"Wetlands" is the collective term for habitats that are too wet to be upland and not wet enough to be fully aquatic. They occur in areas of transition between dry upland and open water or in low areas where drainage water collects or the water table is at the ground's surface.
Wetlands are characterized by:
- • the presence of surface water, at least part of the year
- • unique soils that differ from adjacent uplands (due to the influence of waterlogging)
- • plants adapted to wet soil conditions (hydrophytic vegetation)
There are many types of wetlands, differing in water chemistry, hydrology, soils, topography, climate, and vegetation. The broadest categories are coastal and inland wetlands. Coastal wetlands experience periodic flooding by saltwater or brackish water, and include estuaries (tidal marshes), mud flats, and mangrove swamps. They are nurseries for crustaceans, such as shrimp, and many fish species, and are also important habitat for birds and other wildlife. The presence of coastal wetlands can reduce inland erosion and other damage from hurricanes and winter storms.
Inland wetlands are freshwater wetlands and occur throughout the interior of a continent. These wetlands include: cattail marshes and wet meadows dominated by grasses, sedges, and herbs; swamps dominated by woody vegetation such as shrubs and trees; and peatlands (fens and bogs) that contain a buildup of peat, which forms as plants die and fall into the water and are not completely decomposed. The Florida Everglades are a vast inland wetland system.
A key factor determining what kind of soil and plant community develops in a wetland is the depth and duration of waterlogging and its effect on oxygen (O 2 ) in the soil. Soils that are waterlogged for any length of time become depleted of O 2 because soil microbes and plant roots use it during cellular respiration. The oxygen is not quickly replaced by O 2 from the atmosphere because O 2 diffuses very slowly through water. The anoxic (low oxygen) conditions influence soil development. Decomposition of plant litter and other organic matter is slowed in absence of O 2 and the wetland soils become high in organic matter. If decomposition is much slower than the production of plant matter, peat will form. Peatlands typically occur in northern climates where low average temperatures further slow decomposition.
Since O 2 availability is a limiting factor for plants growing in wetlands, most wetland plants have structural adaptations that increase gas exchange. Some have spongy tissues, called aerenchyma, in their stems and roots that conduct O 2 within the plant from the aboveground shoot down to the roots. Others produce adventitious roots above the anoxic zone or have prop roots with pores that let in oxygen from the atmosphere.
In the past, many people viewed wetlands as mosquito-infested wastelands needing to be drained. More than one-half of the original wetlands of the United States have been drained or otherwise altered. Now there is a public consciousness that wetlands are important and valuable natural resources. Wetlands improve water quality by removing and retaining nutrients from surface waters and trapping sediments. They reduce flood and storm damage, and act to control erosion of shorelines. They provide important habitat for fish, crustaceans, and other wildlife and produce natural products such as blueberries, cranberries, rice, mink, and beaver. They support hunting and fishing activities and provide other recreational and educational opportunities.
Mitsch, William J., and James G. Gosselink. Wetlands. New York: Van Nostrand Reinhold, 1986.
Williams, Michael, ed. "Understanding Wetlands." In Wetlands: A Threatened Landscape. Cambridge, MA: Basil Blackwell, Inc., 1990.
U.S. Environmental Protection Agency. America's Wetlands: Our Vital Link Between Land and Water. Washington, DC: U.S. Environmental Protection Agency, 1988.