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Wetland Loss and Degradation
In the 1600s, over 220 million acres of wetlands are thought to have existed in the lower 48 states. Since then, extensive losses have occurred, and over half of our original wetlands have been drained and converted to other uses (Dahl, 1990). The years from the mid-1950s to the mid-1970s were a time of major wetland loss, but since then the rate of loss has decreased.
In addition to these losses, many other wetlands have been degraded, although calculating the magnitude of the degradation is difficult. These losses, as well as degradation, have greatly diminished our nation's wetlands resources; as a result, we no longer have the benefits they provided. Recent increases in flood damages, drought damages, and the declining bird populations are, in part, the result of wetlands degradation and destruction.
Wetlands have been degraded in ways that are not as obvious as direct physical destruction or alteration. Other threats have included chemical contamination, increased nutrient inputs and eutrophication (accelerated succession from low to high primary productivity rates), hydrologic modification, and sediment from air and water. Global climate change could affect wetlands through increased air temperature; shifts in precipitation; increased frequency of storms, droughts, and floods; increased atmospheric carbon dioxide concentration; and sea level rise. All of these impacts could affect species composition and wetland functions.
Human Development and Landscape Alteration
Human alterations to the natural landscape have the potential to exert significant direct and indirect influence on wetland ontogeny and processes. Changes to natural hydrological, chemical, and physical regimes have been documented as affecting the production and succession of a wetland's ecology, and therefore its functions and values (Mitsch and Gosselink,1993; Booth and Reinelt 1993; Preston and Bedford, 1988.).
During urbanization or development, pervious areas-those that permit the infiltration of precipitation through the ground-including vegetated and forested land, are lost. These natural areas are converted to land uses that increase the amount of impervious surfaces, such as roads, parking lots, and buildings. Impervious surfaces transform watershed hydrology by changing the rate and volume of runoff and altering natural drainage features, including groundwater levels. This, in turn, alters wetland hydrology and may adversely affect aquatic and riparian wetland habitat. Increases in population pressures from urbanization results in corresponding increases in pollutant loadings generated from a wide array of human activities.
Impacts to Water Quality: Pollutant Constituents
Both nationally and in Massachusetts, urban runoff and discharges from stormwater outfalls are some of the largest sources responsible for the non-attainment of water quality standards. The following is a breakdown of the individual pollutant constituents typically found in urban stormwater and the principle sources of runoff pollutants:
Impacts to Hydrology
Urban development of the natural landscape changes both the form and function of the natural downstream drainage system. Data from a host of sources demonstrate that the shift from undeveloped to developed areas results in substantial increases in runoff volume, thereby reducing the amount of rainfall available for groundwater recharge. Increases in peak runoff rates and volumes to stream channels intensifies streambank erosion and alters the natural deposition regimes (USEPA, 1983). Physical, chemical, and biological data from King County, Washington demonstrate that consistent thresholds exist for aquatic ecosystem impacts from urbanization (Booth, 1993). Approximately 10 to 15 percent impervious area in a watershed typically yields demonstrable loss of aquatic system functioning, as measured by changes in channel morphology, fish and amphibian populations, vegetation succession, and water chemistry. The following is a list of the major causes of wetland loss and degradation:
New England Wetlands: Ecology, Functions, and Degradation