Human development activity during the past century had more of an impact than climate change on most coastal regions (Scavia et al, 2002; Nicholls et al, 2007). The Gulf Coast region comprises some of the most valuable and heavily populated coastal landscape in North America. The total population of the coastal counties in the five Gulf Coast states has roughly doubled since 1950 and is projected to grow another 40 per cent or more by 2025, compared with an average of 23 per cent for the entire US (Twilley et al, 2001; Census Bureau, 2000).
Natural resources in the region are under intense pressure from navigation and flood control projects, and from large-scale land use change associated with industrial, agricultural and municipal development. In addition, this coastal region is being impacted by the development of its oil, gas and mineral resources. The northern Gulf of Mexico Outer Continental Shelf produces roughly one quarter of US domestic oil and gas, and Gulf coastal counties receive 60 per cent of the nation's oil imports (EIA, 2005). The Texas and Louisiana coasts are heavily impacted by oilfield access canals, navigation channels and onshore facilities that support oil and gas development in state and federal waters. Elevation has not been a constraint to development and climate change and, with rare exception, is not currently a consideration in coastal infrastructure planning. Approximately 25 per cent of the roads, 72 per cent of the port facilities and 9 per cent of the rail lines in the coastal counties between Mobile and Galveston are on land below 1.5 m in elevation (Savonis et al, in review).
The large and growing pressures of development are responsible for most of the current stresses on Gulf Coast natural resources, which include: water quality and sediment pollution, increased flooding, loss of barrier islands and wetlands, declining fisheries and other factors that are altering biodiversity, productivity and the resilience of coastal habitats (EPA, 1999). Human alterations to freshwater inflows through upstream dams and impoundments, dredging of natural rivers and man-made waterways, and flood control levees have also affected the quantity of water and sediment delivered to the Gulf coastal zone. The pressure on freshwater resources was particularly evident in the summer drought of2000, when cities and agricultural water users suffered from salt-water intrusion and reduced availability of water resources (Twilley et al, 2001). In contrast to salt-water intrusion, which is a problem in the central and eastern Gulf region, human activity has led to a lowering of salinity in the Laguna Madre since 1949. The dredging of the Gulf Intracoastal Waterway (which allowed less saline Gulf waters to enter the lagoon) and increased drainage from agricultural lands has shifted seagrass species from the highly salt-tolerant shoalgrass (Halodule wrightii) to manatee grass (Syringodium filiforme), which has a lower salinity tolerance (Quammen and Onuf, 1993).
Roughly 80 per cent of US coastal wetland losses have occurred in the Gulf Coast region since 1940, and predictions of future population growth portend increasing pressure on Gulf Coast communities and their environment. Upper Mobile Bay marshes, for example, have declined by 34 per cent since the 1950s due to commercial and residential development, disposal of dredged material from navigation projects, industrial development, erosion and subsidence (Roach et al, 1987). Wetland loss in south Louisiana is the highest in the region and is attributed to a combination of human activity and natural factors including sediment and fresh water deprivation due to the construction of levees and dams along the Mississippi River and its tributaries, groundwater and oil and gas withdrawals, drainage of organic soils, natural compaction and dewatering of deltaic sediments, dredge and fill operations, channelization of wetlands and waterways, and sea level rise. The net effect of these various stressors has been the loss of over 1565 km2 of intertidal coastal marshes and adjacent lands to open water between 1978 and 2000 (Barras et al, 2003; and for a popular account, see Tidwell, 2003).
Increased urbanization, industrialization and agricultural land use have significantly altered coastal water quality in all five Gulf Coast states. Over 25 per cent of the Gulf Coast's shellfish growing areas are closed during an average year due to pollution (EPA, 1999), and increased nutrient loading of the Mississippi River can lead to hypoxia in a near-shore area of the Gulf of Mexico covering roughly 15,000-20,000 km2 and chronic algal blooms in many of the regions inshore coastal waters (Rabalais et al 1996). Salt-water intrusion is a serious problem threatening water supplies in some parts of the region, such as Houma, Louisiana.
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