Invasive zebra mussels — the bane of water systems and power plants in the Great Lakes area —appear to be proliferating in the lower Susquehanna River and the upper Chesapeake Bay, just a couple years after they were first spotted there.
While not abundant enough yet to cause problems, they're increasing "exponentially," according to Matthew Ashton, a biologist with the Maryland Department of Natural Resources keeping tabs on the alien invasion. Water systems, power plants, boaters, anglers and divers need to step up their vigilance, he said, to try to prevent what he called a "growing threat" that the troublesome mussels could spread to lakes, reservoirs and other rivers in Maryland.
"The numbers are going up and they're going up rather quickly," Ashton said.
An annual check recently found more than 500 zebra mussels clinging to 14 concrete blocks used to anchor navigational buoys in the lower Susquehanna and upper bay around Havre de Grace. That's more than 10 times the number found in the past two years combined, according to the biologist.
A couple of fishermen also reported seeing the distinctive mussels for the first time this fall on gear they pulled from the water near the Susquehanna Flats, a vast meadow of underwater grass where the river flows into the Chesapeake Bay.
Steve Lay, a waterman in Havre de Grace, said he was astonished to find 25 zebra mussels clinging to the inside of a small wooden box he keeps in the water to hold the eels he catches. He said while out fishing for sport one day this fall he even reeled in a mussel snagged on his line.
Not far away, Mike Benjamin, a charter captain and commercial fisherman in North East, said he found 1,500 zebra mussels in clumps on a 3,000-foot line he left in the water through the spring and summer.
"They're just everywhere out here," he said.
The mussels, accidental imports from Eurasia, grow so prolifically that if unchecked they clog intake pipes for water systems and power plants and foul boat motors. According to the Ohio Sea Grant program, water users in the Great Lakes region spend more than $30 million each year on treatments to control the troublesome mussels. The U.S. Fish and Wildlife Service has put their overall impact on human and natural systems in the billions of dollars.
Aside from the cost to water users, zebra mussels also wreak significant ecological havoc, crowding out native mussels and altering the food chain. Zebra mussels have virtually eliminated native bivalves from the Great Lakes and may be affecting native fish populations there. In the lower Susquehanna, Ashton said, there are at least four species of native fresh-water mussels at risk from the recent zebra mussel infestation.
Dreissena polymorpha, as they're known to scientists, are tiny, ranging in size from a little larger than a speck to two inches across. Drawing their common name from their striped shells, the bivalve mollusks inhabit fresh to brackish waters, like those found in the upper Chesapeake.
They weren't seen in the United States until 1988, when they showed up in the Great Lakes.
They spread quickly throughout the Great Lakes, and into the Hudson, Mississippi and Susquehanna rivers, among others. They've also leapt west to Colorado, Utah and California.
Scientists suspect zebra mussels came to North America in the ballast water of ships that sailed from Europe into the Great Lakes. They've been able to spread so rapidly because their larvae, called veligers, can be carried by currents. The adults can attach themselves to boat hulls and survive out of water for at least a short time, so if a vessel is hauled by a trailer, they can leapfrog from one body of water to another.
In 2008, the first two zebra mussels — one of them dead — turned up in Maryland, just upriver of Conowingo Dam, according to Ashton. Two years later, state biologists looking for native mussels and other fish just below the dam found a dozen more intruders. In 2012, about 20 were plucked off buoy anchor blocks in Havre de Grace.
Ashton said live mussels haven't been reported south of the Susquehanna Flats, though a dead shell turned up on a jet ski mooring in the Sassafras River in 2011. The water there was saltier than what the mussels usually can tolerate, the biologist pointed out.
Zebra mussels have turned up "occasionally" at Conowingo Dam since 2008, Exelon Corp. spokesman Robert Judge said, but so far "aren't an issue" with hydroelectric generation there.
"We have no specific monitoring program in place," Judge said.
At Peach Bottom nuclear power plant, on the Susquehanna just north of the Maryland line, Exelon has a contractor make weekly checks during the spring and summer, a spokeswoman said.
Exelon's environmental consultants, Normandeau Associates, have retrieved only a handful of live adult mussels clinging to the plant's intake canals over the years, but the number of tiny larvae found in water samples has mushroomed from 47 in 2009 to nearly 14,000 this year.
Normandeau also checked Harford County's water system this year, and counted more than 1,000 zebra mussel larvae swimming in samples taken inside the water treatment plant, according to Ashton.
Some area water systems haven't been checking that closely.
"We haven't seen any yet, not at the water plant," said Donna Geiger, deputy director of public works for the city of Havre de Grace. When told hundreds were found nearby this fall, she said, "Oh, boy. We may have to take a peek out there."
Zebra mussels were found last year and a few again this fall at Baltimore's water intake on the Susquehanna just upriver of Conowingo Dam, according to Ashton. The city draws from the river only occasionally, when its local reservoirs run dry.
No mussels have been reported in Liberty, Loch Raven or Prettyboy reservoirs, the city's three main sources of raw water, according to Jeffrey Raymond, communications chief for Baltimore's Department of Public Works. The city spent $3.6 million in the early 1990s in a bid to prevent them from clogging Baltimore's water system, which serves 2 million area residents.
The in-takes at the Susquehanna and at the city's three reservoirs were fitted with equipment that could inject potassium permanganate into the water, Raymond said. Previously used to prevent corrosion in water system, the chemical also has proved effective at preventing mussels from attaching to pipes.
In addition, the city has restricted boating on the reservoirs to keep out vessels that might have been in mussel-infested waters. Anglers fishing the reservoirs may use only live bait that has been certified zebra-mussel-free.
Options for removing mussel infestations are limited, which is why experts counsel prevention.
Chlorine and some other toxic chemicals may be used on pumps and other equipment that might have gotten mussels inside, but experts caution that care should be taken not to poison other aquatic organisms.
Boaters, canoeists, jet skiers, anglers and divers are urged to take precautions against unwittingly transporting zebra mussels — draining vessels and gear that's been in the water, flushing inside and out with hot water and then drying everything for five days. That's generally long enough to kill off any tiny mussels that might have been missed.
A new water treatment has been discovered and approved: A dead soil bacteria that appears to be both safe and effective in killing zebra mussels. It's been tested successfully on a lake in Minnesota, and holds promise for killing or controlling the invaders in such settings. But experts say it's unlikely to work in open, flowing waters such as the Susquehanna or the bay.
"They're here to stay," said Amy Benson, a fishery biologist with the U.S. Geological Survey.
Meanwhile, she said, zebra mussels might not be the last word in aquatic invaders. The bigger quagga mussel, also an import, appears to be replacing zebra mussels in the Great Lakes. And if anything, she said, they look to be more troublesome, in part because they favor warmer waters where they can grow year-round.
Quagga mussels have not turned up in Maryland.