August 3, 2008
Underwater, a Disturbing New World
OFF ATWATER BEACH, Wis. _ This place should be an underwater desert.
But as the trio of researchers wearing scuba tanks and lead weights drops through the water, the landscape of rounded stones 30 feet below is disturbingly full of strange, new life.
In just a few years, the gravel and white boulders that for centuries covered the bottom of Lake Michigan between Chicago and the Door County peninsula have disappeared under a carpet of mussels and primitive plant life.
The change is not merely cosmetic. In the last three years or so, scientists say, invasive species have upended the ecology of the lake, shifting the distribution of species and starving familiar fishes of their usual food supply.
Signs of the shift have been hard to ignore. Mats of dead, smelly algae wash ashore on Lake Michigan from Chicago to the Straits of Mackinac, castoffs of a vast underwater expanse seen from boat decks and from hilltops at Sleeping Bear Dunes in Michigan. Fishermen haul it up in their nets, dubbing it "lake moss."
Multiple strains of E. coli bacteria and botulism spores thrive in the new underwater garden, leading scientists to suspect it is contributing to beach closings and the widespread deaths of migratory birds. Meanwhile, fishermen notice the lake trout, salmon and whitefish are getting skinnier each season.
The rapid shift has researchers scrambling to understand what is happening and how widely the impact will be felt.
"The lake is changing faster than we can study it," said University of Wisconsin-Milwaukee researcher Harvey Bootsma, whose small team of researchers hunts explanations from this new lake bottom in weekly dives off the Wisconsin shore.
Some ecologists and fishery managers say the Great Lakes may adapt, noting that some fish seem to be eating the most common invasive species. But experts also say the species are fueling change in the lakes at a rate far faster than they have ever seen.
"We don't necessarily know all the impacts, but we know enough to know that they are being catastrophic," said Alliance for the Great Lakes executive director Cameron Davis. "The ecological balance of the Great Lakes is at a tipping point. And the question is, can they recover? Or can we act quickly enough to help them recover?"
None of the key species leading the change _ mussels, algae and round gobies _ are new arrivals. The zebra mussel famously invaded Lake Michigan two decades ago, and its cousin, the quagga mussel, wasn't far behind.
But in the last handful of years the quagga has taken off with alarming speed, exploding across the lake floor.
While zebra mussels like to attach themselves to rocks and man-made structures, the quaggas also can colonize sandy bottoms deeper in the lakes. Between them, the species filter lake water ceaselessly, making it so crystal clear that light can penetrate far deeper than before.
That change has allowed a native species of algae called cladophora to run rampant. It now can grow in 30 feet of water, twice as deep as a decade ago, and its waving tendrils cover vast offshore areas.
Round gobies, an invasive fish species from the Black Sea willing to eat the mussels, love this new environment. They breed in prolific numbers and are now the most abundant fish species found in many parts of the lake.
Together, these species have not only altered the clarity of the water but also devoured and filtered out the nutrients that used to sustain plankton and shrimplike diporeia at the base of the lake's food chain, starving what larger fish are left.
To be sure, the Great Lakes ceased to be a wholly natural ecosystem long ago. Alewives sneaked into the lakes in 1873. People began stocking rainbow trout and chinook salmon shortly after that, and added brown trout and coho salmon to the mix by 1933.
By the 1950s, the most important fish in the native food chain _ lake trout, ciscoes and spiny sculpins _ were nearly gone in the lower lakes and severely reduced in Lakes Michigan and Huron. Still, scientists say perch, salmon and the alewives on which they foraged formed a relatively stable ecosystem until the invasive mussels began devouring key microscopic nutrients.
"Now all the forage fish are way down in Lake Michigan and Lake Huron," said Henry Vanderploeg, a Great Lakes research ecologist for the National Oceanic and Atmospheric Administration. "There's a crisis. The mussels are really messing up the food chain."
It's possible fish will weather the changes. Fishermen have caught lake trout with gobies in their stomachs, and smallmouth bass in Lake Erie have doubled their size in 10 years by feasting on gobies, said Marc Gaden, spokesman for the Great Lakes Fisheries Commission.
There also are signs that Lake Michigan whitefish have been eating zebra and quagga mussels. "That shouldn't surprise anyone," Gaden said. "That's now the new food source."
Still, fishermen are uneasy about the fish they catch, said Dan Thomas, president of the Great Lakes Sportfishing Council.
"They're one half the size they were in the 1970s and 1980s," he said. Big chinook might be a dozen pounds now instead of 30, he said. Coho are 10 pounds, not 18. Whitefish are numerous enough, but still much skinnier.
Even alewives, a once-plentiful invasive species eaten by stocked salmon, have retreated to the north end of Lake Michigan, Thomas said. He blames mussels for chasing them off. "The food chain is gone," Thomas said.
To be more exact, a new food chain has settled in, says Brenda Moraska Lafrancois, an aquatic ecologist in Minnesota with the National Park Service.
The changes are easy to see. From 20 feet of water out to 40 feet or more, mussels cover the lake floor in a crunching layer as brittle as breakfast cereal. On their shells fronds of algae wave in the water, forming a carpet the lush green of a tropical forest. Darting sand-colored gobies complete the picture.
"The first time that I dove in Lake Michigan, I was shocked at the amount of biomass down there," Moraska Lafrancois said. It just "wasn't native. Nearly everything I looked at was an invasive species in and of itself, or was facilitated by one."
A half-mile from the Wisconsin shore, Bootsma dove into 30 feet of water from his research boat, the R/V Osprey, to visit experiments on the lake floor with marine biology students Julie Barker of Niles and Jim Weselowski of Winnipeg, Manitoba.
The gray-green water is a strange world, suffused in a swaying glow; Barker said the team learned last year that cladophora followed the light. Now they want to know what feeds the algae. Nourishment may pour into the lake from rivers, or it may come from the mussels, which produce waste like any living thing.
There is urgency to learn.
"Every year we have new species," Bootsma said. "That changes the way the lake works."
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