Wetlands

“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 (O₂) in the soil. Soils that are waterlogged for any length of time be­come depleted of O₂ because soil microbes and plant roots use it during cel­lular respiration. The oxygen is not quickly replaced by O₂ from the atmosphere because O₂ 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 O₂ 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 O₂ 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 O₂ 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.

References

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.