Sent out to In-depthangling by John Pitlo – Iowa Department of Natural Resources
LaCrosse Tribune
Published – Sunday, August 03, 2003
Zebra mussels dying off
By BETSY BLOOM of the Tribune staff
Researchers working on the Upper Mississippi River last week finally had something good to say about zebra mussels. “It was nice,” said Teresa Newton, a fishery biologist with the U.S. Geological Survey, “to see them dead.”
In Pool 8, just south of the Interstate 90 bridge near Dresbach, Minn., a diving team checking native mussel beds found sections of the river bottom layered with zebra shells, up to a foot deep at some points. That the zebra mussels had colonized the area so extensively was no surprise. Since it first appeared in the Upper Mississippi River in 1991, this small Eurasian mollusk had become the poster child of harmful invasive species. It reproduced rapidly and had the ability to anchor itself to objects, so it could be carried to other sites upstream or downstream. Within a decade, anything left sitting in the river wound up wearing a dense coat of zebra mussels.
But the Godzilla-like advance of this rogue species appears to have hit a wall. Virtually every shell scooped up from the bottom of Pool 8 last week was empty, the husk of a deceased adult mussel.
Though thrilled with the development, biologists are still trying to figure out what might have triggered the zebra mussels’ mass demise.
Flooding two years ago raised the river’s sediment load and turbidity, throwing more grit and stress at the filter-feeding mussels, said John Thiel, environmental biologist for Dairyland Power Co. Then the river’s water temperature went up. Zebra mussels have a low tolerance for warmer water; power plants had been able to kill them by flushing intake valves with water heated to 103 degrees, Thiel said.
In the fall of 2001, the first mass die-offs of zebra mussels were reported in spots from Lake Pepin, Wis., to Cordova, Ill. Native mussels, adapted to such river fluctuations, seemed unaffected.
“Zebras really are kind of wimpy,” said Heidi Dunn, an aquatic biologist with Ecological Specialists Inc., the St. Louis-area company doing the mussel diving in the La Crosse area last week. The company has seen underwater graveyards of zebra mussels up and down the river, she said.
Parasites, predation, exhausted food supply, old age —all could have been factors. Yet none of the theories seems to fully explain the eradication of all adult mussels in parts of the river.
The zebras don’t seem to be rebounding, either, Thiel said. Checks done at the company’s power plants at Alma and Genoa, Wis., have shown some new mussels, but not the rapid colonization seen before the die-off.
In the Pool 8 survey, almost every live zebra mussel seen was a juvenile produced within the last year, said Newton, who works out of the Upper Midwest Environmental Sciences Center in La Crosse and has studied mussels since 1985. A week of sorting buckets of river bottom in Pool 8, which extends from Dresbach, Minn., to Genoa, turned up only about a dozen live adults, she said. Yet no one is willing to declare the zebra mussels vanquished. The simplest explanation for the die-off might be that the mussels, like other prolific species, have natural population peaks and crashes.
“If the conditions turn right again, you could see the zebra mussels go up again,” Newton predicted.
“I would be really hesitant to say that we can say, ‘Hurrah, this is over,'” said Thiel. “But it has given us a little bit of a breather.”
Betsy Bloom can be reached at [email protected] or (608) 791-8236.
* Caption with photo to read: Jenny Sauer sifts through Mississippi River bottom substrate samples identifying fresh-water mussel species. This sample consists primarily of dead Zebra Mussels. Danielle Gardner photo *
