Chesapeake Bay ‘dead zone’ predicted to be larger than average, researchers say

In spite of above-average rainfall this spring, the Chesapeake Bay’s low-oxygen “dead zone” is predicted to be slightly larger than average this year.

Though this year’s increased rainfall led to higher water flows entering the Chesapeake across the watershed, the dead zone — called that because there is little to no oxygen for creatures such as crabs, fish and oysters — is only estimated to be 4% larger than the long-term average, according to a June 21 prediction from researchers at the Chesapeake Bay Program, University of Maryland Center for Environmental Science, University of Michigan and U.S. Geological Survey.

Oxygen-poor dead zones are created when algae blooms die, stripping oxygen from the water as they decompose and making it difficult for marine life to survive. The dead zone is most pronounced in the deep waters of the mainstem of the bay due to less oxygen toward the bottom, according to the Chesapeake Bay Program. Experts say the Chesapeake’s dead zone likely began to form earlier this year due to warmer-than-normal temperatures and weaker winds.

Higher water flows mean that more pollutants such as nitrogen and phosphorus, nutrients that spur the growth of algae blooms, run off into the bay. Though river monitoring stations at the bay’s nine largest tributaries measured 131 million pounds of nitrogen entering the Chesapeake from January to May — 77% higher than last year — the nitrogen pollution entering the waterway is consistent with the historic average of data taken from 1985 to 2023, the report said.

In 2023, the state saw lower-than-average rainfall, which contributed to last year’s dead zone being the smallest since scientists started monitoring it in 1985.

Parts of Maryland experienced heavy rainfall earlier this year, contributing to the higher water flows. In January, Annapolis saw historic coastal flooding, with waters reaching 5.1 feet above normal levels.

Throughout the year, researchers measure oxygen and nutrient levels in the Chesapeake. In Maryland, the Department of Natural Resources conducts cruises between May and October to track summer oxygen levels in the bay. Hot temperatures and weak winds also provide ideal conditions for the dead zone to grow larger and last longer.

“Dissolved oxygen levels are a key measure of bay health, as sufficient oxygen is needed to support vital fish, crab and oyster populations, as well as a healthy ecosystem,” said Mark Trice, the program chief of water quality informatics at the Maryland Department of Natural Resources. “Bay monitoring data collected by the Chesapeake Bay Program’s state and federal partners informs us of habitat conditions, advances our knowledge of bay ecology and measures the progress of our efforts toward restoration and nutrient reduction goals.”

Experts say t this year’s slight increase, despite higher flows, can be attributed to efforts to keep nutrient runoff out of the Chesapeake Bay watershed, including buffers to stabilize streams and riverbanks, cover crop planting and use of permeable pavement.

In the fall, researchers will follow up on their forecast with an assessment of the bay’s dead zone size and duration.

While some progress has been made, the Chesapeake Bay Foundation warned a changing climate could make it more challenging to reduce runoff.

“The latest dead zone forecast reflects that the longstanding partnership between the federal government and the Chesapeake Bay watershed states has made some strides, particularly from wastewater treatment plant upgrades,” Alison Prost, the foundation’s vice president for environmental protection and restoration, said in a statement. “In the face of a warmer and wetter climate, there is an urgent need for strong leadership to develop new solutions for pollution from agriculture and developed areas.”