The Chesapeake last year gained nearly 6,000 acres of the “underwater meadows” that provide the critical food and habitat for the fish, waterfowl and crabs that depend on the Bay.

In general, scientists credited the rebound with better water quality, which stemmed in large part from lower-than-normal flows into the Bay during 1997.

Overall, grass coverage increased by 5,770 acres — or 9 percent — to 69,238 acres, according to an annual aerial survey conduced for the Bay Program by the Virginia Institute of Marine Science.

That’s a huge increase from the record low of slightly less than 40,000 acres observed in 1984, but still only a fraction of the estimated 600,000 acres of grasses that once covered the bottom of the Bay.

The increases were not evenly divided. Grass acreage increased by 27 percent in the upper Bay, 8 percent in the mid-Bay, and 3 percent in the lower Bay. But in the Tangier Sound area, grasses declined by 14 percent — the fifth straight year of losses in that location.

During those years, water-quality monitoring has shown a worsening trend for sunlight-blocking sediment in Tangier Sound, said Peter Bergstrom, a biologist with the U.S Fish & Wildlife Service’s Chesapeake Bay Field Office and the chair of the Bay Program’s Submerged Aquatic Vegetation Workgroup.

“Most of that could be locally generated from shoreline erosion,” Bergstrom said. “That certainly seems to be part of the problem.”

Elsewhere in the Bay, grass recovery was aided by lower-than-normal flows into the Bay from its major tributaries. Low flows generally result in improved water quality because they carry less sediment and less nutrients.

Excess amounts of the nutrients phosphorus and nitrogen spur algae blooms which, like sediment, block the amount of sunlight reaching the grasses.

The amount of underwater grasses in the Bay declined sharply in recent decades as the amount of nutrients and sediment in the water increased. Grasses have rebounded slowly since 1984, but remain short of the Bay Program’s goal of restoring 114,000 acres by 2005.

Underwater grasses have increasingly been viewed as one of the Bay’s most important habitats. They are important food for waterfowl and provide habitat for juvenile fish, crabs and other species. In addition, they help protect shorelines from erosion and can help filter the water.

Because of the increased importance being placed on grass beds, both Maryland and Virginia have taken action to protect the beds from certain clam-harvesting techniques that can damage grass beds. SAV planting efforts are also under way in both states.