Scientists expect this year’s summer Chesapeake Bay hypoxic or “dead zone”—an area of low to no oxygen that can kill fish and aquatic life—will be larger than average, approximately 1.89 cubic miles, or nearly the volume of 3.2 million Olympic-size swimming pools. This is due to spring rainfall amounts in New York and Pennsylvania that led to an above average Susquehanna River nitrogen load (81.4 million pounds) to the Chesapeake Bay this spring.
"The forecast is a reminder that the improvements such as we saw last year are subject to reversal depending on weather conditions—two steps forward, one step back," said Don Boesch, president of the University of Maryland Center for Environmental Science. "This underscores the critical importance of continued investments by federal agencies in science and monitoring as the states continue to implement the Bay's pollution diet."
Expectations are for an above average hypoxic zone, an average early summer anoxic zone, and a slightly-above-average late summer anoxic zone. The anoxic portion of the zone, which contains no oxygen at all, is predicted to be 0.35 cubic miles in early summer, growing to 0.49 cubic miles by late summer—both of which are at or slightly above average. Above average nutrient loading from the Susquehanna River this spring accounts for the overall slightly larger-than-average predicted size of the anoxic portion.
"Although the higher forecasts for this summer seem to buck a recent trend toward lower anoxic volumes in Chesapeake Bay, they are consistent with known links between high river flows and oxygen depletion,” said Jeremy Testa, assistant professor at the University of Maryland Center for Environmental Science’s Chesapeake Biological Laboratory.
Measurements for the Bay’s dead zone go back to 1950, and the 30-year mean maximum dead zone volume is 1.74 cubic miles.
The Bay’s hypoxic (low-oxygen) and anoxic (oxygen-free) zones are caused by excess nutrient pollution, primarily from human activities such as agriculture and wastewater. The excess nutrients stimulate an overgrowth of algae, which then sinks and decomposes 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.
Spring rainfall plays an important role in determining the size of the hypoxic zone. In 2017, the Susquehanna River delivered 81.4 million pounds of nitrogen into the bay slightly greater than the long term average. Rainfall amounts were greatest in New York and Pennsylvania, leading to higher than average streamflow into the Bay from the Susquehanna.
“Despite this year’s forecast, we’ve made great strides in reducing nutrient pollution from various sources entering the Chesapeake Bay, and we are starting to see positive long-term signs,” said Rob Magnien, director of NOAA’s Center for Sponsored Coastal Ocean Research. “However, more work needs to be done to address non-point nutrient pollution from farms and other developed lands, to make the Bay cleaner for its communities and economic interests.”
The Bay outlook is based on models developed by NOAA-sponsored researchers at the University of Maryland Center for Environmental Science and the University of Michigan. They rely on nutrient loading estimates from the U.S. Geological Survey. Throughout the year, researchers measure oxygen and nutrient levels as part of the Chesapeake Bay Monitoring Program, run by the Maryland Department of Natural Resources and the Virginia Department of Environmental Quality. This year’s findings will be released in the fall.