Summer oxygen levels in the Bay were better than last year, though not quite as good as scientists had expected, according to monitoring data.
Through early August, the size of the oxygen-starved "dead zone" at the bottom of the Bay averaged about 1 cubic kilometer. That was sharply less than the size of last year's dead zone, which was about 2.5 cubic kilometers, and better than the average of about 1.4 cubic kilometers.
But it was larger than predicted by two separate groups of scientists, one from the University of Michigan and one composed of Bay scientists from the University of Maryland Center for Environmental Science and the National Oceanic and Atmospheric Administration's Chesapeake Bay Office.
Their projections were based largely on unusually low river flows into the Bay during the late winter and early spring. That time is typically critical for setting up anoxic (no oxygen) conditions in the Bay.
Those flows bring in large amounts of nutrients that spur the excess growth of algae which die, sink to the bottom and are decomposed in a process that depletes oxygen in the water.
Strong flows also set up a barrier between fresh water on the Bay's surface, and saltier water on the bottom. High flows create a stronger pycnocline, or barrier, than low flows, preventing the bottom layer from getting oxygen from the surface.
River flows from January through April were all significantly below average, according to monitoring data from the U.S. Geological Survey. But May and June were both rainier than normal, particularly in parts of Maryland and Virginia close to the Bay. A final analysis of the data has not been completed, but scientists say the unusual flow pattern may have contributed to a larger dead zone than anticipated.
Both groups' predictions for hypoxia-water that contains oxygen, but at levels low enough to be harmful-was closer to the mark. They anticipated about 5.1 cubic kilometers of hypoxia, and monitoring data suggests that the July hypoxic volume will fall close to that mark, said Peter Tango, monitoring coordinator for the EPA's Chesapeake Bay Program Office. That would be a bit better than the average of about 7.5 cubic kilometers of hypoxia since 1985. The mainstem Bay, which doesn't include its tidal tributaries, holds a bit more than 50 cubic kilometers of water.