Estuaries are unique and complex environments located between oceans and river mouths. As freshwater flows into the sea from land, it dilutes the salty water in a small area around the shore. This relatively small space is the site of sediment build-up resulting from fluvial (stream or river) erosion along the riverbanks. The organic sediments and brackish (slightly salty, but undrinkable) water make a unique environment that supports a diverse community of plants and animals. The sediments themselves also form characteristic types of deposits and bed forms (the appearance of the horizontal layers of sand ) that are easily seen in cross-section.
It is well known that rivers often carry tremendous amounts of sediment which, when emptied into the oceans, construct distinctive patterns in the underlying sediments. In an estuary, the deposition of sediments is greatly influenced by tidal currents and ocean waves. Even climate is a factor in how the sands and muds settle into distinct patterns. During seasonal storms, erosion is increased and the waters become heavily laden with a wide variety of sediments. Unlike deltas, in which the finer sediments are often carried far out to sea, the estuary is bordered on the deeper ocean side by heavy sands while clays and muds are dropped at river mouths. As tidal forces work the sediments by tumbling and rolling them, the lighter and finer particles are left near the river mouth. The build-up of coarse-grained (larger particle size) sands at the estuary edge often makes a barrier at the outer edge of the estuary that contains the bulk of the fine sediment and diluted water. The sediment structures in these ridges are defined as longitudinal or oblique bars. The structures in the upper reaches of the estuary are described as asymmetric and longitudinal bars become point bars similar to those observed in rivers. A dendritic (tree-shaped) pattern of channels occurs in these finer, flat lying sediments.
The greatest force at work shaping and changing the estuary comes from moving water. Daily tidal fluctuation brings saltier water into the estuary and pushes medium-grained sands into the main body of the tidal flats. On the ocean side of the estuary, the sand bars are penetrated by channels through which the flow of water into and out of the estuary is restrained. These containment structures close the general water and sediment circulation paths around the main body of the estuary. Water and sediment flow is greatly restricted and additional build-up of medium and fine-grained sediments occurs. Water is forced to leave the estuary by these well-defined channels.
The tidal or exit channels of the estuary can be dangerous places for some life forms. As low tide occurs, the ebb of general sea level reverses the oceanic flow into the estuary. Water laden sediments release their burden and contribute to the general volume of water leaving the flats. The force of water exiting through the channels becomes great. The velocity of the water can reach dangerously high levels. In well-established estuaries, large animals can be swept to sea. However, for many marine animals this is a benefit.
The estuaries are safe places for many creatures such as crabs and other crustaceans to lay their eggs. The hatched larvae live in the estuary until they are ready to join the zoo-plankton community of the larger sea. The swift release of water from the tidal flats helps the floating larvae to jet far out into the ocean where they will spend the next phase of their lives. If not for the tidal currents, the larvae would have to live dangerously exposed to shore birds and other marine carnivores. The estuarine water channels help them grow and gain a slight advantage for survival that would not be found in typical shore lines.
When high tide returns, the encroaching water brings oxygen to the anaerobic (without oxygen) sediments. The water also brings marine organic food for mud dwelling inhabitants. The sediments are refreshed with salt water and another cycle of replenishment occurs. During stormy seasons , this effect can be exaggerated and actually quite harmful to inhabitants as sediments are churned and redeposited. However, this continuous recycling of sediments and resources keeps the estuary healthy and flourishing.
Because the water plays such a physical and active role near the outer borders of the estuary, the inner regions of the estuary are more protected. By containing the general flow of water to the channels, the finer sediments, such as silt and clay , are left relatively undisturbed near the river mouths. They build up into areas of fine muds and contribute to the distinctive tidal flats. Organic debris is carried along by the rivers as they carve through valleys and plains of the terrestrial environment. This lightweight material comes to rest in the tidal flat as the velocity of the water is drastically reduced in the tidal flats. The decay and spreading of organic material throughout the flats makes them rich in nutrients. Subsequently, clams and other burrowing animals thrive in the rich sediments of the upper estuary. In turn, birds are lured to this feast where they are able to rear young on the nearby shore. These life forms are relatively protected because the muds make it difficult for heavier predators to walk out into the estuary with any stealth. There are even places where the muds act as a sort of quicksand and can be very dangerous.
Estuaries are fragile environments that are becoming increasingly threatened. They are being geologically altered as sediments are trapped upstream by dams. Diversion of water and sediments by agriculture is also reducing flow to the estuary. As a direct result, the life forms that rely on the dynamics of the estuary are decreasing in numbers. Many people are realizing the importance of estuarine environments and the important role they play in both marine and terrestrial ecosystems.
See also Oceanography; Sedimentation; Tides
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.
[Eugene C. Beckham ]
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.
Horton, T. "Chesapeake Bay: Hanging in the Balance." National Geographic 183 (1993): 2–35.
es·tu·ar·y / ˈeschoōˌerē/ • n. (pl. -ar·ies) the tidal mouth of a large river, where the tide meets the stream.DERIVATIVES: es·tu·ar·i·al / ˌeschoōˈe(ə)rēəl/ adj.es·tu·a·rine / ˈeschoōəˌrīn; -əˌrēn/ adj.