Satellite (Landsat) picture of Chesapeake Bay (center) and Delaware Bay (upper right) – and Atlantic coast of the central-eastern United States. Image credit: Landsat/NASA, via Wikimedia Commons

Researchers from the University of Michigan, the Chesapeake Bay Program and the University of Maryland Center for Environmental Science are forecasting a slightly smaller-than-average Chesapeake Bay “dead zone” this year, due to reduced rainfall and less nutrient-rich runoff flowing into the bay from the
watershed this spring.

The bay’s hypoxic (low oxygen) and anoxic (no oxygen) zones are caused by excess nutrient pollution, primarily from agriculture and wastewater. The excess nutrients stimulate an overgrowth of algae, which then sink and decompose in the water. The resulting low oxygen levels are insufficient to support most marine life and habitats in near-bottom waters, threatening the bay’s crabs, oysters and other fisheries.

This year’s Chesapeake Bay hypoxic volume, known as a dead zone, is expected to be 9% lower than the average measured over the past 34 years. The volume of water with no oxygen is predicted to be 4% lower than the average, the researchers announced today.

“Each year, the forecasts are reported to be bigger or smaller than some long-term average, when in fact the long-term average is not the goal,” said University of Michigan aquatic ecologist Don Scavia, professor emeritus at the School for Environment and Sustainability. “This year’s forecast calls for a dead zone that is still larger than that implied by the targets set under the Chesapeake Bay load agreement.”

Although different types of nutrients contribute to the annual Chesapeake Bay dead zone, it is nitrogen—which enters the bay from January through May—that is a key driver in how hypoxic conditions can vary from year to year.

In spring 2020, Chesapeake Bay received levels of nitrogen pollution that were 17% below the long-term average. The nitrogen loads included 111 million pounds recorded at nine river-input monitoring stations, along with 7.3 million pounds from treated wastewater.

“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 the Resource Assessment Service at the 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.”

Jeremy Testa of the University of Maryland Center for Environmental Science said the annual forecasts “continue to help scientists understand what controls long-term changes in hypoxia in Chesapeake Bay, improving our ability to predict them and to identify actions to mitigate them.”

Since 2007, a model developed by University of Michigan researchers has been used to forecast the volume of summer hypoxia for the main stem of the Chesapeake Bay by using the amount of nitrogen pollution flowing into the bay from the Susquehanna River during the previous January through 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 that are already showing substantially less hypoxia in 2020 than in recent years.

Funding for the models came from the National Oceanic and Atmospheric Administration. Data used in the models are provided by the U.S. Geological Survey, Maryland Department of Natural Resources, Virginia Department of Environmental Quality, and Chesapeake Bay Program.

In 2020, the hypoxia model was updated, refined and later transferred to the Chesapeake Bay Program through a collaborative effort led by modelers at the University of Michigan, University of Maryland Center for Environmental Science, Virginia Institute of Marine Science and Chesapeake Bay Program.

The enhanced model now provides hypoxia projections for an average July, average summer and the total annual average hypoxia volume, based on the measure of nitrogen pollution and river flow captured at the nine U.S. Geological Survey river input monitoring stations, through partnerships with Maryland and Virginia.

Together these stations—which are located on the Appomattox, Choptank, James, Mattaponi, Pamunkey, Patuxent, Potomac, Rappahannock and Susquehanna rivers—reflect nitrogen loads flowing into the bay from 78% of its 64,000-square-mile watershed. The load estimates also include point sources such as wastewater treatment plants that enter the rivers downstream of the monitoring stations.

“We are excited to see our research forecast model transferred into operations at the Chesapeake Bay Program, ensuring its continuity, updates and refinements,” said Michigan’s Scavia.

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 more than 30 scientists.

Among these institutions, the Maryland Department of Natural Resources and the Virginia Department of Environmental Quality conduct eight to 10 cruises between May and October—depending on weather conditions—to track summer hypoxia conditions in the bay.

Results from each monitoring cruise can be accessed on 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 website.

A bay-wide assessment of the 2020 dead zone will be available in the fall.

Pollution-reducing practices used in backyards, in cities and on farms can reduce the flow of nutrients into waterways. Management actions taken to decrease loads from point sources such as wastewater treatment plants may immediately show detectable pollution changes, but there is often a lag in measuring their impact on improving water quality and the health of the bay.

Weather conditions also play a role in the size and duration of the annual dead zone. Heavy rainfall can lead to strong river flows entering the bay, which carries along increased amounts of nutrient pollution. Hot temperatures and weak winds provide the ideal conditions for the dead zone to grow larger and last longer, as occurred in 2019.

This press release was written by Rachel Felver of the Chesapeake Bay Program.

More information:

• Don Scavia’s hypoxia forecast page