Dead zones have become a yearly occurrence in the Chesapeake Bay and other estuaries. Dead zones form in summers when higher temperatures reduce the oxygen holding capacity of the water, the air is still and especially in years of heavy rains that carry excess nutrient pollution from cities and farms. The excess nutrient pollution combined with mild weather encourages the explosive growth of phytoplankton, which is a single-celled algae. While the phytoplankton produces oxygen during photosynthesis, when there is excessive growth of algae the light is chocked out and the algae die and fall from the warmer fresh water into the colder sea water. The phytoplankton is decomposed by bacteria, which consumes the already depleted oxygen in the lower salt level, leaving dead oysters, clams, fish and crabs in their wake.
In a wedge estuary such as Chesapeake Bay where the layers of fresh and salt water are not well mixed, there are several sources of dissolved oxygen. The most important is the atmosphere. At sea level, air contains about 21% oxygen, while the Bay’s waters contain only a small fraction of a percent. This large difference between the amount of oxygen results in oxygen naturally dissolving into the water. This process is further enhanced by the wind, which mixes the surface of the water. Scientists are still studying the impact of the winds in delivering oxygen to various water layers. The other important sources of oxygen in the water are phytoplankton and aquatic grasses which produce oxygen during photosynthesis, but when they die consume oxygen during decomposition by bacteria. Finally, dissolved oxygen flows into the Bay with the water coming from streams, rivers, and the Atlantic Ocean.
Dead zones have become common summer events caused by man, human waste, and the waste and excess nutrients from agriculture necessary to feed us and ornamental gardens to please us. It has be predicted by Researchers from Texas A&M University that the Gulf of Mexico dead zone currently estimated at 3,300 square miles will exceed the typical summer average of 5,600 square miles. The scientists are predicting more than 9,400 square miles of dead zone in the coastal waters of the estuary due to the heavy rains in the upper Mississippi that flooded fields and towns during the spring carrying with the flood waters the excess nutrients from farms, yards, septic systems and sewage treatment plants in its wake. The Gulf of Mexico Dead Zone is not expected to peak until late August.
|From IAN UMCES source of nitrogen pollution in Chesapeake|