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

June 26, 2023

A significantly smaller dead zone is due in part to below-average rainfall

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.