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Background Information: Dead Zones Endanger the Chesapeake Bay |
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bridges vol. 21, April 2009 / Feature Articles
Sixteen million people live in the Chesapeake Bay watershed, 6 million of them in the DC-Baltimore area. This concentration of human
Chesapeake Bay Watershed.
population in a coastal watershed is a major source of increased nutrient loading. Therefore, the wastewater treatment plants have to meet particularly strict regulations before the sewage can be discharged. Sediment investigations show that the impact of anthropogenic nutrients - nitrogen and phosphorus - have changed the ecology of the mainly shallow, and therefore sensitive, Chesapeake Bay over the past 200 years.
Dr. Walter Boynton, at the Chesapeake Biological Laboratory of the University of Maryland, Center for Environmental Science, has worked in this field for more than 30 years and follows the trends. The main tributaries to the Bay are the Susquehanna River, the Potomac River, and the James River, contributing 80 percent of the Bay's water inflow and also the nutrient load. The lighter fresh water stays on top of the heavier salt water, as their difference in density largely inhibits mixing. The nutrients cause an algal bloom in the upper water layers, thereby reducing the clarity of the water. Because sunlight cannot penetrate the water thoroughly, less sea grass grows on the bottom and the food chain changes substantially. Once the algae die, they sink to the bottom, where biological degradation of this organic matter causes oxygen depletion. Hypoxic (low oxygen) or anoxic (no oxygen) conditions result, killing organisms on the floor of the Bay such as clams, worms, and oysters - the extent and duration of these so-called "dead zones" have increased since the 1950s, and can persist for years.
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