America's Wetland Resources

From an ecological and societal perspective, wetlands are among the most important natural habitats in the United States. Their importance stems from aesthetic values like a sense of open space. Wetlands also serve as ecologically important habitat and spawning grounds for a multitude of fish and wildlife species. From a societal perspective, wetlands are even more important for the economic value they provide in the form of sustainable fisheries, natural flood control, groundwater cleansing and replenishment, natural cleansing of agricultural and urban runoff, and countless other services and functions.

Wetlands occupied 11 percent of the surface area of the lower 48 states just 200 years ago. Today, they cover about 5 percent of the surface area. Data from the Fish and Wildlife Service's National Wetlands Inventory also indicate that interior wetland water regimes are more numerous than coastal wetlands, although both are uniquely important from an ecological and social or resource perspective. In its first wetland report to Congress in 1982, the National Wetlands Inventory (NWI) found that 9.2 million acres of wetlands had been lost nationwide between the 1950s and 1970s. The average annual net loss rate during this time was 458,000 acres annually; 87 percent of that loss was due to agriculture. In its second report to Congress in 1991, NWI reported that 2.6 million wetland acres were lost nationwide between the 1970s and 1980s. The net loss rate over this interval of time was 290,000 acres annually; agriculture remained the single largest reason for the loss. In its most recent analysis, NWI found that the rate of wetland loss continues to decline.

Various federal laws and policies dating from the mid-1800s have encouraged and facilitated wetland loss. Only in the past 30 years have there been serious efforts on the part of government to curtail the net annual wetland loss rate. In recent years, a concerted effort has been directed at wetland restoration as part of a national goal of "No Net Loss" of wetland resources. In most instances, resource managers have been successful in restoring water to the land, but have only been successful about half the time in replacing all the functions and services provided by the original wetlands.

Coastal Wetlands and Global Climate Change

The potential impacts of climate change on wetlands are of great practical concern. Among the coastal areas of greatest risk in the United States are those low-lying coastal habitats that are easily eroded and which occur along the northern Gulf of Mexico and southern Atlantic coasts of the U.S. These coastal wetlands are especially vulnerable to direct, large-scale impacts of climate change, primarily because of their sensitivity to sea-level rise.

Observational records indicate that sea level has already risen between 10 and 25 cm globally over the past 100 years. In addition, the Intergovernmental Panel on Climate Change (1995) has projected a sea-level rise of 15-95 cm as a consequence of global warming. Sea-level rise will also increase the depth of coastal waters and increase inland and upstream salinity intrusion, both of which affect fresh and brackish water wetlands. While the influence of global warming on the frequency and intensity of storm events is uncertain, sea-level rise alone has the potential for increasing the severity of storm surges, particularly in areas where coastal habitats and barrier shorelines are rapidly deteriorating. These direct consequences of global- and regional-scale changes will increase the vulnerability of coastal wetlands (including mangrove and salt marshes) which are already heavily impacted by human activities.

Predicted changes in sea level will also dramatically alter the community composition and aerial coverage of submerged aquatic seagrass beds. Field studies off the coasts of Mississippi and Louisiana have shown, for example, that barrier island overwash events are a major determinant of seagrass community composition in the Chandeleur Sound, which contains some of the largest grassbeds in the Gulf region. Because different species of seagrass support different organisms, changes in the composition of seagrasses will be felt at every level of the food web, which includes shellfish, finfish, waterfowl (particularly redhead ducks, which depend almost entirely on seagrasses for their winter diet), and people. Farther inland, salinity intrusion becomes more important than disturbance in controlling the species in submerged grassbeds. Some species have a higher tolerance to salt water and will quickly outcompete those that are salt-intolerant. Other species may be completely eliminated, or may possibly be displaced inland depending on the rate at which sea level rises. Most of the southeastern coastal zone is highly developed, however, and the inland migration of seagrasses and salt marshes will be prevented in many areas by the existing infrastructure of roads, seawalls, and other human constructions.

Analyses of sites in five coastal states indicate that many marshes and mangrove ecosystems receive adequate mineral sediments to produce enough organic sediment and root material to remain above sea level at the present rate of sea-level rise (1-2 mm per year globally). However, three of the twelve wetlands studied were not keeping pace with the current rate of sea-level rise. If sea-level rise accelerates, some additional sites would also begin to slowly deteriorate and submerge. In some areas, the sinking or subsidence of the marsh system, coupled with human development, is the major cause of wetland loss.

In Florida, Louisiana, and South Carolina, submergence and/or salt water intrusion have been implicated in the decline in productivity, and death, of certain coastal forests. However, restoration of salinity-impacted baldcypress swamps, for example, may be possible by cultivating more salt-tolerant strains of baldcypress. Studies in Texas have also shown that changes in global climate would affect bottomland hardwood forests and other forests of the coastal plain by possibly influencing patterns of disturbances, such as fires or storms, and by altering the regional moisture balance. Such changes would affect resources in both preserved and commercial forests of the coastal plain.

Model simulations of the combined influence of sea-level rise, storms, and floods suggest that large areas of the southeastern United States could be converted from coastal marsh to open water, and from forest to marsh. Hurricane model simulations also suggest that if hurricanes or the effects of hurricanes become more intense over the next century, wetland community structure and composition would be altered. For example, historical simulations suggest that the occurrence of major hurricanes is the most important factor controlling mangrove community dynamics.