Wetland losses involve conversion of wetlands to open water either by expansion of existing water bodies (channels, lakes, ponds) or creation of new water bodies. This strict definition does not include burial of wetlands by spoil material or draining wetlands for agricultural or urban uses because these activities do not cause land loss, but rather a gain in land area.
There are opposing opinions as to whether natural causes or human activities are responsible for most of the wetland losses in the U.S. This is because there have been few studies that measured wetland losses and also determined the actual causes. In New England, marshes are small enough that the causes of wetlands deterioration and destruction can often be isolated and identified, but in vast wetland areas such as the Louisiana delta plain and the Florida Everglades where losses are caused by many factors, a comparison of natural land losses to anthropogenic losses has not been made.
Louisiana wetlands are unique in their extent and they coincide with valuable fuel and mineral resources. They form the surface of very thick and young sediments that are weak and compressible because of their deltaic origin. Today organic production in the Louisiana wetlands is incapable of keeping up with submergence because the influx of inorganic sediments has been eliminated primarily by human activities. On a geological time scale, sediment deposited by the Mississippi River compensated for the relative rise in sea level and new land was constructed because of abundant sediment supply. After each delta lobe was abandoned, natural land losses were initially high, especially those losses caused by submergence and shoreline erosion. Subsequent land losses gradually declined as each delta lobe grew older (Craig et al., 1979; DeLaune et al., 1983).
Correlation of land loss rates in south Louisiana with the regional geologic framework and delta age is clear evidence that natural processes are a predominant cause of wetland loss on the delta plain. The interconnected network of dredged canals in the Louisiana wetlands (Fig. 11) account for about 10% of the land losses, but they may be responsible indirectly for additional losses of vegetation caused by intrusion of saltwater into freshwater marshes, changes in marsh hydrology, changes in sediment dispersion, and changes in nutrient distribution (Scaife et al., 1983).
Most interior wetland losses along the Gulf Coast are caused indirectly by stream control that reduces the magnitudes and frequencies of flooding. These alterations to the coastal hydrologic system have two adverse effects on wetlands productivity and viability. First, the reduced flooding prevents deposition of overbank sediments that are vital to land-surface aggradation and wetlands maintenance. Second, the reduced freshwater inflow allows saltwater encroachment, which kills the fresh and brackish water marshes and swamps.
Many marshes in New England and the southeastern coastal states are building upward as fast as the land is subsiding so these marshes are being maintained. However in the Gulf Coast, most interior marshes away from the streams are not receiving sufficient sediment to prevent their submergence and deterioration. Deposition of sediment across the marshes is seasonal; deposition is greatest in the winter when strong winds immediately preceding passage of cold fronts transport suspended sediments into the marsh (Reed, 1989). Except for infrequent episodic deposition associated with hurricanes, marsh deposition rates are lowest in the summer and also in the spring. In Louisiana, low rates of sediment deposition are a result of flood control structures and levees on the Mississippi River and its distributaries. Before river control, spring floods annually delivered large volumes of suspended sediment across the delta plain helping maintain marsh elevations even on inactive delta lobes.
The powerful forces that devastate developed coasts can actually be beneficial to maintenance of some disintegrating wetlands and the creation of new wetlands (Conner et al., 1989). Storms import new sediment for wetland construction and circulate nutrient-rich water that stimulates new plant growth. These same storm currents redistribute seeds and can establish new stands of marsh or mangroves while the reflux of storm waters exports organic detritus out of the marsh.
Compared to rocky shores and sandy beaches, wetlands are much more susceptible to deterioration and destruction. In addition to losses associated with storms and subsidence, wetlands also can be destroyed by physical contact, such as infestations of herbivores, bacterial infections, freezes, fires, and pollution (Fig. 1). Marshes are particularly vulnerable to repeated or heavy traffic of marsh buggies and tracked vehicles that depress the ground surface and kill the vegetation. Large populations of muskrats and nutria, an imported rodent accidentally introduced in Louisiana in the 1920s, have destroyed some marshes. Prolonged or unusually cold temperatures or accidental burning caused by lightning can also kill wetlands vegetation. Numerous wetland areas have declined or have been destroyed as a result of oil spills caused by pipeline breaks or tanker accidents. The discharge of saltwater and drilling fluids associated with petroleum exploitation has been responsible for the decline or death of some marshes.
Other wetland losses are associated with old land reclamation projects that failed. In Louisiana, land reclamation began in the early 18th century and reached its zenith in the early 1900s. However, these efforts eventually resulted in land loss as levees sank, organic soils decomposed, and the wetlands subsided (Craig et al., 1979). Land reclamation in the Florida Everglades has also had disastrous consequences causing as much as 2 m of land subsidence (Holzer, 1984). Draining of marshes and swamps typically causes subsidence because water saturated peats and organic-rich soils initially lose as much as 75% of their volume when they are exposed to the air and allowed to dry (Snowden et al, 1977).