Underwater grasses love it, and so do jellyfish. It's giving plankton a longer shelf life and could be luring porpoises farther north. The drought that's gripping Washington these days is bringing both gain and pain to the Chesapeake Bay and its rivers.

Despite the rainfall this week, most of the region remains under a severe drought, according to the National Weather Service. Conditions actually worsened this month in Western Maryland and the eastern panhandle of West Virginia.

However, the dry conditions bring some good to the Chesapeake Bay and its rivers, for this reason: Rain washes pollution into waterways. The less rain, the less pollution--everything from dirt to motor oil to fertilizer.

In May, river flows into the Chesapeake were the lowest on record for that month, and they have been below average for the past 10 months, according to the U.S. Geological Survey. Rivers usually supply more than half the water to the bay; the Atlantic Ocean supplies the rest.

"We have been seeing nice clear water throughout the bay this spring," said Bill Goldsborough, a senior scientist with the Chesapeake Bay Foundation.

The Potomac River, a tributary of the bay, is being flushed less often by rain, which gives plankton longer to grow before being washed away downstream.

"It's a good thing," said Curtis Dalpra, spokesman for the Interstate Commission on the Potomac River Basin. "Plankton serve as one of the base parts of the food web."

The lack of rain is helping hydrilla and other underwater grasses, the scourge of boaters but an important source of food and living space for fish. Nancy Rybicki, a hydrologist with the U.S. Geological Survey, said three inspections this year along the Potomac found the plants "are doing good. They had a real good spring."

Scientists emphasize, though, that the drought is not good news for all species.

Because there is less fresh water from rain, the tidal Potomac River near Washington is saltier than usual--a less friendly environment for striped bass, herring and shad. That could mean lower counts of young fish this year, Dalpra said.

And some species that thrive when water is saltier are not species that people like very much--jellyfish, for example.

"I've seen two or three this year already," said Ann Manning, who lives on the South River in Anne Arundel County. "Usually, we don't see them before the Fourth of July."

But other bay news has been good. Usually by now, there has been at least one big bloom of algae, fed by spring floods that carry pollution. This year, said the Chesapeake Bay Foundation's Goldsborough, there have been none.

But, Rybicki said, algae seem to be doing well in the Potomac River this year.

That's important because algae thrive in water that has little oxygen, and when algae die and rot, even more waterborne oxygen gets soaked up. (When algae die in the Potomac, drinking water tastes musty.) Oxygen is vital for other species, though. Scientists hope that this year's low-oxygen "anoxic zone" in the bay will be smaller than usual.

"If there's less of a big anoxic zone in the bay, there's a larger area for crabs and fish not to be affected," said Scott Phillips, a Geological Service hydrologist in Baltimore.

The extra oxygen will benefit fish that feed and live on the bottom of the bay--such as spot, croaker, drum, sea trout, flounder and perch, Goldsborough said.

It also could help oysters, which live on the bay floor, but the oyster story is more complicated.

The oyster harvest is rebounding after several years of record lows in the mid-1990s. But the saltier water in the bay this year encourages oyster diseases, especially MSX, which can kill an oyster in a season. The extent of the damage it does, Goldsborough said, should be evident by late summer.

"It really will be a telling year with regard to whatever progress [oysters] will have made," he said.

Another major bay species, blue crabs, thrive in salty water. But Goldsborough said the bay's crab population is so low there may not be enough of them to benefit.

Dry weather sometimes brings more unusual species to local waters. A dead whale washed up this month on Love Point, just north of the Bay Bridge. Scientists determined it had choked on a fish. Goldsborough said it may have been following its food supply, croaker, which is present in greater numbers this year.

Porpoises, another saltwater lover, are being spotted in Maryland's Dorchester County on the Eastern Shore, he said, which is farther north than usual.

Meteorologists say that unless the summer is far wetter than usual, the current conditions are likely to persist. A Weather Service update issued this week said that the evaporation rate from land and water would exceed normal summer rainfall and that the "drought could continue to worsen."