Mussels harming Lake Michigan food web

February 18, 2009 at 04:31 PM

TRAVERSE CITY, Mich. (AP) - A tiny, shrimplike creature that forms a crucial link in the Great Lakes food web has all but disappeared from Lake Michigan because of competition from invasive foreign mussels, scientists reported Wednesday.

Observations over a decade have documented a 96-percent drop-off of the amphipod species known as diporeia, according to scientists with NOAA’s Great Lakes Environmental Research Laboratory in Ann Arbor.

During that period, the lake’s population of quagga mussels rose dramatically, said the researchers, who described their findings in the journal Freshwater Biology.

“It’s pretty astounding what changes occurred the lake in just 10 years,” said Tom Nalepa, a biologist who led the study for the National Oceanic and Atmospheric Administration lab.
The quagga mussel, a thumbnail-sized invader from Eastern Europe, is believed to have hitched a ride to the Great Lakes in ballast water of a trans-Atlantic car go ship in 1989 - three years after the arrival of its better-known cousin, the zebra mussel.

Both have spread rapidly across most of the lakes, with the quaggas inhabiting cold, dark waters that zebra mussels avoid. They eat the same types of algae on which diporeia relies. Diporeia is a dietary staple for bottom feeders such as whitefish - standard fare in many of the region’s restaurants.

Previously, diporeia made up 70-80 percent of the whitefish diet. Now they are getting skinnier and less abundant as they subsist on the mussels, which are the aquatic equivalent of junk food - lower in calories than diporeia, their shells devoid of nutritional value.

“The quagga mussels are basically sucking all the energy out of the lake,” Napela said. “Not only is diporeia declining. The lake cannot support all this biomass of quagga mussels without other components of the food web losing populations.”

Also slumping amid the mussel onslaught are prey fish such as the alewife, bloater and sculpin, which are crucial food for salmon, trout and other popular sport species.

Similar trends have been noted in Lakes Huron and Ontario and parts of Lake Erie. The mussels have a far lesser presence in Lake Superior, probably because its calcium concentrations are too low to meet their needs, Nalepa said.

Biologists first noted a diporeia drop-off in southern Lake Michigan in the early 1990s, as the zebra mussel rapidly colonized the area to a depth of about 55 yards.

The latest study documents the virtual lakewide replacement of a native species by an invader and suggests the change will fundamentally alter the ecosystem, the scientists said.

Napela and colleagues began taking sediment samples from dozens of sites around the lake in 1994-95 and repeated the operation in 2000 and 2005. They did extra sampling annually near the southern end.

Diporeia were particularly abundant in lower depths. Its biggest declines happened after the quagga mussel’s rapid expansion in the late 1990s, Napela said.

The scientists have continued their annual sampling in selected locations, turning up no reason evidence that the trends are slowing. Only when the mussel populations finally stabilize will their long-term effects on fish and the rest of the ecosystem be known, Napela said.

Other studies are exploring whether food competition is the only reason for the mussels’ devastating effect on diporeia.

Environmentalists have long campaigned for stronger laws to prevent exotic species invasions in the lakes - particularly from ship ballast. The study illustrates the continuing fallout from governments’ failure to act sooner, said Joel Brammeier, vice president for policy of the Chicago-based Alliance for the Great Lakes.

“The lesson here is that if we don’t focus on prevention, there will always be an invader around the corner that will be 10 or 100 times more dangerous than the one we saw before,” Brammeier said.