What is a dead zone?
A “dead zone” is an area of the Bay where marine life cannot survive. In the Chesapeake Bay, the dead zone is usually in the deepest waters of the bay where the water has little or no oxygen. Without oxygen, there is no life. Fish and crabs leave the area or have only small areas in which to live. Oysters and other bottom animals that cannot move away die.
A dead zone routinely forms in the bay each year from late spring through early fall. Polluted runoff that enters the bay contributes to the formation of the dead zone. Nutrient pollution acts as a fertilizer and stimulates the excessive growth of microscopic plants called algae. When the algae die, they settle to the bottom of the bay and decay. Decomposition of the algae uses up oxygen; and the bottom waters become depleted of oxygen.
Water circulation patterns in the Chesapeake Bay make the problem worse. Fresh water entering the Bay from the rivers forms a layer on top of the heavier salt water entering from the ocean. Since mixing between the layers is difficult, the bottom waters become isolated and low oxygen conditions in these areas can quickly worsen. The result is a persistent dead zone.
Although a dead zone forms in the Chesapeake Bay each year, the size of the dead zone can change from year to year. Monitoring the size of the dead zone can help scientists and managers track progress in Chesapeake Bay health and restoration.
- Diana Domotor