Quagga mussel the latest dangerous invader
Written by Green Bay Press-Gazette   
Thursday, 23 February 2006 15:25

Even as Lake Michigan became home to popular exotic species of trout and salmon the past 40 years, it also was accumulating a growing inventory of infamous exotics such as the ruffe, alewife, round goby, white perch, sea lamprey and zebra mussel.

The impacts of the more villainous exotics on native species range from irritating to devastating, but some scientists think the sea lamprey has competition for the "most dreaded invader" title.

That designation might soon be shared with quagga mussels — a ship-ballast stowaway from Eurasia's Caspian Sea drainage — which are booming in Lake Michigan. They already blanket the lake's silt and sand bottoms at depths more than twice as deep as what their smaller cousin, the zebra mussel, can tolerate.

Quagga mussels were found in Lake Michigan in 1997, and initially were confused with zebra mussels. That's because even though quaggas are twice as large as zebra mussels, they're still only as big as your thumbnail.

In addition to the larger size, lighter color and more rounded shells, other traits set quagga mussels apart. Their colonies are thriving from warm, shallow water to cold, dark depths of 330 feet, and they're probably pushing even deeper. The zebra mussel, meanwhile, remains rare at depths exceeding 150 feet.

Quagga mussels also eat year-round, while zebra mussels are dormant in winter. Quagga mussels can survive turbulence and almost any temperature, and live on almost any surface — silt, sand, rock or shipwreck — while zebra mussels require a hard surface.

Ongoing work at the University of Wisconsin-Milwaukee and the National Oceanic and Atmospheric Administration suggests quagga mussels even are killing off, or at least squeezing out, zebra mussels. Between 2000 and 2005, quaggas almost totally replaced zebra mussels in several sampling areas.

Both mussels, which filter water for their nutrition, disrupt the food chain by depleting plankton and other nutrients. This denies young and small fish vital food at a crucial point in their life. One species hit hard is diporeia, a quarter-inch long shrimp-like species that's a basic food source for small fish. Between 1992 and 2000, diporeia declined 66 percent in Lake Michigan.

This process probably explains the crash of yellow perch and whitefish populations in Lake Michigan. Recently, alewife numbers also declined, which has made Lake Michigan's salmon skinnier, hungrier and easier to catch the past couple of years.

Some researchers also believe there's a link between zebra and quagga mussels and Lake Erie's recently discovered dead zone, a large region in the lake's central basin where oxygen levels drop so low in summer that almost nothing can live there by August. Professor Hunter Carrick at Penn State University suggests mussels filter such large volumes of water and excrete so much organic materials on the bottom it rapidly depletes oxygen while decomposing.

Although quagga mussels are taking over lakes Michigan, Erie, Huron and Ontario, they've yet to be found in Lake Superior. Unfortunately, that seems only a matter of time. They've also been found in the Mississippi River near St. Louis, and they're expected to follow zebra mussels into Wisconsin's inland lakes and waterways, even some thought unsuitable for zebra mussels.

Even if quagga mussels don't prove as destructive as sea lampreys, we'll soon have a difficult time ignoring them. When the NOAA sampled Lake Michigan sites during recent summers, the average density of quagga mussels jumped from 2,200 to 7,700 per square meter from 2002 to 2005.

Tom Nalepa, a research biologist at NOAA's lab in Ann Arbor, Mich., believes it's only a matter of time before quagga mussels colonize Lake Michigan's greatest depths, which reach 925 feet.

Whether the infestation crashes overnight, spreads rapidly for years or just slows and stabilizes, no one predicts. All we know is the Great Lakes' food chain is disrupted, and the changes aren't subtle.

Don't bet on a quick fix. Blown ballasts and other random acts of ignorance always seem to baffle science's best minds.

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