Although large amounts of the Bay suffered from low-oxygen conditions this year, the situation for most of the summer was greatly improved from 2003, with August showing the best oxygen conditions on record.

Last year, with river flows into the Bay approaching record levels—and carrying large amounts of nutrients—dissolved oxygen conditions were worse than normal for much of the summer. July 2003 rivaled some of the worst conditions ever seen as nearly 40 percent of the Chesapeake suffered from hypoxia—low oxygen.

Although things improved in August, monitoring during late fall and winter also detected lower than normal oxygen conditions—something unusual for winter months as there is typically little bacterial activity during cold months to use oxygen as it breaks down organic matter.

Scientists are still trying to determine what caused those unusual dissolved oxygen conditions, said Dave Jasinski, a monitoring analyst with the University of Maryland’s Center for Environmental Science.

But with the year starting poorly, many scientists went into this summer fearing the worst. “We expected things to be as bad, if not worse, than last year,” Jasinski said.

Instead, for most of the summer, the volume of the Bay with less than 5 milligrams of oxygen per liter of water was near the long-term average observed since the Baywide monitoring program was established in 1985.

Helping the situation was that river flows during the first half of the summer were near normal. Strong river flows create a barrier that can prevent the mixing of oxygen-rich surface water and oxygen-poor bottom areas.

Hypoxic conditions peaked in late July when about 18 cubic kilometers of the Bay—slightly more than a third of its total volume—contained water with less than 5 mg/l of oxygen. That was slightly worse than average for the month. But conditions improved rapidly the next month, and by late August less than 6 cubic kilometers of the Bay was hypoxic —the best reading ever observed for that period and less than half the normal hypoxic volume present in late summer.

Jasinski said it’s normal to begin seeing improvements in August because much of the algae and other organic material, which drive low-oxygen conditions when they are consumed by bacteria, is used up. “The fuel to keep depleting oxygen sort of gets burned out in July, so it’s natural to see an upturn. But this is a pretty serious upturn this year.”

Jasinski said a series of storms that passed through the area in August probably helped to mix the surface and bottom layers.
The story was not so good for areas with severe hypoxia—dissolved oxygen concentrations of less than 2 mg/l. Severe hypoxia was worse than normal from late June through late July before getting better-than-normal readings in August.

At its worse in late July, about 10 cubic kilometers, or about a fifth of the Bay, had severely hypoxic water. That was better than the worst conditions reported for the month—exceeding 14 cubic kilometers—but significantly worse than the long-term average of 7 cubic kilometers for July.

That pattern for anoxic water—areas devoid of any oxygen—was similar to that of severe hypoxia, with conditions being worse than normal from late June through late July, then improving to better-than-average conditions. At its worst in late July, about 5 cubic kilometers—or roughly 10 percent of the Bay’s total volume—was devoid of oxygen.

It’s unclear why severe hypoxia and anoxia were worse than normal for much of the summer, Jasinski said. It might have stemmed from excessive amounts of organic material driven into the Bay late last year by Hurricane Isabel, which left added “fuel” for the decomposition process, Jasinski said, although he added that was speculation.