Below Average Dead Zone Forecast for 2021

 

from VIMS

The “Dead Zone” of the Chesapeake Bay refers to a volume of hypoxic water that is characterized by dissolved oxygen concentrations less than 2 mg/L, which is too low for aquatic organisms such as fish and blue crabs to thrive. Within the hypoxic area life of the bay dies and a “Dead Zone” forms. The Chesapeake Bay experiences hypoxic conditions every year, with the severity varying from year to year, depending on nutrient and freshwater flows into the bay, wind, and temperature.

In late-June, the EPA Chesapeake Bay Program, United StatesGeological Survey, University of Maryland Center for Environmental Science andUniversity of Michigan scientists released their prediction for a smaller thanaverage 2021 Dead Zone. This prediction was based on lower than average water and nitrogen flows into the bay from January – May 2021. Though different types of nutrients contribute to the annual dead zone, history has shown that the amount of nitrogen that enters the Bay from January-May is a key driver in the size of the dead zone.  In spring 2021, river flows entering the Bay were 13% below average, but still fell within the normal range.

That’s the forecast and at various times each summer the Maryland Department of Natural Resources will measure the actual dissolved oxygen in the Maryland portion of the Chesapeake Bay main stem and the size of the Dead Zone. While the Virginia Institute of Marine Science (VIMS), Anchor QEA and collaborators at UMCES, operate a real-time three-dimensional hypoxia forecast model using measured inputs that predicts daily dissolved oxygen concentrations throughout the Bay (www.vims.edu/hypoxia) using the National Weather Service wind monitoring data.

The peak of oxygen depletion typically occurs in July or August. Water temperatures are highest during these months and the days are longest accelerating the growth of phytoplankton that ultimately consumes all the dissolved oxygen. The dead zone is typically gone by late fall. Cooler air temperatures at that time of year chill the surface waters, while the deeper water remains warm and allows more mixing of the layers during storms. Cooler water also will hold more oxygen. The size and shape of the dead zone is variable from month to month during the summer.

At the end of the season the Virginia Institute of Marine Science (VIMS), Anchor QEA and collaborators at UMCES compile all the collected data to report the actual results. Though last year’s prediction was that the Dead Zone in 2020 would similarly be slightly less than average, the actual Dead Zone experienced last year was significantly smaller and or shorter duration than expected.  Springtime nitrogen inflows in 2020 were 17% below the long-term average, resulting in the original forecast.  The actual ChesapeakeBay Dead Zone report from VIMS says: “However, cool windy weather helped mix and aerate Bay water in the spring, resulting in hypoxia starting later than usual. As summer arrived, weak winds and very high temperatures allowed hypoxia to increase considerably, resulting in a very large dead zone in late July... In 2020, hypoxia decreased quickly in early August in response to Hurricane Isaias; however, hypoxia returned in early September until stronger winds and cooler temperatures prevailed, ending hypoxia in the mainstem of the Bay earlier than in previous years.”

from VIMS