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|A new invasion. Quagga mussels take over lake|
|Written by Milwaukee Journal Sentinel - Jamoke|
|Sunday, 14 May 2006 11:28|
When the Army Corps of Engineers explored the potential benefits of expanding the St. Lawrence Seaway a few years back, the agency pooh-poohed worries that more overseas ships carrying more unwanted hitchhiking organisms would, logically, lead to more ecological trouble for the Great Lakes.
The Corps did acknowledge that opening the once-isolated lakes to overseas ships already had resulted in the arrival of some troublesome species. But it told the public not to fret the environmental consequences of a Seaway expansion.
"The most dramatic impacts to the ecosystem have likely already occurred," the Corps concluded in a more than 400-page report, released in June 2002.
What, after all, could cause more havoc than the zebra mussel?
Meet the quagga mussel.
Less than four years after that reassuring report, quagga mussels have gone from a rare find on the bottom of Lake Michigan to its dominant invasive mussel. Along the way, they have done what many invasion biologists thought would be impossible: They have nearly annihilated Lake Michigan's zebra mussel population.
That is not necessarily a good thing.
Five months after the Corps report surfaced, a researcher at the University of Wisconsin-Milwaukee brought back a fistful of suspicious mollusks found during a fish survey on a mid-lake reef that lies about 20 miles northeast of downtown Milwaukee.
The fingernail-size shells looked much like those of the zebra mussel, a native of the Caspian and Black Sea region first found in the Great Lakes in the late 1980s.
But they were found in an area of the lake typically too deep for zebras. Further analysis revealed that they were quagga mussels, a close cousin to the zebra, and a species that scientists also believe invaded the Great Lakes from the Caspian and Black Sea region via contaminated ballast water discharged from overseas freighters.
That cluster of mussels brought to the UWM labs in November 2002 turned out to be a harbinger of a stunningly rapid ecological revolution.
"Right now, if you go out and suck up 1,000 mussels, you're lucky to find a couple of zebra mussels," says Russell Cuhel, a senior scientist at the UWM Great Lakes WATER Institute.
Quaggas have not just out-competed zebra mussels for food everywhere their ranges overlap - Quaggas also are blanketing the lake bottom in many of the deep, cold places that the more delicate zebras could not.
"Everybody used to say, 'Oh no - zebra mussels!' " Cuhel says. "Well, zebras don't hold a candle to what these guys are going to do to Lake Michigan."
Like zebras, only worse
At the same time, they have turned Lake Michigan's delicate food chain upside down by hogging the plankton upon which so many fish species directly or indirectly depend.
They have also sucked the nearshore waters unnaturally clear, leading to algae outbreaks on the lake bottom. This material washes ashore and rots in a noxious sludge that renders once-popular Lake Michigan beaches at times unusable.
Quaggas, which were first documented in Lake Michigan in 1997, are doing the same thing, but on a super-sized scale.
The reason: Zebra mussels depend on relatively warm water and must attach themselves to hard surfaces. That typically limits their range in Lake Michigan to the rocky nearshore areas, and they do most of their filtering work during the warm summer months. But quaggas can thrive in both warm water and near-freezing conditions. They are flourishing at depths of 300 feet and have been found at depths as deep as 540 feet in the lake, and filtering year-round.
They can also colonize sand, clay, pebbles - anything but soft mud.
Cuhel points to a map of Lake Michigan and traces the zebra mussel's range. It is basically a necklace ringing the U-shaped lake. Then he points to areas where the quaggas are being found - most everywhere but the patches of soft bottom in a body of water that is 307 miles long and 118 miles wide.
If zebra mussels have added an ugly border to the lake's ecological tapestry, quaggas are giving it a whitewash.
"I knew right from the beginning this was going to be huge, way huge, in comparison to the zebra mussel, in terms of change in the ecosystem structure of Lake Michigan," Cuhel says.
Others agree the lake could be headed for ecological shock the likes of which it has never experienced.
"With quagga mussels getting in and becoming established, it's a much different situation, and I don't think most people realize that," says Tom Nalepa, a researcher at the National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory. "The changes occurring to Lake Michigan are going to be more significant because the mussel biomass is much greater."
Nalepa says a recent survey of 160 sites across Lake Michigan show a complete flip-flop in the invasive mussel population. In 2000, zebra mussels made up 98.3%. In 2005, quaggas made up 97.7%.
But Nalepa says a more worrisome figure is the "stunning increase" in the number of overall invasive mussels now found on the lake bottom.
In 2000, Nalepa says, his survey showed an average of 899 zebra mussels per square meter of lake bottom at those 160 sites. Quagga mussels now average 7,790 per square meter of lake bottom.
"It's amazing," Nalepa says. "It doesn't matter whether it's shallow depths or deeper depths, its pretty much all quaggas."
Biologists in other fields are already documenting changes in the lake.
University of Michigan researcher David Jude says a research expedition netting fish off Lake Michigan's eastern shore last month yielded a surprise catch.
"We probably had a half-pound of fish and 600 or 700 pounds of quaggas. . . . I could barely get it in the boat," Jude says.
That came after dragging a net for 10 minutes across the bottom of the lake at a depth of about 300 feet - too deep for other invasive mussels.
About the same time on the other side of the lake, UWM's Cuhel was taking his own survey on water clarity, a key indicator of the amount of plankton hanging in the water.
Water clarity is measured by lowering a saucer-sized disc into the water, and measuring how far down it can be seen from the surface.
Cuhel says a couple of decades ago, that disc would have disappeared at about 15 feet this time of year. His test, taken a few weeks ago, showed it was still visible at a depth of 74.5 feet - an appropriate clarity for the waters off the Virgin Islands' St. Croix, perhaps, but not Milwaukee County's St. Francis.
That reading is about 15 feet deeper than just a few years ago, when zebra mussels dominated.
Light, however, penetrates well beyond 74.5 feet. John Janssen, another senior scientist at the UWM water institute, says he can operate a remote video camera at a depth of 200 feet using only the ambient light pouring down from the lake surface.
Fish food disappearing
Another important fish food, a shrimp-like creature called diporeia that once blanketed the bottom of the lake, is continuing to crash. The average number of diporeia found at his sampling sites dropped by more than half between 1994 and 2000, and the spiral continues. Five years ago, the average density was 1,836 per square meter. Today, the average is 293. Nobody has ever made the direct link between the rise of invasive mussels and the decline of diporeia, but few doubt there is a connection.
This change is echoing up the food chain. Whitefish historically relied on diporeia for as much as 75% of its diet, and the loss of that food source is evidently taking its toll. In 1988, the year before zebra mussels were discovered in Lake Michigan, the average 7-year-old whitefish weighed more than 5 pounds. Last year, it weighed 1.6 pounds.
Nobody knows where all this is headed, but the scientists whose job it is to study the lake say it is folly to think the worst is over.
"Lake Michigan is such a moving target. It just changes so much from one year to the next," says UWM researcher Harvey Bootsma, an expert on the mussel-fueled algae outbreaks plaguing Milwaukee's beaches. "We try to answer questions, and the questions keep changing."
And the invasions keep coming, with a new species discovered in the Great Lakes, on average, every 6 1/2 months. The overwhelming majority are arriving in the bellies of overseas freighters. Legislation to better control contaminated ballast water spills has been stalled in Congress for more than three years.
Other species on the way
"There are other (invaders) coming. Just because they haven't made a name for themselves here doesn't mean they won't be high-impact," he says.
Ricciardi particularly has his eye on two species of shrimp. One grows to almost an inch long and can have a profound impact on an ecosystem because it kills far more than it eats. The other blankets lake bottoms at densities up to 700,000 per square meter and builds wasp-like honeycombed homes that smother fish spawning beds.
Both have exceptionally long names that are hard to remember. At least for now.
"How many people today have heard about the quagga mussel?" he asks. "Well, they're going to."
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