Monday, July 27, 2015

2015 Dead Zone

The recently released 2015 NOAA-funded forecast calls for a smaller than average dead zone in the Chesapeake Bay this summer. Scientists are predicting that the dead zone in the nation's largest estuary will cover a volume of 1.37 cubic miles, 10% lower than the long term average. The University of Maryland Center for Environmental Science attributes this smaller dead zone to the cool and relatively dry spring in Pennsylvania followed by late arriving rains the same thing that happened in 2013. 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 there is hope that this also reflects that the overall condition of the bay may be improving in response to the Chesapeake Bay Program coordinates U.S. Environmental Protection Agency mandated TMDL.

The predicted “dead zone” size is based on models that forecast the zone based on midsummer volume of the low-oxygen hypoxic zone, early-summer oxygen-free anoxic zone, and late-summer oxygen-free anoxic zone. The models were developed by NOAA-sponsored research at the University of Maryland Center for Environmental Science and the University of Michigan. They rely on nutrient loading estimates supplied by the U. S. Geological Survey. USGS estimates that 58 million pounds of nitrogen were transported to the Chesapeake Bay from January to May 2015, which is 29 % below average.

Later this year researchers will measure oxygen levels in the Chesapeake Bay. The model forecasts are then combined with the oxygen measurements taken during summer monitoring cruises to improve our understanding of how nutrients, hydrology, and other factors affect the size of the hypoxic zone. Improved understanding will result in improved the models which are used in turn to develop effective strategies for reducing dead zones.

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 shaped 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. 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.
Nitrogen loads into the bay

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