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The average life span of a zebra mussel is four to five years. In Lake Erie it begins to spawn in its second year, when the water temperature reaches 59� F. According to Ronald Griffiths, a biologist for the Ontario Ministry of the Environment, by the time a mussel is four years old, it will produce a million eggs a year. The fertilized eggs develop into larvae known as veligers, which are too small to be seen with the naked eye. Veligers are carried about by currents for eight to 14 days before they settle and form shells. To build those shells, the veligers require water with at least 12 milligrams of calcium per liter, which means that they are unlikely to colonize North American lakes and rivers that have become acidified—small comfort, considering the adverse effects of acid rain. In addition, water temperatures above 90� are lethal to the mussels.
Apart from that, the zebra mussel is an extraordinarily robust creature. In Schloesser's lab, colonies of them remained alive after being starved for 11 months. They can live out of water for a considerable period of time—as much as two weeks under humid conditions. It is such hardiness that virtually guarantees that the zebra mussel will eventually infiltrate most freshwater systems in the U.S. and Canada. Simply by attaching itself to the hull of a trailerable boat, a mussel can make the leap from one landlocked body of water to another.
In June 1988, while searching for worms in Lake St. Clair, part of the water system that links Lake Huron and Lake Erie, a research team associated with the University of Windsor (Ont.) discovered the zebra mussel in North America. The team members, Sonya Gutschi, Bernie Muncaster and Ronald Allison, brought up a stone that, Gutschi says, "looked like it had a wart on it." Muncaster took the stone to his former advisor, Dr. Gerry Mackie, a zoologist at the University of Guelph near Toronto, whose specialty is freshwater bivalves. Mackie identified the "wart" as a zebra mussel. Judging by the quantity and the age of the mussels that others soon found, scientists now believe that in late 1985 or early '86 a ship from Europe discharged ballast water containing veligers into Lake St. Clair.
During the summer of 1988, zebra mussels migrated south through the Detroit River into Lake Erie. Their presence has now also been confirmed in Lake Ontario, Lake Huron, Lake Michigan and Lake Superior. In March '89, Schloesser, Tom Nalepa, a biologist with the National Oceanic and Atmospheric Administration Great Lakes Laboratory in Ann Arbor, and biologist William Kovalak of Detroit Edison met with nine Canadian scientists in London, Ont., to discuss the zebra mussel invasion. The group came to call itself the Dirty Dozen. Schloesser says, "We thought there was a problem, but few people knew about it."
Three months later, in Ann Arbor, after the Dirty Dozen had spread the word of the impending threat, the U.S. Fish and Wildlife Service and Detroit Edison co-sponsored the first North American conference on the environmental threat posed by the zebra mussel. It did not attract much attention. "It was difficult to get people to come," says Schloesser. "We finally got 80 people, mainly scientists and some water-treatment people, to show up. Then the Chicken Little story came true. The sky fell on Monroe, Michigan."
Monroe is situated on Lake Erie, 30 miles south of Detroit. Its previous claim to fame was being the hometown of General George Armstrong Custer, but now it had its own showdown—one that occurred at the town's water-treatment plant and Detroit Edison's nearby coal-fired power plant. The attackers were millions of zebra mussels. In February 1989, Kovalak and Tim Walsh, the environmental specialist at the power plant, had counted 500 to 1,000 mussels per square meter on the walls of the plant's intake canal. By July, after the spawning season, there were more than 700,000 mussels per square meter, a 700-fold increase in six months.
Mussels plugged 75% to 85% of the trash-collector bars, the coarse strainers that serve as the initial filtering system in the power plant's intake canal. Water essential for the plant's operation could enter only through the top couple of feet of a 20-foot deep canal. Any further blockage might shut down Monroe's 3,000-megawatt power plant, the biggest in Detroit Edison's system.
By hydroblasting with a high-pressure water gun for two weeks, workers were able to remove the deposit of mussels in the canal. However, further inspection this year revealed that the mussels had heavily infested the interior of the plant. The plant's steam condensers were also partially blocked, and a service-water system was affected. The infestation threatened the water supply that the power plant needed for cooling, fire protection and coal-dust suppression.
These mussels would have to be removed as well. This summer, Detroit Edison sought permission from the Michigan Department of Natural Resources to use chlorine, a substance toxic to many forms of aquatic life, to kill the mussels in the plant. Permission was granted and the first chlorine treatment was done in September. But even dead mussels present a problem. The putrefying remains are now being chipped and scraped out of the system by hand.
Detroit Edison spends $500,000 a year on mussel monitoring and control at its Monroe facility. "Two years ago we weren't spending anything," says Rick Smithee, a senior engineering technician, "and the unknown is that these things are still peaking."