Zebra Mussels That Damaged Great Lakes Are Found in Maryland Waters

By Ashley Halsey III
Washington Post Staff Writer
Sunday, May 24, 2009

The discovery of eight shells no bigger than a fingernail in Maryland waters has signaled the arrival of the exotic zebra mussels that have caused an estimated $5 billion in damage to the Great Lakes.

If they spread, the invasive fresh-water mussels could threaten the less-salty waters of the Chesapeake Bay northward from Annapolis.

The zebra mussels found in Maryland apparently were transported on a recreational fishing boat that was plopped from a car trailer into the fresh waters of the Susquehanna River above Conowingo Dam. Whether that handful can get past the Harford County dam and into the Chesapeake may be a multibillion-dollar question.

"If a bit of debris with a zebra mussel on it gets to the dam, it goes through," said Merrie Street, spokeswoman for Conowingo Dam. "There is no filter."

Power generation at the dam, which produces 572 megawatts of electricity for 13 states, including Maryland and Virginia, as well as the District, could be the first to suffer if the mussel population explodes.

"I'm knocking on wood that we don't have a zillion of them by fall," said Jon McKnight, associate director for habitat conservation at the Maryland Department of Natural Resources.

Once zebra mussels become established in a large body of water, they are virtually impossible to eradicate. They are prolific breeders that overwhelm native species, but the more-expensive threat comes when they clog the valves on machinery including outboard motors, as well as nuclear power plants.

"You prevent the spread for as long as you can, and then you just suck it up," McKnight said. "Typically with an invasive species you have an explosion in the population, and then the ecosystem begins to change."

Zebra mussels can survive in water in which the salt ratio is 10 parts per thousand or less. Routine tests taken in Baltimore Harbor this month found salt at six parts per thousand, said Beth McGee, senior water-quality expert at the Chesapeake Bay Foundation.

"I don't think they'll get much farther south than Baltimore or, perhaps, Annapolis, in a normal year," McGee said. "But they certainly could take hold farther up the bay in places like the Bohemia and Sassafras" rivers where the water is fresher.

The effort to stem the spread of the mussels from the lower Susquehanna has consisted of festooning the riverbanks with signs asking boaters to inspect boat bottoms before they launch.

Given that zebra mussels have been compared to rats and roaches -- if you see one, there are many more nearby -- the robust education campaign may be too late to contain a mussel that can lay as many as a million eggs each year.

"If you find one, there's more," Street said. "We are looking at our options for treating and protecting against them, but we haven't settled on anything yet."

It is hard to imagine that such an expensive ruckus could be caused by a tiny bivalve that snuck over here in the 1980s on ships bound from Eastern European ports. The mussels native to Maryland and Virginia do little more than hang around waiting to be eaten.

Most of those dinner-table mussels are from the Mytilidae family, which lives in saltwater, and the zebras are from the fresh-water Dreissenidae clan. The two are no more related than the Capulets and Montagues or the Hatfields and McCoys.

The sharp-edged zebra mussels are an expensive and painful nuisance to boaters and swimmers, covering the undersides of docks, boats and virtually every other man-made object placed underwater. They become a potential major expense to taxpayers and rate payers who ultimately subsidize the cost of clearing them from valves at water treatment plants, electric power plants and all manner of commercial operations that rely on water sources.

Power plants, which often have six-foot-wide intake valves, fight back by using chlorination, heat, electrical shocks and sonic vibrations to keep their pipes mussel-free. On Lake Erie, which is among 100 lakes infested by zebra muscles, the Monroe Power Plant spent more than $500,000 to clear the pipes and $50 million more for repairs and pipe replacement.

If there is an upside to zebra mussels, it is that they filter sediment from the water. When their bivalve distant cousin, the oyster, was populous in the Chesapeake, bay waters were dramatically cleaner. In some areas of Lake Erie, where densely layered colonies contain as many as 70,000 mussels per square yard, water clarity has increased from six inches to 30 feet.

But even the virtue of water made cleaner by zebra mussels ultimately becomes a curse.

"Suddenly you get light getting down deeper where it's not supposed to be," McKnight said, devastating the habitat of fish that feed there. "These mussels have changed the biological content of the Great Lakes."

When zebra mussels mysteriously appeared in a privately owned, fenced-in Prince William County quarry seven years ago, there was some speculation that scuba divers introduced the mussels to create more crystalline water. The state brought in a Great Lakes firm, at a cost of $365,000, that used potassium to kill them off without otherwise damaging the quarry water.

That won't work in waters that flow like the Susquehanna River or the Chesapeake Bay, said Brian Watson, the Virginia biologist who helped supervise the cleanup of the 12-acre quarry.

"There's no inflow or outflow in a quarry," Watson said. "Also, in a much larger body of water, the cost would be prohibitive."

© 2009 The Washington Post Company