As Hurricane Floyd swept through the Chesapeake Bay, it washed as much pollution-laden sediment into creeks and rivers as normally lands over the course of a year, burying fragile oyster beds and threatening seagrasses vital for young fish and crabs, scientists say.

But scientists cautioned that the Chesapeake, a vibrant system governed by crosscurrents of natural and human forces, rarely allows for tidy calculations of harm and benefit. The storm washed pollution off the land--much of it from chicken farms that dominate the Eastern Shore--but it also flushed algae out to sea, extinguishing a cycle of fish kills wrought by oxygen shortages, and probably ending the season's threat of outbreaks of the toxic microbe Pfiesteria piscicida. The fish-killing microbe feasts on algae.

Some oysters may suffocate under layers of silt, something authorities will be watching for when they begin surveying reefs later this month in preparation for October's commercial harvest. But some oysters may have taken relief from the rains, which lowered the high salinity that encourages oyster diseases.

"So many of these organisms are affected by so many different factors throughout the year," said Robert Magnien, a water quality expert with the Maryland Department of Natural Resources. "You really have to wait until all is said and done to tally up the effects."

So intense was the storm, so abundant its rains, that Maryland scientists on Friday recorded the second-highest flow ever on the Choptank River, a major bay tributary on the state's Eastern Shore.

Normally, 34 cubic feet of water slips down the Choptank every second. But a day after some nine inches of rain fell, the Choptank flowed at 6,420 cubic feet a second, according to the state's Department of Natural Resources. In short, 189 times as much water as usual.

Donald Boesch, president of the University of Maryland's Center for Environmental Science, spent Thursday driving across the Eastern Shore.

"Let me tell you, I've never seen as much water running off the land," he said. "All of these creeks were absolutely full to the banks with turbid water. You could just see it, how much sediment was being washed off the land."

Of most immediate concern is the impact on seagrasses. Nurseries for fish and crabs, the grasses also nurture food for waterfowl.

Big storms have devastated seagrasses before. One, in August 1933, buried grasses in sediment, at a time when many already were thinning from disease.

"In seaside lagoons, where eelgrass was king, in 1934 there wasn't a speck of grass," said Robert J. Orth, a professor at the Virginia Institute of Marine Science, and a leading expert on seagrasses. "It was gone."

In 1972, Hurricane Agnes rumbled up the Atlantic and buried seagrasses in drifts of sediment. Cloudy water lingered for months. "There were huge losses of seagrasses that we've never really recovered from," Boesch said.

Floyd arrived at a time of mounting concern over seagrasses. Although grass beds have grown back nicely in some areas of the Chesapeake, particularly in the lower bay where high salinity favors them, grasses in Tangier Sound--prime crabbing grounds for watermen--have declined six years in a row. In May, the U.S. Environmental Protection Agency Bay Program announced the sound's grasses have receded 63 percent since 1992.

Whether Floyd significantly hurt seagrasses can't be known until next spring, the start of another season of growth, Orth said. Although it's reasonable to assume that some plants were washed away by wind-driven waves that hammered shallow areas, it's also possible that the storm scattered seeds to places where the grasses have yet to recover, he added.

But Floyd clearly exacerbated the underlying trend that attacks seagrasses, oysters, crabs and most sea life: It dumped more nutrient pollutants in the water--excess nitrogen and phosphorus--thereby encouraging more algae which deny seagrasses the light they need to grow. Much food for algae washes down the Nanticoke and Pocomoke rivers from chicken farms in Maryland and Delaware, according to EPA data.

Many farmers spread chicken manure on their fields as fertilizer. But in key areas of the Eastern Shore, poultry production has exceeded the land's ability to absorb its waste. Soils are saturated with nutrients. When rains come, and as soils erode, some of the excess washes into the water.

Big storms are particularly effective polluters. While some of the effects are diluted by increased flows of water, the dominant factor is the extra pollution that a major storm washes in.

Data won't be available for at least several more days, but Magnien surmised that the runoff wrought by Floyd was particularly rich in nutrient pollution, because of the summer drought that preceded it. The absence of rain stymied the growth of crops that normally digest nutrients in the soil, leaving excesses in the fields, he said. In one great pulse, Floyd likely deposited the extra in the water.

"What was on the land is now in the bay," said Michael F. Hirshfield, a water quality expert and vice president of the Chesapeake Bay Foundation, an advocacy group. "We're really lucky this didn't happen in June."