Coastal Wetlands

Coastal wetlands are plant-dominated communities that occur along the shoreline of bays and where rivers meet the sea. Coastal wetlands, because they occur at the interface between terrestrial and marine environments, provide important and unique ecological services. They are highly productive, and supply food and habitat for fish, shellfish, and birds. In addition, they play a major role in decreasing coastal erosion and buffering marine environments from nutrient loading and pollution.

There are two types of coastal wetland communities: salt marshes and mangrove swamps. Salt marshes occur in temperate climates, whereas mangroves are found in tropical regions, between 30 degrees north and south latitude, where surface water temperatures are greater than 16 degrees centigrade. Mangrove

Salt marsh in the Pacific Northwest. Note different bands or zones of vegetation with elevation. (Sally Hacker)

swamps do not occur at higher latitudes, on account of frost sensitivity.

Salt marshes are low-species-diversity communities characterized by halophytic (salt-loving) grasses, herbs, and small shrubs.

Commonly associated animals include shellfish, insects, and birds. Salt marshes are some of the most productive communities on earth. Productivity can be as high as 3,000 grams dry plant weight per square meter per year, higher than that of most agricultural crops. Nearly 50 percent of the productivity can be exported into adjacent marine habitats and used in aquatic food webs.

Mangrove swamp communities are dominated by halophytic trees that have large, branching prop roots, creating an ideal underwater habitat for fish, shrimp, and root-encrusting animals, including sponges and oysters.

The above-ground portion of mangroves provides extensive habitat for birds, alligators, and crocodiles, and for mammals such as bears, pumas, and wildcats. Mangrove communities have similar productivity but higher species diversity than salt marshes.

Coastal wetlands are influenced by tides, which move water up and down across their surface at least once a day. Daily tidal inundation combined with a natural elevational

gradient, from low elevation near the water's edge to high elevation near the terrestrial border, creates bands or zones of varying species associations and physical conditions. Low zones are frequently flooded, producing salty, low-oxygen conditions with high sedimentation. High zones are less frequently flooded and have low salinity and sedimentation but higher oxygen levels. Wetland plants play a major functional role in mediating physical conditions. Plants efficiently shade the soil, causing decreased evaporation and salt accumulation, and extensive and specialized roots increase sedimentation and oxygen concentration.

Coastal wetlands have experienced wide-scale human disturbance for many centuries. Close to 90 percent of North American coastal wetlands have been lost, and salt marshes are used for grazing livestock, agriculture, and development. Some major cities, including Boston, San Francisco, and London, are built on filled or drained wetlands. In addition, salt marshes and mangrove swamps reside at the discharge end of rivers, where they receive high levels of nutrients, heavy metals, and other pollutants. Conservation has increased in recent times, and many remaining wetlands are protected under law.

—Sally Hacker

See also: Communities; Draining of Wetlands; Ecosystems; Erosion; Estuaries; Extinction, Direct Causes of; Interior Wetlands; Intertidal Zone; Lagoons; Nurseries; Preservation of Habitats; Reptiles; Sponges; Tides; Topsoil Formation; Topsoil, Loss of

Bibliography

Bertness, Mark D. 1999. The Ecology of Atlantic Shorelines. Sunderland, MA: Sinauer; Molles, Manuel C., Jr. 1999. Ecology: Concepts and Applications. Boston: WCB McGraw-Hill; Smith, Robert L. 1996. Ecology and Field Biology, 5th ed. New York: HarperCollins.

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