|
The wetlands of Louisiana are water-saturated coastal and swamp regions of southern Louisiana. The Environmental Protection Agency defines wetlands as "those areas that are inundated or saturated by surface or groundwater at a frequency and duration water to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions(e.g. swamps, bogs, fens, marshes, and estuaries)."〔(EPA: Term : )〕 Different kinds of wetland arise due to a few key factors, principally water levels, fertility, natural disturbance and salinity.〔Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p.〕 Around Lake Pontchartrain, for example, these few factors produce wetlands including bottomland hardwoods, cypress swamp, freshwater marsh and brackish marsh. High levels of flooding reduce the abundance of trees, leaving four principle marsh types: saline, brackish, intermediate and fresh〔Chabreck, R. H. 1972. Vegetation, Water and Soil Characteristics of the Louisiana Coastal Region. Louisiana State University, Agricultural Experiment Station Bulletin No. 664〕 Although these areas make up a very small percentage of the total land found in the United States, southern Louisiana contains 40 to 45 percent of the wetlands found in the lower states. This is because Louisiana is the drainage gateway to the Gulf of Mexico for the Lower Mississippi Regional Watershed. The Lower Mississippi Regional Watershed drains more than 24 million acres (97,000 km2) in seven states from southern Illinois to the Gulf of Mexico. Hence the wetlands of this area are important at the national scale.〔Gosselink, J. G., J. M. Coleman, and R. E. Stewart, Jr. 1998. Coastal Louisiana. pp. 385–436. In M. J. Mac, P. A. Opler, C. E. Puckett Haecker, and P. D. Doran 1998. Status and Trends of the Nation's Biological Resources, 2 Vols. Reston: U.S. Department of the Interior, U.S. Geological Survey.〕 On the east side of Louisiana, coastal wetlands intergrade with long leaf pine savannas, which support many rare and unusual species such as pitcher plants and gopher tortoises.〔Keddy, P.A. 2008. Water, Earth, Fire: Louisiana's Natural Heritage. Xlibris, Philadelphia. 229 p.〕 On the western side, they intergrade with wet prairies, an ecosystem type that was once vast, and now has been all but eliminated.〔 The larger vertebrate fauna such as wolves and bison was exterminated. The eastern coastline of Louisiana is much more susceptible to erosion than the western coastline because much of the eastern coastline was created by silt deposits from the Mississippi River. This natural process of sediment deposition has been blocked by an extensive levee system that directs flood water past wetlands〔Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Figure 2.25, p. 73.〕 The western coastline is marshy, but the marshes only extend inland by at the most, then the elevation begins to increase and the marshes fade into solid grounded prairies. Therefore, rising sea levels due to global warming and coastal erosion, may not affect the western coastline as profoundly as it will the eastern half, which may be replaced in open water over substantial areas. == Loss of the wetlands == The many benefits of the wetlands found in this region were not recognized by a majority of policy makers early in the 20th century. Wetlands provide many important ecological services including flood control, fisheries production, carbon storage, water filtration and enhanced disagreement over the relative importance of these factors, although it is probably safe to say that the two major factors now acting are subsidence, mostly from lack of sediment, and salt water intrusion from canals dredged to service oil and gas wells and facilitate oil and gas exploration. Further wetland loss is attributed to the construction of the now-closed Mississippi River Gulf Outlet, which introduced salt water into freshwater and intermediate marshes and facilitated significant erosion.〔"Closing the Mississippi River Gulf Outlet: Environmental and Economic Considerations." Coastal Wetlands Planning Protection and Restoration Act. Louisiana State Government. Web. 27 Feb 2013. Subsidence of the coast is certainly occurring. Some people blame the direct effects of oil and gas extraction. The logic is that as billions of barrels of oil and saltwater and as trillions of cubic feet of gas were removed from the subterranean structures in which they had accumulated over millions of years, these structures lost their ability to support the weight of the earth above. As these structures slowly collapsed, the soil above gradually subsided. The wetlands on the surface began to sink into the gulf waters. Others argue that subsidence is a natural process in deltas, as sediments compress, and that the real problem is the lack of flood waters that would normally deposit new layers of sediment.〔Boesch, D. F., Josselyn, M. N., Mehta, A. J., Morris, J. T., Nuttle, W. K., Simenstad, C. A., and Swift, D. P. J. (1994). Scientific assessment of coastal wetland loss, restoration and management in Louisiana. Journal of Coastal Research, Special Issue No. 20.〕 The role of hurricanes is also a matter of disagreement; some studies show that hurricanes actually build elevation in marshes.〔Liu, K. and Fearn, M. L. (2000). Holocene history of catastrophic hurricane landfalls along the Gulf of Mexico coast reconstructed from coastal lake and marsh sediments. In Current Stresses and Potential Vulnerabilities: Implications of Global Change for the Gulf Coast Region of the United States, eds. Z. H. Ning and K. K. Abdollhai, pp. 38–47. Baton Rouge, LA: Franklin Press for Gulf Coast Regional Climate Change Council.〕 A new and important factor is rising sea levels associated with global warming. An extensive levee system aided by locks and dams has been developed in the waterways of the lower Mississippi River.〔Reuss, M. (1998). Designing the Bayous: The Control of Water in the Atchafalaya Basin 1800–1995. Alexandria, VA: U.S. Army Corps of Engineers Office of History.〕 The levees, designed to prevent flooding along the waterways, on one hand, prevent needed silt from draining into the river, yet also prevent it and fresh water from being distributed into the marshes downriver. With no new accretion and with steady subsidence, the wetlands slowly are replaced by encroaching saltwater from the Gulf. As a result of this apparent engineering dilemma, large areas of marsh are being lost to the ocean. Since 1930 water has consumed more than 1,900 square miles (4,900 km2) of the state's land. This loss equates to the disappearance of 25 square miles (65 km2) of wetlands each year or a football field sized area every 30 minutes. This loss can be reversed, at least in some areas, but only with large scale restoration, including the removal of levees to allow the Mississippi River to carry silt into these areas〔〔Turner, R. E. and Streever, B. 2002. Approaches to Coastal Wetland Restoration: Northern Gulf of Mexico. The Hague, the Netherlands: SPB Academic Publishing.〕 Another factor that damaged wetlands was large scale logging, particularly the extensive logging of cypress forests in the early 1900s. One early logger described it this way: "We just use the old method of going in and cutting down the swamp and tearing it up and bringing the cypress out. When a man's in here with all the heavy equipment, he might as well cut everything he can make a board foot out of; we're not ever coming back in here again"〔Keddy, P.A. 2008. Water, Earth, Fire: Louisiana's Natural Heritage. Xlibris, Philadelphia. 229 p. P. 122.〕 This logging often required construction of canals, which, once the logging was finished, allowed salt water to enter the wetlands and prevent regeneration of the cypress.〔 As if these problems were not enough, the introduction of nutria from South America in the 1930s provided an entirely new species of grazing mammal. Although only a few escaped, there are now millions.〔Keddy, P.A. 2008. Water, Earth, Fire: Louisiana's Natural Heritage. Xlibris, Philadelphia. 229 p. P. 148-150.〕 Natural grazing by muskrat was now accelerated by grazing from nutria. By removing plants, nutria both cause loss of vegetation, and, perhaps more seriously, a loss of dead organic matter which would otherwise accumulate as peat and raise the level of the marsh〔Keddy, P.A. 2000. Wetland Ecology: Principles and Conservation. Cambridge University Press, Cambridge, UK. 614 p. P. 163-165.〕 One of the most important natural controls on nutria is large alligators, which may provide a useful tool for biological control of nutria, and therefore for reduced impacts of grazing.〔Keddy, P.A., L. Gough, J.A. Nyman, T. McFalls, J. Carter and J. Siegrist. 2009. Alligator hunters, pelt traders, and runaway consumption of Gulf coast marshes: A trophic cascade perspective on coastal wetland losses. p. 115-133 in B.R. Silliman, E.D. Grosholz, and M.D. Bertness (eds.) Human Impacts on Salt Marshes. A Global Perspective. University of California Press, Berkeley, CA.〕 Southern Louisiana's disappearing wetlands have a broad impact ranging from cultural to economic. Commercial fishing in Louisiana accounts for more than 300 million dollars of the state's economy. More than 70% of that amount stems from species such as shrimp, oysters and blue crabs that count on the coastal wetlands as a nursery for their young. Annually Louisiana sells more than 330,000 hunting licenses and 900,000 fishing licenses to men and women who depend on the wetlands as a habitat for their game. Additional recreational activities such as boating, swimming, camping, hiking, birding, photography and painting are abundant in wetland areas. Wetlands host a variety of trees such as the bald cypress, tupelo gum and cottonwood. Other plants such as the dwarf palmetto and wax myrtle and submerged aquatic plants such as ''Vallisneria'' and ''Ruppia'' are native to Louisiana wetlands. Wetland plants act as natural filters, helping to remove heavy metals, sewage, and pesticides from polluted water before reaching the Gulf of Mexico. Animal species native to these areas include osprey, herons, egrets, alligators, and beavers. Although there are several naturally occurring forces that adversely affect the wetland regions of Louisiana, many believe it is human intervention that has caused the majority of the decline.〔Tidwell, Mike. ''The Ravaging Tide: Strange Weather, Future Katrinas, and the Coming Death of America's Coastal Cities'', Free Press, 2006. ISBN 0-7432-9470-X〕 In summary, prior to the building of levees on the Mississippi River, the wetlands were kept in balance by occasional floods, which fill the area with sediment, and subsidence, the sinking of land. After the levees were built, however, flood sediment flowed directly into the Gulf of Mexico. This subsidence along with the recent sea level rise tipped the balance toward subsidence rather than marsh growth. This, along with the canals built in the area, caused decline of the wetlands and also caused less weakening of and less protection from recent hurricanes such as Hurricane Katrina.〔 The Lake Pontchartrain Basin Foundation has developed a (comprehensive management plan ) for the eastern regions of the Louisiana coast, placing emphasis upon restoration of riverine habitats, cypress swamps and fringing marsh. This could be a model applied to other coastal regions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Wetlands of Louisiana」の詳細全文を読む スポンサード リンク
|