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An estuary is a zone where a river flows into the sea, mixing freshwater with saltwater. The river brings nutrient-rich sediment into an estuary that can support a high level of biological productivity. Estuaries are usually relatively warm and calm, protected from the force of the ocean, but the constant ebb and flow of the tides, together with the presence of both freshwater and saltwater can make it a challenging habitat. Species living in estuaries, like salt-resistant grasses, are those that can adapt to the margins of the sea.

Many fish and shellfish spawn and develop in estuaries, protected from the open ocean. Estuaries have traditionally provided a rich hunting ground for species such as crabs, clams, and oysters. However, many estuaries are now polluted from sources such as agricultural runoff arising from human overuse of pesticides and fertilizers, and industrial waste. Efforts are being made to restore some of the world’s great estuaries, including Chesapeake Bay, but their recovery is a slow process.

Historical Background and Scientific Foundations

An estuary is the place where a river, stream, or creek inlet meets the sea or ocean, creating a complex physical environment and a set of ecosystems that is productive, although not necessarily diverse. There may be many water inlets, giving the estuary a complex shape, or just one. An estuary is partially enclosed by some combination of coastland, marshes, wetlands, and barrier islands, and is thus protected from the direct force of ocean wind and waves.

Freshwater from the inlets bring nutrients to the estuary in its sediments and dilutes the saltwater. In some temperate estuaries, the less dense freshwater floats on top of the more dense saltwater. Estuarine ecology is shaped by the tides. The saltwater layer picks up nutrients from the muddy bottom as it moves with the tides and then mixes these with the freshwater layer. Estuarine water is defined as brackish, meaning having a salt content in between that of freshwater and seawater. Typically, its salt content is not constant, but will vary with location in the estuary and over time. Estuaries are usually shallow and so the euphoric zone, which is the layer receiving enough sunlight for photosynthesis, tends to extend to the bottom, which gives the whole zone high biological productivity.

Both terrestrial and aquatic plants and animals are found in estuaries. Terrestrial species include storks, pelicans, coniferous and deciduous trees, wildflowers, and butterflies. Aquatic species include sea grasses, crabs and lobsters, turtles, marine birds, and marine mammals like sea lions. Filter feeders like the horseshoe crab pull larvae and plankton out of the water moving past them. Oysters adhere to the ocean floor and are a source of food not just for humans but also for fish, sponges, and snails. These species are all closely linked to one another in a complex food web.

There are four types of estuary, classed according to geological formation. The coastal plain estuary is created when rising sea levels fill an existing river valley, as in the Chesapeake Bay in Maryland or the harbor in Charleston, South Carolina. Tectonic estuaries are formed by the folding of land surfaces and are found along major fault lines. Such estuaries include San Francisco Bay. The bar-built estuary is one in which a shallow bay or lagoon is protected from the ocean by a barrier island or a sand bar. One example is the Eastern seaboard or the Gulf Coast of North America. Fjords are U-shaped valleys that have been formed by glacial action. They are found in Norway, Alaska, and Canada.

The tides and the flow of freshwater into saltwater create three main types of habitat within an estuary. The salt marsh is farthest from the sea and is a shallow wetland that is occasionally or regularly flooded with saltwater. Notable salt-marsh locations in the United States include the eastern coast from Maine to Florida and continuing on to Louisiana and Texas along the Gulf of Mexico. Some of these marshes are brackish or freshwater rather than saltwater marshes. They are all influenced by ocean tides that create two zones. The lower or inter-tidal marsh is generally covered and exposed every day by the tide, while the upper or high marsh is only covered sporadically. This affects the plant ecology in the two zones. Tall cordgrass tends to grow in the lower marsh, while a shorter version grows in the upper marsh, along with various other salt-resistant grasses. Salt marshes also provide food and shelter for clams, crabs, juvenile fish, and migratory wildfowl.

Mudflats and sandflats are found closer to the sea. They comprise large zones of mud, which is a fine-grained sediment, or sand, which is coarser; both do not support vegetation. Mudflats are exposed to the air at low tide and submerged at high tide. They are, therefore, a challenging habitat because of the constant switch between land and aquatic conditions, from fresh to saltwater and changes in temperatures. Therefore, the biodiversity of mudflats is not great, but there may be large populations of the few species that can survive here. They are rich in invertebrates such as worms, clams, and other shellfish and often support commercial oysterbeds. Mudflats are also good foraging grounds at low tides for shorebirds and, at high tide, for birds and fish.

The mudflats of the San Francisco Bay area are extensive and there are smaller ones throughout California and some Pacific areas. Because they are shal-low, and near the shore, they are at risk from dredging, filling for development, sewage pollution, and agricultural runoff. The final main habitat in an estuary is the pathways full of water known as channels, which connect the salt marshes and mud flats to the sea or ocean. These serve as nurseries for fish and shellfish, many of which have commercial value, because of the protection and food supplies they offer. Other habitats found in estuaries include rock pools, mangrove swamps, and coral reefs.

Every estuary is unique. An estuary may be known as a bay, harbor, lagoon, or sound. However, not all areas given these names are estuaries. The defining feature of an estuary is the mixing of fresh and salt water. Some well-known examples of estuaries in the United States include Mobile Bay, Puget Sound, Boston Harbor, the New York/New Jersey Harbor, and Tampa Bay.

Chesapeake Bay, which extends for 200 mi (322 km) from Havre de Grace in Maryland to Virginia Beach, Virginia, illustrates many of the typical features of an estuary. It is 34 mi (55 km) wide at its widest point, which is the mouth of the Potomac River, and its average depth is about 21 ft (6.5 m). It is home to more than


CHANNEL: A water-filled path which connects mudflats and salt marshes to the ocean in an estuary.

FLYWAY: A flying route regularly taken by migratory birds.

FOOD WEB: An interconnected set of all the food chains in the same ecosystem.

MUDFLAT: An area of low-lying muddy land that is covered at high tide and exposed at low tide.

SALT MARSH: Wetland which is sometimes flooded with seawater.

SEDIMENT: Solid unconsolidated rock and mineral fragments that come from the weathering of rocks and are transported by water, air, or ice and form layers on Earth’s surface. Sediments can also result from chemical precipitation or secretion by organisms.

WATERSHED: The expanse of terrain from which water flows into a wetland, water body, or stream.

3,600 species of animal and plant including 348 species of fish and 173 species of shellfish. There are 2,700 different plant species and 29 species of waterfowl in Chesapeake Bay, which is an important resting ground for birds along the Atlantic flyway. A million waterfowl winter in the bay every year.

Impacts and Issues

Estuaries are among Earth’s most biologically productive ecosystems. They produce more organic matter every year than a forest, grassland, or agricultural land of comparable size. More than two thirds of commercial fish and shellfish spend part of their lives in estuaries, and about 80 to 90% of all recreational fishing is done here, Many migratory birds rest and refuel in estuaries during their long journeys, thanks to the abundant food supplies and protection from the ocean. Estuaries also act as filters for sediments and pollutants running off the land toward the sea or ocean, creating clearer, cleaner water, which benefits both people and wildlife. Estuarine soils and plants have a spongelike capacity that helps prevent flooding and storm surges. Salt-marsh grasses and other estuarine plants also help to prevent erosion of the sea shore.

Many estuaries are places of outstanding natural beauty or scientific interest, where people like to go to bird watch, fish, surf, swim, or sail. They also have important commercial value in terms of the fishing and related industry. Estuarine fishery trade is worth billions of dollars in the United States; many estuaries include ports or harbors of importance in shipping and transportation. Together commercial and recreational fishing, boating, tourism, and other coastal industries in and around estuaries provide more than 30 million jobs in the United States. For instance, two of the major North Atlantic ports, Baltimore and Hampton Roads, are situated on Chesapeake Bay.

Most estuaries in the United States and elsewhere, however, are now under threat because of human activities. This is mainly because coastal communities have grown in recent years. The population of the Chesapeake Bay area is 16.6 million and it is growing by 170,000 people a year. Therefore, estuarine channels have been dredged, waters polluted, and marshes filled in to create housing, roads, and industry. This has led to polluted water and beaches, development of harmful algal blooms, and loss of habitat for formerly abundant species.

The damage to estuaries worldwide has been going on for many years. The most comprehensive study ever of 12 important temperate estuaries and coastlines in

Europe, North America, and Australia, reported in 2006, found a high degree of degradation and depletion at all sites. The international team of researchers used paleontological, archaeological, historical, and ecological records to discover what the estuaries were like in the past so as to plot the changes that have occurred. The exploitation and decline of estuaries began as far back as the Roman Empire, but have accelerated dramatically in the last 150 to 300 years. Over 90% of formerly important species have been depleted to at least some degree and 65% of seagrass and wetland habitat is now destroyed, while water quality has been degraded. Most of the damage can be attributed to rapid population growth and the accompanying increase in demand. Commercialization of resource use and the growing demand for luxury foods like salmon, lobster, and oysters, have accelerated decline.

This study found that estuarine locations with the longest history of human impact and the highest human populations were among the most degraded, and these include the Adriatic, Wadden, and Baltic Seas. The outer Bay of Fundy, Southern Gulf of St. Lawrence, and Massachusetts Bay were the least degraded estuaries. Stocks of most mammals, birds, and reptiles were already depleted by 1900 and had declined still further by 1950 because of over exploitation for food or increasing demand for luxury items like furs.

For food, salmon and sturgeon stocks were depleted first, then tuna, shark, halibut, herrings, and sardines, demonstrating a shift from high- to lower-value species. The study also showed that oysters were the first invertebrate to be depleted because of their high value and destructive exploitation methods. The reef-forming and filter-feeding capacity of oysters has meant that their depletion had a deleterious effect on the estuarine ecosystem by reducing its ability to provide high water quality and a complex habitat. The advent of seafloor trawling hit previously unaffected filter feeders like corals and sponges. Mussels and crabs went into decline because, although they have long been used for food, they only recently become the target of commercial fish-ries.

The report also found that loss of coastal vegetation in estuarine zones has been a factor in the loss of nursery habitats for fish and also in protecting coastlines from erosion. However, although many species have been depleted, only 7% have actually become extinct. Restoration efforts are working, but slowly. There has been a partial recovery, although estuaries today remain far from their original ecosystem structure and function. Reducing exploitation, safeguarding water quality, and protection of habitats are the key to restoring estuaries.

Restoration of estuaries is going on all around the world. The United States National Estuary Program was established in 1987 following amendments to the Clean Water Act. Its mission, however, extends far beyond

merely ensuring water quality and is more about protecting and restoring the nation’s estuaries through maintaining the integrity of the whole system in terms of its physical, chemical, and biological properties. The program encourages local communities to take the lead in managing their own estuaries. Currently, 28 estuaries are participating in the program. They include San Francisco Bay, Puget Sound, Indian River Lagoon, and the New Hampshire estuaries.

A separate program is underway to protect the largest estuary in the United States, Chesapeake Bay, which was the first to be targeted for protection and restoration in 1983. This is proving to be a long-term project, as the damage of the last 100 years or so cannot be reversed in only a couple of decades. In a recent assessment, the overall health of the Bay area was still judged to be degraded. Just 12% of the Bay and its tidal tributaries met dissolved oxygen standards. This means that pollution with nutrients is still in the water, causing overgrowth of algae and so encouraging the growth of microorganisms that use up oxygen. Lower oxygen levels will decrease the population of fish and other species. The area of grasses increased to 65,000 acres, but this is only about a third of the ultimate restoration goal. Native oyster and blue crab populations continue to be lower than their targets. However, striped bass populations remained high, although some fish are not healthy.

The main factors that are impeding the recovery of Chesapeake Bay include population growth, agricultural runoff, and climate variability, which are the same as those affecting estuaries around the world. The program has achieved only part of its goals of controlling nitrogen, phosphorus, and sediment coming from agricultural lands and wastewater. But progress has been made. A total of 2,266 mi (3,647 km) of freshwater stream habitat has been re opened to migratory fish. Maryland, Virginia, Pennsylvania, and the District of Columbia have permanently preserved 6.88 million acres of land, which brings them close to their 2010 goal of preserving 20% of their combined land within the Chesapeake watershed.

See Also Bays and Estuaries; Coastal Ecosystems; Oceans and Coastlines



Kaufmann R., and C. Cleveland. Environmental Science. New York: McGraw-Hill International Edition, 2008.

Web Sites

Chesapeake Bay Program. “About the Bay.” March 2008. (accessed March 31, 2008). “About Estuaries.” August 8, 2006. (accessed March 31, 2008).

Lenfest Ocean Program. June 23, 2006. (accessed May 22, 2008).

U.S. Environmental Protection Agency. “National Estuaries Program.” January 16, 2008. (accessed March 31, 2008).

Susan Aldridge


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Estuaries are the areas where rivers run into oceans. They often exist where the opening to the sea is somehow obstructed, for example by a sandbar or a lagoon (sandbars are ridges of sand built up by water; lagoons are shallow areas of water separated from the ocean by sandbars or coral). The water in estuaries is dominated by the flow of the tides. When tides are high, the ocean water washes through the estuary bringing with it sediments (particles of sand, silt, and gravel), nutrients, and organisms from the ocean. When the tide is low, the freshwater of the river floods the area, releasing its load into the estuary. Because estuaries exist where two different types of water come together and where the land meets the water, estuaries provide many different types of habitats for animals and plants. In addition, both the river and the ocean bring estuaries nutrients such as nitrate and phosphate, which plants need to grow. This results in a complex range of plants and animals that thrive there. Estuaries are also important to human settlement and economics. As a result, estuaries are often subject to pollution and other environmental stresses.

General structure of an estuary

The part of the estuary farthest from the ocean is often called a salt marsh. (A marsh is a wetland dominated by grasses.) Water usually flows through salt marshes in tidal creeks. Unlike river water, the water in tidal creeks can flow in two directions. When the tide comes in, the water runs into the salt marsh and when the tide goes out, the water runs the opposite direction, away from the salt marsh.

The part of an estuary closer to the ocean may contain mudflats (a thick, flat layer of mud or sand that is usually underwater at high tide) and sandbars. These areas are exposed when the tide is out and may be covered with water when the tide is in. They are often covered with a layer of thin algae, which are tiny rootless plants that grow in sunlit water. Many different types of burrowing (digging holes or tunnels) creatures, like clams and worms, live on mudflats and sandbars. Birds often walk along mudflats and sandbars when the tide is out, hunting for prey (animals hunted for food) buried in the ground.

The ocean edge of the estuary is almost always covered with water, although its depth changes with the tides. In this region, river water and ocean water mix and the resulting water has a salinity (the concentration of salt in water) that is neither fresh nor seawater. This type of water is called brackish. Brackish water includes water of a large range of salinities, from freshwater, which is about 0.5 part salt per thousand parts of water (ppt) to seawater, which is about 35 ppt.

The ways that the freshwater and the ocean water mix within the estuary is often very complicated. Sometimes the freshwater sits on top of the ocean water, because it is less dense. When this occurs, a halocline forms between the two types of water. (The root word halo means "salt" and the root word cline means "change.") A halocline is a layer of water where the salinity changes very quickly. The halocline can act as a physical barrier between the freshwater on top and the saline water below, blocking the exchange of nutrients, and even organisms, between them.

Life in estuaries

Brackish waters pose one of the most important challenges for many animals and plants living in estuaries. Because the salinity of the water is constantly changing, their cells must be able to handle osmotic changes. Osmosis is the tendency of water to have the same concentration on both sides of a material that allows liquid to pass (like a cell membrane, the structure surrounding a cell). When exposed to fresher water, cells that have grown accustomed to waters that are more saline will take in water, expand and even burst. When exposed to more saline conditions, cells that have grown accustomed to fresher water will release water, shrivel, and perhaps die. There are a variety of animals and plants that have special adaptations so that they can live in waters with changing salinities and these organisms thrive in estuaries.

A second problem facing organisms that live in estuaries is the ever-changing water level. Because the tide goes in and out, animals and plants must be able to handle waterlogged environments as well as environments that are dry. Many animals burrow in the sand and mud in estuaries. For example, sea cucumbers and polychaete worms live in holes in the mud. They expose their tentacles to the water where they capture plankton (free-floating organisms) and small prey that float into their reach. When the tide goes out, they burrow into their holes where they can stay moist.

Plant life in estuaries The salt marsh region of the estuary is characterized by plants that are adapted to salty conditions. The high salt marsh cordgrass has special organs on its leaves that remove the salt it takes up from its roots. The eelgrass, Spartina, looks like a grass with very tough leaves and stems that help it retain moisture in saltwater. It can be found in salt marshes throughout the East Coast of the United States. Other common salt marsh plants are sea-lavender, scurvy grass, salt marsh grass and sea-aster.

Farther out in the deeper waters of the estuary, microscopic phytoplankton (tiny plants that float in fresh or saltwater) are some of the most important plants. These single-celled algae-type plants float near the surface of the water where sunlight is available. Because the ocean water and the river water both deposit the nutrients that phytoplankton needs to grow quickly, phytoplankton in estuaries flourish. The large populations of phytoplankton are food for zooplankton (free-floating animals, often microscopic). In turn, the phytoplankton and zooplankton are meals for worms, clams, scallops, oysters and crustaceans (aquatic animals with jointed limbs and a hard shell).

Animal life in estuaries Because the types of habitats in estuaries are so diverse, estuaries are home to many different species of animals. Worms, clams, oysters, sea cucumbers, sea anemones and crabs all make their homes in the muddy floor of the estuary. Many of them burrow in the mud and filter the water for plankton and small fish that swim within the grasp of their tentacles and claws.

In some places, the clams and oysters become so numerous that their shells provide special habitats for other small animals. Barnacles grow on oyster shells in oyster beds. Small fish, snails, and crabs will hide from larger predators in the crevasses between clamshells. Mosses and algae will grow on the surfaces of some molluscs, providing food for the animals that take refuge there.

A variety of fish live in estuaries. Very small fish called gobies hunt along muddy and rocky surfaces for small crustaceans like shrimp. Long slender fish called pipefish swim among the grasses in the marsh, their shape blending in with the long blades of the plants. Larger fish like halibut and flounder, swim along the muddy floor, their flattened shape allowing them to move into the shallow regions of the estuary. Large predatory fish like redfish, snook, striped bass, mullet, jack, and grouper make their way into estuaries to feed on the rich supply of fish that can be found. Salmon pass through estuaries on their way up rivers to breed.

Many fish and invertebrates (animals without a backbone) use the estuary as a nursery ground for their young. For example, in Florida, a variety of species of shrimp spawn in the ocean, and their larvae (immature young) travel to the mouth of the estuary, where they develop into young shrimp. At a certain stage of their development, they ride the tide into the estuary, where they live among the eelgrass. The eelgrass provides them with protection from predators and the rich nutrients in the estuary produce plenty of food for them to eat. Once the shrimp become adults, they swim back to the ocean, where they spawn, producing young that will move back to the estuaries again.

Birds are extremely numerous in estuaries. During low tides, a variety of shorebirds walk along mudflats, pecking their beaks into holes where worms, crabs and clams are buried. Herons scour the shallow waters for shrimp and small fish. Brown pelicans, an endangered species, use estuaries as breeding grounds and nesting areas for their young.

Importance of estuaries

Estuaries are a unique habitat for a large variety of animals and plants. Because of their complexity a broad variety of species live in estuaries, either for part of their lives or for their entire life. The U.S. Department of Fisheries estimates that three-quarters of the fish and shellfish that people eat depend on estuaries at some point during their lives. Oysters, clams, flounder, and striped bass may live their entire lives within estuaries.

Estuaries serve as a buffer from flooding and storm surges. The soil and mud in estuaries is absorbent and can absorb large quantities of water. In addition, the roots of the grasses and sedges (grass-like plants) in estuaries are able to hold together sediments and protect against erosion (wearing away of land). Estuaries provide important protection to the real estate in many coastal communities.

As water moves through an estuary it is naturally filtered and cleaned. The many plants and bacteria that live in the estuary use pollutants, like agricultural fertilizers, to grow. Sediments that are transported to estuaries by rivers tend to settle into the estuary, where they act as filters, allowing cleaner water to flow into the ocean.

Danger to estuaries

Bacteria can break down some, but not all pollutants and many pollutants are not taken in by plants. Pollutants can build up to harmful concentrations within estuaries that threaten the health of the birds, fish, and humans that live nearby.

Chesapeake Bay

The largest estuary in the United States is Chesapeake Bay. It is an environment that has affected and been affected by humans for hundreds of years. Native Americans lived on the estuary and used it for its rich resources for thousands of years before Europeans came to North America. Once the colonists arrived, they began changing the landscape. By 1750, about one-third of the forests surrounding the estuary had been cleared. By 1865, more than half were gone. As cities and towns grew up along the Bay in the 1900s and into the 2000s, even more land was cleared for houses and commercial developments. With more and more people living near the Bay, the environmental stresses have become increasingly harmful.

Since the 1970s, both legislators and the people who live near the Chesapeake Bay have been actively involved in protecting the bay from environmental stresses. The Chesapeake Bay Program has worked to reduce pollution, to restore water quality and habitat, to manage the fisheries, to monitor the Bay ecosystems (the network of interactions between living organisms and their environment), and to develop practices that use the land in the best possible ways.

There are four major types of environmental stresses that affect Chesapeake Bay, the largest estuary in the United States. The most damaging type of pollution to the Bay is the input of nutrients like phosphate and nitrate, which are fertilizers used in agriculture. High concentrations of nutrients enter the Bay as rainwater runoff from land and from sewage treatment facilities. Although they are required for plants to grow, high concentrations can cause overgrowth of algae and marsh plants. This overgrowth can result in the plants using up all the oxygen in the water, causing the fish to die.

A second type of pollution is the input of sediments like clay, sand, and gravel that enter the Bay through river runoff. Although sedimentation is a natural occurrence, increased rates of erosion sometimes cause large amounts of sediments to be deposited in the Bay. Sediments can clog the feeding apparatus of filter-feeding animals and can cloud the water making it more difficult for plants to get light.

Air pollution is a third source of stress on the bay. Pollutants released from factories and cars as exhaust eventually make their way to the bay. Some of these pollutants produce acid rain, which changes the acidity of the bay, while others contribute to the concentration of nitrogen in the bay. Although evidence shows that dangerous pollutants, a fourth stress on the environment of the bay, are currently not as damaging as the other forms of pollution, the release of chemicals into the bay from some of the industries in the region can be deadly to both animals and plants.

Juli Berwald, Ph.D.

For More Information


Challand, Helen J. Disappearing Wetlands. Chicago: Childrens Press, 1992.

Cunningham, William P., and Barbara Woodworth Saigo. Environmental Science: A Global Concern. Boston: WCB/McGraw-Hill, 1999.

Haslett, Simon K. Coastal Systems. New York: Routledge, 2000.

McLeish, Ewan. Wetlands. New York: Thompson Learning, 1996.

Sayre, April Pulley. River and Stream. New York: Twenty-First Century Books, 1996.


"Chesapeake Bay Program." Chesapeake Bay Program. (accessed on August 16, 2004).

"Estuaries: The Cradle of the Ocean." Florida Department ofEnvironmental Protection. (accessed on August 16, 2004).

"Estuary Live." The North Carolina National Estuarine Research Reserve. (accessed on August 16, 2004).

"Information on Estuaries." National Estuarine Research Reserve System. (accessed on August 16, 2004).


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Estuaries are defined as semi-enclosed coastal bodies of water that have a free connection with the sea and within which sea water is measurably diluted by fresh water. The fresh-water sources for most estuaries are streams, rivers, and even groundwater for some areas.

In the continental United States, estuaries comprise more than 80 percent of the coastline along the Atlantic Ocean and Gulf of Mexico, and more than 10 percent of the Pacific coast. Geographical features such as bays, inlets, harbors, lagoons, bayous, and sounds usually are one of several different types of estuaries. Although there are many ways to classify estuaries, geomorphology and water circulation are features commonly used for characterizations.

Estuary Formation

Most estuaries can be grouped into four geomorphic categories based on the physical processes responsible for their formation: (1) rising sea level; (2) movement of sand and sandbars; (3) glacial processes; and (4) tectonic processes. Drowned river valley or coastal plain estuaries were formed by rising sea level during the last interglacial period (about 15,000 years ago) which flooded river valleys that were cut into the landscape when sea level was lower. Chesapeake Bay (Virginia and Maryland) and Galveston Bay (Texas) are classical examples of this type of estuary.

The movement of sand and formation of sandbars along the coastline can enclose bodies of water and form lagoon-type or bar-built estuaries such as Laguna Madre, Texas.

In colder climates, glaciers cut deep valleys in the landscape. When glaciers recede during warmer climate periods, coastal waters fill the valley to form fjord-type estuaries, which are common in New England and Alaska.

Finally, earthquakes and faulting (the development of faults ) may cause the rapid sinking of coastal areas below sea level to form tectonically produced estuaries such as San Francisco Bay, California.*

Water Circulation

Water circulation characteristics may also be used to classify different types of estuaries. The movement of water in estuaries is regulated by the ebb and flow of tides; differences in the density of water; and wind. Because most estuaries are influenced by lunar tides, the once-daily (diurnal) or twice-daily (semidiurnal) rise and fall of water results in a net flow out of the estuary. In the strictest sense, estuarine circulation usually refers to the residual water movement after the short-term tidal effects are removed. Thus, circulation is the time-averaged current in an estuary and is sometimes described as net current, nontidal flow, or tidal residual.

The density of water also plays a major role in the movement of water in estuaries. Density, which is the weight per unit volume of water, increases with increasing salinity and decreasing temperature. In an estuary, the lighter fresh water mixes with the heavier salt water from coastal waters and creates a gradient in water density in the estuary. As the fresh water gains salt, becomes heavier, and sinks, the resulting movement of water is known as gravitational circulation, and is caused by density and elevation differences between the fresh-water runoff and saltier coastal waters. In some estuaries, large differences in water temperatures can also drive gravitational circulation.

Circulation Modes.

Although several classification systems based on estuarine circulation modes have been proposed, most estuaries can be placed in one of four different categories: (1) classical; (2) reverse; (3) discharge; and (4) storage.

Classical estuarine circulation involves the net movement of lighter, lower-salinity water along the surface and out of the estuary while heavier, higher-salinity water flows into the estuary along the bottom. This results in a two-layered circulation system with net water flow out at the surface and in along the bottom.

Reverse circulation occurs when evaporation rates exceed fresh-water input rates, which usually happens in arid climates. In these estuaries, the salinity of the water in the estuary may be higher than in the coastal waters. Thus the net circulation in a reverse estuary is in at the surface and out along the bottom, which is opposite that of classical estuarine circulation.

Some estuaries, such as the mouth of the Mississippi River, receive such large volumes of fresh-water input that the net flow is out of the estuary at all depths. This type of circulation is referred to as discharge circulation.

Storage circulation is an infrequent mode that occurs when the net flow is into the estuary at all depths. In reality, many estuaries shift circulation modes depending on the season, fresh-water input, and major meteorological events (e.g., hurricanes and other large storms).

Influence of Winds and Rivers.

Strong winds that blow across the surface waters of large estuaries may cause nontidal circulation. Friction between the wind and surface of the water results in the movement of the surface waters. Persistent winds can push large volumes of water to one side of the estuary and raise the water level along the downwind shoreline. Gravity acts upon the mound of water so that water flows in the opposite direction (away from the shoreline) along the bottom (downwelling). Similarly, the water level along the upwind shoreline may fall as the wind blows water offshore. Deeper waters flow in to replace the water that is pushed away from the shoreline (upwelling).

Rivers bring fresh water into the estuary. The fresh-water travel time slows as it mixes with estuarine water. Thus, the water spends some time in the estuary before it gradually exits into coastal waters. The hydraulic residence time of an estuary is the time required to replace the equivalent amount of fresh water in the estuary by fresh-water inputs. In short, it is the time that a molecule of water spends in the estuary. Estimates of residence time are useful for calculating the movements and concentrations of dissolved substances, such as nutrients or pollutants , in the estuary. Of course, the residence times within a particular estuary vary depending on many factors, including fresh-water input, circulation, and bathymetry (depth characteristics).

Common Features

Each estuary is unique with respect to physical, chemical, and biological characteristics, but estuaries share many common features. For example, rivers provide a continuous input of sediment into the estuary. Turbidity is a measure of the amount of suspended particles, which includes fine sediments, in the water. Circulation within the estuary redistributes the suspended sediment and a turbidity maximum usually is located in the region of the estuary where fresh water from the rivers contacts the more saline coastal waters.

Chemical interactions between the sediment particles and dissolved ions in sea water result in an attraction between particles that causes the particles to stick together (flocculate). The heavier and larger particles settle out of the water column and are deposited on the bottom. Over time, these deposits may fill navigation channels.

Dredging is the periodic manual removal of sediment to keep channels open for navigation. In some estuaries, dredging is controversial because it may have negative effects on the biota (living plants and animals). Once the sediment is dredged, it is usually deposited nearby in specially designed diked areas. The removed sediment deposit is called dredge spoil.

Traps for Pollutants and Sediments.

Most estuaries are very efficient at retaining dissolved and particulate matter. In this respect, estuaries are often thought of as filters or traps that are located between the land and the sea. Because these systems are so efficient at retaining these substances, they are very susceptible to pollutants that are washed into the estuary. Organic and heavy-metal pollutants typically have a long residence time in estuaries and tend to accumulate over time.

Organisms in the estuary are exposed to elevated concentrations of these pollutants, and many species accumulate these compounds within their organs and tissues by the processes of bioconcentration and bioaccumulation. Human consumption of marine life is prohibited in many estuaries due to the toxins which have accumulated in seafood. Once pollutants are introduced into an estuary, they are very difficult (or expensive) to remove. Therefore the best strategy is to minimize the input of these compounds.

Ecosystem Productivity.

Estuaries are ecosystems that play an important role in coastal food webs . High rates of primary productivity supply the food and energy to support valuable fisheries for oysters, crabs, shrimp, and a variety of fishes. The diverse habitats and water characteristics in estuaries provide refuge for the larval and juvenile stages of many fresh-water and marine species, and therefore are considered important nursery areas.

The wide range of salinity and chemical conditions, coupled with high primary productivity and assorted habitats, creates environments that support many different species. Therefore estuaries are noted for their high biological diversity.

Eutrophication Concerns.

Many estuaries in the continental United States are experiencing eutrophication, which is an increase in the rate of organic matter input into an estuary, and the associated biological responses. The sources of organic matter may be from outside the estuary in the form of terrestrial plant material or dissolved organic matter. Organic matter also is produced from within the estuary by phytoplankton , macroalgae, and sea-grasses. High rates of nutrient inputs from land runoff stimulate phytoplankton production and thereby increase the rate of organic matter input in the estuary.

One of the primary symptoms of eutrophication is low dissolved oxygen concentrations in the water, which may lead to large fish and shellfish kills due to insufficient oxygen concentrations for aerobic (oxygen-dependent) respiration. Eutrophication also has been blamed for an increase in the number of harmful and sometimes toxic phytoplankton blooms in estuarine and coastal waters.

Numerous studies are underway to determine ways to assess the health and integrity of estuarine ecosystems to help resource managers develop more effective ways to protect and preserve this valuable resource.

see also Algal Blooms, Harmful; Algal Blooms in the OCEAN; Bays, Gulfs, and Straits; Biodiversity; Chesapeake Bay; Coastal Ocean; Coastal Waters Management; Ecology, Marine; Ocean-Floor Bathymetry; Ocean-Floor Sediments; Sea Level; Tides; Wetlands.

James L. Pinckney


Alongi, Daniel M. Coastal Ecosystem Processes. Boca Raton, FL: CRC Press, 1998.

Hobbie, John E., ed. Estuarine Science: A Synthetic Approach to Research and Practice. Washington, D.C.: Island Press, 2000.

Mann, K.H. Ecology of Coastal Waters with Implications for Management, 2nd ed. Malden, MA: Blackwell Science, 2000.

National Research Council. Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution. Washington, D.C.: National Academy Press, 2000.

* See "Bays, Gulfs, and Straits" for a photograph of San Francisco Bay.


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Estuaries are partially enclosed bodies of water that occur where the land meets the ocean. The world's largest estuaries are at the ocean ends of rivers that deliver freshwater from surrounding and sometimes remote upland areas. Estuaries may be configured as sounds, bays, lagoons, or networks of tidal creeks and marshes. Many estuaries are separated from the ocean by barrier islands and do not have major sources of freshwater inflow. Estuaries are more common on coasts with wide and shallow continental shelves than on coasts close to tectonic plate boundaries.

Estuaries are physically and chemically dynamic and complex ecosystems . Annual, seasonal, and daily fluctuations in freshwater input, tidal inundation, temperature, wind, and other hydrological and meteorological factors are responsible for the highly changeable character of estuaries. Due to variations in tidal height, currents, wave exposure, sediment types, salinity, and depth within estuaries, many different types of submerged and intertidal habitats exist. The diversity and interrelatedness of habitats contributes to the biological richness of estuaries.

Temperate and tropical estuaries are among the most biologically productive ecosystems on Earth. Salt marshes dominated by Spartina grasses can produce 5 to 10 tons of organic matter per acre per year, which is more than most agricultural crops. In tropical estuaries, mangroves are the dominant producers. Submerged seagrass beds, macroalgae ("seaweeds"), and phytoplankton also produce organic material that supports abundant and diverse populations of animals. Direct consumption of estuarine plants is important, but many small estuarine animals process decomposing plant material and associated microbes known as detritus. Rich populations of invertebrates living in the sediments and water provide food for shrimps, crabs, fishes, birds, and mammals.

Estuaries support large commercial and recreational fisheries. Crabs, clams, oysters, herrings, drums, striped bass, and other harvested species reproduce and grow within estuaries and rivers. In addition, major fishery species such as shrimps, flounders, mullets, and menhaden, which spawn in the ocean, rely on estuaries as nurseries for juveniles. At least 70 percent of the species harvested in the United States requires a period of estuarine residency to complete their life cycles. Adult fish, marine mammal, and bird migrations are often timed to coincide with best conditions for reproduction and feeding in estuaries.

Coastal areas, especially estuaries, have always attracted and supported human populations. About 40 percent of the world's population lives within 60 miles of the coast, and 22 of the 32 largest cities are located on estuaries. Human impacts associated with agricultural, industrial, and residential development in coastal watersheds have resulted in changes in freshwater inflow, increases in nutrients, and the destruction of wetlands. Dredging, diversion, and damming have also altered estuarine habitats. Reductions in water and habitat quality and overharvesting have reduced resources and changed biological communities.

Healthy estuaries help to regulate flooding and decompose contaminants. Increasing awareness of impacts and advances in scientific knowledge and technology have led to some success in reducing impacts and restoring water quality. Education and long-term planning are keys to achieving a balance between sustaining economies and preserving the ecological integrity of estuaries.

see also Biodiversity; Ocean Ecosystems: Hard Bottoms; Ocean Ecosystems: Soft Bottoms; Plankton; Rivers and Streams; Wetlands

Dennis M. Allen


Day, John W., Jr., Charles A. S. Hall, W. Michael Kemp, and Alejandro Yanez-Arancibia. Estuarine Ecology. New York: John Wiley & Sons, 1989.

Teal, John, and Mildred Teal. Life and Death of the Salt Marsh. New York: Ballantine Books, 1969.

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