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VALUES of WETLANDS


Illustration of several of the potential wetland values for riparian wetlands during a. flood season b. dry season (Mitsch and Gosselink, 2000).

Water Quality

Wetlands help maintain and improve the water quality of our nation's streams, rivers, lakes, and estuaries. Since wetlands are located between uplands and water resources, many can intercept runoff from the land before it reaches open water. As runoff and surface water pass through, wetlands remove or transform pollutants through physical, chemical, and biological processes. For example, the Congaree Bottomland Hardwood Swamp in South Carolina removes a quantity of pollutants from watershed water resources equivalent to that which would be removed by a $5 million water treatment plant (USEPA 1995). In another case, scientists estimate that a 2,500 acre wetland in Georgia saves $1 million in water pollution control costs annually (OTA 1993).

Nutrient Removal

Scientists have estimated that wetlands may remove between 70% and 90% of entering nitrogen (Reilly 1991; Gilliam 1994). Riparian forests can reduce nitrogen concentrations in runoff and floodwater by up to 90% and phosphate concentrations by 50% (Gilliam 1994). The estimated mean retention of phosphorus by wetlands is 45% (Johnston 1991). Wetlands with high soil concentrations of aluminum may remove up to 80% of total phosphorus (Peterjohn and Correll 1984; Richardson 1985; Gale et al. 1994; Walbridge and Struthers 1993).

Ranchers and watershed managers in the West are utilizing beaver-created wetlands to improve water quality (USEPA 1993b; SCS 1989). Beaver impoundments can be extremely useful in agricultural watersheds because they may retain up to 1000 times more nitrogen than streams that are not impounded (Whigham et al. 1988).

Removal of Biological Oxygen Demand from Surface Water

Biological oxygen demand (BOD) is a measure of the oxygen required for the decomposition of organic matter and oxidation of inorganics such as sulfide. BOD is introduced into surface water through inputs of organic matter such as sewage effluent, surface runoff, and natural biotic processes. If BOD is high, low dissolved oxygen levels result. Low dissolved oxygen

levels can lead to mortality of aquatic life. Wetlands remove BOD from surface water through decomposition of organic matter or oxidation of inorganics (Hemond and Benoit 1988). BOD removal by wetlands may approach 100% (Hemond and Benoit 1988).

Removal of Suspended Solids and Associated Pollutants from Surface Water

Suspended solids (such as sediment and organic matter) may enter wetlands in runoff, as particulate litterfall, or with inflow from associated water bodies. Sediment deposition in wetlands depends upon water velocity, flooding regimes, vegetated area of the wetland, and water retention time (Gilliam 1994; Johnston 1991). Sediment deposition in wetlands prevents a source of turbidity from entering downstream ecosystems. Typically wetland vegetation traps 80-90% of sediment from runoff (Gilliam 1994; Johnston 1991). Less than 65% of the sediment eroded from uplands exits watersheds that contain wetlands (Johnston 1991).

Other pollutants that impact water quality such as nutrients, organics, metals and radionuclides are often adsorbed onto suspended solids. Deposition of suspended solids, to which such substances are adsorbed, removes these pollutants from the water. Thus sediment deposition provides multiple benefits to downstream water quality (Johnston 1991; Hemond and Benoit 1988; Hupp et al. 1993; Puckett et al. 1993).

Removal of Metals

Certain wetlands play an important role in removing metals from other water resources, runoff, and ground water (Owen 1992; Gambrell 1994; Puckett et al. 1993). Wetlands remove 20% - 100% of metals in the water, depending on the specific metal and the individual wetland (Taylor et al. 1990). Forested wetlands play a critical role in removing metals downstream of urbanized areas (Hupp et al. 1993).

Delfino and Odum (1993) found that lead leaking from a Florida hazardous waste site was retained at high levels by a wetland; less than 20 - 25% of the total lead in the soil and sediments was readily bioavailable. The majority of the lead was bound to soil and sediments through adsorption, chelation, and precipitation. Bioavailable lead was absorbed primarily by eel grass, which had bioaccumulated the majority of the lead. In another case, researchers found that wetland vegetation and organic (muck) substrate retained 98% of lead entering the wetland (Gambrell 1994 ).

Removal of Pathogens

Fecal coliform bacteria and protozoans, which are indicators of threats to human health, enter wetlands through municipal sewage, urban stormwater, leaking septic tanks, and agricultural runoff. Bacteria attach to suspended solids that are then trapped by wetland vegetation (Hemond and Benoit 1988). These organisms die: after remaining outside their host organisms, through degradation by sunlight, from the low pH of wetlands, by protozoan consumption, and from toxins excreted from the roots of some wetland plants (Hemond and Benoit 1988; Kennish 1992). In this way wetlands have an important role in removing pathogens from surface water.

Water Supply

Wetlands act as reservoirs for the watershed. Wetlands release the water they retain (from precipitation, surface water, and ground water) into associated surface water and ground water. In Wisconsin watersheds composed of 40% lakes and wetlands, spring stream outflows from the watersheds were 140% of those in watersheds without any wetlands or lakes (Mitsch and Gosselink 1993). Forested wetlands, kettle lakes and prairie potholes have significant water storage and ground water recharge (Brown and Sullivan 1988; Weller 1981). Forested wetlands overlying permeable soil may release up to 100,000 gallons/acre/day into the ground water (Anderson and Rockel 1991). Verry and Timmons (1982) studied a Minnesota bog which released 55% of the entering water to stream and ground water.

Ground water can be adversely affected by activities that alter wetland hydrology (Winter 1988). Drainage of wetlands lowers the water table and reduces the hydraulic head providing the force for ground water discharge (O'Brien 1988; Winter 1988). If a recharge wetland is drained, the water resources into which ground water discharges will receive less inflow, potentially changing the hydrology of a watershed (Brinson 1993; Winter 1988). Ewel (1990) calculated that if 80 percent of a 5-acre Florida cypress swamp were drained, available ground water would be reduced by an estimated 45 percent.

Flood Protection

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Photo courtesy of USDA NRCS

Wetlands help protect adjacent and downstream properties from potential flood damage. The value of flood control by wetlands increases with: (1) wetland area, (2) proximity of the wetland to flood waters, (3) location of the wetland (along a river, lake, or stream), (4) amount of flooding that would occur without the presence of the wetlands, and, (5) lack of other upstream storage areas such as ponds, lakes, and reservoirs (Mitsch and Gosselink 1993). The cost of replacing the flood control function of the 5,000 acres of wetlands drained each year in Minnesota was determined to be $1.5 million (USEPA 1995).

Wetlands within and upstream of urban areas are particularly valuable for flood protection. The impervious surface in urban areas greatly increases the rate and volume of runoff, thereby increasing the risk of flood damage. The drainage of wetlands, the diversion of the Mississippi and Missouri Rivers from their original floodplains, and the development allowed in the floodplains over the past 100 years were partly responsible for the billions of dollars in damage to businesses, homes, crops, and property that occurred as a result of the Midwest flood of 1993 (OEP 1993).

Erosion Control

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Photo courtesy of USDA NRCS

By virtue of their place in the landscape, riparian wetlands, salt marshes, and marshes located at the margin of lakes protect shorelines and streambanks against erosion. Wetland plants hold the soil in place with their roots, absorb wave energy, and reduce the velocity of stream or river currents. Coastal wetlands buffer shorelines against the wave action produced by hurricanes and tropical storms (Mitsch and Gosselink 1993). The ability of wetlands to control erosion is so valuable that states and landowners are restoring wetlands to control shoreline erosion in coastal areas (Lewis 1990).

Fish and Wildlife Habitat

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Photo courtesy of
US Army Corps of Engineers

Photo courtesy of
US Army Corps of Engineers

Photo courtesy of
US Army Corps of Engineers

Diverse species of plants, insects, amphibians, reptiles, birds, fish, and mammals depend on wetlands for food, habitat, or temporary shelter. Although wetlands make up only about 3.5 percent of U.S. land area, more than one-third of the United States' threatened and endangered species live only in wetlands (Mitsch and Gosselink 1993). An additional 20% of the United States' threatened and endangered species use or inhabit wetlands at some time in their life.

Coastal and estuarine wetlands provide food and habitat for estuarine and marine fish and shellfish, bird species, and some mammals (NOAA 1990a; NOAA 1990b). Most commercial and game fish breed, and their young develop, in coastal marshes and estuaries. Menhaden, flounder, salmon, sea trout, and striped bass are among the more familiar fish that depend on estuaries during their life cycles. Shrimp, oyster, clams, and blue and Dungeness crabs likewise rely on coastal wetlands and estuaries for food and habitat.

Many of America's bird species utilize wetlands as sources of food, water, nesting material, or shelter. Migratory waterbirds rely on wetlands for staging areas, resting, feeding, breeding, or nesting grounds.

Recreation, Aesthetics, Culture, and Science

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Photo courtesy of
US Army Corps of Engineers

Wetlands have archeological, historical, cultural, recreational, and scientific values. Societies have traditionally formed along bodies of water and artifacts found in wetlands provide information about these societies. The culture of the Louisiana bayou and the Chesapeake Bay formed as a result of their wetland ecosystems.

Historically, painters and writers have used wetlands as their subject matter. Today, such artists are often joined by others with cameras and camcorders. The monetary value derived from the observation and photography of wetland-dependent birds by more than 50 million Americans is at least $10 billion per year (USEPA 1994b).

More than half of all U.S. adults hunt, fish, birdwatch or photograph wildlife, spending a total of $59.5 billion annually (USEPA 1995). Waterfowl hunters spend over $630 million annually to harvest wetland-dependent birds (OTA 1993). Coastal areas alone attract at least 100 million Americans annually (NOAA 1995a). The coastal wetlands-dependent recreational fishing of 17 million Americans generates at least $18 billion in economic activity annually (NOAA 1995a).

Scientists value the processes of wetlands individually, particularly the role of wetlands in the global cycles of carbon, nitrogen, and water. Many scientists consider the removal of carbon dioxide from the atmosphere into plant matter and its burial as peat (sequestration) the most valuable function of wetlands (OTA 1993). Carbon sequestration is thought to be an important process in reducing the greenhouse effect and the threat of global warming.

Commercial Benefits

Commercially important products harvested from wetlands include fish, shellfish, cranberries, timber, and wild rice, as well as some medicines derived from wetland soils and plants. Fish and shellfish species dependent on wetlands for food or habitat comprise more than 75% of the commercial and 90% of the recreational harvest (USEPA 1994b; Feierabend and Zelazny 1987). Seafood is a $50 billion industry (NOAA 1995a). In the Southeast, fish and shellfish that depend on coastal and estuarine wetlands comprise nearly all of the commercial catch (USEPA 1994b). Louisiana's coastal marshes alone produce an annual commercial fish and shellfish harvest amounting to 1.2 billion pounds, worth $244 million in 1991 (USEPA 1995). The U.S. commercial fisheries harvest is valued at more than $2 billion annually and is the basis for a $26.8 billion fishery processing and sales industry (USEPA 1995).

Many mammals and reptiles harvested for their skins, including muskrat, beaver, mink, otter, and alligator, require wetland habitats. The nation's harvest of muskrat pelts alone is worth over $70 million annually, while the alligator industry is valued at $16 million (Mitsch and Gosselink 1993; OTA 1993).

Wetlands containing timber comprise approximately 55 million acres (22 million hectares), with two-thirds of the acreage east of the Rocky Mountains (Mitsch and Gosselink 1993). Although historically the practice has been to clear-cut and drain the forests of the bottomland hardwood swamps, with proper management, the timber industry can harvest wetland timber with minimal adverse effect (Conner 1994; Shepard 1994). In addition, replicating wetland conditions may improve production of desired flood-tolerant pine and hardwood species by preventing competition by non-wetland species (Conner 1994).

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