When this eutrophication is extensive and persistent, bottom waters may become hypoxic, or even anoxic (no dissolved oxygen), while surface waters can be completely normal and full of life. Human activities that increase nutrient loading can increase the intensity, spatial extent, and duration of hypoxic events. Although the extent of effects of hypoxic events on U.S. coastal ecosystems is still uncertain, the phenomenon is of increasing concern in coastal areas. Hypoxia has become more frequent and widespread in shallow coastal and estuarine areas. About 21% to 43% of the area of the United States' estuaries have experienced a hypoxic event; more than half of the affected area is in the Mississippi/Atchafalaya River plume. The area of hypoxia extends westward from the mouth of the Mississippi River to the upper Texas coast. Nutrient enrichment is the primary cause of eutrophication, of some algal blooms, and of hypoxia, and is believed to be a major factor in areas such as the northern Gulf of Mexico. Evidence suggests that the dead zone forces fish and shrimp further offshore as well as into shallow nearshore areas, and reduces the area of essential habitat. In response to an integrated scientific assessment of hypoxia in the northern Gulf of Mexico by the multi-agency Watershed Nutrient Task Force, a Plan of Action for addressing hypoxia was released in January 2001. Near the end of the 105 th Congress, provisions of the Harmful Algal Bloom and Hypoxia Research and Control Act of 1998 were incorporated into the Coast Guard Authorization Act of 1998. This measure was signed into law as P.L. 105-383 on November 13, 1998; Title VI authorized appropriations through NOAA to conduct research, monitoring, education, and management activities for the prevention, reduction, and control of hypoxia, harmful algal blooms, Pfiesteria , and other aquatic toxins. In 2004, Title I of P.L. 108-456 , the Harmful Algal Bloom and Hypoxia Amendments Act of 2004, expanded this authority and reauthorized appropriations through FY2008. Legislation has been introduced in the 110 th Congress to reauthorize and amend the Harmful Algal Bloom and Hypoxia Research and Control Act.
An adequate level of dissolved oxygen is necessary to support most forms of aquatic life. While very low levels of dissolved oxygen (hypoxia) can be natural, especially in deep ocean basins and fjords, hypoxia in coastal waters is mostly the result of human activities that have modified landscapes or increased nutrients entering these waters. Hypoxic areas are more widespread during the summer, when algal blooms stimulated by spring runoff decompose to diminish oxygen. Such hypoxic areas may drive out or kill animal life, and usually dissipate by winter. In many places where hypoxia has occurred previously, it is now more severe and longer lasting; in others where hypoxia did not exist historically, it now does, and these areas are becoming more prevalent.
The largest hypoxic area affecting the United States is in the northern Gulf of Mexico near the mouth of the Mississippi River, but there are others as well. Most U.S. coastal estuaries and many developed nearshore areas suffer from varying degrees of hypoxia, causing various environmental damages. Research has been conducted to better identify the human activities that affect the intensity and duration of, as well as the area affected by, hypoxic events, and to begin formulating control strategies.
Near the end of the 105th Congress, the Harmful Algal Bloom and Hypoxia Research and Control Act of 1998 was signed into law as Title VI of P.L. 105-383. Provisions of this act authorize appropriations through NOAA for research, monitoring, education, and management activities to prevent, reduce, and control hypoxia. Under this legislation, an integrated Gulf of Mexico hypoxia assessment was completed in the late 1990s. In 2004, Title I of P.L. 108-456, the Harmful Algal Bloom and Hypoxia Amendments Act of 2004, expanded this authority and reauthorized appropriations through FY2008. Legislation has been introduced in the 110th Congress to reauthorize and amend the Harmful Algal Bloom and Hypoxia Research and Control Act.
As knowledge and understanding have increased concerning the possible impacts of hypoxia, congressional interest in monitoring and addressing the problem has grown. The issue of hypoxia is seen as a search for (1) increased scientific knowledge and understanding of the phenomenon, as well as (2) cost-effective actions that might diminish the size of hypoxic areas by changing practices that promote their growth and development. This report presents an overview of the causes of hypoxia, the U.S. areas of most concern, federal legislation, and relevant federal research programs. This report will be updated as circumstances warrant.