W. Michael Kemp

W. Michael Kemp, a professor at The University of Maryland Center of Environmental Science, examines grasses underneath the surface of the water at the Susquehanna Flats. (Lloyd Fox, Baltimore Sun / August 26, 2014)

There weren't any keepers yet, but the fish were definitely biting for Willie Edwards one day last week as he trolled along the edge of the Susquehanna Flats. The 72-year-old fisherman from North East said he'd caught "a lot of little rock," or striped bass.

The Flats — a vast, grass-covered shoal at the mouth of the Susquehanna River — are a magnet for fish and the anglers who pursue them. But they're also a symbol to scientists of the Chesapeake Bay's resilience, and of its ability to rebound, if given a chance, from decades of pollution and periodic battering by storms.

"This part of the bay is healthy, at least," said Cassie Gurbisz, a graduate research assistant with the University of Maryland Center for Environmental Science, as she stood waist-deep in the water to get a closer look at the grasses carpeting the bottom. "Underwater plants could be considered sentinels of environmental quality," she added, because they can only grow in relatively clear, unpolluted water.

Covering nearly 10 square miles, the grass bed on the Flats is one of the biggest and lushest in the entire bay. The water is calm and clear in the heart of this underwater prairie, revealing tiny yellow water star grass flowers reaching toward the sunlight amid a mane of wild celery, its green, ribbon-like shoots bent over by the tidal current.

"This habitat for hundreds of years provided a place for migratory ducks and waterfowl to stop over and feed," said W. Michael Kemp, ecologist and professor at the UM center's Horn Point laboratory. It draws hunters as well as fishermen, he said, because "it's teeming with life."

Until three years ago, the grass bed was nearly twice as large as it is now. That's when Hurricane Irene and Tropical Storm Lee hit the Mid-Atlantic region within two weeks of each other, flushing 19 million tons of sediment and tens of thousands of tons of pollution down the Susquehanna into the upper bay.

But the fact the Flats still are here at all is remarkable. They were virtually wiped out after an even bigger tropical storm, Agnes, inundated the region in 1972, dumping an estimated 30 million tons of sediment into an already ailing bay. For nearly three decades afterward only scattered clumps of grass remained beyond the river's mouth.

Then, starting in the early 2000s, the Flats began an abrupt resurgence. Kemp said he and other scientists were impressed at the time — and mystified — by what he calls this "magnificent and totally unexpected" development.

Gurbisz and Kemp think they've figured out now how it happened: A modest reduction in pollution, coupled with some favorable weather patterns, sparked the comeback, and then momentum took over. They laid out their hypothesis in a paper published earlier this year in the journal Limnology and Oceanography.

Like plants above ground, underwater grasses need nutrients to grow, but too many can stress and even kill them. The bay has been choking for decades now on an overdose of nutrients and sediment from sewage plants, farm and suburban runoff, and air pollution, resulting in algae blooms, an annual "dead zone" and a decline baywide in grasses.

But starting in the 1990s, the scientists say, there was a small decline in the amount of one nutrient, nitrogen, flowing from the Susquehanna. That alone was not enough to spur a rebound in underwater grasses, but a prolonged dry spell from 1997 to 2002 proved the tipping point, clearing up the water enough to allow more sunlight to reach the bottom and fuel new plant growth.

Nutrient levels coming from the Susquehanna are still excessive, but once the grass bed's revival had begun, the researchers say, it fed upon itself. The grasses slowed the water's flow enough so sediment dropped out of the water, clearing it up. And with more sunlight penetrating the water, more plants could sprout.

The 2011 storms, especially Lee, dealt a severe blow to the Flats, one which scientists at first feared would lead to its demise again.

"There was so much sediment dumped on some of the Flats that it actually buried the grass," said Robert J. Orth, a Virginia Institute of Marine Science biologist who coordinates an annual survey of grasses throughout the bay. The Susquehanna's raging flow also uprooted and washed away plants in the outer portion of the bed.

As a result, the extent of underwater vegetation seen from the Conowingo Dam in the lower Susquehanna to just south of the Flats shrank by more than half, from more than 15,000 acres before the storm to 6,500 acres in 2012, according to Lee Karrh, a biologist with the Maryland Department of Natural Resources.

What survived was the densest part of the bed, Karrh said. Grasses covering at least 70 percent of the bottom were able to withstand the pummeling of mud and flood.

Since then, the Flats appear to be coming back again, albeit slowly. Last year's aerial survey of bay grasses found vegetation covering 7,000 acres of the bottom, an increase of about 8 percent. This year's canvass isn't complete.

Even in its somewhat reduced state, the Flats serve as both lungs and kidneys for the upper bay. The sprawling grass bed breathes fish-sustaining oxygen into the water while filtering out sediment and nutrients.

"You can see a visibly clear plume of water coming out of the south end of the bed," Kemp said.

Nitrate levels are also orders of magnitude lower, Gurbisz added.

The Flats' resurgence a decade ago spurred a similar rebound in numbers of waterfowl and at least some fish frequenting the grass bed, according to state biologists. Though there's been a drop-off since the 2011 storms, Joseph Love, the DNR's tidal bass manager, said it's still a "wonderful area" for largemouth and smallmouth bass to spawn in and forage for food. The area is also a popular site for tournaments, he said.

Likewise, in fall and winter, thousands of ducks, Canada geese and tundra swans flock to the Flats to chow down on the dormant vegetation and the critters hiding in it. DNR waterfowl manager Larry Hindman calls their return "a welcome sight."

"It's an incredibly important part of the economic and social structure of this region of the bay," Kemp said.

The scientists hope their research on the Flats can inform efforts to restore degraded waters throughout the bay and elsewhere. It required an extra boost from nature, they say, but reducing nutrient pollution did help bring back one of the bay's most important grass beds.

"It shows that the bay responds," Kemp said.

tim.wheeler@baltsun.com