15.3.2.7. Southeast United States

The Gulf of Mexico has a problem with hypoxia (low oxygen) during summer over an area of approximately 15,000 km² (Turner and Rabalais, 1991; Rabalais et al., 1996). The hypoxia has been shown to be a result of excess nutrients—primarily nitrogen—transported to the Gulf from the Mississippi River basin. This basin is heavily fertilized for crop production. The causes of, and proposed solutions for, this hypoxia problem are an excellent example of a complex interaction between climate and other human stresses on natural ecosystems. Specific "lessons" from this problem include the following:

• The hypoxia problem was greatly exacerbated by the 1993 Mississippi basin flood that appeared to "flush" large amounts of residual nitrogen from agriculture to the Gulf (Rabalais et al., 1996). Increases in extreme precipitation events, which are likely to occur with climate change, are likely to exacerbate coastal eutrophication problems in many locations.
• Recommended solutions to the hypoxia problem include creation of approximately 60,000 km² of wetlands to remove ~1,000 t yr^-1 of nitrogen by denitrification (Mitsch et al., 2001). If such "eco-technologies" become a common solution to eutrophication problems on a global basis, they could result in a significant flux of N₂O to the atmosphere.

Management practices designed to reduce nitrogen losses from agricultural watersheds—from improved fertilizer management to the construction of wetlands—will be strongly affected by climate (NRC, 1993). Climate change will decrease the reliability of these practices. Increases in climate extremes almost certainly will decrease their long-term performance.

Table 15-6: Climate change adaptation issues in North American subregions. Unique issues for certain locations also are indicated.
North American Subregions	Development Context	Climate Change Adaptation Options and Challenges
Most or all subregions	Changing commodity markets Intensive water resources development over large areas—domestic and transboundary Lengthy entitlement/land claim/treaty agreements—domestic and transboundary Urban expansion Transportation expansion	Role of water/environmental markets Changing design and operations of water and energy systems New technology/practices in agriculture and forestry Protection of threatened ecosystems or adaptation to new landscapes Increased role for summer (warm weather) tourism Risks to water quality from extreme events Managing community health for changing risk factors Changing roles of public emergency assistance and private insurance
Arctic border	Winter transport system Indigenous lifestyles	Design for changing permafrost and ice conditions Role of two economies and co-management bodies
Coastal regions	Declines in some commercial marine resources (cod, salmon) Intensive coastal zone development	Aquaculture, habitat protection, fleet reductions Coastal zone planning in high-demand areas
Great Lakes	Sensitivity to lake-level fluctuations	Managing for reduction in mean levels without increased shoreline encroachment

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