay's waters. These grasses provide protective cover and food for a
    diverse community of organisms which share this zone. Epiphytic
    plants use them as a place to attach. They provide support for the
    larval stages of small marine snails as well as a variety of other
    invertebrates which feed on the decaying grasses. In turn, many of the
    invertebrates provide food for small blue crabs, striped bass, perch and
    other fish. Ducks and geese also feed on the bay grasses and other
    small inhabitants of this community. Wading birds such as herons
    often feed on small fish in this area.
       Another important ecological function of bay grasses is their
    ability to slow down water velocities, causing particulate matter to
    settle at the base of their stems. Water thus tends to be more dear near
    grass beds. Finally, like marsh grasses, bay grasses can act as nutrient
    buffers, taking up nitrogen and phosphorus and releasing them later
    when the plants decay
       Surveys of bay grasses have revealed a serious decline in popu-
    lations throughout the Bay Previously, fluctuations in abundance of
    certain species, such as eelgrass (reduced in i93o's) and Eurasian milfoil

                                                                                                                                21

      Plankton Community                                                                 phytoplankton
                                                          zooplankton                      and bacteria

                                                        (abundant growth, then reduction in the I960's) have occurred, but the
                                                       current situation may be a cause for concern because it is so
                                                       geographically widespread and involves all species.

                                                        Plankton
                                                       The plankton form another important and representative community
                                                        in the Chesapeake. This predominantly microscopic community
                                                        includes phytoplankton, zooplankton and bacteria. These organisms
                                                        exist largely at the mercy of the current and tides, floating and drifting
                                                       with the water's movements. Some of the tiny creatures, however,
                                                        have been shown to move up or down in the water to take advantage
                                                        of light or to drop below the halocline to avoid being washed out to
                                                        sea.
                                                           Phytoplankton are tiny one-celled plants often occurring in
                                                       colonies known also as algae. Although there are numerous varieties,
                                                       the major types found in the Bay are diatoms, dinoflagellates, golden
                                                        algae, green algae, and blue-green algae. Some phytoplankton, called
                                                       nannoplankton, are extremely small. What they lack in size, however,
                                                       the nannoplankton make up for in numbers. They are responsible for
                                                        as much as two-thirds of the phytoplankton tissue produced in the
                                                        Bay.
                                                           Like all plants, phytoplankton require light to live and to
                                                       reproduce. Therefore, the largest colonies are found near the surface.
                                                        Salinity is another important factor in determining phytoplankton
                                                       distribution. The largest number of species are found in the more
                                                       saline waters at the mouth of the Bay. Weather is also a major
                                                       determinant in the life of these plants. The greatest concentrations
                                                       occur in the late summer or fall, when the days are long and the water
                                                        is warm. However, during some years the lower Bay shows a spring
                                                        pulse (bloom), with large numbers of phytoplankton in evidence.
                                                       These high concentrations of phytoplankton produce the characteristic
                                                        green color of estuarine and near-shore waters. Under certain condi-
                                                       tions, phytoplankton with red or brown pigments will dominate the
                                                        water, producing a red-tinted bloom, sometimes referred to as "red
                                                        tide."
                                                           Because phytoplankton reproduce quickly, changes in chemical
                                                        conditions, such as addition of nutrients, may cause rapid changes in
                                                       the number and diversity of species. Such changes may result in the

    22

    presence of a single species, like the type causing "red tides." These
    blooms can have serious consequences. Once they begin to decay,
    decomposing organisms can completely deplete all dissolved oxygen
    reserved, suffocating other estuarine animals.
        Phytoplankton are the major food source for microscopic animals
    called zooplankton. Most zooplankton are copepods, a particular type
    of crustacean that is only about a millimeter long. One study found
    three million in a single cubic meter of water. Their distribution is
    related to salinity levels and matches that of their favorite meal, the
    phytoplankton. Copepods also feed on plant matter and bacteria.
       The tiny larvae of benthic animals and fish are also considered to
    be zooplankton, although they remain so only temporarily. These
    larvae may be consumed by larger animals, and may themselves, as
    they grow, consume copepods.
       The final group of zooplankton found in the Bay are the protozoa.
    These single-cell creatures feed on detritus and bacteria. They, in turn,
    become food for larvae, copepods and larger protozoa.
       Bacteria have an important function in the Bay. They are essentially
    the undertakers or decomposers. Their primary function is to break
    down dead matter, particularly plants. They feed on the detritus and
    are then eaten by zooplankton and so on up the food chain. This
    process makes the nutrients in dead plant and animal matter available
    for consumption by larger organisms.
       Bacteria are either normal residents of the Bay or can be
    introduced through various pathways including human sewage and
    precipitation runoff from the land. The variety that specializes in
    feeding on human waste are called coliform bacteria. Coliforms in
    themselves are not normally harmful. They are, however, an indicator
    that pathogens (disease-producing bacteria) may be present. More
    coliforms are likely to be found near large population centers.
       A final group of plankton dwellers is dearly visible to the unaided
    eye. These are the jellyfish, sea nettles and comb-jellies (ctenophores)
    which move with the water currents.

    Life at the Bottom
    The organisms that live on and in the bottom of the Bay form a
    complex assemblage of communities. Commonly termed benthos, they
    are considered in terms of the animal components. However, the plant
    and bacterial constituents should not be overlooked. The roots and
    lower portions of submerged aquatic vegetation supply physical
    support for a wide variety of "epibiotic" organisms. An oyster bar that
    supports many small organisms is an example of a benthic community.
    Benthic communities also exist on bare, unvegetated sediments, but
    numbers may be more limited because there is less protection against
    predators.
       Salinity and sediment type are the two major determinants to the
    distribution of benthos. These two factors result in a gradient of
    benthic organisms within the Chesapeake. With respect to sediments,
    neither coarse sands nor soft, slurry muds usually possess rich or
    diverse benthic populations. Sediments containing a significant
    amount of silts and clays are considered optimum.
       Prefixes are useful to differentiate the various types of benthic
    organisms. Epifauna, dwelling upon the surface, are distinct from
    infauna, which form their own community structure under the
    sediment surface. Other prefixes categorize benthic organisms by size.       5A
    For example, mega- or macro-fauna are the largest, most visible
    animals, while meio- and micro-fauna range downward in size.

                                                                                                                            23

                                                        The benthic community has an important effect on the physical
                                                    and chemical condition of the sediments, especially the upper eight to
                                                    ten inches. Filter feeders, such as oysters and clams, pump large
                                                    volumes of water through their bodies and extract food from it.
                                                    Infaunal deposit feeders, such as worms, plough through the sedi-
                                                    ments in search of food. Predators such as the blue crab scurry across
                                                    the sediment surface. These activities all help to keep the sediments
                                                    stirred up, increasing the rate of diffusion, or exchange, of materials
                                                    into the water and facilitating the passage of oxygen into the
                                                    sediments.
                                                        Benthic animals also affect the structure of the sediments. Some
                                                    build tubes or burrows through which they pump water. Many
                                                    benthic animals bind sediments together in fecal pellets which settle
                                                    more readily. If toxic chemicals are present in the sediments, they can
                                                     be taken up by benthic fauna, and in some cases harm them.
                                                        Some commercially valuable benthic organisms, such as oysters and
                                                    blue crabs, are widely distributed. Others are more limited by salinity.
                                                     For example, hard-shell clams require highly saline (greater than Is
                                                    ppt) waters, while soft-shell clams can thrive in lower salinity levels.
                                                     Less commercially important benthic species include barnacles and
                                                     sponges, which live in higher salinities; mysid shrimp and mud crabs
                                                     in mid-salinity ranges; and brackish water clams in lower salinities.
                                                    Intolerance to lower salinities can also limit the distribution of certain
                                                     benthic predators, parasites and diseases. Oyster drills, which feed on
                                                    oysters, are far less of a problem in upper Bay waters than they are in
                                                     the lower Bay because of their intolerance to low salinities.

                                                     The Swimmers
                                                     Nekton are the swimmers of the Bay. They control and direct their
                                                     movements, and thus their own distribution, throughout the Bay. This
                                                     group includes fish, certain crustaceans, squid and other invertebrates.
                                                        Approximately 200 species of fish live in the Chesapeake. They can
                                                     be divided into permanent residents and migratory fish. The residents
                                                     tend to be smaller in size, therefore less capable of negotiating the
                                                     distances often covered by the larger migratory species.
    Benthic Community

                      '~: ,~- ~.%7~:-

       Smaller resident species include killifishes, anchovies, silversides,
    hogchokers and gobies. They are normally found in shallow water        Migratory Pathways of Fish
    where submerged vegetation provides cover. Here they feed on a
    variety of invertebrates such as zooplankton and amphipods. Larger             Spavming
    residents also tend to make their home in these areas, feeding on the
    invertebrates and the smaller resident fish.
       The migratory fish generally fall into two categories: those who
    spawn in the Bay or its tributaries, and those who spawn on the ocean  hwater
    shelf. The members of the Bay spawning category migrate varying               _
    distances to spawn in fresh water. This group includes a few species
    that really could be considered Bay residents. For instance, during the
    spawning season, yellow and white perch travel relatively short
    distances from their residence areas in the brackish water of the Bay to
    freshwater areas in the upper Chesapeake. Striped bass also spawn in            e\\\
    the low salinity areas of the Bay. Some remain in the Chesapeake to
    feed, while others migrate to the ocean waters. Shad and herring are
    truly migratory, traveling from the ocean to fresh water to spawn and
    returning to the ocean to feed.
       Other migratory fish spawn on the ocean shelf and use the Bay
    strictly for feeding. Some journey into the Bay while still in the larval
    stage and use the shallow waters of the Bay as a nursery Croakers,
    drum, menhaden, weakfish and spot fall into this group. The
    menhaden deserve special note. They occupy the Bay in such
    abundance that they support a major commercial fishing enterprise.
    The adults of this category feed on the abundant supply of phyto-
    plankton. Bluefish only enter the Bay as young adults or mature fish.
       One species that must be considered a migratory fish because of
    spawning practices is the eel. Eels reside in the Bay for long periods,
    but eventually migrate to their ocean spawning grounds in the
    Sargasso Sea.
       Other organisms appearing in the nektonic food web are some of
    the "lesser' members of the nekton mentioned earlier. Swimming
    crustaceans include shrimp, which spend most of their adult life near
    the bottom. Usually thought of as a "creeper, the blue crab has                               water
    developed a swimming capacity with one pair of powerful legs that
    enable it to travel considerable distances in the Bay. Finally, numerous
    members of the shark family enter the Bay as do several marine
    mammals including the porpoise.

    Ecological Succession
    The replacement of one Bay species in a community by another is a
    natural and usually long-term process. This process is called ecological
    succession, and the rate of succession depends on the type of
    community. In the Bay the rate may not be constant because of the
    powerful influence of physical factors affecting distribution and
    abundance of species. Where sediments accumulate quickly, replace-
    ment of open water plankton and fish by marsh, and finally by
    terrestrial communities, will eventually occur.
       Ecological recovery, which has some features in common with
    ecological succession, also occurs on newly formed bottom deposits.
    Such deposits can result from dumping of wastes. Natural causes, such
    as hurricanes, can damage bottom communities through increased
    sedimentation and associated low levels of dissolved oxygen. If the
    environment is altered by humans, say by introducing an excessive
    supply of nutrients to Bay waters, then the combinations of species in
    the Bay communities will shift in favor of those which can better
    utilize these nutrients.

                                                       Food Production &
                                                       Consumption

                                                                 embers of the Bay ecosystem are related to each other in
                                                                  many ways. Perhaps the most important relationship
                                                                   among Bay species is their dependence upon each other as
                                                       a food. All biological material ultimately serves as food for some
                                                       organism in an ecosystem food web (a term used to describe the
                                                       relationships between food production and consumption). The proc-
                                                       ess of obtaining and using food is, to a great extent, a process for
                                                       transferring different compounds containing the key element carbon.
                                                           We are all carbon-based creatures. It is the basic element in all
                                                       life-giving organic nutrients such as proteins, carbohydrates, fats and
                  r_                         -=- _ ,~'nucleic acids.
                                                           The processes by which organisms acquire, utilize and ultimately
                                                       lose this important element form the basis of the Chesapeake's life
                           ./~ 7., ,. .~! :. (  ~support system. All the complex processes and interactions involved in
                                                       this system are essentially part of a transfer cycle in which sunlight,
                                                       carbon and oxygen play major roles.
                                                           Plants and some bacteria have the ability to produce their own
                                                       food by combining carbon dioxide and water into high-energy organic
                                                       compounds (usually sugar) through photosynthesis, using energy
                                                        from the sun. These organic compounds form the plant's cellular
                                                       structure, permitting growth to occur. Because of this ability to use
                                                       inorganic carbon dioxide to produce their own food, plants are
                               5s              5        referred to as autotrophs -self feeders. Autotrophs are the only
                                                       organisms able to produce new food from carbon dioxide, water and
                                                       light energy, and are the ultimate producers of all food on the earth; all
                                                       other organisms must feed directly or indirectly on organic material
                                                       produced by autotrophs.
                                                           Animals don't have the ability to photosynthesize inorganic
                                                       carbon. Instead, they acquire carbon in its organic form by ingesting
                                                       the organic matter contained in plant and animal tissue. The animal
                                                       then breaks this organic material down into components from which
                                                       to derive energy and the material needed for action and growth. Thus
                                                        animals, including protozoa, fungi and most bacteria are heterotrophs,
                                                       or other-feeders. To grow they must obtain their food from already
                                                       existing organic material. This organic substance may be in the form
                                                       of plants, other heterotrophs, or dissolved in water.
                                                           Whether they produce them themselves (autotrophs) or ingest
                                                       them from other sources (heterotrophs), all organisms must break
                                                       down organic molecules to use the carbon and energy contained
                                                        therein. This process is called respiration. Every life-supporting activity
                                                        (growth, heartbeat, swimming) requires energy. This energy comes
                                                        from respiration. Respiration requires oxygen, and during this process
                                                        organic carbon is given off as carbon dioxide.
                                                           Respiration and photosynthesis are complementary, and comprise
                                                        the carbon-oxygen cycle. Plants release more oxygen than they
                                                        consume, while animals use that excess oxygen for respiration. In turn,
                                                        animals release carbon dioxide, which plants require.

    26

       This basic system indicates that each organism supplies the others
    with the requisites of life. Most biological material ultimately serves as
    food for some organism and, through this process, biological
    molecules and their inorganic constituents are continuously cycled
    through the ecosystem. Although this carbon/oxygen cycling system
    appears simple, its application in the Chesapeake ecosystem incorpo-
    rates an array of complex interrelationships and dependencies.
       While carbon and oxygen are the most prevalent elements in our
    physical make-up, many others play important roles in the creation,
    growth and functions of organisms. Nitrogen and phosphorus are two
    such elements. They are crucial to the operation of the Bay's life
    support system.
       Nitrogen is a major component of all organisms, primarily as a key
    ingredient in protein. When an organism dies, bacteria and fungi
    break the protein down into amino acids. Bacteria then remove the
    carbon, converting the acids into ammonia. Plants are able to use this
    ammonia as a source of nitrogen. In the presence of oxygen, other
    bacteria can convert the ammonia to nitrite and nitrate, which are also
    good nitrogen sources for plant life. However, under low dissolved
    oxygen conditions the bacteria reduce nitrate to elemental nitrogen
    which, as a dissolved gas, is not available to most aquatic organisms.
    In tidal freshwater, some blue-green algae are able to use elemen-
    tal/gaseous nitrogen directly and, thus, have no requirements for
    combined nitrogen.
       Phosphorus is another element essential to plant growth. Decom-
    posing plants and animals yield organic phosphorus. During the
    decomposition process, bacteria convert this organic phosphorus to
    phosphate when oxygen is present. In this form, it is readily employed
    by plants. However, phosphate settles out of water very quickly,         Nutrient Mix for
    sometimes resulting in situations where sufficient quantities are not    Phytoplankton Growth
    available to assure adequate plant growth.
       Temperature plus the overall availability of sunlight and carbon
    dioxide, along with usable nitrogen and phosphorus, control the rate
    of photosynthesis in the Bay Since plants are the only organisms                                � �
    capable of producing 'new" food from inorganic matter, the rate of
    photosynthesis largely determines the production of organic carbon
    (biomass) and thus the ultimate availability of food in the Chesapeake. *
       To illustrate how these factors affect the productivity of the Bay,        .         - .               �
    let's look at the Chesapeake's most copious food producer, the    .
    phytoplankton. Like all plants, phytoplankton require sunlight, nutri-  :
    ents and water. In the Bay, water is never a limiting factor. However,           �                � �  �
    the others may limit potential phytoplankton growth. The amount of
    sunlight available to an aquatic plant depends on factors such as
    cloudiness, the sun's latitude, choppiness of the water, turbidity and
    depth. Temperature is another factor which can reduce the rate of
    photosynthesis.
       Nutrients in the form of carbon dioxide, usable nitrogen and
    usable phosphorus must also be available in the proper proportions.
    Studies have established that phytoplankton, the Chesapeake's most
    copious food producer, require an approximate carbon:nitrogen:phos-
    phorus ratio oflo6a6:L These nutrients are rarely available in the exact
    ratio that is required. Normally, one nutrient is in short supply
    compared to the others. That one is referred to as the 'limiting
    nutrient." If it is added to the water, a growth spurt may occur in one
    or more species. Conversely, if its availability is further reduced, a
    decline in plant production will occur. Additional increases in the
    non-limiting nutrients will not increase production.

                                                                                                                          27

    Major Nutrient Cycles:
    Carbon (C)                                                                                     0fihn

          carbon    remiineralizing                      phMsyte=i

                            (stor~~~~~~~~~~~~~~~~~~ecange &bra

      *C02 represents all forms of inorganic carbon

    Nitrogen (N)rurgnfxgfihg
                   r~~~~~~~~~~~~~~~~~ain daiae bacteria

                       &    w~~~~~~~~~astesi~V.

          ntorganc    remineralizin gaocn
           phosphoricr MObe

                Pho~~~~~~~~~~~~~~~hsphorus(P ihn

                    rain,  drainage~~~~~~~~~4%

     & wst

       The role of these limiting factors on food production in the
    Chesapeake and its tributaries is currently being studied. Some
    evidence suggests phosphorus may control the growth of some
    phytoplankton species in the spring, especially in the tidal freshwater
    and brackish areas. Nitrogen may be a limiting factor at higher
    salinities, particularly during warm months. Carbon dioxide limi-
    tations may control the rate of further photosynthesis during algae
    blooms. Sunlight and possibly temperature determines the rate of
    photosynthesis during the colder months. Turbidity also plays an
    important role in affecting the quality and quantity of light that
    reaches the plant.
       Normally, organic carbon (or biomass) production and decomposi-
    tion keep pace with one another over an annual cycle, causing the total
    amount of biomass in an ecosystem to remain fairly constant.
    However, biomass production at all trophic levels is ultimately
    dependent on the production of new food by plants (autotrophs).
    This production is, in turn, linked to the availability of sunlight and
    nutrients. Sunlight availability depends largely on weather conditions.
    Nutrient availability depends on the carbon, nitrogen and phosphorus
    cycles, which are extremely complex and very sensitive to changes
    caused by pollutants. Since substantial quantities of nutrients are
    washed into the Bay from its tributaries, they also have a significant
    effect on the overall nutrient availability within the Bay proper.
       As you can see, the Bay's entire life support system is balanced on
    some rather complex underpinnings. Even though the Chesapeake's
    production capacity is massive, it is also finite. A look at the Bay's food
    web should provide an understanding of how food production
    problems at even the lowest and most broadly based trophic level can
    have dramatic effects on higher trophic levels.

    The Food Web
    The Chesapeake is noted for its ability to produce food. However, the
    production of economically important foods like fish and shellfish
    depends on the production of plant biomass in the Bay. The animals,
    plants and microbes of the Bay are connected by a complex network of
    feeding interactions called the food web.
       The food web has both direct and indirect linkages to higher
    trophic levels. Typically, the direct food web encompasses four key
    linkages -five, if we humans are included. For example, a predomi-
    nant feeding pattern in the open waters of the Bay starts with
    phytoplankton converting sunlight and nutrients into living tissue.
    They are, in turn, eaten by copepods, members of the zooplankton
    family. The copepods are then swallowed by anchovies, which are later
    eaten by bluefish. This illustrates how the organic carbon originally
    produced by the plant is passed through successively higher trophic
    levels. The indirect food web encompasses the feeding on dead
    organic matter by detritivores.
       The amount of energy available decreases at each successively
    higher trophic level. This is due to the fact that the transfer of energy
    (required to produce biomass) from one trophic level to the next is
    inefficient. Several factors account for this. For example, of the total  :
    available phytoplankton carbon/energy, only a portion is ingested by
    zooplankton. Even the portion that is ingested is not fully utilized.
    Some is not assimilated by the herbivore's digestive system. Another       .    .
    portion of the carbon/energy goes to the maintenance of cellular
    respiration to provide energy for food collection and locomotion.                       A      G
    Only a small fraction is allocated to growth and reproduction. Since

                                                                                                                           29

                                                     these are the only functions that produce additional tissue, the
                                                     carbon/energy assigned to them is all that is available to the predator
                                                     at the next trophic level.
                                                         The efficiency of the carbon/energy transfer from one trophic level
                                                     to the next is approximately Io percent. This lack of efficiency shows
                                                     why there is only a limited amount of organic carbon/energy available
                                                     at the top carnivore level. For example, for every pound of commercial
                                                     fish taken from the Chesapeake, almost four tons of organic material
                                                     had to be produced at the plankton trophic level.
                                                         Several important conclusions can be drawn. First, because plants
                                                     are the source of all food, their rate of biomass formation determines
                                                     the quantity of animals and all other organisms in the Bay.
                                                         Second, through the food web concept we can see that energy
                                                     flows through an ecosystem. It is provided by the sun and is dissipated
                                                     at every transfer between trophic levels. (This flow of energy contrasts
                                                     with the cycling of nutrients, which change in form but are not
                                                     degraded to heat in every transfer as is true of energy.)
                                                         Third, an ecosystem can support relatively few animals at the
                                                     highest trophic level. In the Chesapeake Bay, massive quantities of
                                                     plants are required to support relatively few carnivores such as the
                                                     striped bass or bluefish.
                                                         Fourth, because the amount of food energy available decreases
                                                     with each rise in trophic level, feeding at the lowest trophic level
                                                     means that more food (energy) is available. Some animals are known
                                                     to switch from carnivory to herbivory when food becomes scarce. This
                                                     switch theoretically increases the amount of food available to them
                                                     five- to tenfold.

    The Bay Food Web

                 K -   algae z3~~~~~~ooplsh and

                                       filter            invertebrates          invertebrates
               SAV                   feeders              and larvae

                                                     Decomposer Community

        Finally, high-level carnivores consume many times their weight in
    food. If this food contains a toxic chemical, even in small amounts, the
    fish or animal may be exposed over time to high levels of the chemical.
    Heavy metals and organic chemicals may be stored in the fatty tissues
    of the animal and concentrate there. As a result, the body may contain
    a much higher concentration of the chemical than did its food. This
    phenomenon is called biological magnification.

    Direct & Indirect Food Webs
    As mentioned earlier, two basic pathways dominate the estuarine food
    web. The direct pathway leads from living plants to higher animals.
    The indirect, or detritus pathway leads from dead organic matter to
    lower animals then to higher animals. The marsh and bay grass
    communities are strongly dominated by the detritus pathway.
       The importance of the detrital food web has not always been easy
    to demonstrate experimentally. The higher plants, like eelgrass,                                               i
    saltmarsh, cordgrass and widgeongrass contribute most of their carbon
    content as detritus. However, the small algae, like diatoms and
    filamentous green algae that grow as epiphytes on the grasses, are
    usually eaten by grazers, which puts them in the direct predatory
    chain.
       In deeper waters, detritus resulting from dead phytoplankton, .
    zooplankton and larger animals, as well as that washed in from upland
    drainage, marsh and bay grass communities, continuously "rains"
    down on the benthos. Here, bottom-dwelling animals such as oysters,
    clams, crustaceans, tube worms, shrimp and blue crabs feed on it.
    These animals then provide food for fish (and people). To the extent
    that live phytoplankton reach the benthos where filter feeders can
    directly remove the cells or small colonies from the water, they
    contribute directly to the benthic food web.
       The plankton community is dominated by a direct food web,
    especially seaward of the turbidity maximum. Relatively little is known
    about the importance of individual phytoplankton species in the food
    web. This is partially due to their great variability from season to
    season and varied distribution within the Bay. The role of zooplankton
    is better defined. Knowledge of specific details pertaining to food web
    relations involving bacteria and other microbes and protozoa is
    severely limited.
       Phytoplankton are divided by size. The larger species are often
    called "net" plankton, because they are traditionally collected with fine
    nets. Many diatoms and large dinoflagellates are in this group. Species
    that pass through fine nets are known as nannoplankton. The
    nannoplankton are believed to be very important as food, and may
    contribute from 50o to 75 percent of the primary organic carbon
    production attributed to phytoplankton. Microzooplankton, including
    rotifers and tintinids (a group of protozoa), are probably the major
    consumers of nannoplankton. "Net' phytoplankton provide food for
    larger zooplankton and some fish. Bacteria, fungi, phytoplankton, and
    possibly small protozoa provide food for oysters and clams. In
    addition, all plankton contribute to detritus.
       Copepods, the dominant form among zooplankton, are a key link
    in the food web between phytoplankton and larger animals. Larger
    copepods feed on most phytoplankton species and occasionally on the
    juvenile stages of smaller copepods. Small copepods concentrate on
    the smaller members of the phytoplankton species.
       Copepods often fill the waters of the Bay, devouring enormous
    quantities of phytoplankton. Most production of animal protein from

                                                                                                                            3'

                                                         plant materials in marine waters is carried out by copepods. On a
                                                         larger scale, copepods are the world's largest stock of living animal
                                                         protein. Larger carnivores feed voraciously on copepods. One herring,
                                                         for example, may consume thousands of the tiny creatures in a single
                                                         day.
                                                            Most of the Bay's fish are part of the direct food web but their
                                                         feeding habits are complex. Some Bay experts contend that menhaden
                                                         are the dominant fish in the Bay as far as food consumption is
                                                         concerned. As juveniles they consume large quantities of zooplankton.
                                    ,IRMSTRONG          Upon reaching adulthood, they switch to phytoplankton. The gill
                                                         rakers of the adult are extremely fine and act as a filtering net. Adult
                                                         menhaden swim with their mouths open and 'sample' the plankton
                                                         that appear in their paths. Menhaden are a major food of striped bass
                                                         and bluefish. They also support a large commercial fishery that
                                                         incorporates them into poultry and animal feed.
                                                            Like menhaden, anchovies and all fish larvae are primarily plankton
                                                        feeders. Adult striped bass, bluefish and weakfish (sea trout) feed
                                                        mainly on other fish. Striped bass and other predators may feed upon
                                                        their own young, thus contributing to this species' periodic popula-
                                                        tion growth/reduction cycles. Finally, there is a large group of fish that
                                                        are omnivorous, such as eels and croakers. Their diets are often
                                                        composed of planktonic copepods, amphipods, crabs, shrimp, small
                                                        bivalves and small forage fish. Small forage fish, like killifish and
                                                        silversides, often feed upon the epifauna and epiphytes of the marsh
                                                        and littoral communities.
                                                            Some fish may be "specialty' feeders, but most species subsist on a
                                                        variety of prey, or have alternative foods. This ability to switch from an
                                                        item in low abundance to a more available food contributes to the
                                                        overall stability of the fish community by assuring long-term surviva-
                                                        bility and providing a buffer against major changes.

                                                        Why is the Chesapeake Bay So Productive?
                                                        The physical and ecological processes of the Bay make it possible for
                                                        us to understand why the Bay is such a tremendous food source.
                                                            The Bay sustains several juxtaposed habitats, which exchange
                                                        materials and complement one another's resources. For example,
                                                        detritus (dead organic matter) derived from marsh and bay grass
                                                        communities is a major source of food for benthic animals. On the
                                                        other hand, nutrient-laden sediments from the Bay's waters are
                                                        deposited in marshes, increasing productivity there.
                                                            The existence of two major food webs, the detrital (indirect)
                                                        pathway and the predatory (direct) pathway, promotes overall stability.
                                                        If one pathway falters, resources can be used via the other. Some
                                                        organisms can even switch food sources.
                                                            The estuarine circulation patterns result in accumulation and
                                                        retention of suspended sediments and nutrients. This yields the Bay's
                                                        high productivity. The sediments and nutrients that accumulate in the
                                                        zones of maximum turbidity result in increased phytoplankton
                                                        growth, although the turbidity itself can limit growth by blocking out
                                                        light. Detritus is also retained in the estuary because of the Bay's
                                                        circulation. This longer retention increases its availability to animals. It
                                                         also permits complete decomposition by microbes, providing a source
                                                        of inorganic nutrients for plants.
                                                            This brief look at the Bay's life support system should convey some
                                                         idea of its true complexity. Even the smallest of creatures plays a vital
                                                         role in the overall success of this system. The dual food webs provide a
                                                        modicum of resilience, but by no means guarantee continued levels of
                                                         high productivity.

    32

                                  The Future

                                            hus far, the Bay and tidal tributaries have absorbed
                                            considerable pressure from both natural and human causes.
                                            They have remained highly productive. But we cannot
                                  count on this natural carrying capacity to endure any and all future
                                  pressures. In recent years, the Bay has been showing signs of wear and
                                  tear. Sewage and land run-off have markedly altered the nutrient
                                  balance of some of the Bay's tributaries and possibly of the upper Bay
                                  itself. Carbon, nitrogen and phosphorus loadings have increased and,
                                  more importantly, the ratios of one material to another have changed.
                                  The results of some of these changes are obvious - for example, the
                                  pea green blankets of algae and the decline in submerged aquatic
                                  vegetation.
                                      Other effects are not as obvious. They may show up over time,
                                  gradually becoming apparent among the processes taking place within
                                  the Bay ecosystem. Circulation, for instance, transports nutrients and
                                  organisms to all parts of the Bay It also carries pollutants. Excess
                                  nutrients can cause an overabundance of algae which can rob oxygen
                                  resources from other organisms as well as the algae themselves.
    A%_ ~              ~          Benthic communities stir up bottom sediments, making nutrients and
                                  other material available to organisms in the water column. Their
                                  activities can also increase the exchange of toxic chemicals. Some of
                                  these chemicals, the man-made organics, are used in virtually every
                                  industrial product and process. Unfortunately, little is known about
                     -)~ ~ =~,.    the chronic effects of toxic substances in the environment, and
                                  especially little about how they affect living beings.
                                      We have tried to convey to you the major components and dynamic
                                  processes governing the Bay Understanding these interacting principles
                                  and how pollutants are affected by them, will allow us to begin to
                                  understand the effects of individual, apparently insignificant actions.
                                      We have already begun to use the "ecosystem approach" to manage
                                  parts of the Bay Controls such as the Upper Chesapeake Bay
                                  Phosphorus Limitation Policy which limits amounts'of phosphorus
                                  released from some sewage treatment plants discharging into the Bay
                                  above Baltimore, will help to curb the domino effect of pollutants on
                                    an ecosystem. Future controls, like this policy, should be chosen
                                      with the ecosystem in mind. Failure to recognize this basic principle
                                        can only lead to frustration and loss of management options.

                                                                                                          33

    Technical Reviewers:
    Scott W. Nixon
    Richard L. Wetzel
    Walter R. Boynton
    Robert B. Biggs
    David A. Flemer
    Willa C. Nehlsen
    C. John Mei                                     N      AA   RVI          IR
    Bert S. Brun                                   PU[

                                                     3 6668 00001 7865