Scientists estimate this year's Chesapeake Bay low-oxygen "dead zone" will be roughly the volume of 2.3 million Olympic-size swimming pools — or about average for the summertime months.
The National Oceanic and Atmospheric Administration expects 1.58 cubic miles of the bay to become hypoxic, meaning it contains low levels of oxygen. That level is close to the long-term average since 1950.
"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," NOAA said.
The oxygen levels are influenced by low river flow, nutrient loading and agricultural wastewater, NOAA said. Nutrients fertilize large algae blooms that create the dead zones when they die and decompose, stripping the water of oxygen.
Scientists said there are some signs the dead zones are shrinking.
"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, 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."
The portion of the dead zone that contains no oxygen is estimated to reach 0.28 cubic miles in early summer and 0.31 cubic miles by late summer, NOAA said. Both predictions are smaller than average.
The U.S. Geological Survey estimates that the Susquehanna River contributed 66.2 million pounds of nitrogen to the bay from January to May, 17 percent below average conditions, NOAA said.
"The link between Susquehanna pollution and the dead zone underscores the importance of reducing nitrogen pollution from Pennsylvania," said Beth McGee, Chesapeake Bay Foundation Senior Scientist. "CBF believes that an average sized dead zone is still unacceptable, and that Pennsylvania and the other Bay states must implement the plans they developed to reduce pollution and restore water quality in local rivers, streams and the Chesapeake Bay."