Lagoons

Lagoons are almost completely or partially enclosed bodies of water located between the coastline and offshore structures such as sand spits, barrier islands, and coral reefs. They connect with the open ocean through narrow breaks in the barriers. Coastal lagoons are abundant landforms, and like barrier islands, which they are associated with, form a little more than 10 percent of the world's coastlines. Most commonly they are found along coasts bordering lowlands such as the East and Gulf coasts of the United States; they are complex features in that they frequently adjoin one or more estuaries. Because they are protected from attack by ocean waves and currents, lagoons are low-energy environments and accumulate mud, sand, and organic material. Deposition is the dominant action ultimately causing the lagoon to fill up over a long period of time. Occasionally, in coastal lagoons, large storms send seawater over the protective barrier island, bringing large volumes of coarse sand into the lagoon. Large storms may also break through the barrier forming narrow tidal inlets, which results in the creation of a localized delta. Because the gaps in the barriers are narrow, currents flowing through them are usually strong near the inlets. Tides and winds are the most important ecological factors.

Lagoons are water bodies that generally vary greatly in salinity and temperature, even within an individual lagoon. They have a greater temperature range than the adjacent open sea and are warmer in the summer and colder in the winter. As a result, lagoons contain a biota of low diversity, although individuals may be abundant. Lagoons with restricted inlets and large evaporation rates develop highly saline waters and accumulate crystalline salt. When subsidence is slow, thick deposits of salt can be produced, similar to the ancient deposits found in many parts of the world.

Because they are shallow, waves, even small ones, can stir lagoon sediments vigorously. Where sediment is abundant, coastal lagoons fill up, creating intertidal flats around their edges and developing extensive marshes. The decaying plant material as well as skeletal remains enrich the sediment. Under these conditions, deposits of peat can develop. In tropical climates, coastal lagoons are often covered with algal mats that trap sediment. As the algae grow upward more sediment is trapped, eventually producing finely laminated structures called stromatolites. Stromatolites are found frequently as fossils, and were first illustrated in the scientific literature in 1825; it wasn't until 1914, however, that their blue-green algal origin was suggested.

Where there is little or no input of sediment, as in coral reef lagoons, skeletal material covers the lagoon floor; sediment in these environments may also include calcium carbonate mud precipitated under high saline conditions. Shallow coral reef lagoon floors are frequently covered by vast patches of coral, while deeper lagoons are more open. Rocky islands associated with some coral reef lagoons inhibit coral development and create murky water because they are the source of sediment detrimental to coral growth. Many coral reef lagoons have very deep inlets eroded when sea level was lower, during the height of the last glaciation.

—Sidney Horenstein

See also: Barrier Islands; Beaches; Coral Reefs; Estuaries; Oceans

Bibliography

Carson, Rachel. 1955. The Edge of the Sea. Boston: Houghton Mifflin; Keahey, John. 2002. Venice against The Sea: A City Besieged. New York: St. Martins Press; Ketchum, Bostwick H. 1983. Estuaries and Enclosed Seas. New York: Elsevier Science; Kjerfve, Bjorn. 1994. Coastal Lagoon Processes. New York: Elsevier Science.

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