Thursday, September 6, 2018

Dead Zone 2018 Smaller than Predicted

This past spring Ecologists from the University of Michigan and the University of Maryland Center for Environmental Science were forecasting that the Chesapeake Bay would have a larger-than-average “dead zone” in 2018, due to increased rainfall in the watershed this spring.

In the Spring the scientists thought that this summer’s dead zone, an area of low to no oxygen that can kill fish and other aquatic life, would be about 1.9 cubic miles, according to their June forecast.

However, when the Maryland Department of Natural Resources actually performed their July measurements they found the opposite. Due to extreme summer weather, dissolved oxygen conditions in Maryland’s portion of the Chesapeake Bay mainstem were the best ever observed in late July, reported the Maryland Department to Natural Resources. The department tracks hypoxia throughout the summer during twice monthly monitoring cruises.

The hypoxic water volume (areas with less than 2 mg/L oxygen) was 0.26 cubic miles a fraction of the June prediction. This good news was likely the result of the massive rainfall in central and northern Maryland and Pennsylvania (and the whole region), which resulted in near maximum monthly water flows throughout the watershed. This wall of freshwater, accompanied by sustained winds of 20 knots before sampling, reduced stratification of the water column and mixed oxygen well into the system. 

Measurements of the Chesapeake Bay’s dead zone go back to 1950, and the 30-year mean maximum dead zone volume is 1.74 cubic miles. Dead zones are 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 usually 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.

Bottom dissolved oxygen concentrations in the Chesapeake Bay have continued to increase since 2014, and last year we recorded the second-smallest hypoxic volume ever according to the Resource Assessment Service at the Maryland Department of Natural Resources. Great strides have been made in reducing nutrient pollution from point and non-point sources, though more work needs to be done.

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