Estuary

Estuaries represent one of the most biologically productive aquatic ecosystems on Earth. An estuary is a coastal body of water where chemical and physical conditions modulate in an intermediate range between the freshwater rivers that feed into them and the salt water of the ocean beyond them. It is the point of mixture for these two very different aquatic ecosystems. The freshwater of the rivers mix with the salt water pushed by the incoming tides to provide a brackish water habitat ideally suited to a tremendous diversity of coastal marine life.

Estuaries are nursery grounds for the developing young of commercially important fish and shellfish. The young of any species are less tolerant of physical extremes in their environment than adults. Many species of marine life cannot tolerate the concentrations of salt in ocean water as they develop from egg to subadult, and by providing a mixture of fresh and salt water, estuaries give these larval life forms a more moderate environment in which to grow. Because of this, the adults often move directly into estuaries to spawn.

Estuaries are extremely rich in nutrients, and this is another reason for the great diversity of organisms in these ecosystems. The flow of freshwater and the periodic flooding of surrounding marshlands provides an influx of nutrients, as does the daily surges of tidal fluctuations. Constant physical movement in this environment keeps valuable nutrient resources available to all levels within the food chain/web .

Coastal estuaries also provide a major filtration system for waterborne pollutants. This natural water treatment facility helps maintain and protect water quality , and studies have shown that one acre of tidal estuary can be the equivalent of a $75,000 waste treatment plant. When its value for recreation and seafood production are included in this estimate, a single acre of estuary has been valued at $83,000. An acre of farmland in America's Corn Belt, for comparison, has a top value of $1,200 and an annual income through crop production of $600.

Throughout the ages man has settled near bodies of water and utilized the bounty they provide, and the economic value of estuaries, as well as the fact that they are a coastal habitat, has made them vulnerable to exploitation. Chesapeake Bay on the Atlantic coast is the largest estuary in the United States, draining six states and the District of Columbia. It is the largest producer of oysters in the country; it is the single largest producer of blue crabs in the world, and 90 percent of the striped bass found on the East Coast hatch there. It one of the most productive estuaries in the world, yet its productivity has declined in recent decades due to a huge increase in the number of people in the region. Between the 1940s and the 1990s, the population jumped from about three and a half million to over 15 million, bringing with it an increase in pollution and overfishing of the bay. Sewage treatment plants contribute large amounts of phosphates , while agricultural, urban, and suburban discharges deposit nitrates , which in turn contribute to algal blooms and oxygen depletion. Pesticides and industrial toxics also contribute to the bay's problems. Since the early 1980s concerted efforts to clean up the Chesapeake Bay and restore its seafood productivity have been undertaken by state and federal agencies. Progress has been made but there is still much to be done, both to restore this vital ecosystem and to insure prolonged cooperation between government agencies, industry, and the general populace.

See also Agricultural pollution; Aquatic chemistry; Commercial fishing; Dissolved oxygen; Nitrates and nitrites; Nitrogen cycle; Restoration ecology

[Eugene C. Beckham ]

RESOURCES

BOOKS

Baretta, J. W., and P. Ruardij, eds. Tidal Flat Estuaries. New York: Springer-Verlag, 1988.

McLusky, D. The Estuarine Ecosystem. New York: Chapman & Hall, 1989.

PERIODICALS

Horton, T. "Chesapeake Bay: Hanging in the Balance." National Geographic 183 (1993): 2–35.