United States
Environmental Protection
Agency
Washington, D.C. 20460
January, 1982
600982016
&EPA
Chesapeake Bay:
Introduction to an Ecosystem
, ' 01 ""*" i\~t
..»..'.rr:tM, J'A i^xy/
EPA Report Collection
information Resource Center
US EPA Region 3
Philadelphia, PA 19107
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Foreword
In recent years, the well-being of the Chesapeake Bay
ecosystem has been stressed by the activities of the region's
growing population. In 1976, concern for this national
resource prompted Congress to order the Environmental Protection
Agency (EPA) to conduct an intensive five-year study of the environ-
mental quality and management of the Chesapeake Bay resources. This
program was created to coordinate research on the main factors
impacting the Bay's water quality; to analyze, store and distribute
research data; to determine which government agencies have resource
management responsibilities; and to stimulate coordination among
them.
The EPA program encourages state (Maryland, Virginia and
Pennsylvania) participation throughout all its activities including
planning, technical support, data collection and program development
and implementation. The State of Maryland is represented by the
Tidewater Administration of the Department of Natural Resources
and the Office of Environmental Programs in the Department of
Health and Mental Hygiene. Their counterpart in Virginia is the State
Water Control Board. The Department of Environmental Resources,
in conjunction with the Susquehanna River Basin Commission,
represents Pennsylvania.
To ensure citizen input, EPA established a public participation
program as an integral part of its Chesapeake Bay Program. This is the
main mechanism by which information flows between citizens and
managers regarding alternative choices on issues affecting the Bay.
The Chesapeake Bay Program complements environmental studies
being conducted by other institutions and citizen groups. The
program's objectives are to describe historical trends and to determine
the current state of the Bay by evaluating ongoing research and by
providing new research to fill information gaps. The program also
projects future conditions and uses these projections to aid in
identifying alternative strategies for managing Bay resources.
The Bay is, in many ways, like an incredibly complex living
organism. Each of its parts is related to its other parts in a web of
dependencies and support systems. For us to manage the Bay's
resources well, we must first understand how it functions.
By explaining the ecological processes in the Bay, this primer serves
as an introduction to several forthcoming Chesapeake Bay Program
publications. These publications will describe the results of the
scientific investigations and identify management solutions to specific
Bay resource problems. They present, in an easy to understand yet
informative manner, key issues and concerns pertaining to the
Chesapeake Bay.
Tudor Davies
Director
Chesapeake Bay
Program
Thomas DeMoss
Deputy Director
Chesapeake Bay
Program
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Table of Contents
i Foreword
3 Introduction
6 The Geology of
the Chesapeake
10 The Water & Sediments
18 Representative Biological
Communities
Chesapeake Bay:
Introduction to
an Ecosystem
Prcfectron Agency
ioji Resource
••z,FA 19107
26 Food Production &
Consumption
33 The Future
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The Chesapeake Bay
and its Tributaries
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Introduction
Down through the years, residents and visitors alike have
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, die 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 $100 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 24 percent of die 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 $24 billion, were shipped via the
Chesapeake during 1979.
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Shipping Projections:
Chesapeake Bay
I I Baltimore
I j Hampton Roads
millions of short tons
$o
20
10
I98O I99O 2OOO
Bi-state conference on Chesapeake Bay, 1977.
Both Baltimore and Hampton Roads are situated near the
coal-producing regions of Appalachia, making them essential to
promoting the use of U. S. coal abroad. The Hampton Roads
Complex already leads the nation in exporting coal and lignite.
Nearly 50,000 commercial vessel trips are recorded annually from
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
the Chesapeake watershed.
Perhaps one of the Chesapeake's most valuable functions, yet one
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
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
the Bay to feed on its rich food supply.
The hospitable climate, lush vegetation and natural beauty of the
Chesapeake have made it an increasingly popular recreation area.
Boating, fishing, swimming, hunting and camping are the major
attractions. Both power and sail boating have grown dramatically In
1979, more than 122,000 pleasure craft were registered in the State of
Maryland alone.
Sportfishing is another major recreation activity in the Chesapeake.
A1979 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.
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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
which together make up the Chesapeake ecosystem. Lest we forget,
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
complex system along with enough background information to allow
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
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.
Population Projections:
Chesapeake Bay Drainage Basin
millions of people
16
12
ID
1980
1990
2OOO
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The Geology of the Chesapeake
The 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.
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 10,000
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 200 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 2,200 square miles. When its
tributaries are included that figure nearly doubles. On average, the
Chesapeake holds about 18 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 90 percent of the freshwater entering the Bay comes from the
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northern and western sides. The remaining 10 to 15 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 64,000 square-mile drainage
basin which also includes part of the Piedmont Plateau, and the
Allegheny Plateau. More than 50 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 15 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,
and are potential areas for
accumulation of sediments
which may contain toxic
materials.
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miles west of the Bay. This fall line forms the boundary between the
Piedmont Plateau and the coastal plain. Waterfalls and rapids clearly
mark this line, where the elevation sharply increases to approximately
yoo 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 significandy 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.
'3fl
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.
The forces of erosion and sedimentation are continually reshaping
the details of the Bay.
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The Chesapeake Bay Drainage Area
Approximately 64,000 square miles of territory in five states
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
chemical mixture of the Bay.
miles
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The Water & Sediments
Typical Vertical Temperature
Profile in Spring
(April-May)
depth
14° C
THERMOCLINB
Water... four-fifths of the earth's surface is covered by it. It
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
measurably diluted by freshwater from land drainage." Within an
estuary, freshwater mixes with salt water, each contributing its own
variety of chemical and physical characteristics. The mixing of fresh
and salt waters creates unique chemical and physical environments,
each of which supports different communities of organisms particu-
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.
Water: Temperature, Salinity & Circulation
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
together they determine the physical characteristics of the water at any
given point in the Bay
Temperature dramatically affects the rates of chemical and
biochemical reaction within the water. A ro-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 ip-degrees C (?2-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
10
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Autumn Salinity
in parts per thousand
Spring Salinity
in parts per thousand
the deeper waters, due to the warmth of the sun. The water
temperature gradually decreases in relation to depth until a point is
reached where the temperature drops abrupdy. That point is known as
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 diminish. As
the surface water cools, it increases in density, becoming heavier. Once
the surface water becomes colder and denser than the water toward
the bottom, it begins to sink and vertical mixing occurs. Wind may
speed up the process. This mixing action can bring nutrients, materials
essential to the growth of organisms, up from the bottom and into
higher water levels. The turn-over makes the nutrients available to
phytoplankton and other organisms inhabiting the upper water levels.
During the winter, the water temperature becomes relatively
constant from surface to bottom until March, when the process of
surface warming begins again.
Salinity is the second key factor affecting the physical make-up of
the Bay. It is the concentration of dissolved salts in the water, usually
expressed in parts of salts pen,ooo parts of water (ppt). Freshwater
contains few salts (drinking water usually has a salinity of less then 0.5
ppt), while seawater averages 35 ppt.
With the heavy influx of
freshwater in the spring, the
surface of the Bay declines in
salinity. As freshwater flow
declines in the autumn, salinity
from seawater increases.
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Two-Layered Circulation Pattern with
Zone of Maximum Turbidity
•-
>--o.-!.?H
•,vvv--'-' '!"•'••;.
•••.'.• > v.-. ..••••.-.
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 50 percent of the Bay's freshwater, can vary ij-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 eastern 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 surface to the bottom, salinity increases gradually,
but there may be an intermediate layer where the increase in salinity is
abrupt. This layer is known as the halocline. It normally separates the
less saline surface layer from the high saline bottom layer. As such, it
also distinguishes two different types of environments.
Perhaps the most important aspect of the Chesapeake's graduated
salinity levels is their effect on the distribution and well-being of 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.
12
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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, widi
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.
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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 die 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 Bn.
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
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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 17.5 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 Bn 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 slighdy 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 15 degrees Celsius, one liter of
seawater has a saturation level of 5.8 milliliters of dissolved oxygen,
while one liter of freshwater can hold 7.1 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
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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 shirts 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
16
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of 16 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.
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 i96o's and early
1970'$, and algal blooms have decreased. This is at least in part
attributable to the installation of new sewage treatment facilities in the
mid ipyo'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 paniculate 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
sediments on the bottom fills in waterways and ultimately leads to the
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, hi 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.
Chesapeake Bay
• present coastline
- colonial coastline
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Representative Biological
Communities
More than 2,700 species of plants and animals inhabit the
Chesapeake and its shoreline. All depend on the Bay and
their 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
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
water 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 1930'$ and early ip4-o's, while the croaker was abundant.
The bass increased until the early 1970'$, 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.
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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 212,000 acres in Maryland and 213,000 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
that affect sediment transport and deposition patterns,
such as levees and channels, may affect the development
of a marsh in unanticipated ways.
The Marsh
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.
19
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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 die
transmission of light so that, in relatively clear waters, bay grasses can
exist in water depths up to about 10 feet.
Like the marsh grasses, the various species of submerged grasses
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
sand. Turbulence caused by wave action can also restrict their
distribution. Although submerged grasses can act as "baffles" to reduce
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Bay Grass Community
This community typically con-
tains a seasonally abundant
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.
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 diem as a place to attach. They provide support for die
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 dieir
ability to slow down water velocities, causing participate matter to
settle at the base of their stems. Water thus tends to be more clear near
grass beds. Finally, like marsh grasses, bay grasses can act as nutrient
buffers, taking up nitrogen and phosphorus and releasing them later
when die 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 1930'$) and Eurasian milfoil
21
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Plankton Community
zooplankton
phytoplankton
and bacteria
(abundant growth, then reduction in the 1960'$) 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
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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 clearly 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.
For example, mega- or macro-fauna are the largest, most visible
animals, while meio- and micro-fauna range downward in size.
5A
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Benthic Community
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 15
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 offish 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.
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Smaller resident species include killifishes, anchovies, silversides,
hogchokers and gobies. They are normally found in shallow water
where submerged vegetation provides cover. Here they feed on a
variety of invertebrates such as zooplankton and amphipods. Larger
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
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
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
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.
Migratory Pathways of Fish
Non-Spawning
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Food Production &
Consumption
a/i
Members 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
nucleic acids.
The processes by which organisms acquire, utilize and ultimately
lose this important element form the basis of the Chesapeake's life
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
referred to as autotrophs — self feeders. Autotrophs are the osily
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.
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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,
sometimes resulting in situations where sufficient quantities are not
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 of 106:16:1. 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.
Nutrient Mix for
Phytoplankton Growth
nitrogen
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Major Nutrient Cycles:
Carbon (C)
rain, drainage
& wastes
sediments
(storage & burial)
*CCh represents all forms of inorganic carbon
fishing
Nitrogen (N)
rain, drainage
& wastes
nitrogen fixing
algae & bacteria
fishing
organic
nitrogen remineralian
microbes
Phosphorus (P)
rain, drainage
& waste
fishing
organic remineraljang
phosphorus microbes
sediments
28
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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.
Only a small fraction is allocated to growth and reproduction. Since
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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 10 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
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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,
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 purs them in the direct predator}'
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 die 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 die Bay. The role of zooplankton
is better defined. Knowledge of specific details pertaining to food web
relations involving bacteria and odier microbes and protozoa is
severely limited.
Phytoplankton are divided by size. The larger species are often
called "nef5 plankton, because they are traditionally collected with fine
nets. Many diotams 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 50 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 die waters of die Bay, devouring enormous
quantities of phytoplankton. Most production of animal protein from
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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.
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 offish 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.
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The Future
Thus 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.
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.
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Technical Reviewers:
Scott W. Nixon
Richard L. Wetzel
Walter R. Boynton
Robert B. Biggs
David A. Flemmer
Willa C. Nehlsen
C. John Klein
Bert S. Brun
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