Wetland

I

Introduction

Wetland, geographic area with characteristics of both dry land and bodies of water. Wetlands typically occur in low-lying areas that receive fresh water at the edges of lakes, ponds, streams, and rivers, or salt water from tides in coastal areas protected from waves. In wetlands, the surface of the water, called the water table, is usually at, above, or just below the land surface for enough time to restrict the growth of plants to those that are adapted to wet conditions and promote the development of soils characteristic of a wet environment.

Wetlands provide habitat for a wide variety of plants, invertebrates, fish, and larger animals, including many rare, threatened, or endangered species. The plants and animals found in wetlands include both those that are able to live on dry land or in the water and those that can live only in a wet environment.

Wetlands are found on every continent except Antarctica and in climates ranging from the tropics to the tundra. They occupy about 6 percent of the land surface of the world, or approximately 890 million hectares (approximately 2.2 billion acres). The United States contains about 111 million hectares (about 274 million acres) or approximately 12 percent of the world's wetlands.

II

Types of Wetlands

Wetlands can be classified into three general categories: marshes, swamps, and peatlands. Within each of these categories, wetlands can vary widely. Because wetlands depend on water sources, their boundaries can change. The characteristics that describe each category include vegetation, soil type, water supply, and water chemistry.

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A

Marshes

Marshes are periodically or continually flooded wetlands characterized by nonwoody emergent plants, plants that are adapted to living in shallow water or in moisture-saturated soils. Different species of emergent plants often occur in zones within a marsh; zones are determined by the elevation of the soil surface relative to the water level. Water chemistry in marshes depends on the water sources and varies from salt water (from incoming and outgoing ocean tides) to mineralized fresh water (from groundwater, streams, and surface runoff) to poorly mineralized fresh water (mostly from precipitation). Marshes often have mineral soils. Coarser soils such as sand are found in areas subject to waves or flowing water; in more protected areas, silts and clays accumulate with dead plant matter to form organic soils. Examples of marshes include the Everglades in south Florida; the prairie potholes of central North America; numerous coastal areas along the Great Lakes; and salt marshes along middle- and high-latitude ocean coasts around the world.

B

Swamps

Swamps are dominated by trees or shrubs and occur in a variety of flooding conditions. Standing water can be present in swamps during all or just a small part of the year. Water chemistry in swamps can vary greatly, depending on the water source. Swamp soils can be rich or poor in nutrients and vary in mineral or organic content. Swamps often occur along river floodplains, in shallow, quiet waters of lakes, and along subtropical to tropical coasts. Examples of swamps include bottomland hardwood forests in the lower Mississippi River Valley; the Okefenokee Swamp on the Georgia-Florida border; and mangrove forests in Florida Bay, tropical Africa, South America, and Southeast Asia.