Quagga Mussels: Michigan Sea Grant

March 2, 2012

Expansion of Quagga Mussels in Lake Michigan Adds to Food Web Uncertainties

During spring sampling in southern Lake Michigan, University of Michigan biologist David Jude unexpectedly hauled in several trawl nets filled with invasive quagga mussels (Dreissena bugensis). A 10-minute trawl at a depth of 100 meters near Muskegon resulted in 11 tubs of the small mussels weighing an estimated 315 pounds.

At another location 30 miles offshore, underwater cameras allowed Jude and colleagues aboard the R/V Laurentian to see firsthand how the invasive mussels have multiplied there since 1999, when none were present. “It was just astounding,” says Jude. “It was a complete carpet of quagga mussels as far as you could see.”

Quagga mussels, a close relative of zebra mussels, have gradually increased in Lake Michigan over the past five years and have expanded their range to deep, offshore waters uninhabited by zebra mussels. Their abundance and expansion raises concerns about long-term effects on the lake’s deepwater food web.

Like zebra mussels, each quagga mussel filters about 1 liter of water per day, removing vast amounts of phytoplankton (mostly algae) and depleting the food supply for native zooplankton and bottom-dwelling organisms, which in turn provide food for many species of fish.

Denizens of the Deep Quagga mussels can tolerate colder water than zebra mussels and are found more frequently in softer sediments. These qualities have allowed quagga mussels to expand their range, and they’ve been found at depths of 150 meters (450 feet), according to scientist Tom Nalepa of the NOAA Great Lakes Environmental Research Laboratory (GLERL).

Nalepa recently finished quantifying mussels collected in 2005. He notes that out of 160 sites sampled, quagga mussels were dominant at every location where mussels were found. Nalepa estimates that 98 percent of mussels collected in 2000 were zebra mussels; by 2005, 98 percent were quagga mussels. “It was a complete switch,” he says. “They’ve virtually replaced zebra mussels. For them to be that dominant over the entire lake is surprising.”

Their dominance may be due in part to several traits. Quagga mussels appear to have bioenergetic and reproductive advantages over zebra mussels. They have a lower metabolic rate and thinner shells. Scientists suspect they’re able to direct more energy toward growth and reproduction, allowing them to out-compete zebra mussels.

The explosion of quagga mussels comes at a time when the Lake Michigan food web is already stressed. With the introduction of zebra mussels in the early 1990s came a sharp decline in the shrimp-like organism Diporeia, a nutritious food source for many fish. The average abundance of Diporeia dropped from about 5,200 per square meter in 1994/95 to 1,800 by 2000.

Nalepa, who has been monitoring the decline of Diporeia as part of long-term lake wide studies conducted by GLERL, notes that the decline in 2005 is even more dramatic. The average abundance in 2005 is now only 300 per square meter. With quagga mussels becoming more abundant, says Nalepa, Diporeia has declined in deeper waters, and areas of the lake with no Diporeia have expanded greatly.

Food for Thought               Due in part to the disappearance of Diporeia, some fish species have switched to alternate food sources. According to Jude, two important Lake Michigan forage fish, slimy and deepwater sculpin, have shifted their diets from mostly Diporeia toward more fish eggs, insects and Mysis (opossum shrimp).

With Michigan Sea Grant funding, Jude is examining the food web impacts on sculpin in relation to the decline in Diporeia, which began following the introduction of zebra mussels. Jude also reports that slimy sculpin, usually found in waters less than 50 meters deep, are now being found in the deepest parts of the southern Lake Michigan basin.

Commercially important lake whitefish are also consuming alternate food sources, primarily Mysis, chironomids (larval insects), and shelled prey including zebra mussels and now quagga mussels, according to GLERL ecologist Steve Pothoven.

He notes that approximately 40 percent of the diet of lake whitefish in southern Lake Michigan is now composed of quagga mussels. Energetically, says Pothoven, quagga mussels are not any worse than zebra mussels, but “neither is good.” Pothoven, who has been monitoring a decline in the physical condition of whitefish for several years, says that smaller, thinner fish may be due in part to sustained high numbers of whitefish and increased competition for food.

Impacts of quagga mussels on other fish species, particularly larval life stages that depend on a healthy zooplankton population, are still unfolding. “It’s important now to keep an eye on offshore productivity. We now have a filter feeder offshore that wasn’t there before,” says Pothoven. “We need to see how it affects the lower food web.”







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