This may seem tantamount to blasphemy, but Prof. Barton says most organisms driven from the lakes survive in streams, and some day the mussels' time will be up too. He hopes that when that happens, the "lost species" will reclaim their old territory.
Tim Johnson, a research scientist with Ontario's Ministry of Natural Resources, says news of the Dreissenas' "cleansing" property triggered the entrepreneurial spirit in some quarters. He shakes his head as he recalls the day a businessman proposed using the mussels to treat toxic waste. "He planned to fill a rail car with them. He saw it as a semi-portable thing." Sadly, mussels require much time to filter thoroughly, and it was never made clear where their contaminated bodies would wind up.
Mr. Johnson's job is to monitor the health of the Great Lakes, particularly as it affects fish. Formerly based on Lake Erie (whose Canadian commercial fishery is worth roughly $30-million a year), he recently moved to a research station on Lake Ontario's Bay of Quinte. The shelves of his office are stacked with studies on the interaction of native and invasive species while living examples of both can be found in the waters lapping at the jetty just outside his window. In 20 years of study, he says glumly, nobody has come up with a single productive use for Dreissena mussels.
He also believes that the mussels' cleansing effect creates more problems than it solves. Clear water not only drives many fish to greater depths, it promotes the growth of microcystis, a blue-green algae that releases microcystin, a toxin harmful to many creatures, including humans. Sadly, it's one of the few algae unpalatable to Dreissena mussels, and he believes increased levels of microcystin may be partly responsible for Lake Erie's famed "dead zone" -- a lifeless patch of 500 to 1,000 square kilometres primarily attributed to chemical pollutants.
That said, "the single biggest effect of zebra mussels has been their alteration of the physical habitat," Mr. Johnson says. Twenty years ago, scientists knew the mussel preferred a hard bottom to a soft one -- but they didn't count on its ability to grow on its own kind. It is common to find three or four generations bound together in cantaloupe-sized clusters, or "drusses" anchored to something as small as a popsicle stick in the mud.
"Suddenly the earlier maps of where the mussel could live were grossly underestimated," he says, estimating there could be as many as 52 trillion Dreissena mussels -- that's more than 15,000 railcars full -- in the Great Lakes today. Densities as high as 600,000 per square metre have been found.
As the layers of mussels grew thicker, native species of insects, fish, and mollusks -- many of which live or spawn in the shallows -- began to suffer. Among the first casualties were the big clams Prof. Hebert's students were sampling for pollutants back in 1988. Within two years of his study, he says, they had been all but wiped out. "The Lake St. Clair bottom went from being the rich mussel bed it had been for many thousands of years to being stripped. The most charismatic of the big mollusks in the Great Lakes were gone, because of this single invading species."
Charismatic? "I will admit," Mr. Hebert says sheepishly, "there aren't many people who wake up in the night thinking about clams." But the native species could live for centuries (versus five years for the mussels), and they had beautiful iridescent shells once harvested for buttons."They were some of the oldest organisms in the Great Lakes -- certainly the oldest organism on the bottom -- and they were destroyed in one fell swoop."
And then there was the impact on the fish.
Roxy Lancaster is 61 and among the fifth generation of his family to fish the waters of Lake Ontario off Point Traverse in Prince Edward County. (His great-great-grandfather is buried under a black rock near the spot where Mr. Lancaster keeps his boat.)
Like Prof. Hebert, he remembers the thick green algae of the mid-sixties, and the effect it had on his primary catch, the whitefish. "They didn't spawn for five years."And he has seen the impact of the mussels first-hand. "The whitefish are now eating small zebra mussels, which is remarkable," he says. "Our food processor in town says when he splits them open, their gut is full of zebra mussel shells."
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