Oxygen levels in the Bay should recover from last year's dismal lows, according to scientists who expect oxygen conditions in the midrange of what's been observed over the last three decades.

The EcoCheck summer forecast calls for about 12 percent, or 6.4 cubic kilometers, of the Chesapeake to suffer from low oxygen, or hypoxia, in July. That would be the 10th smallest hypoxic area in the last 28 years if the projection holds true.

Hypoxic water is water with less than 2 parts per million of oxygen.

But the extreme weather that helped foul water quality last year — especially the huge amount of nutrients flushed into the Bay by Tropical Storm Lee in September — may also contribute to problems this year.

The hypoxia forecast is based on the amount of nitrogen and freshwater flows entering from the Susquehanna from January through May. In most years, nitrogen that enters the Bay in late summer and fall is largely used up or flushed out of the Chesapeake by spring. That may not be true for 2012.

"The substantial load late in the year last year may have a carry-over effect this year," said Don Scavia, a University of Michigan scientist who makes the annual Bay projection in partnership with EcoCheck. "If that happened, our forecast would underestimate the hypoxic volume."

But several scientists said that sorting out the impact of Tropical Storm Lee on this year's water quality will be difficult because the weather has been abnormal since then.

This winter was unusually warm, the first half of spring was unusually dry, and late spring was wet but cool. Algae blooms and fish kills in the Bay occurred early this spring, but it's unclear whether that was caused by unusual weather this year, or last year's extreme events.

"We have been concerned that the storms in late 2011, Hurricane Irene and Tropical Storm Lee, may impact 2012 conditions and we did have some poor dissolved oxygen and associated fish kills this spring," said Bill Dennison, vice president for science applications with the University of Maryland Center for Environmental Science.

EcoCheck is a partnership between the UMCES Integration and Application Network and the National Oceanic and Atmospheric Administration.

The forecast is available on the EcoCheck website: http://ian.umces.edu/ecocheck/forecast/chesapeake-bay/2012/

Different takes on low oxygen

Dissolved oxygen assessments by EcoCheck and by the state-federal Bay Program partnership may appear different because they define low oxygen differently. EcoCheck's forecast model is designed to estimate the amount of water with less than 2 parts per million of oxygen, which is generally defined as hypoxic.

However, parts of the Bay can have more than 2 ppm of oxygen but still not have enough to support species that live in those areas. Open water areas near the surface, for instance, should have about 6 ppm of oxygen to fully support striped bass.

The Bay Program considers any habitat which does not have sufficient oxygen to fully support species that should be found in that habitat as being out of compliance with water quality standards. For instance, an area that should support striped bass but has only 4 ppm of dissolved oxygen would not meet water quality standards.

As a result, low-oxygen areas calculated by the Bay Program would typically be larger than hypoxia estimates from EcoCheck.