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Hypoxic zone size affected by low river flow and nutrient loading

Scientists expect that this year’s mid-summer Chesapeake Bay hypoxic low-oxygen zone or “dead zone” – an area of low to no oxygen that can kill fish and aquatic life – will be approximately 1.58 cubic miles, about the volume of 2.3 million Olympic-size swimming pools. This is close to the long-term average as measured since 1950.

The anoxic portion of the zone, which contains no oxygen at all, is predicted to be 0.28 cubic miles in early summer, growing to 0.31 cubic miles by late summer. Low river flow and low nutrient loading from the Susquehanna and Potomac rivers this spring account for the smaller predicted size of the anoxic portion.

The Bay’s hypoxic and anoxic zones are caused by excess nutrient pollution, primarily from human activities such as agriculture and wastewater. The low oxygen levels are insufficient to support most marine life and habitats in near-bottom waters and threaten the Bay’s production of crabs, oysters and other fisheries.

The predicted “dead zone” size is based on models that forecast three of its features: midsummer low-oxygen hypoxic zone, early-summer oxygen-free anoxic zone, and late-summer oxygen-free anoxic zone. The models were 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.

"These improved forecasts help groups like the Chesapeake Bay Program protect the Bay for the future,” said Russell Callender, Ph.D., assistant NOAA administrator of the National Ocean Service. “They’re also one of many tools NOAA and its partners are developing to better understand the environmental threats to the Bay, and guide the Bay restoration efforts.”

USGS provides nutrient runoff and river stream data used in the forecast models. USGS estimates that the Susquehanna River delivered 66.2 million pounds of nitrogen to the Bay from January to May 2016, which is 17 percent below average conditions. The data are funded through a cooperative agreement between USGS and the Maryland Department of Natural Resources. USGS operates more than 400 real-time stream gauges and collects water quality data at numerous long-term stations throughout the Chesapeake Bay basin to track how nutrient loads are changing over time.

“Monitoring how nutrient levels may be changing in streams, rivers, and groundwater and how the estuary responds to these changes is vital in assessing our overall progress in improving the health of the Bay,” said Don Cline, USGS associate director for water. “Local, state and regional partners rely on the basic data to inform adaptive management strategies in Bay watersheds.”

"There has been a recent trend toward less hypoxia later in the summer that may signal an emerging response to actual reductions in nutrient pollution," said Donald Boesch, Ph.D., president of the University of Maryland Center for Environmental Science, "But it’s no reason to be complacent — we have a long way to go to finish the job."

Later this year researchers will measure oxygen levels in the Chesapeake Bay, based on surveys by the Chesapeake Bay Program's partners from the Maryland Department of Natural Resources  and the Virginia Department of Environmental Quality. The history of hypoxia in the Chesapeake Bay since 1985 can be found at EcoCheck, a website from the University of Maryland Center for Environmental Science.

Photo: Wendy S. McPherson, USGS

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