The underwater grasses that provide vital shelter to Chesapeake Bay creatures are rebounding in an important blue crab nursery, but they declined sharply in the middle bay last year, according to a new survey.
In Tangier Sound, where juvenile blue crabs rely on the grasses for concealment from predators, the bay's largest underwater meadows expanded by nearly 3,000 acres last year.
It was the second year in a row that the grasses grew back, showing signs of recovery after mysterious, enormous losses in 1998. In Maryland's portion of the sound, the grasses now cover nearly 10,000 acres.
But bay grasses were wiped out in Eastern Bay and other parts of the middle bay, according to an annual aerial survey conducted by bay grass expert Robert J. Orth of the Virginia Institute of Marine Science.
In Eastern Bay, lush beds that covered nearly 5,000 acres in 1999 disappeared completely last year.
"It's unbelievable," said biologist Mike Naylor, chairman of the Chesapeake Bay Program's task force on underwater grasses. "We are concerned any time we see losses that significant."
On the Patapsco and South rivers, small, struggling patches of underwater grasses also died off completely. There were sharp declines in the Chester, Severn and other middle bay rivers.
When the changes were averaged together, the grasses expanded by about 1 percent overall, covering 69,126 acres. That's more than halfway to the multistate bay restoration goal of 113,000 acres baywide, but it's "far, far below" the historic extent of bay grasses, Naylor said. "We're still well below meeting our goal in the vast majority" of the bay's regions, he said.
Scientists blamed a persistent "mahogany tide" algae bloom that afflicted the middle bay last year for the grass losses in those areas.
Naylor, a biologist for the Maryland Department of Natural Resources, said the grass die-offs occurred in exactly the same places where DNR measured worsening water quality caused by the algae blooms last year.
The bay grasses need light to photosynthesize. They can be killed when algae blooms cloud the water, blocking the light.
Naylor said the grasses that grow in the low salinity levels of Eastern Bay are among the "most ephemeral," disappearing and re-growing quickly.
"We have every reason to believe that if we get a good year of water quality in that middle-bay region, the grasses will rebound swiftly," he said.
The grasses increased by about 20 percent in the Potomac River last year, and officials credited improvements at Washington's Blue Plains sewage treatment plant.
The spread of the grasses downriver from the capital is "a perfect example of the system responding" to efforts to reduce nutrient pollution, said Peter Marx of the Environmental Protection Agency-led bay program.