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."
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."
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
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.
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.
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.
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