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The Great Waters Program

Chesapeake Bay

Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.
Introduction to the Issues and Ecosystems

Chesapeake Bay, the largest estuarine system in the contiguous United States, has a watershed of almost 64,000 square miles. The total surface area of the Bay is 3,830 square miles. Of these, 153 square miles are tidal fresh waters, 3,562 square miles constitute the mixing zone, and 115 square miles are salt waters. This unique ecosystem also contains more than 1,500 square miles of wetlands that provide critical habitat for fish, shellfish, and wildlife; filter and process residential, agricultural, and industrial wastes; and buffer coastal areas against storm and wave damage.

Chesapeake Bay

Fish Advisories within the Shaded Area

    PCBs Dioxin Chlordane Kepone
West Virginia Potomac River   *    
Virginia Jackson River   *    
  James River   *   *
Maryland Back River     *  
  Baltimore Harbor     *  
District of Columbia Anacostia River *   *  
  Potomac River *   *  


Chesapeake Bay's watershed stretches from New York State to Virginia and encompasses one-sixth of the Eastern Seaboard. The Bay accounts for almost half the fresh water entering estuaries in the Middle Atlantic Region. Five major tributary systems--the Potomac, Susquehanna, Rappahannock, York, and James Rivers--as well as dozens of smaller rivers supply fresh water to Chesapeake Bay. This freshwater inflow (85,800 cubic feet per second) significantly affects estuarine circulation and combines with tides to create complex circulation patterns that contribute to Chesapeake Bay's vitality.

Atlantic Coastal Plain estuaries such as the Chesapeake Bay are characteristically shallow and are subject to strong tidal circulation, creating ideal conditions for biological productivity. About 25 percent of all approved shellfish waters for oysters and clams in the United States are found in Chesapeake Bay.

In 1991 over 150 million pounds of fish and shellfish were harvested from this highly productive system. The Chesapeake Bay estuarine system is not only a major fishing area but also provides essential nursery areas for a wide variety of commercial and sport fish species. The Bay provides year-round habitat for white perch, bay anchovy, and several catfish species, including the channel catfish and white catfish, and attracts marine predators such as bluefish and Atlantic croaker. The Bay also serves as a nursery for early life stages of migratory species, such as Atlantic menhaden, American shad, American eel, weakfish, spotted sea trout, and striped bass. 

Water Quality Issues

In 1975, Chesapeake Bay became the Nation's first estuary to be targeted for protection and restoration when Congress directed EPA's Office of Research and Development to initiate a study investigating the causes of the environmental declines observed in the Bay. The Clean Water Act Amendments of 1987 required the EPA Administrator to continue the ongoing Chesapeake Bay Program and maintain a Chesapeake Bay Program Office. This Program continues to collect and make available information about the Bay's environmental quality, to coordinate federal and state efforts to improve the Bay, and to determine the impact of natural and man-made environmental changes in the Bay, especially from sediment deposition, nutrients, chlorine, acid precipitation, low dissolved oxygen, and toxic pollutants.

Studies completed in the 1970s documented that increases in agricultural development, population growth, and sewage treatment plant discharges were causing the Bay to become nutrient enriched. Nitrogen and phosphorus are the two primary nutrients required to sustain aquatic biological productivity. Although phosphorus is the limiting nutrient in most freshwater systems, nitrogen is the limiting nutrient in most coastal estuarine and marine waters. As a result of elevated inputs, however, these nutrients are often present at concentrations in excess of basic nutrient requirements, causing excessive growth of phytoplankton and algae. This condition has two effects:

  • In shallow areas, the excess algae block the sunlight that important submerged aquatic grasses need to grow. This degrades the habitat and causes the eventual loss of these grass beds.
  • In deeper areas, the decomposition of dead algae uses up available oxygen in the water. During the warm summer months, oxygen in the bottom waters can only be replenished slowly because little mixing with the high-oxygen surface water occurs. Many bottom-dwelling organisms such as oysters, clams, and worms, which provide food for fish and crabs, cannot survive this prolonged period of low oxygen.

Nutrients in the Chesapeake Bay originate from point sources (e.g., municipal and industrial wastewater), nonpoint sources (e.g., cropland, animal wastes, urban and suburban runoff), and airborne contaminants, including inputs from states within the Bay watershed that are not signatories to the Chesapeake Bay Agreement (New York, West Virginia, and Delaware).

Water Quality Trends and Characterization

Bay water quality monitoring data confirm the significant progress made in reducing phosphorus from nonpoint sources and municipal point sources but indicate that further progress is needed toward reducing nitrogen loadings.

The Bay Program's highest priority is to restore the Bay's living resources. One way to do this is to improve water quality through nutrient reductions.

These reductions will increase dissolved oxygen, improve water clarity, and enhance conditions conducive to the growth of submerged aquatic vegetation that provides critical habitat for many of the Bay's organisms.

Point Source Nutrient Reduction

Municipal wastewater treatment plant discharges contribute the majority of point source loadings. Three elements of the Chesapeake Bay Program's point source control strategy are responsible for reductions in the nutrient loading:

  • pollution prevention actions such as prohibiting the sale of detergents containing phosphorus
  • upgrading wastewater treatment plants
  • improving compliance with permit requirements.

Because the majority of municipal treatment plants discharge into fresh waters where phosphorus is the limiting nutrient, nitrogen concentrations received little attention until recent years. New technologies such as biological nutrient removal are being developed to increase removal of nitrogen from wastewaters, and these are being added to some existing treatment facilities. Upgrading of wastewater treatment plants has strengthened controls for nitrogen as well as for phosphorus.

Nonpoint Source Nutrient Reduction

Nonpoint sources of nutrients contribute about 60 percent of the nitrogen that reaches the Bay. The largest single source is agricultural runoff. Nitrogen loading results from application of chemical fertilizers, livestock manure, and sewage sludge on fields as well as from animal wastes that run off pastures and feedlots. Other nitrogen sources include atmospheric deposition to tidal surface waters, adjacent ocean waters, and the watershed, as well as runoff from urban and suburban lawns, roadways, and other developed areas to creeks and tributary rivers. The Chesapeake Bay Program's nonpoint source control program emphasizes reductions of controllable nonpoint sources including agriculture, paved surfaces, and construction in urban areas. The most important additional control measure is the practice of nutrient management in which animal wastes and fertilizers are applied to farmland in amounts carefully calculated to meet the needs of the crops. This practice reduces the runoff and leaching of nutrients that result from overuse of fertilizers.

Atmospheric Nitrogen Reduction

In addition to point and nonpoint sources of nitrogen loadings to Chesapeake Bay, concern is growing over the atmospheric deposition of nitrogen to the Bay. Atmospheric nitrogen is largely produced from the burning of fossil fuels; its two largest sources are automobiles and fossil fuel electric generating plants throughout the Chesapeake Bay airshed, which extends well beyond the watershed. Computer models indicate that about 10 percent of the Bay's nitrogen load is the result of airborne nitrogen, deposited directly on the surface of the Bay and the tidal portions of its tributaries. When the amount of atmospheric nitrogen deposited throughout the 64,000-square-mile watershed is considered, air pollution could account for nearly 40 percent of the Bay's total nitrogen load. The exact nitrogen load added from air pollution sources is uncertain because of the lack of monitoring data and questions about how and where nitrogen is transported. The EPA is developing a model that will provide a more definitive idea about air pollution sources that impact the Bay.

Reductions in atmospheric deposition are difficult to achieve because the sources of the pollutants, stationary and mobile, are not easily controlled and may be generated within the Chesapeake Bay region or transported a considerable distance to the Bay. To obtain the greatest reductions, the Bay states are considering enacting air pollution controls more stringent than those specifically mandated by the Clean Air Act Amendments, particularly for car emissions. The Governors of Maryland, Virginia, and Pennsylvania recently took a first step in that direction by endorsing a plan to require California-style emissions standards for cars sold after 1995.

Toxics Problem

In recent years, increased attention has been paid to the role that toxics may play in the problems facing Chesapeake Bay. Through a recent reevaluation of a 1989 basinwide toxic reduction strategy, the Bay Program has determined that toxics problems exist in some locations in the Bay. A few well-known areas have serious, localized problems, and some other regions that were previously thought to be uncontaminated have shown toxic effects. No evidence was found of severe, system-wide responses to toxics similar in magnitude to the effects observed throughout the Bay due to excessive nutrients. Widespread areas have measurable levels of toxic substances, below thresholds associated with adverse effects on the Bay's living resources. The long-term effects from these low levels remain unclear. Through efforts to develop a basinwide toxics loading and release inventory, estimates of direct atmospheric deposition to tidal surface waters have been made using data from a sampling network set up in 1990. Atmospheric deposition was found to be a significant source of metals, organics, and pesticides loadings to the Bay's tidal waters, although not the major source. Recent research and assessments of sediment contaminant patterns in Chesapeake Bay indicate that atmospheric deposition may be the major source of sediment contamination, particularly of polynuclear aromatic hydrocarbons resulting from incomplete combustion of fossil fuels.

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