CRS Report: 98-869 - Marine Dead Zones: Understanding the Problem

Abstract

Very low levels of dissolved oxygen (hypoxia) in bottom-water "dead zones" are natural phenomena, but can be intensified by certain human activities. 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 contribute to increasing the intensity and duration of, as well as the area affected by, hypoxic events, and to begin formulating control strategies. As the scientific community has become more concerned about the possible impacts of hypoxia, congressional interest in monitoring and addressing the problem has grown. This report presents an overview of the causes of hypoxia, the U.S. areas of most concern, relevant federal research programs, and legislation in the 105th Congress to authorize and fund additional research. This report will be updated as this issue evolves.

Summary

An adequate level of dissolved oxygen is necessary to support most forms of aquatic life. Very low levels of dissolved oxygen (hypoxia) in bottom-water "dead zones" are natural phenomena, but can he intensified by certain human activities. Hypoxic areas are more widespread during the summer, when they may drive out or kill animal life, and usually dissipate by winter.

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 contribute to increasing 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, provisions of the Harmful Algal Bloom and Hypoxia Research and Control Act of 1998 were incorporated into the Coast Guard Authorization Act of 1998 (H.R. 2204). This measure was signed into law as P.L. 105-383, wherein Title VI authorizes appropriations through NOAA to conduct research, monitoring, education, and management activities for the prevention, reduction, and control of hypoxia.

As the scientific community has become more concerned about possible impacts of hypoxia, congressional interest in monitoring and addressing the problem has grown. This report presents an overview of the causes of hypoxia, the U.S. areas of most concern, relevant federal research programs, and legislation in the 105th Congress to authorize and fund additional research.

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/liter. Normal dissolved oxygen concentrations in nearshore marine waters range between 5 and 8 milligrams/liter, and many fish species begin having respiratory difficulties at concentrations below 5 milligrams/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. 1 The recovery of marine ecosystems following an 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. 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 are completely normal and full of life. This is encouraged by rising bottom-water temperatures in spring that stimulate increased decomposition by microbes, leading to the development of bottom-water hypoxia.

Eutrophication occurs naturally when offshore winds or surface currents cause cold, nutrient-rich, deep marine waters to rise near coasts, resulting in algal blooms and natural hypoxic events. Many of the hypoxic events along the Pacific and Atlantic coasts are caused by this natural upwelling. However, eutrophication can be increased in intensity or frequency by nutrient loading from non-point sources (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 areas frequently occur in coastal areas where rivers enter the ocean (e.g., estuaries). Nutrient-rich fresh water is less dense than saltwater 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. 2 Hypoxia is more likely to occur in estuaries with high nutrient loading and low flushing (i.e., low freshwater turnover). 3 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 Environmental Protection Agency (EPA). Legislation was introduced in the l05th Congress to provide additional authority and funding for research and monitoring to address these concerns.

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. About 60 to 70 hypoxic zones have been identified worldwide. Hypoxia is becoming more frequent and widespread in these shallow coastal and estuarine areas. 4

About 21% to 43% of the area of United States' estuaries have experienced an hypoxic event, more than half of which is the Mississippil/Atchafalaya River plume. 5 In the Mid-Atlantic region, 13 of 22 estuaries have experienced hypoxic/anoxic events. 6 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, 7 with nearly two-thirds of this region's 21 estuaries experiencing some hypoxia/anoxia. 8 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). 9 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). 10

1 Some molluscs and annelid worms are more tolerant of low oxygen conditions and can survive hypoxic episodes that last many weeks.

2 D. Justic, et al. "Seasonal Coupling Between Riverborne Nutrients, Net Productivity and Hypoxia." Marine Pollution Bulletin, v.26 (1993):184-189.

3 R. Turner and N. Rabalais. "Suspended Particulate and Dissolved Nutrient Loadings to Gulf of Mexico Estuaries." In: T. Bianchi, J. Pennock, and R. Twilley (eds.), Biogeochemistry of Gulf of Mexico Estuaries. New York City: John Wiley & Sons. (In press).

4 R.J. Diaz and R. Rosenberg "Marine Benthic Hypoxia: A Review of Its Ecological Effects and the Behavioral Responses of Benthic Macrofauna." Oceanography and Marine Biology An Annual Review, v. 33 (1995) 245-303 (Hereafter referred to as "Marine Benthic Hypoxia.")

5 N. Rabalais. "Oxygen Depletion in Coastal Waters." NOAA ~ State of the Coast Report. NOAA, Silver Spring. MD, 1998. National Picture, p.4, http://state-of-coast.noaa.gov/bulletins/html/hy_09/hyp.html. (Hereafter referred to as "Oxygen Depletion in Coastal Waters.")

6 S. Bricker. "NOAA's National Estuarine Eutrophication Survey: Selected Results for the Mid-Atlantic, South Atlantic and Gulf of Mexico Regions. Estuarine Research Federation Newsletter, v.23, no. 1(1997); 20-21. (Hereafter referred to as "NOAA's Estuarine Eutrophication Survey: Selected Results."); NOAA. NOAA 's Estuarine Eutrophication Survey, vol. 1: South Atlantic Region. Silver Spring, MD: National Ocean Service, Office of Ocean Resources Conservation and Assessment, 1996. p.50.

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