Monday, July 1, 2013

2013 Dead Zone

The NOAA-funded forecast, for the Chesapeake Bay, calls for a smaller than average dead zone in the nation's largest estuary this summer. Professor Bill Dennis of the University of Maryland Center for Environmental Science attributes this smaller dead zone to the cool and relatively dry spring followed by late arriving rains. The spring load of nutrients into the bay was light and locked in a lighter load of nutrients in the water layers within the Chesapeake Bay for the summer. The forecast is based to a large extent on the quantity and timing of rainfall in the Chesapeake Bay watershed, but the overall health of the Chesapeake Bay is also a contributing factor. So, there is hope that this forecast also reflects that the overall condition of the bay may be improving.

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.
Stream flow into the Chesapeake Bay is currently at “normal” levels after a relatively dry early spring. Overall, data from the U.S. Geological Survey, USGS, shows that the dry years of 2000-2004 are behind us and we may be entering a wet period. The Chesapeake Bay Program in partnership with USGS, monitors stream flow, nutrients and sediment in the rivers throughout the Chesapeake Bay watershed. There are 85 sites in the network; currently being monitored; however, only 31 of these sites have enough long-term data to be used to forecast trends. In the mid-1980s, the Chesapeake Bay Program (CBP), a partnership between the Commonwealths of Pennsylvania and Virginia, the State of Maryland, the District of Columbia, the Federal Government, and the Chesapeake Bay Commission, began efforts to reduce nutrients and sediments in the bay. Improvement in water-quality conditions in the bay has been slower than promised; however, and the U.S. Environmental Protection Agency, EPA, stepped in to put the entire region on a pollution diet. The Chesapeake Bay pollution diet, the Total Maximum Daily Load (TMDL) was mandated by the EPA to the six Chesapeake Bay Watershed states and the District of the Columbia. TMDLs for nitrogen, phosphorus and sediment were assigned by the EPA to each segment of the Chesapeake Bay Watershed in all six Chesapeake Bay watershed states.

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

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