Scientists predict the Bay’s oxygen-depleted “dead zone” will be almost exactly average in size this year.

In their third annual forecast, they predict that from June through September, an average of 1.39 cubic kilometers—or 2.6 percent of the Bay’s mainstem, will be anoxic—essentially having no oxygen. That makes it off-limits to almost all Bay creatures.

If that prediction holds up, 2007 would rank 11th when compared with the previous 22 summers, according to Dave Jasinski, a University of Maryland Center for Environmental Science analyst.

That would be worse than last summer when, on average, the amount of anoxic water in the Bay was 0.93 cubic kilometers, or 1.79 percent of the Bay’s total volume. Last year was the seventh best year since Baywide water quality monitoring began in 1985.

But it would be a significant improvement from 2005, which was the fifth worst summer on record, when anoxic water covered 2.35 cubic kilometers along the bottom of the Chesapeake from the Bay Bridge to the Patuxent River.

The anoxia forecast is based on the historic relationship between spring flows from the Susquehanna River, nutrient loads from the upper Bay and the amount of anoxic water in the Bay.

Susquehanna river flows help to create a barrier, known as the pycnocline, between fresh water on the surface and salty ocean water on the bottom of the Bay. The nutrients fuel the growth of algae, much of which die and sink to the bottom where they are consumed by bacteria, which in turn use up oxygen. The pycnocline helps to prevent the bottom water from mixing with oxygen-rich water near the surface.

Studies of sediment cores from the bottom of the Bay indicate that anoxic conditions were once rare, but have become an annual event as the Chesapeake has been overwhelmed by nutrients since the mid-1900s.

Since 1985, the worst year for anoxia was 1993 when an average of 2.72 cubic kilometers (5.2 percent of the mainstem) was anoxic during the summer. The best year was 2002, when an average of 0.49 cubic kilometers (0.9 percent of the mainstem) was anoxic.

The prediction only covers the volume of anoxic water in the mainstem—the main Bay not including its tidal tributaries—because of its strong relationship to springtime flows and nutrient inputs. Predictions from the last two summers have been remarkably accurate.

Because little can be done to influence flows, the Bay Program is seeking huge nutrient reductions from the Chesapeake’s 64,000-square-mile watershed to reduce algae growth and improve water quality.

If the Bay Program’s nutrient reduction goals were achieved, computer models suggest that anoxia would become a rare event, showing up only in the most extreme weather conditions.

A much larger area of the Bay is not anoxic but has less oxygen than is needed to support aquatic life. The Bay Program estimates that only about 37 percent of the Chesapeake and its tidal tributaries met their water quality standards for dissolved oxygen last year.

Scientists in June are planning to make their first attempt to predict the amount of low oxygen, or hypoxic, water in the Bay this year, a more complex prediction.

Also in June, they expect to release predictions about underwater grass coverage and harmful algae blooms.

One wild card in the anoxia and hypoxia forecasts, Jasinski said, is the potential for severe storms or hurricanes during the summer that could alleviate low oxygen conditions by mixing water in the Bay.

Forecasters at Colorado State University anticipate 17 named storms forming in the Atlantic basin between June 1 and Nov. 30. Nine of the 17 storms are predicted to become hurricanes, and of those nine, five are expected to develop into intense or major hurricanes.

The anoxia forecast, launched by a team of scientists from the Bay region in 2005, is intended to educate the public, challenge scientists and help managers plan their activities.

Instead of being shocked by the poor water quality reports that are likely in coming months, the Bay Program hopes the forecast will alert people to the factors that lead to the conditions and spur interest in tracking water quality developments during the summer.

To see the full summer forecast when it is released, and to keep track of field observations made by scientists this summer, visit the Bay Program’s website: www.chesapeakebay.net.