Undercurrents: beneath the obvious

November 27, 2006

Mussel fatigue

Filed under: Great Lakes Issues,Invasive Species — nemo @ 3:44 pm

The Toronto Globe and Mail has this promising outlook on Zebra Mussels; even going so far as saying that “… some even say the invader’s arrival hasn’t been ‘such a bad thing'”:

So what, looking back 20 years after it all began, has really happened?

Early estimates of the financial impact were probably on track — the most recent figures put the cost to Great Lakes utilities alone at $200-million to $500-million (U.S.) a year, on top of the damage to a $4.5-billion fishing industry and tourism in the region.

The environmental toll has been much more difficult to assess. For one thing, the zebra is not acting alone. Scientists now believe that it has been largely muscled out by its own cousin, the quagga mussel, Dreissena bugensis, which looks almost identical and arrived from the same part of the Caspian Sea in the early 1990s. There’s no doubt that, together, they have virtually wiped out several species of insect and native mussels, while seriously jeopardizing certain fish populations. They are also, literally, changing the terrain of the Great Lakes.

It’s true that Dreissena mussels clarify water. Like tiny vacuum cleaners, they suck in vast amounts of algae and other particulates, producing feces from the organic matter while binding the non-organic sand and pollutants with mucous to create an unappealing glop called pseudo-feces. Both wind up on the bottom of the lake, meaning that over time the mussels remove tonnes of material from the water.

The visual impact is stunning. “I’m old enough to remember when Lake Erie was dead — remember those headlines? — but it was, in fact, so alive, it was choking itself,” says David Barton, a biology professor at the University of Waterloo. “The water was so green that sometimes in the summer, the wake from boats was green — it was thick.”

Prof. Barton says, “Things really improved when the mussels came along because they’re so good at pulling stuff out of the water column, and incorporating some of it into their own tissues and the rest on the bottom, where it can be eaten by other organisms that are good fish food. So, ecologically, it’s not such a bad thing.”

Tim Johnson, a research scientist with Ontario’s Ministry of Natural Resources, … 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.

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.

It is that kind of loss to biodiversity that may be the greatest consequence of invasive species. A new invader arrives in the Great Lakes every eight months — 173 have been identified so far — and each creates a chain reaction often too complicated to measure, much less control.

“We have a whole new ecosystem now that we have zebra mussels,” Mr. Johnson says flatly. “And while 50 to 100 years of research on the Great Lakes hasn’t exactly gone out the window, we’re pretty close to starting from scratch.”


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