The Dead Zone Forecast 2023

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.

Last week researchers from the Chesapeake Bay Program, the University of Maryland Center for Environmental Science, University of Michigan and U.S. Geological Survey announced that they are predicting the 2023 dead zone will 33% smaller than the historic average (from 1985-2022), which would be the smallest dead zone on record if the forecast proves accurate.

The significantly smaller than average size is forecast due in large part to a lack of rainfall and mild drought this past spring. Less rainfall means lower flows of the rivers, but also generally means there is a lower amount of nutrients being washed off the land and into the water.

Although different types of nutrients contribute to the annual dead zone, scientist say it is the amount of nitrogen that enters the Bay during spring that is a key driver in how hypoxic conditions can vary from year-to-year. The amount of nitrogen pollution entering the Bay during spring 2023 was 42% lower than the long-term average and included 74 million pounds of nitrogen recorded at nine river input monitoring stations and 5.2 million pounds from treated wastewater. There was 20% less water flowing into the Bay when compared to the long-term average. This is a decrease from last year when researchers noted 102 million pounds from monitoring stations and 5.7 million pounds from wastewater treatment plants.

Each year the Maryland Department of Natural Resources measures 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.

from VIMS actual 2022 report

While rainfall plays a major role in the size of the dead zone, efforts to limit nutrient pollution in the watershed under the Chesapeake Bay TMDL were also cited as a factor. Maryland, Virginia, Pennsylvania, New York, Delaware, West Virginia and Washington, D.C., have been implementing best management practices to reduce nutrient runoff that enters the Bay from sources such as wastewater, agriculture and suburban/urban stormwater. For the past three years, the Bay’s dead zone has been smaller than the long-term average, suggesting that progress is being made to manage nutrient pollution.

A model developed by the University of in 2007 and updated in 2020 is used to forecast the volume of summer hypoxia for the mainstem of the Chesapeake based on the amount of nitrogen pollution flowing into the Bay from nine river monitoring stations and the wastewater treatment plants that are located downstream of them. There are nine river input monitoring stations along the Appomattox, Choptank, James, Mattaponi, Pamunkey, Patuxent, Potomac, Rappahannock and Susquehanna rivers. Together, the U.S. Geological Survey, in partnership with Maryland and Virginia, monitor nitrogen pollution and other important pollutants, flowing into the Bay from 78% of the watershed. In the area not monitored by these stations, additional pollution reported from wastewater treatment plants are also included in the model.

The Chesapeake Bay Monitoring Program is a cooperative effort involving watershed jurisdictions, several federal agencies, 10 academic institutions and over 30 scientists. Among these institutions, the Maryland Department of Natural Resources and Virginia Department of Environmental Quality conduct 8-10 cruises between May— October, depending on weather conditions, to track summer hypoxia in the Bay. The peak of oxygen depletion occurs in July or August when water temperatures are highest 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.

 The actual 2022 Chesapeake Bay Dead Zone report from VIMS says: “Cool and relatively windy conditions in the spring resulted in hypoxia in 2022 starting later in the year (June) than average, like the mild-hypoxia year of 2020. As summer arrived in 2022, moderate winds allowed hypoxia to increase through the beginning of August, resulting in a maximum size of the dead zone similar to the average historical size. This mid-summer peak is similar to what occurred in 2020 and 2021, but smaller than 2019, when hypoxia was quite severe. In 2022, hypoxia quickly decreased from the high in early-August and continued a downward trend until ending in late September to early October. The quick decrease in hypoxia resulted from cooler temperatures and stronger winds, both of which act to limit the amount of hypoxia in the Bay. Overall, the duration of hypoxia in summer 2022 was short and the total annual amount of hypoxia was relatively low, representing a relatively good year for hypoxic conditions in the Bay.”