Little quagga mussel has big impact on Lake Michigan

In his  lab at the Great Lakes WATER Institute, scientist Russell Cuhel places several quagga mussels into a beaker containing water tinted green with tiny algae. Within a half hour, the hungry mussels suck up virtually all the algae in the beaker, leaving the water clear.

 A similar scene plays out year-round on a much larger scale in Lake Michigan, where trillions of the invasive mussels have colonized rocky and sandy areas of the lakebed since the species’ arrival less than a decade ago. These filter feeders’ voracious appetites have transformed vast areas of the lake from cloudy to Caribbean clear. That may be a boon for divers and others who benefit from greater visibility in the water. But to scientists like Cuhel, it’s a symptom of a dramatic shift in the lake’s food web. 

Dramatic Shift

Before the mussel invasion, the lake’s phytoplankton-tiny algae and other organisms that turn sunlight into energy and form the base of the food web-were eaten mainly by tiny animals known as zooplankton. Zooplankton in turn provided food for prey fish such as alewives, which in turn provided food for predator fish such as trout.

But now, in areas where they have a foothold, mussels are sucking up large amounts of phytoplankton and changing the flow of energy through the lake. “Instead of going through one or two steps to fish, [energy is] now going through one step into quagga mussels and quagga mussel poop, and it’s all on the lake bottom,” said Cuhel.

Cuhel has witnessed the change up close. For more than a decade he, scientist Carmen Aguilar, and other colleagues have been regularly visiting several sites off Milwaukee’s shores aboard the research vessel Neeskay. At each site, they collect organisms from the lake bottom and from throughout the water’s depth. Back at the lab, Cuhel and his colleagues count, weigh, measure, and analyze the chemical makeup of the samples.

 

This diagram illustrates how invasive quagga mussels are altering the flow of energy and material through Lake Michigan’s food web. Each box represents the area on and above a square meter of lakebed. Percentages are of total biomass (excluding fish) in that area. In studies by scientists Russell Cuhel and Carmen Aguilar, in areas without mussels (left box), phytoplankton made up most of the biomass. In areas with mussels (right box), the mussels themselves made up most of the biomass. Without mussels, energy and material flow from phytoplankton mainly to zooplankton and then fish, as one consumes the next. With mussels, energy and material flow from phytoplankton mainly to mussels, as mussels consume the phytoplankton. —Jennifer Yauck

That information has helped Cuhel quantify the extent to which mussels alter the food web. He recently estimated how much of the biomass-or living tissue-on or above a square meter of lakebed exists as phytoplankton, zooplankton, mussels, or bottom-dwelling scavengers and decomposers. Those percentages can vary by site and season, but generally, Cuhel found phytoplankton make up the bulk of the biomass-some 70 to 80 percent-in mussel-free areas. In mussel-infested areas, mussels make up 75 to 90 percent of the biomass, while phytoplankton make up only about 5 percent.

That shift reverberates up the food web, said Cuhel. Both the larval fish that are too small to eat anything other than phytoplankton and the zooplankton that sustain adult prey fish have a harder time finding the meals they need to grow and survive. As a result, said Cuhel, prey fish populations are dwindling, and the predator fish that eat them are not growing as large as they once did.

“In general, it’s been a downward trend [in prey fish],” said Brad Eggold, Wisconsin Department of Natural Resources fisheries supervisor. “It seems reasonable that [the mussels] are having an impact.”

According to the U.S. Geological Survey’s most recent lake-wide survey of prey fish, Lake Michigan’s total prey-fish biomass has been trending downward since peaking at an estimated 450,000 tons in 1989. In 2008, it sank to 25,000 tons, the lowest recorded amount since the survey began in 1973. But the report notes that factors other than mussels, such as shifts in habitat and higher predation by salmon, could be contributing to the decline.

Don’t Move a Mussel

The quagga mussel, like the invasive zebra mussel that preceded it, is native to the Caspian Sea and likely came to Lake Michigan in the ballast water of ocean freighters. But quaggas have proven themselves the hardier species in Lake Michigan, virtually wiping out the zebra mussel population in the past few years.

Along with altering the food web, the abundant animal has been blamed for promoting the growth of nuisance algae that wash ashore and foul beaches, and for forming costly clogs in water intake pipes.

And now it appears the quaggas and the problems they cause are poised to infest the West, too. In 2007, quaggas were discovered in the Colorado River at Nevada’s Lake Mead-the first-ever sighting west of the Continental Divide-and have since spread downriver all the way to California. They were likely transported from the Midwest in or on a recreational boat.

The moral of the story? “Stop moving stuff,” said Cuhel. “Don’t encourage unnatural movement of organisms because it can have huge ramifications. A single organism can completely change what you’ve grown to love to where you don’t even recognize it anymore.”

Jennifer Yauck is a science writer at the Great Lakes WATER Institute. GLWI (glwi.uwm.edu) is the largest academic freshwater research facility on the Great Lakes.