Estuaries

Estuaries are known by many different names: bays, coves, inlets, harbors, sounds, and lagoons. Simply put, they are places where rivers meet the sea. Estuaries are, however, far more complex than merely areas where seawater is mixed with fresh. They are also ecotones, places where different ecosystems meet and boundaries overlap: terrestrial and aquatic, freshwater and marine, inland and offshore. They are dynamic regions of transformation, high productivity and species diversity. Estuaries act as buffers and filters, protecting upland habitats from storm surges and, preventing sediments and pollutants from reaching coastal waters. Estuaries contain diverse habitats: sandy beaches, rocky shorelines, mudflats, fringing salt marshes and mangroves, and provide critical habitats for many species.

Diverse definitions of estuaries have been proposed, emphasizing various sets of attributes, delineating landward and seaward ranges differently, but there is general agreement that estuaries are semienclosed bodies of water with freshwater inputs and some open connection to the sea, allowing for dilution and tidal exchange. The proximity of barrier islands, reefs, bars or peninsulas protects them from the full force of the ocean. Several geophysical processes give rise to estuaries and, as a result, estuaries exhibit different forms. Several types of estuaries have been delineated, encompassing those of glacial origin, known as fjords, those of tectonic origin, those formed from river deltas, flooded river mouths and bar-built estuaries where barrier islands or peninsulas form from sand bars, protecting river mouths from the ocean.

Environmental conditions in an estuary may be highly variable, unpredictable and extreme. Considerable variation can exist even within a given estuary. Due to tidal fluxes, river flows, topography, and weather conditions, estuarine waters experience dramatic changes, both regular and irregular, affecting salinity, sediment load, oxygen concentrations, and temperature. Salinity fluctuates, varying from brackish to almost fresh and in some arid areas, hypersaline. Waters may be well-mixed, or strongly stratified, with water layers of different salinities and densities, called a salt wedge.

Environmental conditions in an estuary may be highly variable, unpredictable, and extreme.

These physical extremes present physiological challenges for organisms, but species have adapted to the rigors of the estuarine environment. Estuarine productivities are among the highest in the world, due to high nutrient loads in water and sediments. Estuaries are critically important in the life histories of many species, including recreationally and commercially valuable species. Some species, particularly benthic invertebrates, live their entire lives within the bounds of the estuary. These include species of oyster, clam, and scallop. Some spend only a portion of their lives in the estuary, using them for reproduction, larval, and juvenile rearing. Other species, notably shorebirds, waders and wildfowl, utilize these areas for feeding. Salmon use estuaries as nurseries for juveniles and migrate through them as adults on their way to freshwater tributaries in which they spawn.

Humans settled around estuaries because they provided subsistence opportunities, good harbors, direct linkages between rivers and the sea, facilitating transportation and commerce. Settlements became cities and now several of the world's largest cities are found along estuaries. Estuaries are also considered attractive places to live for recreational and aesthetic reasons. In the United States, over 50 percent of the human population lives along the coast, which constitutes less than 10 percent of the lower 48's land. Globally, coastal density is approximately 40 percent. In either case, human populations are increasing in estuarine areas and, as a result, the ecological footprint of humans on the estuarine environment is significant and growing.

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