Chesapeake Bay’s dead zone predicted to be 33% smaller than long-term average

A significantly smaller dead zone is due in part to below-average rainfall Download
Annapolis, MD ()
Aerial view of river with land to the side.
The Sassafras River separates Cecil County, Md., and Kent County, right, on Nov. 9, 2021. (Photo by Will Parson/Chesapeake Bay Program with aerial support by Southwings)

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 they are predicting the 2023 dead zone to be significantly smaller than the long-term average taken between 1985 and 2022.

During the spring and summer, nutrient pollution spurs on the growth of algae blooms, which remove oxygen from the water when they die off. These low-oxygen sections of the Bay, known as hypoxic areas or “dead zones,” can suffocate marine life and shrink the habitat available to fish, crabs and other critters.

But in 2023, the dead zone is predicted to be 33% smaller than the historic average, which would be the smallest dead zone on record if the forecast proves accurate. The significantly smaller than average forecast size is due in large part to a lack of rainfall in the spring of 2023. Researchers working on the forecast calculated that from November 2022 to May 2023, river flows were 20% lower than the average. Less rainfall generally means there is a lower amount of nutrients being washed off the land and into the water.

As a result, the amount of nitrogen pollution flowing into the Bay from its watershed was 42% lower than the long-term average during January through May 2023. Scientists calculated 74 million pounds of nitrogen at nine river input monitoring (RIM) stations and 5.2 million pounds were tracked from wastewater treatment plants. 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.

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

This year, hypoxic conditions began forming in the Bay in mid-May, which is typical. Warm weather increases the likelihood of hypoxic areas forming which is why dead zones tend to last from late May to early fall.

In the fall of 2023, researchers will follow up on the forecast with a Bay-wide assessment of the 2023 dead zone size and duration.


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. The U.S. Geological Survey monitors river flow, nutrients and sediment entering the Bay at the nine river input monitoring stations.

A model developed by the University of Michigan has been used since 2007 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. The hypoxia forecast model, enhanced in 2020, allows for projections of average July, average summer and the total annual hypoxic volume, and is based on the monitoring of nitrogen pollution and river flow at the 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.

Each of these models and forecasts are supported by the most up-to-date river flow and nutrient inputs from the U.S. Geological Survey. Scientists at the Virginia Institute of Marine Science, in collaboration with Anchor QEA, use a computer model to produce daily real-time estimates of hypoxia volume that show levels beginning in mid-May 2023, consistent with monitoring data.

Funding for the models has come from the National Oceanic and Atmospheric Administration and 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.


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. This drives the growth of algae blooms, which eventually die and decompose, removing oxygen from the surrounding waters faster than it can be replenished. This creates low-oxygen—or hypoxic—conditions. 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”.

Pollution reducing practices used in backyards, cities and on farms can reduce the flow of nutrients into waterways. Management actions taken to decrease loads from point sources (e.g., wastewater treatment plants) may immediately show detectable pollution changes, but the implementation of best management practices for non-point sources often results in a lag before their impact on improving water quality can be detected.

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 and sediment pollution. Above average spring freshwater flows to the Bay, along with hot temperatures and weak winds in the summer, provide the ideal conditions for the dead zone to grow larger and last longer.


“We are pleased to see that the hypoxic dead zone is predicted to diminish again this year, and hope this continues to be a trend. While the changing climate impacts the dissolved oxygen and water temperatures observed in the Bay, so does nutrient pollution. We will continue to support the hard work happening across the partnership to sustain this positive trend for below-average dead zones in the Chesapeake Bay.”

– Dave Campbell, Acting Director, Chesapeake Bay Program, Environmental Protection Agency

“The fact that the forecast shows another low hypoxia year in spite of globally increasing temperatures, is a very good sign for the state of the Bay and its critical habitats.”

- Dr. Marjorie Friedrichs, Research Professor, Virginia Institute of Marine Science

"This spring we've experienced below-average rainfall. Less water moving through the watershed means less nitrogen was carried by the tributaries to the Bay."

- John Wolf, Acting Coordinator for Chesapeake Bay Studies, U.S. Geological Survey

“Forecasts have been within 20% of the measured dead zone in 12 out of the past 15 years.”

- Dr. Donald Scavia, Professor Emeritus, University of Michigan

“It is exciting to again see the seasonal forecast of below-average hypoxia and the daily estimates of hypoxia looking OK. Summer 2022 had relatively little hypoxia and 2023 is looking promising, but we will have to wait and see how the weather during the summer influences the amount of hypoxia.”

- Dr. Aaron Bever, Managing Scientist, Anchor QEA, LLC.