The Chesapeake Bay Foundation said yesterday that new data on water quality in Virginia's major tidal rivers confirm a stark but familiar story: Oxygen levels are so depleted that many fish, crabs and oysters cannot survive.

Based on samples collected in early June by the state Department of Environmental Quality, low levels of oxygen were reported along the entire 40-mile length of the York River and in parts of the nearby Rappahannock and Pamunkey rivers in central Virginia.

John Kennedy, bay program manager for the agency, said he did not believe that the data cited by the foundation were cause for alarm or indicative of the situation throughout the year. Instead, he said the findings are "cause for us to continue what we've been doing for decades" to improve water quality.

Oxygen depletion is linked to nutrient pollution from sewage treatment plants, farm runoff, septic system runoff and other sources.

Chuck Epes, the foundation spokesman in Virginia, said the data indicate a "general pattern" of low oxygen in many Virginia waters. "The 'so-what' to all that is that we have a very sick, stressed system that is very out of whack," Epes said.

Environmentalists and scientists consider the health of the rivers in the Chesapeake watershed as one indicator of the bay's health. The state samples water in the bay's Virginia tributaries throughout the year. The foundation examined the results from early June and raised an alarm, citing the three rivers as examples.

The foundation has documented large algae blooms -- fed by nitrogen and phosphorus runoff -- that deplete oxygen needed by aquatic life. The blooms block sunlight, interfering with the growth of underwater plants that provide oxygen, and as algae die and decay, they reduce the amount of oxygen in the water.

Doug Jenkins, a waterman who works along the Potomac and Rappahannock rivers, said he started noticing water with a brown hue -- a sign of algae blooms -- about three weeks ago, earlier in the summer than usual. As the algae blooms spread, it becomes hard for watermen to avoid them.

"We're fighting a losing battle," Jenkins said.

Some scientists caution against reading too much into data on nutrient pollution and water quality. They say that oxygen levels in water can change frequently, depending on the time and place samples are taken, and that low oxygen levels are by no means new.

Richard Batiuk, lead scientist on the Environmental Protection Agency's Chesapeake Bay Program, said that he would "hate to say this is ho-hum and normal" but that low oxygen levels are a current reality of the Chesapeake and its watershed. He suggested that the data represent another reminder of the seriousness of the problem.

He also said last year's heavy rains, which dumped excessive soil nutrients and sediment into the bay and its tributaries, had a lingering effect and are partly to blame for the low oxygen levels being reported this summer.

Epes, of the Chesapeake Bay Foundation, said the chief culprits in Virginia are sewage treatment plants. Throughout the bay, he said, sewage plants are second only to agricultural runoff as a source of nitrogen pollution.

The foundation has been urging Virginia to require the Department of Environmental Quality to further limit the amount of nutrients that sewage treatment plants are permitted to discharge.

Batiuk said farmers have to pitch in, too, and take more care in using fertilizers and other pollutants. "It's going to take a lot of money, a lot of time and effort," he said.

The Chesapeake Bay Commission, the legislative arm of the EPA's Chesapeake Bay Program, has said the cost of restoring bay water quality is $11.5 billion for Maryland, Virginia and Pennsylvania. The District, Delaware, West Virginia and New York also are bay watershed jurisdictions.

Staff researcher Bob Lyford contributed to this report.

Watermen who work the Chesapeake Bay and its tributaries try to avoid algae blooms, which have been linked to depleted oxygen levels and reduced catches.