Estuaries
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.

An aerial view of the sandy Ravenglass Estuary in Cumbria, England.
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.
References
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.