Hypoxia refers to a depressed concentration of dissolved oxygen in water. While definitions vary somewhat by region, it is generally agreed that hypoxia in a marine environment occurs seasonally when dissolved oxygen levels fall below 2-3 milligrams per liter. Normal dissolved oxygen concentrations in nearshore marine waters range between 5 and 8 milligrams per liter, and many fish species begin having respiratory difficulties at concentrations below 5 milligrams per liter. In extremely low oxygen environments, less tolerant marine animals cannot survive and either leave the area or die. Mortality is especially likely for sedentary species. In addition, spawning areas and other essential habitat can be destroyed by the lack of oxygen. If these conditions persist, a so-called "dead zone" may develop in which little marine life exists. The recovery of marine ecosystems following a hypoxic event has not been extensively studied. Decreased concentrations of dissolved oxygen result in part from natural eutrophication when nutrients (e.g., nitrogen and phosphorus) and sunlight stimulate algal growth (e.g., algae, seaweed, and phytoplankton), increasing the amount of organic matter in an aquatic ecosystem over decades and centuries. As organisms die and sink to the bottom, they are consumed (decomposed) by oxygen-dependent bacteria, depleting the water of oxygen. 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. Hypoxia is more likely to occur in coastal waters where the water column is stratified (i.e., layered) because of differences in temperature or salinity or both. Marine dead zones become most noticeable when and where natural eutrophication has been accelerated by human-influenced increases in nutrient loads. Hypoxia often develops as a result of upwelled ocean waters, particularly along the western coast of the Americas. In many instances, cool, nutrient-rich, deep marine waters rise along the coastal margin and support massive algal blooms that lead to hypoxia. In other instances, upwelled deep water is simply devoid of oxygen. Eutrophication as a result of human activities usually results from non-point sources of nutrients (e.g., runoff from lawns and various agricultural activities including fertilizer use and livestock feedlots), point-source discharge from sewage plants, and emissions from vehicles, power plants, and other industrial sources. Hypoxic zones frequently occur in coastal areas where rivers enter the ocean (e.g., estuaries). Rivers deliver fresh waters that are rich in nutrients to the saltier estuaries and coastal oceans. The fresh water is less dense than the salt water and typically flows across the top of the sea water. The fresh surface water effectively caps the more dense, saline bottom waters, retarding mixing, creating a two-layer system, and promoting hypoxia development in the lower, more saline waters. In the northern Gulf of Mexico, the greatest algal growth in surface waters occurs about a month after maximum river discharge, with hypoxic bottom water developing a month later. Hypoxia is more likely to occur in estuaries with high nutrient loading and low flushing (i.e., low freshwater turnover). Human activities that increase nutrient loading can increase the intensity, spatial extent, and duration of hypoxic events. Storms and tides may mix the hypoxic bottom water and the aerated surface water, dissipating the hypoxia. 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. Several federal agencies are involved in analyzing the problem, including the U.S. Geological Survey (USGS), the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Environmental Protection Agency (EPA). Legislation was enacted by the 105 th Congress to provide additional authority and funding for research and monitoring to address these concerns. This authority was extended by the 108 th Congress. Hypoxic episodes have been recorded in all parts of the world, notably in partially enclosed seas and basins where vertical mixing is minimal, such as the Gulf of Mexico, Chesapeake Bay, the New York Bight, the Baltic Sea, and the Adriatic Sea. In March 2004, the U.N. Environment Program's (UNEP's) Global Environment Outlook (GEO) Year Book 2003 reported 146 dead zones where marine life could not be supported due to depleted oxygen levels. In 2006, UNEP's Global Programme of Action for the Protection of the Marine Environment from Land-based Activities reported that the frequency and intensity of coastal dead zones is rapidly increasing and could reach 200 sites when a full list of newly identified sites is released in early 2007. Hypoxia has become more frequent and widespread in shallow coastal and estuarine areas. In addition, permanently hypoxic water masses (i.e., oxygen minimum zones) occur in the open ocean, affecting large seafloor surface areas along the continental margins of the eastern Pacific, Indian, and western Atlantic Oceans. 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. In the Mid-Atlantic region, 13 of 22 estuaries have experienced hypoxic/anoxic events. Of these, the Long Island Sound, Chesapeake Bay, Choptank River, and the New York Bight experience the most serious annual episodes. In the South Atlantic region, hypoxic/anoxic episodes are generally brief, with nearly two-thirds of this region's 21 estuaries experiencing some hypoxia/anoxia. The Gulf of Mexico region experiences the highest rate of hypoxic/anoxic events, with almost 85% of this region's 38 estuaries experiencing episodes of hypoxia (including the Mississippi/Atchafalaya River plume). The North Atlantic region is not as prone to hypoxic/anoxic events due to the generally low nutrient input (the result of lower population density) and high tidal flushing. However, areas adjacent to high population density (e.g., Cape Cod Bay and Massachusetts Bay) do experience oxygen depletion. In the Pacific region, hypoxia also occurs near population centers (e.g., San Diego Bay, Newport Bay, Alamitos Bay) or in areas of limited circulation, even where water temperatures are cold (e.g., Hood Canal, Whidbey Basin/Skagit Bay). The hypoxic zone in the northern Gulf of Mexico is the largest observed in the estuarine and coastal regions of the western hemisphere. First recognized in the early 1970s, it is the largest and most hypoxic area in the United States. The area of hypoxia extends westward from the mouth of the Mississippi River to the upper Texas coast. The seasonal shape and extent of the dead zone are mostly a function of the Mississippi/Atchafalaya River plume, the combined outflow from these two major rivers, and the biological processes it influences. The most reliable predictor of the size of the hypoxic zone is the nitrate-nitrogen load in the two months before the mid-summer mapping cruise. This hypoxic zone generally occurs from May to September, but varies from year to year. In summer 1993, following massive Mississippi River flooding, the dead zone covered more than 18,000 square kilometers (an area as large as the state of New Jersey); it reached its largest size in summer 2002—22,000 square kilometers (an area as large as the state of Massachusetts). Variation in size can be substantial between years—after reaching a maximum size of 20,000 square kilometers in 1999, the dead zone covered a much smaller 4,400 square kilometers in 2000. Although initially predicted to be the largest in size since measurements began, the hypoxic zone in 2007 covered about 20,500 square kilometers. However, a second smaller hypoxic zone of about 4,500 square kilometers also developed off Texas in 2007. This second dead zone was located near Galveston south to Matagorda Bay, and formed after runoff from heavy rains in the Brazos River drainage discharged into the Gulf. Low velocity winds during the summer result in calm seas that maintain the stratified barrier between surface and bottom water layers. Only during weather disturbances, such as frontal passages, tropical storms, and hurricanes, does vertical mixing of these stratified layers occur. Increased winds and frontal storms in autumn vertically mix the water column, dissipating the hypoxia. In the summer of 1998, this dead zone extended from very near shore (about 10-15 feet water depth) to deeper waters than are normally hypoxic (as much as 160 feet deep off the Mississippi River delta). 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. The Mississippi watershed drains 41% of the land area of the contiguous 48 states, including most of the farmbelt. Studies of the Mississippi and Atchafalaya Rivers indicate that dissolved nitrogen levels have tripled and phosphorus levels have doubled since 1960, fueling algal growth and the resultant dead zone. Research suggests that fertilizer leaching and runoff from upriver agricultural sources may be the main sources of nutrients. For example, USGS states that 56% of the Mississippi River's nutrient loading results from fertilizer runoff, with an additional 25% of the Mississippi River nitrogen coming from animal manure (municipal and solid wastes account for 6%, atmospheric deposition for 4%, and unknown sources for 9%). Analysis of cores of sediments underlying the hypoxic area reveals historic information on the Mississippi River watershed, indicating that surface water productivity has increased and bottom water oxygen stress has worsened since the early 1900s, with the most dramatic changes occurring since the 1950s—a change strongly correlated with increased use of commercial fertilizers in the watershed.