Invader Hydrilla Might Be A Savior

By Tom Pelton
ON THE POTOMAC RIVER--An underwater jungle thrives beneath Nancy Rybicki’s boat, with orange fish and exotic snails living among mounds of green hydrilla and flowering stargrass.

Rybicki, a scientist with the U.S. Geological Survey, plunges in a rake and drags up four species of aquatic plants from the water beside George Washington’s Mount Vernon home.

“Look at all the diversity–it’s good for the fish, good for the birds,” she says, fingering strands as lush as a mermaid’s hair.

More than two decades ago, headlines screamed of dire threats to the Potomac River from hydrilla, a fast-growing Asian plant that began spreading across the United States in the 1980s after being dumped from an aquarium into a Florida river. “Area Governments Unite to Battle Monster Hydrilla,” one 1984 story shouted. “Army to Use Herbicide on Area Hydrilla,” another reported.

The rafts of hydrilla tangled boat propellers and worried elected officials, who saw a hairy green blob creeping across what’s called “the nation’s river.” Local and federal governments tried poisoning the weeds, then attacked them with machines like floating lawn mowers.

But instead of smothering the river’s plants and fish, the invasive species helped the river by stabilizing the bottom, producing oxygen, slowing currents and allowing native plants to grow, said Rybicki, who has been studying the waterway for three decades.

Despite its monstrous appearance, hydrilla has been growing in harmony with other plants and providing food for birds, said Rybicki, a hydrologist and biologist. The surprisingly positive role of the alien species provides a cautionary tale for communities across the U.S. that rush to spray pesticides on invaders instead of first studying them, Rybicki says.

Aliens hardy enough to jump continents are often tough enough to survive in environments too polluted for native fish and plants, she says.

“We should still be cautious about exotic species–but it’s more complex than to just say exotics are all damaging to the environment,” Rybicki said. “We have not seen the exotics displace the native species here on the Potomac River, which is what was feared.”

Canada Geese, Canvasback Ducks, Scaup, Mergansers and other native waterfowl declined on the Potomac River from 1959 to 1982, but then increased significantly starting in the 1980s as hydrilla and other exotic plants spread, according to a study that Rybicki and a partner published in May in the journal of the American Society of Limnology and Oceanography.

Back in the 1970s, the Potomac River, which flows into the Chesapeake Bay, was nearly dead and its bottom bare mud in many places. The river’s rebound was caused by improved sewage filtration at the nearby Blue Plains Waste Treatment Plant, as well as a ban on phosphorus in laundry detergent. The clearer water allowed the growth of hydrilla and other plants, which in turn helped birds and fish.

Despite the improvements, the river remains polluted from Washington’s storm-water drains, as well as from upstream coal mines and farms. A report last week by Potomac Riverkeeper, an environmental group, highlighted continuing problems with fish kills and deformed bass having both male and female sexual organs.

The fish and plants in the Potomac today don’t look anything like they did when George Washington lived in Mount Vernon, atop his stately bluff over the waterway.

During a recent trip to inspect the river, Rybicki idles her 17-foot motor boat into the shadow of the mansion’s white-pillared porch. Grassy hillsides dotted with sheep roll down from the plantation to the riverbank, where geese sit atop a stone wall. A mat of hydrilla and other plants float out into the river, topped by neon-green algae.

“That bright-green appearance scares the public,” says Rybicki. “They look at it, and they say, ‘What is that?’  But it’s not bad. It’s good. We want to increase aquatic vegetation like this, because it’s like a nursery for fish and crabs.”

Her propeller snags as she motors over a dense forest of both exotic and native plants. Barbed wire-like strands of hydrilla, and feathery tufts of watermilfoil (both plants from Asia) grow beside the ribbony leaves of wild celery and yellow blossoms of stargrass (both native).

In a valley between the leafy masses swims an orange koi, about a foot long. This Asian fish is now reproducing in the Potomac, probably because someone dumped them from an aquarium.

Nestled in the hydrilla is an Asian clam, called corbicula, that is also probably helping the river by filtering the water, Rybicki says. Bobbing on the surface is a snail shell, about an inch and a half long, with a swirling brown pattern like that of chocolate soft-serve ice cream. This is another exotic species, an Apple Snail, now common in the river, she notes.

Overhead flaps a Great Blue Heron, and in a tree on the riverbank perches a Bald Eagle. Both are native species whose numbers are growing in a world of aliens.

Rybicki fires up her engine, speeding off across the choppy green waters. Puffy clouds mound up into a blue sky as helicopters thump overhead and traffic roars by on the George Washington Parkway. In a cove called Dyke Marsh beside the highway, a thick blanket of hydrilla makes the Potomac River look a bit like the Sargasso Sea, a section of the Atlantic Ocean notorious as a graveyard for ships.

“It’s not unhealthy for the ecosystem–the plants are making oxygen,” she says.

As the arched spans of the Wilson Bridge rise in the distance, she points out clumps of brownish watermilfoil weeds just below the surface. In smaller water bodies, Asian weeds like these can cause serious problems, because they can stretch from edge to edge, smothering everything. For example, in Texas, the Rio Grande narrows to such a trickle during droughts that hydrilla blankets it, occasionally drowning migrants trying to swim across from Mexico. Texas has joined Virginia, Florida and other states in releasing giant Chinese carp (another exotic species) to devour the weeds.

But the Potomac River is different. It’s a mile wide below Washington, and too broad and powerful to be overwhelmed by the plants, Rybicki said. The hydrilla, a freshwater plant, didn’t spread downstream into the lower Chesapeake Bay, because that’s too salty.

Back on shore at the Belle Haven Marina in Fairfax County, veteran fisherman Charles Bauserman said the once-dreaded hydrilla has proved a boon. As he climbs out of his rowboat near a bed of the weeds, he says he caught at least 15 white perch that morning.

“It helps with all the fish, because they got a place to hide,” said Bauserman, 70. “But those weeds will sure mess up a motor if it gets caught in your propeller.” –Baltimore Sun