The Chesapeake Bay's underwater grasses decreased 7 percent in 2010, according to a report released Thursday by the Chesapeake Bay Program.

The aerial survey by the Virginia Institute of Marine Science found grasses covered 79,675 acres of the bay and tidal rivers, down from 85,914 acres in 2009.

Despite the decline, scientists said, it is the third-highest baywide acreage estimate since 1984.

The grasses are a measure of bay health because the plants serve as food and habitat, absorb excess nutrients and reduce shoreline erosion, the program said.