An unprecedented combination of weather conditions last year led to a severe lack of oxygen at the bottom of the Chesapeake Bay that caused the estuary's worst oyster harvest, according to a team of biologists from Johns Hopkins University.

The biologists called last year's oxygen depletion "catastrophic," noting that the layer of water without oxygen, which normally hugs the deepest channel of the bay, had expanded upward into shallower waters, penetrating major tributaries such as the Potomac, Patuxent and Choptank rivers.

The oxygen-depleted zone, which normally begins about 33 feet below the surface, had expanded upward to water that was only half that depth by last August. In the areas sampled in August, all the oysters that had been living more than 20 feet below the surface died.

The scientists also say they are learning what has been causing the decline over several decades in oyster harvests in the nation's largest estuary, where they were about 10 times as plentiful about a decade ago.

Christopher Bonzek, a Maryland Natural Resources Department official, said seafood packing houses reported buying 980,425 bushels of oysters by last Saturday, the end of the bay oyster season.

The new findings, published in the journal Science last week, were produced by Johns Hopkins biologists Howard H. Seliger, Jeff A. Boggs and William H. Biggley.

"Conditions were so bad last August we thought the oyster mortality was going to be even worse than it was," Seliger said. "We were saved by an early cold snap in September."

The cold weather, which lasted nine days, cooled surface waters to temperatures lower than those of deeper water. As a result, the denser cool water sank, carrying its oxygen down, as the warmer, oxygen-depleted water rose.

Had this so-called turnover come later in the fall, as it normally does, the oxygen-depleted zone would have continued to enlarge for several more weeks, spreading into still shallower waters and killing even more oysters and other bottom dwellers such as clams and crabs, the researchers speculated.

The first factor that led to last year's severe conditions was an unusually large volume of late winter water from the melting of ice and snow flowing into the bay, Seliger said. This fresh water, being less dense than the salty bay water, runs over the top of the bay, carrying its rich load of oxygen out to sea.

The faster the meltwater flows, the less mixing there is of the layers. As living things below the boundary between the layers use up oxygen, it is not replenished.

Last year, the winter runoff was followed by unusually heavy spring and summer streamflows, which maintained the barrier.

Yet another factor was an offshore weather system that drove more deep, low-oxygen seawater upstream along the bottom of the Chesapeake. As a result, the lower layer of water swelled, pushing the boundary upward and into shallower regions of the bay.

Also noted by the researchers was the relative absence of summer storms in which winds can churn the waters enough to mix the two layers.

One long-term cause of the bay's deterioration, Seliger said, appears to be a steady accumulation of organic matter on its bottom, delivered as soil erosion washes it into streams that feed the bay. As the organic matter decomposes, it uses up oxygen.

Another long-term cause, Seliger said, is the related problem of silt deposits. As these form, they alter the bottom's contours in a way that directs the flow of water so as to minimize mixing between the layers.

Seliger said he had planned to monitor bay conditions again this year, but that his funding request to the federal government's Sea Grant program was turned down.

"I'm disappointed," he said. "I think we were finally finding out what's causing the problems in the bay."