Wetlands

The very word "wetland" indicates that water is the first requirement for wetlands to exist. A wetland can be identified by three basic factors: soil, vegetation, and water regime (hydrology). A wetland generally is described as an area where water is the dominant factor in determining the nature of soil development and the types of plant and animal communities living in the soil and on its surface. Specifically, a wetland is an area that is periodically or permanently saturated or covered by surface water or groundwater , that displays hydric soils (unique soils associated with extended saturation), and that typically supports or is capable of supporting hydrophytic (water-loving) vegetation.

Occurrence and Characteristics

Wetlands form in areas where one or more of the following occur:

• The land is flat and water runs off the surface very slowly;
• Water becomes ponded in land-surface depressions;
• Infiltration of precipitation into the soil is slow;
• Groundwater discharges to the land surface; or
• The water table (the upper surface of the saturated zone of an unconfined groundwater system) is at the land surface.

In the broadest sense, all wetlands fall between two extremes with respect to where they get their water. On the one hand, wetlands may receive most of their water supply from precipitation. On the other hand, they may receive most of their water from groundwater discharge. One type of wetland that receives most of its water from precipitation is a vernal pool. These temporary pools dry up when precipitation is lacking. A different type of wetland, and one that receives most of its water from groundwater discharge, is called a fen. Fens usually occur in low areas, such as at the base of hillsides or in land-surface depressions. Wetlands that are present along streams, called riverine or riparian wetlands, fall between these two extremes because the source of water to a stream depends on precipitation in upstream areas and groundwater inflow to the stream–wetland system.

Plant and Animal Communities.

The type of soil development and the plant and animal communities that are supported depend on the depth of water and the length of time it is available, known as its hydroperiod. Where water is present most of the year, wetlands tend to be dominated by softstemmed herbaceous vegetation and grasses. Where water will be present only for a brief time, and where soil surfaces will dry long enough to allow germination of woody species, brush or trees will be the dominant vegetation. In areas of restricted drainage, the accumulation of partially decomposed sphagnum moss and other acid-loving plants builds up deep peat layers.

Submerged aquatic plants, which are those that grow completely underwater, usually grow in the part of the wetland where the water is deepest. Emergent aquatic plants, which are those that have the lower part of the plant underwater but the upper part above the water, grow where the water is shallower. Some wetland plants near the edge of water do not actually grow in water, but they are considered to be wetland plants because their roots are in the saturated soils directly adjacent to the water.

Wetland plant communities generally have distinctive patterns called zones. For example, in the wetlands that are in land-surface depressions, such as in glacial terrain, the zones generally have a concentric pattern. The open-water zone where the submerged plants grow is in the middle. This is sometimes also called the deep-marsh zone. This zone is surrounded by the shallow-marsh zone where the emergent plants grow. Beyond the water's edge is the wet-meadow zone. Wetlands along streams have similar plant zones, also depending on water depth. Wetlands in flat, coastal areas generally do not have such distinct plant zones because the vast flat areas commonly are covered by only one plant type.

Just as water provides the basic conditions for the distribution of plant zones in wetlands, the plant zones themselves provide habitat for animals. Certain microorganisms and invertebrate animals can be found in some plant zones but not in others. Other animals, usually the more mobile ones, such as waterfowl, use all the plant zones, but for different reasons. For example, some plants provide food for waterfowl, but other plant zones provide shelter and nesting sites.

Wetlands contain some plants and aquatic animals that live only in wetlands and that are different from those living in upland areas. However, some upland animals occasionally use wetlands for food and shelter. For these reasons, wetlands are important ecosystems in their own right, and also are important parts of the larger natural environment.

Wetland Classification

"Marsh," "swamp," and "bog" are some names commonly used to identify wetlands. Other names for types of wetlands include bottomland, fen, mangrove, mire, moor, muskeg, peatland, playa, pothole, reedswamp, slough, swamp, vernal pool, wet meadow, and wet prairie.

To wetland scientists, these terms can be used to identify specific wetland types. But because of the diversity of wetlands, descriptive schemes based on landscape position have been developed to identify broad wetland systems. Wetlands at the ocean's edge are marine systems, whereas wetlands in estuaries (where rivers meet the ocean) are estuarine systems. Wetlands along the edges of rivers and streams are riverine or riparian systems, and wetlands along the edges of lakes are lacustrine systems. Upland wetlands not connected to rivers or lakes are palustrine wetlands.

Water supply, and consequently, vegetation and soils vary for each of the systems. Marine systems are dominated by tides. Estuarine systems are influenced by the interaction of tides and river flows. Riverine systems reflect the controlling role of flooding from high flows, while the water supply for lacustrine systems depends on the lake level and the water supply to the lake. Palustrine systems usually are dominated by rain and snowfall. As noted previously, groundwater may play an influential role in any of these systems, depending on the local geological situation.

The different types of places where wetlands can be found can be divided into six groupings of terrain: mountains, plateaus and high plains, playas, river valleys, coastal, and glacial and dune.

Mountains.

Mountains have small uplands and lowlands separated by large, steep valley sides. Wetlands in mountains generally form in the narrow flat uplands and in the narrow lowlands between the base of mountain slopes and the streams. Wetlands in the higher areas of mountains are among the best examples of those that are dependent on precipitation for their source of water. Wetlands in mountain valleys receive their water primarily from groundwater discharge at the base of mountain slopes and from nearby streams.

Plateaus and High Plains.

Plateaus and high plains have broad, extensive uplands and relatively small, narrow lowlands in the river valleys. Wetlands in plateau and high plains landscapes generally are restricted to the valley bottoms along streams. The wetlands in the valleys receive water from the stream and from groundwater discharge to the stream valley. If riparian wetlands are dependent primarily on the stream, they are dependent on precipitation in their upstream watershed. If wetlands in river valleys are dependent primarily on groundwater discharge, the size of the groundwater flow system is the primary consideration in determining their source of water.

Playas.

Playas are extensive, flat lowlands that do not have streams draining them. The source of water to wetlands in playas is largely from stream flow that originates from precipitation in the surrounding uplands. Groundwater and precipitation directly on the wetland generally are much smaller contributors of water.

River Valleys.

Large river valleys have relatively wide lowlands. The source of most water to the riparian wetlands along the river is the river itself, but groundwater also discharges to the wetlands along the river as well as to other wetlands across the valley bottom not connected to the river. Floods also cause some types of wetlands to form in river valleys. Even without floods, other types of wetlands form when high river levels cause water to move sideways through the stream banks and fill depressions in the floodplain. For these types of wetlands, changes in the level of the river cause changes in water level in the wetlands.

Coastal.

Many coastal plains have broad, flat areas. Perhaps the main reason why wetlands occur in flat coastal areas is the low slope of the land surface. Broad, flat lowlands commonly have slow runoff and also tend to have shallow water tables. Groundwater discharges to wetlands across the broad flatlands, but probably most groundwater discharges near the edge where the plains meet regional uplands. Wetlands in flat coastal areas are among the most extensive wetland systems in the world.

Glacial and Dune.

Glacial landscapes are parts of the Earth that were covered by glaciers during the last Ice Age. Glacial landscapes have isolated depressions that can have a wide variety of shapes and sizes. Wetlands often form in the depressions in such landscapes if the water table is close to land surface. Most of the depressions do not have streams entering or leaving them; therefore, streamflow generally is not a major source of water to the wetlands. Wetlands in these areas receive their water supply from precipitation and/or groundwater discharge. In glacial terrain, some wetlands have no groundwater input; some receive groundwater inflow through part of their bed and lose water to groundwater through other parts; and some receive groundwater inflow throughout their bed. Wetlands of all three types can be present on regional uplands as well as on regional lowlands.

Conserving and Mitigating Wetlands

Wetland ecosystems play varied but important roles in the landscape. Depending on their type and location, wetlands can moderate and influence the timing of flows (including flood flows) in streams and rivers. Wetlands play important roles in helping to maintain streamflow and groundwater supplies because they hold water that otherwise would run off the land surface and be "lost" to a downstream watershed. The stored water can be slowly released to streams and to underlying groundwater systems. Wetlands can improve water quality by trapping and removing sediments and nutrients; in fact, wetlands are so effective that some natural and artificial wetlands are used to treat wastewater.

Wetlands are critical habitats for a variety of plant and animal species. About one-third of the species federally listed as endangered or threatened in the United States depend on wetlands. Wetlands provide resting and nesting habitat for more than half of the nation's migratory bird species.

Society is beginning to appreciate the ecological values of wetlands in supporting a variety of wildlife species. In addition to the direct economic benefits from flood control and water quality improvement, wetlands offer significant recreational and educational benefits.

Despite these recent realizations, wetlands historically have been viewed as not valuable, and have been drained or filled at an alarming rate, often to allow agricultural development. Over half of the wetlands in the United States's lower 48 states were lost to various land uses between the late 1700s and the mid-1980s. Although the trend has slowed, wetland loss still continues.

Mitigating Wetland Loss.

Despite ongoing losses of wetlands in the United States (and worldwide), many strides have been made in conserving and restoring wetlands both through private efforts and through legislation and government programs. The Swampbuster provision of the 1985 U.S. Farm Bill, the Conservation Reserve Program, the Wetlands Reserve Program, and the North American Wetlands Conservation Act have helped protect and restore hundreds of thousands of wetland acres across the country. Another help in slowing the loss has been the federal Clean Water Act, which requires that permits be obtained from the U.S. Army Corps of Engineers for developments that will significantly affect wetlands. Wetland mitigation guidelines often include replacement specifications that require replacing an acre of wetland lost to development by one or more acres of wetland developed or enhanced in another area.

If a mitigation offset cannot be accomplished as a part of the development, a developer may "buy" wetland acres at a "wetland bank" in or near the same watershed, to offset the loss at the development site. The wetland bank is an area of restored, constructed, or enhanced wetland maintained specifically for such banking purposes. Under exceptional circumstances, acres of an existing wetland that are specifically preserved from development can be used as "bank credit."

Depending on the situation, the replacement may be a direct replacement, or it may be a "paper" replacement. Direct replacement may involve restoring a wetland lost to other uses, or it may involve converting an area lacking significant wetland characteristics to an area capable of playing the role of a wetland in the landscape. A paper replacement might involve an agreement to protect appropriate amounts of wetland acreage in another area, or it might involve the use of appropriate acreage from a wetland bank.

The best solution to conserving wetlands is to reduce or prevent wetland loss in the first place. Educating the public about the value of wetlands is one step in protecting these natural ecosystems. With 75 percent of the nation's wetlands in private ownership, the future of wetland protection will increasingly rely on volunteer involvement. The key to furthering wetland protection is to motivate communities to value the unique environmental, social, and economic values wetlands provide, and to create practical solutions to protect, enhance, or restore those values.

see also Everglades; Fish and Wildlife Issues; Land-Use Planning; Watershed, Restoration of a; Wetlands.

Thomas C. Winter (terrains)

N. Earl Spangenberg (mitigation)

Bibliography

Dahl, T. E., and C. E. Johnson. Wetlands—Status and Trends in the Conterminous United States Mid-1970s to Mid-1980s. Washington, D.C.: U.S. Department of Interior, Fish and Wildlife Service, 1991.

Kauffman, S. C. Water Matters, Vol. 1. Arlington, VA: National Science Teachers Association, 1994.

Mitsch, William J., and James G. Gosselink. Wetlands, 3rd ed. New York: John Wiley & Sons, 2000.

National Academy Press. Compensating for Wetland Losses Under the Clean Water Act. Washington, D.C.: National Academy Press, 2001.

National Geographic Society. Our Disappearing Wetlands. National Geographic Magazine vol. 182, no. 4 (1992):2-45.

Internet Resources

"America's Wetlands: Our Vital Link Between Land and Water." U.S. Environmental Protection Agency. <http://www.epa.gov/OWOW/wetlands/vital/toc.html>.

National Wetlands Inventory. U.S. Fish and Wildlife Service. <http://wetlands.fws.gov>.

National Wetlands Research Center. U.S. Geological Survey. <http://www.nwrc.usgs.gov>.

ACID BOGS AND ANCIENT CONDITIONS

Bogs are wetlands characterized by the presence of saturated organic soil (peat) and acidic water. The acidity and anoxic (lowoxygen or no-oxygen) conditions of bogs help preserve organic materials (e.g., plants and animals) for hundreds, even thousands of years. Many human artifacts also have been discovered in these wetlands. Human bodies estimated to be 2,000 years old have been found so well preserved that the color of the hair and eyes could be determined, as well as the last meal eaten.

Scientists use anoxic bogs as natural record-keepers that indicate changes in plant communities over time. The sediments can be dated, and preserved pollen and other plant components can be used to examine changes in plant communities and in the overall environment. Data from bogs also are useful to climate modelers interested in reconstructing past climate-change scenarios over thousands of years.

WHAT (AND WHO) DEFINES A WETLAND?

What is legally considered a wetland has particular importance with respect to the requirements for wetland preservation. On the one hand, agricultural interests, developers, and others want more freedom to develop and drain both seasonally wet regions and permanent wetlands; hence, they desire a restrictive definition of wetlands. On the other hand, environmentalists and natural resources managers want a more inclusive definition that affords protection for more lands. As a result of decades of debate, many definitions of wetlands have been developed by scientists and policymakers. The definition used often depends on the requirements of the user.

wetlands Wetlands are areas that are permanently or periodically inundated by water that support ecological habitats adapted to wet conditions. As defined by an international convention on wetland conservation in 1971, wetlands are ‘areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres’. This broad definition covers an extensive range of habitats from coral reefs to upland bogs. Differences in the definitions of wetland environments have often reflected the subjectivity of interested parties and their reason for classification, which has (to the detriment of many wetlands) led to many problems in legislative protection and conservation.

Wetland areas are particularly responsive to processes governing environmental change in adjacent terrestrial and aquatic environments. They are, because of their setting, particularly sensitive to the interaction between terrestrial and aquatic processes (e.g. between land and sea, river channel and floodplain) and they consequently contain many unique habitats.

Interrelated physical, chemical, and biological processes that occur in wetland areas can be viewed as performing a variety of functions. These functions (e.g. the stabilization of sediment by vegetation, the primary production of organic matter) control both internal wetland processes (e.g. nutrient pathways, patterns of vegetation growth) and the relationships between wetland areas and other environmental systems. Wetland areas have had a chequered past so far as human activity is concerned; from the recognition of wetland functions by early human populations as a valuable resource (e.g. for the production of foodstuffs and for defence) in their natural state, to the extensive destruction and replacement of wetland areas for agriculture and industry unrelated to wetland processes.

The physical development of wetland areas is controlled principally by hydrological factors. Wetlands can therefore be broadly divided into two groups: coastal wetlands, including estuaries, lagoons, deltas, and sheltered settings on open coasts (see mud flats, salt marshes, and mangroves), and interior wetlands at the margins of rivers and lakes and areas of restricted drainage or elevated groundwater levels (see peatlands and bogs). Although a seemingly obvious distinction, the broad division between wetland areas being influenced by either salt water or fresh water is often complicated by regional and local differences in climate, hydrology, and geomorphology. For example, wetlands situated inland under a semi-arid environment or fed by a drainage basin containing evaporite sequences can display soil salinity and vegetation characteristics typical of tidally influenced coastal settings.

Viewed on a broad scale, the development of wetlands and their ability to respond to changing environmental conditions depend on the inertia of adjacent aquatic and terrestrial systems. However, the ecology of wetland areas, particularly the growth of wetland vegetation adapted to variable hydrological conditions, means that wetlands are particularly affected by shorter-term variations in environmental change and present-day conditions. In coastal and interior wetlands, successive colonization of available areas by adapted wetland species leads to a distinct zonation of habitats and plant types as, for example, from mud flat to salt marsh/mangrove or open inland water to mire and bog environments. Once colonization has sufficiently altered the hydraulic setting, colonizers are usually replaced by successive communities of plants that are less tolerant to previous growing conditions. This process is an important control in determining pat-terns of wetland establishment and subsequent internal physiographical evolution and biogeochemical processes.

Wetland areas are prime sites for the temporary storage of water in drainage catchments and coastal areas. Surface and subsurface drainage patterns are significantly affected by the growth of wetland vegetation and associated geomorphological changes, such as the development of salt marsh and mangrove creek systems and of valley-bogs. In both interior and coastal wetlands, the absorption of kinetic energy generated by high rates of channel flow or by wave activity is the main physical reason for the use of wetlands in flood mitigation and coastal protection.

The dissipation of energy at the terrestrial–aquatic interface results in wetlands also being preferential settings for the accumulation of organic material and deposition of sediments. The accumulation in situ of organic matter in wetland soils often leads to peat formation, driven by a high rate of primary productivity, a lack of physical disturbance, and a reduced rate of anaerobic decomposition. Peats developed in marshes, bogs, and mires reflect the productivity (nutrient status) of the wetland catchment and the dynamic nature of the water-table. Similarly, the deposition of sediment derived from outside the wetland reflects the broader geomorphological and hydrological processes that control the provenance of sediment and its transport to the wetland area. Because deposited wetland sediments are capable of capturing materials from a broad range of sources and timescales, and because they reflect both internal wetland dynamics and external influences, they provide an extremely valuable resource for palaeoenvironmental research (see peatlands and bogs).

Wetland areas play a critical biogeochemical role in linking terrestrial and aquatic systems. They act both as temporary sinks between coastal, aquatic, and terrestrial environments and as valuable sources of nutrients and organic material for them. Nutrient cycling within wetland areas is related to biogeochemical interchanges between the overlying water column, wetland vegetation, and accumulated sediment. The uptake, temporary storage, and transformation of nutrients (particularly nitrogen and phosphorous) in wetland soils is dominated by the interaction of biotic processes (e.g. plant growth, organic decomposition, and the associated activity of micro-organisms) and abiotic processes (e.g. pH, oxidation– reduction potential).

Biogeochemical conditions encountered in wetlands are also capable of removing contaminants, such as heavy metals, radionuclides, and organic pollutants, from aquatic systems. Pollutants can be temporarily taken out of the system by plant uptake, but their longer-term removal occurs by adsorption on to organic or minerogenic material and subsequent burial in wetland sediments. The natural effectiveness of wetlands in this process has led to the use of artificial and managed wetlands in wastewater treatment works. Within catchment areas they are used to intercept nutrient–pollutant loads from agricultural–industrial and domestic sources.

Archaeological evidence of early occupation sites in wetland areas has shown that communities were able to take full advantage of wetland resources while adapting to hydrological changes and wetland evolution; the Meso- and Neolithic occupation of coastal lowlands in north-west Europe provides an example. Subsequent human modification of wetland landscapes has been caused both by direct human influences (e.g. peat extraction, reclamation) and by indirect influences (e.g. alteration to drainage catchments). It is only in the past few decades that the perception of wetlands has changed. Formerly regarded as areas of low economic value useful only for reclamation or set-aside conservation, they are now seen as providing a wide range of sustainable socio-economic benefits (e.g. for flood control and coastal protection) as well as being of great ecological and cultural importance.

Simon D. Turner

Bibliography

Williams, M. (ed.) (1990) Wetlands: a threatened landscape. Blackwell, Oxford.
Giblet, R. (1996) Postmodern wetlands; culture, history, ecology. Edinburgh University Press.

Wetlands

Wetlands are low-lying ecosystems that are saturated with water at or close to the surface. (An ecosystem consists of all the animals, plants, and microorganisms that make up a particular community living in a certain environment.) The most common types of wetlands are swamps, marshes, and bogs. Wetlands provide habitats for an incredibly wide variety of plants and animals. They also are important because they absorb heavy rainfalls and prevent flooding. In addition, wetlands protect the ground water humans depend on for drinking by capturing and neutralizing surface pollutants.

However, wetlands are rapidly disappearing because they are being drained and filled for farming and urban growth. Wetlands also are being destroyed by pollution, especially the runoff of agricultural fertilizers and sewage dumping.

Swamps

Swamps are shallow bodies of water in a low-lying, poorly drained area. These wetlands support a wide range of plant life, especially trees and high shrubs. In southeastern North America, swamp forests are typically dominated by such tree species as bald cypress, water tupelo, swamp

tupelo, and eastern white cedar. More northern temperate swamps are usually dominated by red maple, silver maple, American elm, and green or swamp ash.

Swamps provide a habitat for numerous species of animals. For example, swamps of bald cypress provide dwelling for the pileated woodpecker, red-shouldered hawk, Carolina wren, and many other small birds. These swamps also provide a nesting habitat for colonies of wading birds such as herons and egrets. Mammals supported by cypress swamps include swamp rabbits, white-tailed deer, and panthers. Many species of amphibians and reptiles—including the American alligator—live in cypress swamps.

Marshes

Marshes are large wetlands dominated by rushes, sedges, and low-lying grasses. Typical plants of North American marshes include cattails, reeds, bulrushes, and saw-grass. Marshes can support relatively large populations of birds and certain mammals such as muskrats. Relatively small, fringing marshes around lakes and ponds are common in the prairies of North America. The borders of these marshy areas, called potholes, have historically provided major breeding habitats for surface-feeding ducks such as mallards, pintails, and blue-winged teals.

Words to Know

Biodiversity: Existence of a variety of plant and animal species in an ecosystem.

Ecosystem: The collection of plants, animals, and microorganisms in an area considered together with their environment.

Peat: Soil composed chiefly of decaying plant matter.

Primary succession: Natural replacement over time of one plant community with another more complex one.

Bogs

Bogs are areas of wet spongy ground composed chiefly of peat (soil composed chiefly of decaying plant matter). The water underneath the surface-floating peat contains very little oxygen and other nutrients. It is also very acidic. As a result, bogs are dominated by acid-loving vegetation such as sphagnums (an order of mosses), sedges, and heaths.

Wetland ecology

Wetlands are dynamic ecosystems that are in transition between land and water habitats. Over time, most wetlands gradually fill in, a natural process known as primary succession. All wetlands were originally lakes or other bodies of water. Tons of plants, animals, and insects grow and die each year. The decaying material from these organisms gradually accumulates in small lakes. After a while, the lake becomes a wetland. The process continues with the wetland filling in more and more. Eventually, the wetland becomes a meadow, which in turn becomes a forest.

Wetlands also are delicate ecosystems. The biodiversity (the existence of a variety of plant and animal species in an ecosystem) of a particular wetland is maintained by the conditions that exist in that wetland. The plants and animals that thrive in a specific wetland have done so by adapting to the soil, water, nutrient supply, and other conditions found there. In general, wetlands that are well supplied with phosphorus (in the form of phosphate) and to a lesser degree nitrogen (as nitrate or ammonium) sustain relatively large populations of plants and animals. This is commonly the case for marshes, which are among the most productive natural ecosystems on Earth. In contrast, wetlands with low supplies of nutrients, such as bogs, sustain only small populations of plants and animals.

Wetland destruction

All wetlands have great value as natural ecosystems, and they all support species of plants and animals that occur nowhere else. Their usefulness in providing essential habitat for fish, birds, and other wildlife cannot be overstated. Similarly, humans gain from wetlands, which control floods and erosion, cleanse the water that flows through them, and extend supplies of water for drinking or irrigation. In addition, wetlands have an aesthetic (beauty) value that is priceless.

Unfortunately, most of the world's wetlands are being lost rapidly. Land developers drain and fill them in. Since the beginning of European settlement in America, more than 65 million acres have been lost. Often, wetlands are used for the disposal of municipal solid wastes and sewage. Run-offs of chemical pollutants from farmland further pollute wetlands, disturbing their delicate soil-water balance and endangering their many plant and animal species.

[See also Biodiversity; Water ]

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.

see also Estuaries; Global Climate Change; Limnologist

Martha Phillips

Bibliography

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.

WETLANDS

WETLANDS are any of an array of habitats—including marshes, bogs, swamps, estuaries, and prairie potholes—in which land is saturated or flooded for some part of the growing season. According to the U.S. Fish and Wildlife Service, wetlands contain water-loving plants (hydrophytes) and hydric soils. They serve many ecological and practical purposes. Wetlands provide habitat and breeding sites for fish, shellfish, birds, and other wildlife; help maintain biological diversity (biodiversity); reduce the effect of floods by diverting and storing floodwaters;

provide protection from storm waves and erosion; recharge ground waters; and improve water quality by filtering out sediments, excess nutrients, and many chemical contaminants. Wetlands provide recreational, research, and aesthetic opportunities such as fishing, boating, hunting, and observing and studying wildlife.

Since 1780 human activity has destroyed more than half the wetlands of the United States, which now make up only 5 percent of the land surface of the contiguous forty-eight states, or 104 million acres. Nevertheless, they are extremely productive, exceeding even the best agricultural lands and rivaling rain forests in quantity and diversity of plant and animal life. More than half of the saltwater fish and shellfish harvested in the United States—and most of the freshwater sport fish—require wetlands for food, reproduction, or both. At least half of the waterfowl that nest in the contiguous states use the midwestern prairie potholes as breeding grounds. Wetland dependent animals include bald eagles, ospreys, beaver, otter, moose, and the Florida panther.

Few people recognized the value of wetlands until the 1970s. Before then, most people considered wetlands to be wastelands. In an effort to make them more productive—primarily through agriculture or development—people destroyed them by draining, ditching, diking, or filling. Early legislation, such as the Swamp Lands acts of 1849, 1850, and 1860, allowed fifteen states on the Mississippi River to "reclaim" wetlands for cultivation. By the mid-twentieth century, accumulating evidence, including U.S. Fish and Wildlife wetlands inventories in 1954 and 1973, made clear that destruction of wetlands was causing declines in fish and waterfowl. Federal, state, and local laws—notably the federal Clean Water Act of 1972 and amendments in 1977—attempted to regulate destruction.

Development, agriculture, and increasing pollution still threaten U.S. wetlands. One-third of wetland losses have occurred in midwestern farmbelt states. All but three states (Alaska, Hawaii, and New Hampshire) have lost more than 20 percent of their wetlands. Biologists and economists agree that preserving wetlands is less expensive than attempting to restore those that have been damaged, and experts still argue whether it is even possible to restore wetlands and how scientists might measure restoration. The economic and biological feasibility of restoration is debated each time a developer seeks permission to build on a wetland, thus destroying it, and offers (or is required) to attempt to rehabilitate a second site in return. Many biologists feel that because damaged sites cannot be returned to their previous states, it may not be acceptable to allow this tradeoff.

BIBLIOGRAPHY

Council of Environmental Quality. Environmental Trends. Washington, D.C.: Executive Office of the President, 1989.

National Research Council. Restoration of Aquatic Ecosystems: Science, Technology, and Public Policy. Washington, D.C.: National Academy Press, 1992.

Susan J.Cooper/c. w.

See alsoConservation ; Floods and Flood Control ; Reclamation ; Water Pollution ; Wildlife Preservation .

wetlands low-lying ecosystem where the water table is always at or near the surface. It is divided into estuarine and freshwater systems, which may be further subdivided by soil type and plant life into bogs , swamps , and marshes. Because wetlands have poor drainage, the area is characterized by sluggish or standing water that can create an open-water habitat for wildlife. Wetlands help to regulate the water cycle, filter the water supply, prevent soil erosion, and absorb floodwaters. More significantly, however, wetlands serve as spawning and feeding grounds for nearly one third of the endangered animal and plant species in the United States, and their ecological value in most other countries is comparable.

Many wetlands were destroyed by urban growth and farming before their value was recognized. More than half of U.S. wetlands in the lower 48 states have been lost since colonial times. Federal wetlands policy today is based on the wetlands provisions (1987) of the Clean Water Act. The working concept is that of "no net loss," a concept that has been interpreted in various ways by each federal administration. Although the U.S. Fish and Wildlife Service has estimated that more than one million acres (about 400,000 hectares) of wetlands were lost in the decade from 1985 to 1995, this assessment was down from nearly 3 million acres (1.2 million hectares) lost in the previous decade, before the wetlands preservation policy was in force. As part of the "no net loss" policy, developers who fill wetlands may create new ones, but a 2001 National Academy of Sciences report found that new wetlands were not always created and when they were they were often of lesser value, both to the environment and to people, than the wetlands they replaced. The report recommended that replacement wetlands be designed to recreate the function of the developed wetlands.

Because of the restrictions wetlands protection can place on development and agriculture, it has become a political battleground between property rights activists and environmentalists. In 2001 the Supreme Court ruled that the Clean Water Act did not authorize federal regulation of so-called isolated wetlands (wetlands that do not abut navigable waters or their tributaries); as much as a fifth of the nation's wetlands are potentially affected by the ruling.

Wetlands

Wetlands are habitats characterized by saturated (waterlogged) soils for at least part of the year and plants that are adapted to grow under wet conditions. They may be completely covered by water or the water may be just below the ground. There are many different types of wetlands, such as swamps (wetlands dominated by trees), marshes (wetlands dominated by nonwoody plants such as grasses and sedges), wet meadows, bogs, fens, flood-plain forests, lakes, and ponds.

Wetlands are to a large extent the product of the topography of the land. They develop wherever there is a depression in the land that brings the water table (groundwater) close to or even above ground. The type of wetland that will develop in a particular area depends on the rate of water flow, the length of the season of soil saturation, latitude (polar versus temperate versus tropical), proximity to the coast (marine versus freshwater wet-lands), and the surrounding geology.

Plant Adaptations to Wetlands

It is challenging for a plants to grow in constantly damp conditions. The saturated soil contains little or no oxygen compared to upland soil, therefore the roots of wetland plants require special adaptations to enable them to survive. Water lilies have air channels that run from their leaves, which are in constant contact with the air, to their roots under water. Water lilies also have their stomata only on the upper surface of their leaves (most plants have stomata on the lower surface) so that water can not enter when these pores open to allow carbon dioxide in for photosynthesis. Salt marsh grasses (Spartina species) also transport oxygen to their roots, where it may be excreted into the surrounding soil and create a small oxygenated zone around their roots. Plants that grow completely underwater, such as seagrasses and pondweeds, use the oxygen created as a by-product of photosynthesis to aerate their roots. Most wetland plants have also adapted to wet conditions through changes in their metabolism. Non-wetland plants, for example, typically produce an alcohol (ethanol) as a breakdown product of sugar metabolism when the soil is saturated with water. Ethanol is toxic to plants. Wetland plants have different enzymes that prevent the formation of the alcohol.

Although only a limited number of species can thrive in the constantly saturated soils of wetlands, those plants that have adapted are often extremely productive. As anyone who has planted a garden knows, one of the major factors limiting the growth of terrestrial plants is water. Having adapted to life in constantly damp conditions, wetland plants never have to worry about getting water. As a result, their growth rates can be very high.

The Value of Wetlands to Humans

Wetlands are very important features in the landscape and provide humans with some tangible benefits. They act like a sponge helping to reduce the impacts of floods by absorbing water and serving as a reservoir for groundwater. As water flows through a wetland, pollutants such as excess silt and harmful nutrients are trapped; thus, the wetland acts as a filter of pollutants and helps to maintain clean water. Wetlands serve as a vital habitat to many different species of wildlife, including many that are very rare and in danger of extinction.

The value of wetlands has not always been appreciated. A conservative estimate is that over 30 percent of the original wetlands in the United States have been lost forever. These were filled in the past to make way for farms, houses, highways, businesses, and other human activity. Since the early 1970s, the attitude toward wetlands has changed. Not only are there now strong efforts in most states to protect the remaining wetlands, many environmental agencies and land conservation groups are working to restore wetlands damaged by past human activities.

Current Threats to Wetlands

Even with a greater sense of the value of wetlands among much of the public, there are still pressures on these habitats. The ability of wetlands to

absorb pollutants is not unlimited. Excessive amounts of pollution entering a wetland over a long period of time is likely to cause long-term changes in the wetland. One of the world's most famous wetlands, the Everglades of southern Florida, has suffered for years from pollution from fertilizers used by farms upstream from it. The pollution has resulted in some major changes in the plant community and suspected declines in the diversity of animals it supports.

Another major threat to wetlands is changes in hydrology (the flow of water). Water is the lifeblood of wetlands. If too much water is removed for human consumption or to irrigate cropland, the wetland may be degraded into a less-valuable habitat or even disappear completely. The Florida Everglades has to compete with the farms and rapidly growing cities of southern Florida for this precious resource and has suffered as a result. Not only is the quantity of water important to maintaining wetlands, but so is the timing. Many wetlands depend on seasonal flooding followed by a dry period. This type of natural cycle may be altered by dams, which may hold back the water during the wet season.

Direct filling of wetlands, although less common than it was twenty years ago, still occurs, particularly with smaller wetlands that may be perceived as less valuable than larger ones. Some small wetland types contain rare species of animals precisely because they are too small and temporary in existence to support fish predators, pointing out that size is not always a good indicator of value.

Finally, many wetlands throughout the world are threatened by invasions of nonnative plant species. Purple loosestrife, a European garden plant, has taken over many freshwater marshes in the northeastern United States. West Coast salt marshes are threatened with being overrun by tall cord-grass, an East Coast salt marsh species. These are only two examples of a very widespread problem.

see also Aquatic Ecosystems; Aquatic Plants; Carnivorous Plants; Endangered Plants; Invasive Species; Peat Bogs.

Robert Buchsbaum

Bibliography

Mitsch, William J., and James G. Gosselink. Wetlands, 3rd ed. New York: John Wiley & Sons, 2000.

Niering, William A., and Charles Elliot, eds. Wetlands. National Audubon Society Nature Guides, 1983.

Tiner, Ralph W. In Search of Swampland: A Wetland Sourcebook and Field Guide. New Brunswick, NJ: Rutgers University Press, 1998.

wetlands A general term applied to open-water habitats and seasonally or permanently waterlogged land areas, including lakes, rivers, and estuarine and freshwater marshes. Wetland habitats, especially marsh and bog areas, are among the most vulnerable to destruction since they can be drained and reclaimed for agriculture or forestry, drained for pest control (e.g. to eliminate breeding grounds for malaria-carrying mosquitoes), or modified for water supply, flood control, hydroelectric power schemes, waste disposal, etc.

wetlands A general term applied to openwater habitats and seasonally or permanently waterlogged land areas, including lakes, rivers, and estuarine and freshwater marshes. Wetland habitats, especially marsh and bog areas, are among the most vulnerable to destruction since they can be drained and reclaimed for agriculture or forestry, drained for pest control (e.g. to eliminate breeding grounds for malaria-carrying mosquitoes), or modified for water supply, flood control, hydroelectric power schemes, waste disposal, etc.

wetland Ecosystem where the water table lies close to the surface for much of the year. Wetlands include bogs, marshes, swamps and fens. There are both saltwater and freshwater wetlands. Coastal wetlands are said to contain a greater concentration of flora and fauna than any other ecosystem. They are also ecologically valuable as regulators of flooding and the water cycle. Many of the world's wetlands have been drained for farming or housing.

wet·land / ˈwetˌland; -lənd/ • n. (also wetlands) land consisting of marshes or swamps; saturated land.

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