Invasion of the quagga mussels: A food-chain story

Russell Cuhel (right) with Chris Ney (left, piloting the Remotely Operated Vehicle) and Aquanaut teacher Neil Dullinger of Mukwonago High School.

by Laura L. Hunt

The appearance of a relatively new invasive species in Lake Michigan has given Russell Cuhel of UWM’s Great Lakes WATER Institute an unparalleled opportunity to teach people about the balance of nature – and the consequences of tipping it.

The intruder, the quagga mussel, is potentially more threatening to the health of the lake than its nuisance cousin, the zebra mussel, scientists fear.

"In winter, when the waters mix themselves, phytoplankton gets stirred around, and then in the spring, young fish and tiny shrimp can eat the swirling nutrients,” says Cuhel. “But when the mixing starts, the quaggas will take the plankton out before the little guys can get it.”

Zebra mussels have populated the Great Lakes since the late 1980s, disrupting the native ecosystem and attaching themselves in huge numbers to anything hard and stationary – including the city’s water intake pipes.

Like zebra mussels, quaggas also filter plankton from the water at an astounding rate, allowing more sunlight to penetrate the water and causing a boom in aquatic algae that is inedible to fish. But while zebra mussels are sensitive to water temperature and need a hard surface on which to colonize, quaggas can adapt quickly to extreme environmental changes. They thrive in any temperature, can survive turbulence, and are able to colonize both hard and soft substrate, even sand.

A pair of quagga mussels side by side, showing the change in coloration that has occurred as populations moved into shallower water.

WATER Institute scientist John Janssen noticed a few quagga mussels in the colder depths of the lake in late 2002. Equipment at the bottom of the lake’s Linnwood reef, north of the city harbor, is covered in what used to be all zebra mussels. Today, it’s an equal measure of each, says Cuhel.

The same thing is happening at shipwreck locations around Manitowoc and Sheboygan reefs, he says, except the quaggas also are filling in the deeper lake bottom that was previously large tracts of sand.

“I can’t think of a sample that I worked with this year where I’ve had more zebra mussels than quaggas,” says Kelsey Poulson, a junior biological sciences major who works at the WATER Institute as a technician.

Poulson has been measuring the size of the live quaggas that have been collected at various locations. Cuhel and fellow WATER Institute scientist Carmen Aguilar already have amassed a sea of data on the mussels, from average weight to shell thickness, and they have used it to create a model that determines the mussel’s age by measuring its length.

Student technician Kelsey Poulson.

Cuhel and Aguilar have enlisted not only many UWM undergraduates, but also the Institute’s Aquanauts to help with the sampling. Formed in 2003 and funded by the National Science Foundation, the Aquanauts consist of K-12 teachers and museum educators who are trained at the WATER Institute during the summers to test ecological questions and gather ideas to take back to their classrooms.

Aquanaut Jenny Hensgen, a seventh grade teacher from the Waterford Graded School District, went out on the research vessel Neeksay multiple times during the summer, collecting quagga samples.

Back at Fox River Middle School, Hensgen and her students are conducting an ongoing study of species that inhabit the Fox Point River watershed, as well as an interdisciplinary study of the river’s place in nature. The quagga mussel infestation “provides an opportunity to present test-able questions and run experiments for the answers,” she says.

Middle school teacher Jenny Hensgen aboard the Neeksay.

Scientists and teachers filmed a video to show the quagga encroachment at various depths in Lake Michigan .

“The deeper you go, the more flat sand you find – and lots of dead (empty) zebra mussel shells,” says Cuhel about the video. “The shells of the dead ones are not as big as in past collections. They’re probably only at middle age. So what we’re asking is, ‘Are the quaggas squeezing the zebra mussels out?’”

The invasion of quaggas is not good news, even if they eventually wipe out the zebra mussels, says Cuhel, because the effects on the food chain could be even more pronounced, given the new invader’s hardiness, and because it is likely that quaggas will follow the zebra mussels into other freshwater lakes.

“Owners and managers of inland lakes that have sandy bottoms now have to worry about the next infestation of exotics [non-native species],” says Cuhel, who recently gave a briefing on quagga mussels to City of Milwaukee and MMSD officials.

Already they’ve discovered that quaggas are advancing into the lake’s coastal areas, which have been dominated by zebra mussels. And in an odd twist, the normally white quaggas are sometimes popping up in shallow waters with striped shells.

“We’re at that rare ecological moment when you’ve got the first brotherhood (of adult quaggas) and the second generation just came on,” Cuhel says of the shell evidence the team has found. “We can now see whether they are surviving better in one location as opposed to another.”

It is still unclear how many other species that eat plankton may be threatened, or how a double dose of more filter-feeders will impact the way native species feed and spawn. But one thing is certain: The more than 100 invasive species now inhabiting the lake make managing the aquatic ecosystem an ever-changing proposition.

“It’s a great food-chain story,” says Cuhel, “but one without an ending.”

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