The amount of grasses in the Chesapeake declined 10 percent last year, reversing a decade-long trend toward increasing amounts of submerged aquatic vegetation, which is considered a key benchmark of the Bay's health.

The Bay Program's annual aerial survey found about 64,000 acres of grass beds last year, down from about 73,000 acres in 1993, according to a recently completed analysis by Bob Orth, a scientist with the Virginia Institute of Marine Science. The survey also found that grass beds in the upper Bay generally expanded while those in the lower Bay shrunk - in some cases dramatically.

Scientists say the decline most likely is linked to the back-to-back freshets that took place in 1993 and 1994 and flushed huge amounts of water - and nutrients - into the Bay.

"The runoff and the timing of this runoff seems to be, as I see it, the critical element that dictates how well different areas do," said Orth, who conducts the annual survey. "I'm going to be very anxious to see what happens in 1995. We had much less runoff so far this year than we had the last two years."

It was only the second time since the annual survey began in 1984 that grass coverage declined from the previous year. The other drop came in 1986, but the numbers began rising again the following year.

Still, grass coverage remains significantly higher than its low point in 1984, when grasses covered only 38,135 acres of the Bay. Some scientists believe that up to 600,000 acres of grasses once filled the Chesapeake.

Submerged aquatic vegetation - or SAV - is one of the most important parts of the Bay ecosystem, and its abundance is closely linked to water quality. SAV provides food for waterfowl and important habitat for many species of fish and shellfish, particularly blue crabs. It also helps filter sediments and pollutants out of the water, while pumping in oxygen that is needed by many aquatic species.

Excess nutrients directly impact grass abundance because they trigger large algae blooms that block sunlight the plants need to survive. Reducing the nutrients phosphorus and nitrogen has been the cornerstone of the Bay cleanup effort. The goal is a 40 percent reduction from 1985 levels by the turn of the century.

While progress has been made in reducing nutrient flows from sewage plants, the largest source of nutrients is runoff from agricultural fields, city streets, suburban lawns and other land uses. When there are heavy spring rains or large snowmelts, as in the past two years, the increased runoff flushes huge amounts of nutrients into the Bay, spurring large algae blooms.

"There was definitely a correlation between declines in the lower Bay in SAV and declines in water quality in 1994," said Peter Bergstrom of the U.S. Fish and Wildlife Service, who chairs the Bay Program's SAV Workgroup. "Usually in the lower Bay the water quality is great for SAV. For SAV, water quality problems tend to be in the tributaries and in Maryland."

But in the last two years, strong springtime flows washed the nutrients - and the algae - downstream. "The algae blooms that usually occur in Maryland were pushed down into Virginia," Bergstrom said.

As a result, while the overall trend was down in 1994, different parts of the Bay reacted differently. Grass beds expanded in the upper Bay, rebounding dramatically in some places, such as the upper Patuxent River, where extensive grass beds had not been seen in years but where SAV had earlier been reported in the numerous small creeks opening into the river.

The Severn, lower Choptank and Chester rivers all experienced large gains last year, as did an area at the head of the Bay known as Susquehanna flats. Not only did the amount increase, the gains were largely of native species - particularly redhead grass - rather than hydrilla, a nonnative species which had spread through the Bay in recent decades.

But grass coverage in many areas of the lower Bay, including portions of the Potomac River, Tangier Sound, the lower Rappahannock River and Barren Island, all declined. Not only did the size of grass beds decline, the beds were also much less dense than in earlier years, Orth said.

Orth said grasses in the lower Bay begin growing earlier in the year, a time when the spring flows were at their peak. Also, grass species that grow in the upper Bay, form large "canopies" near the surface of the water. Once they reach the surface, they are better able to collect sunlight - and are less affected by algae blooms - than species in the lower Bay, Orth said.

Orth cautioned that factors not related to the freshet may have contributed to the lower Bay declines as well. "Following those freshets we also had very warm summers," Orth said. "And that could have been a double whammy for eelgrass [the dominant species in the lower Bay] because eelgrass does not do very well when water temperatures exceed 25 degrees centigrade."

In addition, mute swans and cownose rays may have contributed to the decline as both species dig up grass beds in the hunt for food. Orth said walking through a grass bed after a school of cownose rays had been through was like "walking through a minefield."

"We're still not sure about some of the biological factors," he said. "We're still not sure about mute swans and what role they might be playing in some of these areas; we're not sure what role cownose rays are playing in some of these beds. We certainly know they are very abundant. But their overall impact is still hard to tease out from all of these other things."

The decline could delay when the Bay Program meets its Baywide SAV restoration goal of 114,000 acres. When that goal was set in 1993, it was anticipated that it could be met by 2005 at the rate of recovery taking place before last year.

Still, Orth noted that the rapid expansion of grasses in the upper Chesapeake was good news because it showed that small remnants of the huge grass beds that once blanketed the Bay could rapidly rebound as water quality improves. Many remnant populations remain throughout the Bay, Orth said. "It is just a matter of getting established and persisting and growing and forming dense beds," he said.

In the meantime, Orth said the 1995 aerial survey would probably begin in the first two weeks of May.

"It looks like 1995 water quality may be better," Bergstrom said, noting the low flows into the Bay so far this year. But, he added, "if it was knocked back enough in 94, it may not immediately bounce back in 1995."