Restoring the Chesapeake Bay's dwindling oyster beds would accelerate efforts to clean the bay's waters, reducing nitrogen and algae problems and fostering the growth of underwater plant life, Maryland researchers said yesterday.

The bay's oyster population, a natural filter for pollution, has been diminished in recent decades by disease and overfishing. If it were revived to 25 times its current size -- about the level found between 1920 and 1970 -- excessive nitrogen levels in the bay would fall by 11 million pounds each year, and underwater grasses would increase by more than 20 percent, said the researchers, who are studying the oyster restoration methods for the state Department of Natural Resources.

"We don't know if it's a 10-year period or 25 years, but if we could find an oyster that could survive the disease, then we'd be moving in the right direction," said Tom O'Connell, a marine fisheries restoration manager at the department.

Reducing disease would not, however, eliminate the need for improving the region's sewage treatment plants and limiting runoff from farms, two key sources of pollution. "Oyster restoration should be viewed as a supplement to bay restoration activities, not as a substitute," O'Connell said.

The research released yesterday was the latest component of an ongoing environmental impact study that focuses on ways to bring more oysters to the bay. Once plentiful, oysters have been killed off by parasitic plagues, pollution and overfishing. Today, their numbers have plunged to an estimated 1 percent of what they were in the 17th century, when the region's oyster harvesting industry began.

During the past decade, some scientists, watermen and politicians have suggested introducing a fast-growing, disease-tolerant Asian variety, Crassostrea ariakensis, into the Chesapeake's waters. But the plan has raised fears that releasing a nonnative species might have unexpected consequences for the bay's fragile ecosystem.

In response, Maryland and Virginia have undertaken an environmental impact study of oyster restoration methods, due to be completed and released for public comment in January.

The section of the study released yesterday did not address merits and drawbacks of native and nonnative species. Rather, it focused on what effect restoration would have on the ecosystem in general.

Oysters filter sediments and eat microscopic algae that cloud the water and kill off underwater plants, the study said. When oysters are plentiful, sunlight reaches greater depths in the bay, nurturing sea grasses and plants, which help improve water quality. Oyster restoration initiatives have set as their goal a 25-fold increase in population. Such growth, the study said, would produce a 21 percent increase in the underwater plant population.

Oysters also help rid the bay of excess nitrogen, which robs the water of oxygen, killing animal and plant life. Maryland is seeking to reduce the nitrogen dumped into the bay by 19 million pounds a year before 2010. Increasing the oyster population would reduce nitrogen by more than half that figure.