A more normal spring rainfall brings less pollution into the Bay
Researchers from the Chesapeake Bay Program, the University of Maryland Center for Environmental Science, University of Michigan and U.S. Geological Survey announced today that a slightly smaller than average Chesapeake Bay “dead zone” is forecasted for this year, due to reduced rainfall and less nutrient-rich runoff flowing into the Bay from the watershed this spring.
"These forecasts continue to help scientists understand what controls long-term changes in oxygen conditions in Chesapeake Bay, improving our ability to predict them and to identify actions to mitigate them,” said Jeremy Testa, Associate Professor at the University of Maryland Center for Environmental Science. “This year’s forecasts indicate modest low-oxygen volumes, which would be much smaller than the large volumes we saw in 2019 following exceptionally high freshwater and nutrient inputs.”
The dead zone is an area of little to no oxygen that forms when excess nutrients, including both nitrogen and phosphorus, enter the water through polluted runoff and feed naturally-occurring algae in the water. When these algae eventually die and sink to deeper water, they are decomposed by bacteria in a process that consumes oxygen from the surrounding waters faster than it can be replenished. This creates low-oxygen—or hypoxic—conditions. The majority of plant and animal life are often unable to survive in this environment, which is why the area is sometimes referred to as a “dead zone.”
Although different types of nutrients contribute to the annual dead zone, the forecasts predict hypoxic conditions based upon the year-to-year changes in the amount of nitrogen that enters the Bay from January-May. In spring 2020, the Bay received a slightly below-average amount of nitrogen pollution (17%) compared to the long-term average. Overall, nitrogen loads entering the Bay included 111 million pounds recorded at nine river input monitoring stations and 7.3 million pounds from treated wastewater.
Weather conditions also play a role in the size and duration of the annual dead zone. Heavy rainfall can lead to high levels of freshwater flows entering the Bay, which carries along increased amounts of nutrient pollution. Above average spring freshwater flows to the Bay, along with hot temperatures and weak winds provide the ideal conditions for the dead zone to grow larger and last longer, as occurred in 2019.
“The river flows and nutrient loads were closer to average conditions in spring 2020,” said Scott Phillips, Co-Chair, Scientific Technical Assessment and Reporting Workgroup, Chesapeake Bay Program. “The more normal spring flows, along with the nutrient reduction efforts of the Chesapeake Bay Program, should result in smaller amounts of hypoxia this summer compared to 2018 and 2019, when near-record levels of spring rains washed more nutrients into the Bay.”
Compared to the past 34 years, this summer’s Chesapeake Bay hypoxic volume, or “dead zone,” is expected to be nine percent lower than the long-term average of 1.5 cubic miles during the June-September period. This volume is equal to roughly 700 M&T Bank Stadiums. The Bay’s anoxic volume - where no oxygen is present - is predicted to be four percent lower than the long-term average of 0.34 cubic miles. The predicted volumes are smaller than the dead zone in 2019, a year with exceptionally high freshwater flows and nutrient pollutants entering the Bay.
"The hypoxic forecast is a critical component to tracking progress of our Chesapeake Bay restoration efforts. Dissolved oxygen levels are a key indicator of Bay health as sufficient oxygen is needed to support our iconic Chesapeake species such as oysters, crabs and fin fish,” said Bruce Michael, Director of Resource Assessment Service, Maryland Department of Natural Resources. “The forecast brings attention to our continued need to implement our nutrient reduction strategies. We look forward to working with our Bay Program partners to monitor and report on hypoxic levels throughout the summer.”
Throughout the year, researchers measure oxygen and nutrient levels as part of the Chesapeake Bay Monitoring Program, a Bay-wide 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. Results from each monitoring cruise can be accessed through the Eyes on the Bay website for the Maryland portion of the Bay and the VECOS website for the Virginia portion. Estimates of river flow and nutrients entering the Bay can be accessed on the U.S. Geological Survey’s website.
Since 2007, a model developed by the University of Michigan has been used to forecast the volume of summer hypoxia for the mainstem of the Chesapeake Bay by using the amount of nitrogen pollution flowing into the Bay from the Susquehanna River from the previous January-May. A companion model from the University of Maryland Center for Environmental Science forecasts summer volumes of oxygen-free water. Scientists at the Virginia Institute of Marine Science, in collaboration with Anchor QEA, produce daily real-time estimates of hypoxia volume.
The data used by the models are provided by the U.S. Geological Survey, Maryland Department of Natural Resources, Virginia Department of Environmental Quality and Chesapeake Bay Program.
