But the picture is completely different in Anne Arundel County's rivers: There were dramatic declines in grass beds in the Magothy and Severn rivers, and no grass beds were documented at all in the South, West and Rhode rivers.

The poor grass showings were a black mark on an otherwise cheerful report on the state of underwater grasses in the bay. The annual report was issued Wednesday by the federal-state Chesapeake Bay Program.

"We had improvements in every other area," said Becky Raves Golden, a biologist with the Maryland Department of Natural Resources.

Underwater grasses - technically called "submerged aquatic vegetation," or SAV - are a key part of the Chesapeake's ecosystem.

They serve as refuge for fish and baby blue crabs, and they're a food source for ducks and other waterfowl. The grasses also buffet waves that come crashing into the shorelines.

Grasses suffer when sediment pollution clouds the waters, blocking sunlight and halting photosynthesis. Grasses also are harmed by summertime algae blooms that are fueled by nitrogen and phosphorus pollution.

Researchers from the Virginia Institute of Marine Science measure underwater grasses by flying over the entire bay, taking pictures from the air, and mapping the grass beds they find.

Biologists can't pinpoint exactly what is troubling underwater grasses in Anne Arundel, but the likely culprits are dirty stormwater runoff and pollution from septic systems and the public sewage system.

Erik Michelsen, executive director of the South River Federation, was not surprised to see a zero for the river again this year. He said the South isn't completely devoid of grasses, as springtime usually sees a brief bloom of horned pondweed.

Michelsen said he was struck looking at the statewide data and seeing grass increases on the Eastern Shore and other rural areas, but declines or zeros on the urban and suburban Western Shore.

He said the survey results underscore the need to combat pollution from stormwater runoff. During storms, rain hits the land and picks up dirt and other pollutants that are washed into streams and rivers that flow into the bay.

Michelsen said the dirt makes the water too cloudy and the river bottom too mucky for grasses to survive. He said past attempts to plant grasses in the river haven't done well. "If the water quality isn't good enough, you're not going to get anything to grow," he said.

The West and Rhode rivers face the same problems, said Chris Trumbauer, who patrols those waterways as riverkeeper.

"We're not going to get SAV in the rivers until the water clarity increases," Trumbauer said.

Like the South, the West and Rhode get brief bursts of pondweed in the spring, but it's not much to be proud of.

If pollution can be reduced, then maybe redhead grass, widgeon grass and other species that last longer will be spurred to grow, he said.

"The condition in our rivers are unfortunately not good enough to sustain the more robust grasses," Trumbauer said.

The grass survey found such a decline in the Magothy River - just 12 acres, compared to nearly 90 in 2008 - that the nonprofit Magothy River Association announced earlier this year it would halt its volunteer grass-growing program.

Dozens of volunteers grew grasses in their homes and then planted them in the river, but the grasses inevitably died after a few years.

The best area for underwater grasses is the northernmost section of the bay. The Susquehanna Flats - where the Susquehanna River pours into the bay near Havre de Grace - has lush grass beds featuring a dozen species.