The Chesapeake Bay and its tributaries have been the scene of "perhaps the most famous, and certainly the most ambitious" efforts to control overenrichment -- scientifically known as eutrophication -- of coastal waters, according to Dr. Donald F. Boesch, president of the marine studies institute at the University of Maryland.
The absence of dissolved oxygen in the deeper waters of the bay had been observed since measurements were first made, but it was not until 1984 that scientists began to observe that there was an increase in oxygen depletion between 1950 and 1980 and to look for an explanation, Boesch wrote in his recent study "Science, Policy and Coastal Eutrophication: The Chesapeake Experience."
Studies by S. R. Cooper and G. S. Brush in the early '90s demonstrated from biological and chemical indicators in core sampling that oxygen depletion has been a part of the bay's ecology for a long time; that overenrichment through human activities began with extensive land-clearing in the late 18th century, and that low-oxygen conditions became more frequent and persistent during the mid-20th century in association with rapid population growth and the widespread use of artificial fertilizers, Boesh wrote.
Current estimates of the origin of excess bay nutrients indicate that nonpoint sources such as agricultural lands, developed areas and the atmosphere contribute 2.5 times more nitrogen to the bay than point sources such as sewage treatment plants and industries.
Agriculture is the leading source of the bay's nitrogen enrichment, with a 39 percent share, according to a study by R. D. Magnien and others published in "The State of the Chesapeake 1995." Other sources of nitrogen enrichment identified by Magnien were point sources, 23 percent; forests, 18 percent; atmosphere, 11 percent; and development, 9 percent.
Excessive amounts of nitrogen lead to rapid growth of phytoplankton, "creating dense populations, or blooms. These blooms become so dense that they reduce the amount of sunlight available to submerged aquatic vegetation," wrote Kathryn Reshetiloff, editor of "Chesapeake Bay: Introduction to an Ecosystem," in 1995.
"Without sufficient light, plants cannot photosynthesize and produce the food they need to survive. ... Another hazard of nutrient-enriched algal blooms that are not consumed by zooplankton comes after the algae die. As the blooms decay, oxygen is used up in decomposition. This can lead to dangerously low oxygen levels that can harm or even kill aquatic organisms," Reshetiloff wrote.
In an effort to control pollution by nitrogen and other nutrients, Maryland, Pennsylvania, Virginia and the District of Columbia agreed in the 1983 Chesapeake Bay Agreement to reduce the total amount of nutrients entering the Chesapeake Bay by 40 percent by the year 2000. The bay is the nation's first estuary to be targeted for restoration and protection.
Pub Date: 10/06/96