Estuaries are found throughout the world in coastal environments.
An estuary is defined as a semi-enclosed coastal body of water with freshwater flowing into it and a connection to the open sea. An estuary typically forms at the tidal mouth of a river, and receives sediment or silt carried in from terrestrial runoff. Types of estuaries include bays and sounds. Large estuaries, like Chesapeake Bay and Puget Sound, may have many rivers flowing into them and have complex shapes. Because of the freshwater input, the salinity of an estuary is lower than that of sea water, and is called brackish. Estuaries are environments whose salinity and water level vary, depending on the freshwater input and the nearby ocean water.
Estuaries have a characteristic pattern of water circulation. Estuarine circulation occurs as lighter, less dense freshwater flows out near the surface, while denser saline water flows inward from the sea near the bottom. The time it takes for all of the water in an estuary to completely cycle is called flushing time. Depending on the relative rates of flow, estuaries can be classified into various types: In a salt wedge estuary, the river output greatly exceeds input of marine water, there is little mixing, and thus there is a sharp contrast between the freshwater near the surface and saline water at the bottom. In a highly stratified estuary, the rates of river output and marine input are more similar, with river flow still dominant. Turbulence causes more mixing of salt water upward into the freshwater. A slightly stratified estuary occurs where river output is less than the marine input, and turbulence causes mixing of the whole water column, such that vertical salinity is more consistent. Finally, a vertically mixed estuary is characterized by river output that is much less than marine input, so that the freshwater contribution is negligible and salinity does not vary vertically at all.
The unique combination of low-salinity water and variable physical conditions characteristics of estuaries hasa produced a number of unique habitat types. Many types of estuarine habitats exist along the United States coast. In New England, salt marshes are found along the shores of tidal rivers. Further south, the coast becomes sandier, and barrier island beaches enclose huge bays or sounds. Here estuarine habitats are very extensive and salt marshes can reach far inland. Along the southern coast of Florida are extensive mangrove forests, in which the characteristic intertidal plants are trees rather than grasses. Along the Texas coast, barrier islands protect estuaries that have formed narrow lagoons with small openings to the Gulf of Mexico. These estuaries get very little freshwater input and may become hypersaline or "super salty." Along the Pacific Coast, rivers flow quickly out of the mountains to a relatively steeply sloping continental shelf; here the estuaries are small and the coast tends to be dominated by rocky shorelines. San Francisco Bay and Puget Sound are the major estuaries on the West Coast.
Biology of estuaries
Estuaries are often associated with high rates of biological productivity. They are among the most productive ecosystems in the world and are home to unique plant and animal communities. Many animal species rely on estuaries for food and as places to nest and breed. An estuary has very little wave action, so it provides a calm refuge from the open sea. Some of the animals, such as flounder, eels, and striped bass are temporary residents, while fishes such as sticklebacks, silversides, and mummichogs, as well as mud snails, fiddler crabs, ribbed mussels, and oysters may spend their entire lives there. In almost all estuaries the salinity of the water changes over the tidal cycle. To survive in these conditions, plants and animals must be able to respond quickly to these changes and tolerate a wide range of salinities. Relatively few organsms have evolved adaptations to such stressful conditions, so estuaries tend to have lower biodiversity than other coastal habitats in the same region. Some organisms have evolved special physical structures to cope with changing salinity. The smooth cordgrass in salt marshes, for example, has special filters on its roots to remove salts from the water it absorbs and salt glands on its leaves that expel excess salt.
Estuarine ecosystem services
Humans also rely on estuaries for food, recreation, commerce, and jobs, and most of the large cities in the world are located on them. Habitats associated with estuaries, such as salt marshes and mangrove forests, act as filters. As water flows through a salt marsh, marsh grasses and peat filter pollutants out of the water, as well as excess sediments and nutrients. One reason that estuaries are so productive is that the water brings in nutrients from the watershed upstream, which support plant and animal growth in the estuary. In addition to nutrients, that same water often brings with it the excess fertilizers and pesticides that were applied to the lands in the watershed, in some cases hundreds of miles away. For this reason, estuaries, while being some of the most fertile ecosystems, are also some of the most polluted. They are under great threat from human activities which have led to a decline in the health of estuaries, making them one of the most threatened ecosystems. People have historically viewed estuaries as places to discard their wastes. Poor water quality affects most estuarine organisms, including commercially important fish and shellfish. The pollutants that have the greatest impact on the health of estuaries include excess nutrients (or eutrophication, which can cause low oxygen levels, or "dead zones", in the deeper water), toxic substances like metals, oil, and pesticides, and pathogens, such as bacteria or viruses. Because of concern over the state of estuaries, many governmental and non-governmental organizations are involved in restoration projects for estuaries around the world in attempts to improve their condition.