Zebras, quaggas muscling their way into Chesapeake watershed
Zebra mussels, the ecosystem-altering invader blamed for billions of dollars of damage in the Great Lakes, is inching closer to the Chesapeake Bay.
Scientists this summer found the zebra mussel at a second lake within the New York portion of the Susquehanna watershed. Meanwhile, a close relative, the quagga mussel, turned up in a gravel pit just a few miles outside the watershed in central Pennsylvania.
In response, Bay officials are scrambling to assemble a task force that will try to come up with ways to stem the spread of the mollusks.
“Zebra mussels have a pretty serious track record,” said Frank Dawson, of the Maryland Department of Natural Resources, and chair of the Bay Program’s Living Resources Subcommittee. “The idea is to try to delay, hopefully for a long time, both the ecological and economic impacts.”
Scientists have long feared that the foreign species, first discovered in the Great Lakes in 1988, could wreak havoc if it were to become established in the Bay watershed.
While scientists say high salinities would make most of the Bay off-limits for the freshwater species, it may find suitable habitat in many rivers, potentially altering food webs and increasing nutrient flows down the tributaries.
Zebra mussels quickly spread from the Great Lakes to many watersheds near the Chesapeake — including the Ohio, Tennessee and Hudson — but the Bay watershed avoided any introductions for more than a decade.
That changed last year when scientists discovered adults living in, and just below, the Eaton Brook Reservoir at the headwaters of the Chenango River, a New York tributary of the Susquehanna. This year, they found veligers — zebra mussel larvae — throughout Eaton Brook between the reservoir and the Chenango, but adults were still confined to the area near the dam.
Scientists this year also found the mussels in the 2,000-acre Canadarago Lake, which is near the headwaters of the Susquehanna in New York’s Otsego County.
Tom Horvath, a scientist with the State University of New York’s Oneonta Biological Field Station, said the Canadarago population consisted of a small number of adults scattered in several areas of the lake. Veligers were detected in water monitoring, indicating the adults were reproducing.
“The population is probably big enough to start filling the water column up,” Horvath said.
Canadarago is connected to the Susquehanna through a small stream, Oaks Creek. “If they can make it through Oaks Creek, then they pretty much have a direct route into the Susquehanna,” Horvath said.
Of equal or greater concern, said Dawson, is a third population that turned up in the Richland Quarry, just a few miles outside the Susquehanna watershed in Pennsylvania’s Lebanon County.
“It’s geographically a lot closer to the Bay proper even though it is not in the drainage,” Dawson said. “The opportunity for it to be accidentally transported from there is probably great.”
The quarry population was discovered by a professional diver who found a fist-sized rock encrusted with a number of adult mussels. The diver removed the rock, but it is not known if the mussels reproduced. Officials believe the rock was transported to the quarry, a popular diving location, to help clear the water.
Dawson said he hopes to create a task force of state officials, biologists, watershed groups and others interested in controlling the mussel in hopes of finding ways to halt, or at least slow, its spread.
That, in turn, may buy time for new zebra mussel control methods to be developed. Researchers at the New York State Museum, for instance, recently reported that a common soil bacteria contains a toxin that can kill zebra mussels when it is consumed. Preliminary results suggest it will not harm fish and native mussels.
“We really want to prevent them from spreading around as much as possible at this point,” Horvath said. “If the mussels get into a lake or reservoir later, maybe we will have the technology to control them at that time.”
Earlier this year, the Bay Program identified the zebra mussels as one of six nonnative species that pose the greatest potential threat to the Bay ecosystem. [See “Invaders cropping up in the watershed,” Bay Journal March 2001.] Nonetheless, many consider its spread inevitable, and are reluctant to commit significant funds to control its movement.
This spring, Dawson wrote New York officials on behalf of the Bay Program expressing “grave concern” about the presence of zebra mussels in Eaton Brook Reservoir and urged “aggressive management” to eradicate the foreign species. New York officials had little enthusiasm.
“Despite more than 10 years’ experience with zebra mussels, I am aware of no practical method of eradicating established zebra mussel populations,” Gerald Barnhart, director of the New York Department of Environmental Conservation’s Division of Fish, Wildlife & Marine Resources wrote in reply.
“I am also unaware of any practical method of preventing the downstream distribution of zebra mussel veligers through connected waterways,” he wrote.
When the letter was written, some thought the Eaton Brook population might be eliminated, perhaps by partially draining the reservoir. The discovery of zebra mussels at Canadarago, a large, natural lake, eliminates potential eradication options such as draining. “It’s there to stay,” Horvath said.
He said populations were likely established in both Eaton Brook and Canadarago through recreational boating. Zebra mussels are easily transported from place to place by boaters and other water users because they can attach themselves to surfaces and live for several days out of water. Live zebra mussels have been reported in California attached to boats trailered from the Great Lakes.
Horvath and Dawson said stepped-up education efforts aimed at boaters could help confine the mussels, although Barnhart, in his letter, said intensive boat cleaning and inspection programs have shown that they only delayed colonization of nearby lakes by a year or two.
Zebra mussels, named for the striped pattern of their shells, are thought to have arrived in the Great Lakes in 1988 in the ballast water holds of ships from Europe.
An individual zebra mussel can filter up to a gallon of water per day while feeding on algae. That has had dramatic impacts on water clarity — visibility in Lake Erie increased from 6 feet to 30 feet after the mussels invaded.
That, however, has reduced the food supply for some fish, causing their populations to fall. Competition with the zebra mussels has nearly wiped out native mussels in some places.
Layers of the mussels have grown several inches thick on substrates in lakes Erie and Ontario. They have been a major nuisance for industrial and municipal water uses as their dense colonies can block water intake pipes. According to the U.S. Fish and Wildlife Service, zebra mussels are expected to cause about $5 billion in economic impacts to just the Great Lakes region over the next decade.
While zebra mussels thrive in lakes, they often fare poorly in fast-flowing stream and small rivers. Yet they have done well in some larger river systems.
Zebra mussels were discovered in the Hudson River in 1991, and their populations expanded rapidly, dramatically altering the river’s ecology. They increased the filtering of river water from 3 percent to 100 percent a day. As a result, the phytoplankton biomass decreased 80–90 percent. Dissolved oxygen levels declined as respiration by the mussels consumed oxygen faster than it could be replaced. Water clarity increased sharply.
In the Hudson River, soluble reactive phosphorus in the water nearly doubled because of the lack of phytoplankton caused by zebra mussels. Nitrate levels also increased, though not as dramatically.
“Pretty much everything we measure about the river has changed,” said David Strayer, a freshwater ecologist with the Institute of Ecosystem Studies who has worked with a team of scientists examining zebra mussel impacts on the river. But, he cautioned, “you have to be careful extrapolating from one system to the next.”
Right now, the zebra mussel’s toeholds in New York’s part of the watershed are more than 400 miles from the Chesapeake, and Strayer expressed doubt that they would easily move down the Susquehanna.
“A lot of the Susquehanna won’t be suitable,” he said. “I think it will take human intervention, likely, to move them from those very headwater infestations farther down river.”
If they eventually get farther downstream, Strayer said the zebra mussels may find pockets of suitable habitat, but it is impossible to predict what impact they will have.
“My question with the Chesapeake is, are you going to get enough zebra mussels to matter, and where are they going to occur?” Strayer said. “If you end up having a few, they will be a nuisance to water users and things but, ecologically, they won’t do anything. If you have gadzillians of them, depending on where they are, you could have some pretty big effects.”
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