You might think the scientists would have been distressed by their findings, and in a way they were. They had detected one of the largest volumes of oxygen-depleted water ever recorded in the Chesapeake Bay -- more than 900 billion gallons that could not sustain a rockfish, blue crab or any of the bay's other cherished species.

Yet the bad news was, in a sense, good as well. Researchers dedicated to untangling the complexities of the nation's largest estuary had found more or less what they predicted in their first attempt at ecological forecasting.

"We're pretty happy in a macabre sense that our actual forecast is coming true," said Bill Dennison, a professor at the University of Maryland Center for Environmental Science. "We're not happy with the condition of the bay. We're just satisfied that we got it right."

The scientists found last month that 6.7 percent of the bay's main section is anoxic, meaning it contains no more than 0.2 milligrams of dissolved oxygen per liter and, therefore, is lethal to almost all fish and shellfish.

The forecast was issued in early May by researchers working with the Chesapeake Bay Program, a restoration effort that includes federal and state agencies and other partners. They predicted that, in terms of anoxia, this summer would rank among the five worst in the past 20 years and that it would mirror the summer of 1998 for the level of anoxic water.

Indeed, the level detected during a survey from July 11 to 15 was greater than in comparable July surveys in all but three of the past 20 years. The numbers are "definitely not good," said David Jasinski, a data analyst with the Chesapeake Bay Program. "But our prediction basically said this was not going to be a good year for dissolved oxygen."

He and others said the ability to forecast conditions in the bay might one day have applications in guiding restoration efforts. For now, they said, the forecast's accuracy merely demonstrated a central truth in dramatic fashion: Low oxygen levels in the summer are linked to the amount of pollution flowing into the bay in the spring.

But, in an interview, Larry Simns, the president of the Maryland Watermen's Association, questioned the value of forecasts. He said public money should be directed to solving the problem, not studying it further.

"We need someone to do something about it," Simns said. "Everybody knows the source of the problems. No one wants to do anything about it because it costs too much money."

In the 20 years that data have been kept, the percentage of the bay's main stem, which excludes tributaries, deemed anoxic during similar July surveys has ranged from nearly zero to 12.5, in 1993. The amount of low-oxygen water tends to be greater in July. The highest average for a summer in the past two decades was 4.8 percent, also in 1993.

Donald F. Boesch, president of the Center for Environmental Science, said an accurate forecast might one day be useful for those who work on the bay. "It would tell watermen and so on to be prepared for large or not-so-large dead zones, which may affect where they can harvest shellfish," he said.

Regardless of the forecasts' value, Simns and the researchers agree on the causes of the problem: farms, human and animal waste, vehicle emissions, and other sources of pollutants high in nitrogen and phosphorous.

Those nutrients feed enormous amounts of algae, which then die and are consumed by bacteria. The bacteria use large amounts of oxygen, rendering vast tracts of the bay inhospitable to familiar species.

The amount of nutrients flowing into the northern Chesapeake Bay during the spring was the measure researchers used to predict anoxic levels this summer.

Some sources of the nutrients, such as waste treatment plants, tend to remain consistent throughout the year. But each spring, rain flushes nutrients off farms and urban areas and into the Susquehanna River and other tributaries. More rain means more nutrients, more algae, more bacteria and, finally, less oxygen.

Dennison said that in the years ahead forecasters may attempt to apply the predictions more specifically to crabs and fish, possibly including jellyfish.

"We're going to do it and try to get better every time," Dennison said. If that information is not used to improve the estuary, he said, "we're going to document the decline of the bay in ever-increasing detail."