Chesapeake Bay Dead Zone Forecast

Right behind the NOAA announcement for the Gulf dead zone last week, came the forecast for the Chesapeake Bay dead zone. Scientists from the University of Maryland Center for Environmental Science and the University of Michigan are forecasting that this summer’s Chesapeake Bay hypoxic or dead zone is expected to be about 2.1 cubic miles, while the volume of water with no oxygen is predicted to be between 0.49 and 0.63 cubic miles during early and late summer. If accurate, this is one of the four largest dead zones in the past 20 years. The 2019 forecast is due to significantly above average rainfall last fall and this spring that have produced above average river flows. 

According to the report’s co-author Jeremy Testa of the University of Maryland Center for Environmental Science. “The high flows observed this spring, in combination with very high flows late last fall, are expected to result in large volumes of hypoxic and anoxic water.”

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, a single-celled algae. 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.

Spring rainfall plays an important role in determining the size of the Chesapeake Bay “dead zone.” This year, exceptionally high spring rainfall and streamflow carried nitrogen, phosphorus and sediment to the Chesapeake Bay in amounts above the long-term average, according to the U.S. Geological Survey (USGS), which provides the nitrogen-loading data used to generate the annual hypoxia forecast. The USGS reports that this past spring the Susquehanna River delivered 102.6 million pounds of nitrogen into the Chesapeake Bay. The Potomac River, as measured near Washington, D.C., supplied an additional 47.7 million pounds of nitrogen.

These measured nitrogen levels are well-above the long-term averages of 80.6 million pounds from the Susquehanna and 31.8 million pounds from the Potomac. Loads from the Susquehanna have not been this high since 2011 and are due to an extent to the Conowingo Dam’s sediment reservoir is nearly full. There are around 200 million tons of sediment, nutrients, and other pollutants from the Susquehanna River trapped behind the dam and it can no longer trap additional sediment. What arrives in the Susquehanna River is delivered to the Chesapeake Bay. Pennsylvania has been slow to implement agricultural best management practices to control nitrogen pollution.

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 strong winds associated with last summer’s storms served to deliver additional oxygen and prevent the Dead Zone from reaching the projected level.

The dead zone forecast is based on models developed at the University of Michigan and the University of Maryland Center for Environmental Science, with funding provided by the National Oceanic and Atmospheric Administration (NOAA) and data generated by the USGS and Maryland Department of Natural Resources will begin conduct bimonthly Chesapeake Bay water quality monitoring cruises June through August to measure and track Bay summer dead zone. Results can be found on the Department's Eyes on the Bay website .