River-Lab 6 Guide Manual
River-Lab 6
GUIDE BACKGROUND MATERIAL
What Is An Estuary?
Many rivers flow down to the coasts of the Earth's continents and islands. What happens
upstream in these rivers can affect life along the coast. Rivers that flow to the coast form wide
mouths, called estuaries, and are important to life along the coast.
Most estuaries are the mouths of main rivers that empty at the coast or shore of a continent.
In estuaries, the fresh water flowing off the land through the river mixes with the salt water
coming in from the sea.
The great gravel deposits of the last Ice Age blocked some rivers from their old estuarine mouths
and forced them to make new mouths. Fresh water continues to flow into the old blocked
estuaries from the basin's underground system.
Most of the estuarine areas at the ends of the rivers are wide, irregular openings. This shaping is
the result of thousands of years of wearing away of the land and depositing of soil by the river
flowing out and the sea flowing in. Both river and sea have greater power to erode and deposit
soil during severe weather such as storms and floods.
River Basins of New England
All of New England is made up of river basins, most of which empty along the New
England coast. Some exceptions are a few basins which empty into the Atlantic Ocean
north of New England and Canada.
Each basin empties through the estuarine area of its main river. The estuarine areas are in
bays and inlets of the coastline. Estuaries of many different shapes and sizes are found
along coasts of all continents, as they are along the New England coast. They range in
size from the great estuary of the Yangtze River in China to little estuaries all over the
world that are even smaller than the blocked estuaries of New England.
River Basins of Southwest Connecticut
There are many estuaries in the southwest part of New England in Connecticut and a
corner of New York State. Most of these estuaries are the mouths of rivers. Several, like
the Pine Creek and the Great Meadows areas, are blocked estuaries. All these estuaries
are important to coastal life. All of them can be affected by what people do to them, or to
the rivers that flow out through them.
The Parts of an Estuary
There are four important areas of an estuarine system: channels, salt marshes, barrier beach, and
marginal area. The two main parts within the estuary are the channels and salt marshes. These
areas provide most of the habitats and food for the great numbers of living organisms in this very
productive area.
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Channels
The main channel of an estuary is usually the end of the main river of a river basin. The
main channel usually has smaller side channels or creeks. These channels flow around
the many salt marsh islands.
Main channels or creeks are widened by the churning collisions of incoming or storm
tides with the out-flowing fresh water. The mixture of salt and fresh water in an estuary
is called brackish water.
Salt Marshes
Salt marshes are sometimes called tidal marshes or tide marshes. They are low, flat,
spongy areas made of peat soil, covered with grasses that can grow in salt water. Salt
marsh peat soil is built up by partly decayed salt marsh organisms (especially the salt
marsh grasses), mixed with sand and other deposited material.
Salt marshes build up in an estuary until they begin to squeeze or constrict the channels.
Water in these channels will then make room for its flow by eroding part of the marsh. A
flooding river flowing into an estuary can cause much erosion.
The channels and salt marshes of the estuary are the most productive parts of the
coastline. This means that they are the areas that produce the most forms of life,
including algae, plants and animals.
The barrier beach and marginal area are the boundaries of the estuary. Barrier beaches stretch
part way across the end of an estuary protecting it from heavy tides and ocean storms. The
marginal areas border the inland sides of the estuary, providing habitats for important predators
that help to keep the balance of life in the estuarine system.
Barrier Beaches
Barrier beaches extend from one or both sides of the end of an estuary across the opening
to the salt water. Many of the beaches people enjoy in the summer are barrier beaches.
Most estuaries exist because a barrier beach developed long ago, limiting the amount of
salt water that could flow into the estuary and protecting it from storms. Barrier beaches
continue to be formed by the collision of incoming seawater in tides and storms with outflowing river water, including floods. The out-flowing river carries sand from the basin.
As these waters collide, the river drops the sand at its mouth.
Marginal Area
The land bordering the inland sides of the estuary is also important to its life. This border
is called the margin, marginal area or upland area. The marginal area of the estuary is
always somewhat higher than the salt marshes, but often not a great deal higher. This
area often has upland vegetation such as trees, which makes it easy to distinguish from
the salt marsh.
It is in these margins around the salt marsh that the top-level predators of the system,
such as the marsh hawk, the osprey, and the fox live. These predators eat small animals
in the estuary that might otherwise overpopulate and upset the balance of the web of life
in the system.
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© 2007 Mill River Wetland Committee, Inc.
River-Lab 6 Guide Manual – Background Material
SALT MARSHES ARE THE MOST PRODUCTIVE AREAS ON EARTH
Life In An Estuary
Life in an estuary is affected by both biotic (living) and abiotic (non-living) factors. Biotic
factors include plants, animals, bacteria, and fungi. Some examples of abiotic factors that may
affect estuarine life are temperature, water, and sunlight.
When you visit an estuary it may look calm, but there is always a lot of action under that calm
surface. Water is going in or out, currents are changing, and life is being moved or moving
about. Organisms living in the estuary have adaptations that help them exist under its everchanging differences in tides, temperature, moisture, and salinity.
Estuarine habitats, like all other habitats, provide living organisms with four basic needs: food,
water, oxygen, and shelter.
FOOD
One reason the estuary is such a productive area is that it can produce an abundance of nutrients.
These nutrients are linked in complex ways in an estuarine food web.
Estuarine Food Web
The estuarine food web is different from a food web in the forest. In the forest, much of
the plant material at the base of the food web is eaten when it is alive (green leaves,
seeds, flowers, etc.).
In an estuary, the base of the food web is still green plants—the salt marsh grasses
(Spartina alterniflora and Spartina patens), the producers—but these grasses are not eaten
when they are alive. They are mostly eaten after they have died.
Bacteria and fungi (decomposers) break down the dead grasses into smaller and smaller
particles (detritus). This detritus (decomposed material plus the decomposers) is a food
source for many animals in the estuary.
Plankton and algae are two other very important food sources in the estuary. Plankton,
algae, and detritus are the main ingredients that make the water of the estuary into a
nutritious “estuarine soup.” This “soup” is very nourishing food for many estuarine
animals. Detritus is also found in the mud and the marsh soil. Many organisms (called
detritivores) eat the detritus in the estuarine soup, mud, and marsh soil.
Some animals, such as the fiddler crab, eat the marsh soil, use some of the detritus in the
soil for food, and excrete out what they do not use.
Some animals eat the detritus in the estuarine soup. Ribbed mussels are called filter
feeders. They take in the estuarine water (with detritus and plankton) and their bodies
filter the detritus and plankton out of the water and expel the “cleaned” water.
Many animals would eat a ribbed mussel or fiddler crab in an estuarine food web.
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Once any of these consumers (animals) die, the decomposers (bacteria and fungi) will
break down the bodies of these animals, eventually into detritus. This adds nutrients to
the mud, marsh soil or estuarine soup.
There are many connections in the food web of an estuary. Remember that in the estuary
the chains begin with the producers (salt marsh plants) being eaten by the decomposers,
not the herbivores. Scientists call this type of food web a detritus food web.
Below is an overview diagram of estuarine productivity:
But what makes the detritus so nutritious?
When detritivores (such as mud snails) eat the detritus, the main nutrients actually come
from the bacteria and fungi. The detritivore excretes what it cannot use. Scientists call
the waste dung. The dung consists mainly of the plant parts (complex carbohydrates—
cellulose) that the animal cannot digest. BUT the decomposers can breakdown (digest)
the cellulose that is in the dung.
Bacteria and fungi start to grow on the dung and breakdown (decompose) some of the
cellulose to use for energy; the mud snail eats the dung with the new bacteria that has
grown on it, uses the nutrients from the consumed bacteria, and excretes what it can not
use.
The whole cycle keeps repeating until the cellulose is broken down all the way into
nutrients. The nutrients in detritus are:
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
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Complex carbohydrates (plant parts)
Microorganisms (bacteria and fungi)
Nutrients released from decomposition (nitrogen and phosphorous)
© 2007 Mill River Wetland Committee, Inc.
River-Lab 6 Guide Manual – Background Material
Where does all the detritus in an estuary come from?
Besides the detritus that is produced in the estuary, the river carries down its detritus from
the inland basin, and tides carry in detritus manufactured by saltwater organisms. This
triple delivery makes estuarine waters the greatest collecting places for nutrients of any
waters on Earth. It is why these areas are so productive. The estuary is like a giant "soup
bowl" of nourishment for many forms of life.
Summary
The detritus food web is vital to the productivity of the estuary.
 The huge amount of grasses in an estuary is at the base of the estuarine food web.
 BUT it is the decomposers (bacteria and fungi) that make the nutrients available
to the food web by decomposing the grasses.
WATER
The need for water is essential for all organisms. Cells cannot function without water.
Organisms each have different tolerances for amounts of salt in the water. The water in an
estuary changes with tides and storms.
The ocean rises and falls against the land in tides that occur about twelve hours apart each day.
These tides bring salt water into the estuary twice a day. Rivers flow constantly to and through
the estuary, carrying fresh water. Organisms living in the estuary are adapted to withstand these
constant changes in salinity.
What happens to aquatic organisms when the tide goes out?
The aquatic organisms of the estuary must be adapted to periods of low, or no, water.
They may be exposed twice daily when the tide goes out. In order to keep their cells
functioning, they need mechanisms to stay wet until the tide comes back in. They have
various adaptations that help them preserve what moisture they can. Those organisms
with shells may close them up, sealing some water inside. Other organisms are adapted
to bury themselves in the moist soil.
OXYGEN
Most organisms, including plants, need oxygen for respiration. Aquatic organisms use oxygen
from the air that has been dissolved in the water of their environment. The tides bring freshlyoxygenated water into the estuary. When the tide goes out, many aquatic organisms will no
longer be covered with water. In order to survive, they will use the oxygen from whatever water
they were able to preserve, as noted above: they may close up (if possible) or bury themselves.
SHELTER
Another basic need for survival is shelter, or protection. Some estuarine organisms provide their
own shelter: their shells. Hard shells provide good protection from the elements and from
predators (like these Oysters). Organisms with two shells can close them tightly for protection.
Organisms with a single shell may have a hard plate that covers the opening. In addition, some
of these shelled animals can secrete mucus to stick themselves down firmly to a hard surface.
Still other organisms seek protection in the holes or tunnels they themselves dig. This is a safe
and speedy option in the soft soils of the estuary.
The dense grasses of the salt marsh provide safe habitat for many other organisms. The grasses
also provide something for small organisms to cling to; otherwise, these organisms could be
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dragged out to sea with the tide, or even by the regular flow of the river through the channels.
Storms increase the amount of water coming down the river or in from the ocean. This may
significantly increase the movement of colliding salt and fresh water.
Organisms that exist in the estuary have adaptations to help them stay put and not be
overwhelmed when faced with increased water volume or receding tides. Some may find
something to cling to; some may burrow into the soil; and some may seal themselves to a hard
surface (temporarily or permanently). This ensures that they will remain in the specific habitat
suited to them.
Biodiversity
Estuaries are complex ecosystems. The four main parts ―barrier beach, salt marsh, channels,
and margin ― provide a tremendous variety of habitats: sandy beaches and mud flats; salt
marshes with dense grasses; winding channels with a mixture of fresh and salt water; and dry
upland areas at the higher margins. Within any of these areas, there are countless variations of
conditions, creating countless numbers of distinct habitats.
Because estuarine ecosystems have a great variety of habitats, it is possible for a great number of
different species to live there. Scientists call this variety of species biodiversity. High
biodiversity is an indicator of the estuary’s productivity.
The HANDBOOK OF ESTUARINE ORGANISMS profiles over 21 estuarine species. The
following section provides a brief preview of several of the categories of these species. More
specific information is provided in the handbook.
Decomposers
Decomposers are vital to the estuarine ecosystem. Certain bacteria and fungi break down
the waste and dead bodies of other estuarine organisms. This recycles nutrients for the
producers and consumers to use.
Plankton
The plankton community includes microscopic protists, plants, and animals.
Phytoplankton, such as algae, can photosynthesize; these producers are the base of many
aquatic food chains. Zooplankton includes organisms that will always be microscopic, as
well as the tiny larvae of many estuarine animals.
Macro-Algae (Seaweeds)
As opposed to micro-organisms, macro-organisms are large enough to be seen. Macroalgae are large forms of algae, commonly known as seaweeds. These macro-algae are
protists, not plants: they do not have true roots, stems, or flowering parts. Although
seaweeds, like plants, make food by photosynthesis, not all are used primarily for food
(many are too tough to be easily consumed). However, many provide shelter and
protection for small marine organisms (fish, snails, crabs), especially juveniles. Humans
have found many uses for these large algae. We process seaweeds to extract substances
that help make many foods thicker and smoother.
Plants
Many of the estuary’s plants are grasses that grow in the salt marshes. Grasses such as
smooth cord grass and salt marsh hay provide many of the nutrients that make the estuary
so productive. Plants such as Phragmites, can invade an estuary, compete with the native
grasses, and replace them. This replacement reduces productivity.
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© 2007 Mill River Wetland Committee, Inc.
River-Lab 6 Guide Manual – Background Material
Marine Worms
Worms such as sandworms and clamworms have segmented bodies like earthworms.
Their burrowing habits benefit the estuary as earthworm digging benefits other
ecosystems: loosening the soil to allow water and oxygen to penetrate.
Crustaceans
This group of organisms is characterized by a segmented body (head, thorax, and
abdomen), stiff carapace (exoskeleton), and multiple appendages. Estuarine examples
include amphipods, barnacles, and crabs. This group also includes lobsters and shrimp,
as well as planktonic species: water fleas, copepods, and seed shrimp.
Mollusks
Mollusks are animals with soft bodies often encased in shells. Mollusks with a single
shell are called univalves. Estuarine univalves include the mud snail, periwinkle, and salt
marsh snail. Bivalves have 2 shells hinged at one side. Estuarine examples are oysters,
clams, and mussels. Mollusks move around using a muscular “foot” extended outside the
shell.
Fish
Many species of fish live in or use the estuary during some part of their life cycles. The
estuary is often used for spawning and as a “nursery” for the young of many saltwater
fish, including the striped bass. Bluefish young often spend their first summer in the
estuary, and adult blues come into the estuary at high tide to eat the menhaden and
killifish that live there.
Birds
Many species of birds seen in the estuary will also be found in more inland habitats, near
rivers, lakes, ponds, and wetlands. Kingfishers, herons, egrets, gulls, and osprey are
well-known for their fishing abilities. As top predators in an estuary, they help keep fish
(and other smaller marine organism) populations in balance. Other top predators, such as
the red-tailed hawk, help control rodent populations. Since rodents feed on vegetation,
controlling them protects the estuary’s valuable plant life.
Mammals
Mammals such as muskrats, raccoons, fox, and skunks live all over the river basin. In an
estuary, they make their homes in the dry upland areas at the higher margins. The estuary
provides mammals many opportunities to find prey. Most mammals are top predators.
They eat small animals such as fish, crustaceans, and rodents. In doing so, these predators
keep their prey species from over-populating and upsetting the balance of life in the
estuary.
Man’s Impact
The protist, plant, and animal life described in this book and the Handbook are examples of
many kinds of coastal life that could not exist without estuaries. When river basins including
estuaries are disturbed by human activities, they become less productive.
Man’s Use of the Estuary
Throughout history, people have used the estuary in many ways. Native Americans, and then
colonists, and now contemporary people, have been able to find a wealth of food such as fish,
oysters, clams, and mussels. In the past, the waters of the estuary also provided power to work
various kinds of mills. More recently, estuaries, particularly those with accessible beaches, have
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attracted heavy recreational use. All of our activities, both at the coast and in upstream parts of
the basin, have an impact on the estuary.
When we destroy estuarine areas to make a marina and use the estuary's margins for parks,
treatment plants, or other development needs, we reduce estuarine and coastal productivity.
Many other aspects of modern life have direct and indirect effects on productivity. These human
activities do not have to take place in ─or even near ─the estuary. Whatever happens anywhere
in the river basin can potentially affect the estuary and the coastal waters.
Today, fast runoff, increased by human activities, is a cause of major impacts on estuarine life.
A river carrying increased basin runoff brings more silt down to the estuary. In many estuaries,
silt has buried the shallow, rocky oyster habitat, reducing or totally destroying the oyster
population. Increased fresh water runoff from every size storm also decreases salt content in
many parts of the estuary. Because each kind of estuarine organism needs a certain level of salt,
this change forces many of them to seek a new habitat area. Some habitat changes make it easier
for predators to reduce the population of an organism. Bigger crests of floodwater from a basin
with increased fast runoff speeds the normal rate of salt marsh erosion, further reducing estuarine
productivity.
Another serious problem related to basin runoff is the pollution it may carry to the estuary and
coast. Basin runoff can pick up animal waste and chemicals, such as pesticides, car oils, and
lawn fertilizers. This contaminated runoff ends up in our rivers, which carry it to the estuary and
coast. This type of pollution, which cannot be linked to a specific source, is called non-point
source pollution. It is much more common, and often harder to remedy, than point source
pollution. Point sources are easier to identify and fix – you can see them. Wastewater coming
out of a pipe or ditch directly into a body of water from a building such as a factory is considered
point source pollution.
For more detailed information about how towns and cities handle wastewater of all types—from
runoff to storm drains and sanitary sewers—consult “GARDY LOO!” (WATCH OUT FOR THE
WATER!).
Estuary: A Link to the Sea
A healthy estuary is one of the most productive places on Earth. As the meeting-and-mixing
place of fresh river water and salty ocean water, estuaries provide an abundance of different
habitats for many forms of life. The brackish waters abound with nutrients brought by the river
and the sea, as well as those produced in the estuary itself. The grassy plants of the salt marsh
are a primary source of nutrients and also offer shelter and protection for many organisms.
Even many organisms that normally live in salt water benefit from the estuary’s productivity at
some point in their lives. In fact, seventy-five percent (75%) of all saltwater life use the estuary
during some part of their life cycles: for feeding, spawning, and protecting their young.
Our coastal waters cannot be productive unless our estuaries are productive. But the health of
an estuary also depends on the health and productivity of the entire river basin. Managing our
river basins carefully is crucial to protect our estuaries; protecting our estuaries will also protect
our oceans.
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© 2007 Mill River Wetland Committee, Inc.