Flexing Their Mussel

The bivalves that ate the Great Lakes are fueling toxic algal blooms, too. Can’t we get rid of them already?

Photo: NOAA

David “Bo” Bunnell trawls the bottom of Lake Michigan for fish every summer, conducting a survey that began 40 years ago. Around the mid-aughts, the fisheries ecologist started dragging tiny mussels in with his net. These weren’t zebra mussels, whose stripes he’d become familiar with ever since the stowaways from Russia filled the lake with their kind in the late 1980s. These were smaller, with comma-shaped shells. At first they popped up here and there, but over the years they appeared in more and more samples.

Eventually the brown bivalves, called quagga mussels, were clogging Bunnell’s trawl, 400 to 500 pounds at a time, preventing him from catching the fish he needed to count and identify. His curiosity about the creatures soured.

“It became sort of a hatred,” says Bunnell, who works for the U.S. Geological Survey. In some places, so many mussels filled his net that he couldn’t haul it on board, a problem that continues to plague his surveys. Quaggas blanket the lakebed, as many as 10,000 in one square meter. “The lake for 10,000 years was sand on the bottom or sediments or rocks, and now, because of this new invasive species, it is completely different,” he says. “That makes me a little sad.”

Just like their zebra cousins, quagga mussels hitched a ride in the ballast water of ships and invaded all five of the Great Lakes. But hard, bumpy lakebeds aren’t the only havoc they have wrought. Quaggas devour huge quantities of phytoplankton. The loss of these microorganisms makes offshore waters crystal clear at certain times of the year, allowing sunlight to penetrate to the bottom and encourage plant growth. The effects of fewer phytoplankton also ripple up the food chain, reducing populations of zooplankton, larger bottom-dwellers, and eventually, fish.

Photo: Mary Anne EvansIn just three months, quagga mussels in Lake Erie covered these cinderblocks.

“There’s no component of the food web—the biological components of the Great Lakes—that has not been affected one way or another by what the mussels have done,” says Tom Nalepa, who studied mussels for 35 years at the Great Lakes Environmental Research Laboratory before joining the University of Michigan as a research scientist.

The quaggas’ most dramatic contribution to the lakes is probably toxic algal blooms. Remember the bloom that polluted Toledo’s drinking water last summer? Thick mats of microcystis, a type of cyanobacteria, have plagued the western basin of Lake Erie for the last 10 years or so.

Green, soupy blooms in Erie are nothing new. The lake experienced lots of them in the ’60s and ’70s, thanks to nutrients such as phosphorus and nitrogen in sewage, industrial waste, and agricultural runoff. Since then, nutrient loads in the lake have gone down. A larger portion of the runoff nowadays, however, is fertilizer in the form of dissolved phosphorus. Algae love to feed off dissolved phosphorous. The millions of quagga mussels then feast on their favorite phytoplankton. The mussels enjoy some algae types more than others and, unfortunately for us, they like the harmless varieties best. This preference gives toxic algae a leg up on the competition. 

So, Nalepa says, quaggas have caused the lakes to become even more sensitive to the phosphorus that flushes into them—what he calls “the mussel effect.” The good news is that like many other Great Lake invaders—of which 180 exist—mussel populations may have hit their peak. Mollusk-munching fish, such as the invasive gobi and native lake whitefish, are partly to thank.

A mussel bust would also be welcome news for salmon, which is another lake invasive, albeit an intentional one. Wildlife officials stock the Great Lakes with Pacific salmon every year, a multimillion-dollar industry. When mussels leave no phytoplankton for fish like alewives to gulp up, salmon have fewer alewives to thrive on. As a result, fishery managers have been decreasing their salmonid stocks. To help the alewives in Lake Michigan, managers have cut the number of Chinook salmon they put into the lake by more than half, from 4.5 million to 1.7 million, since 2001.

Even as mussel populations begin to contract, scientists are looking to a pesticide to beat them back them further. Zequanox contains a patented bacterium that kills mussels by dissolving their digestive tracts and—this is key—only affects quagga and zebra mussels. The U.S. Environmental Protection Agency recently issued a permit for lake managers to release the powdery mixture into open water. Alas…it probably won’t work.

Zequanox inventor Daniel Molloy, an aquatic invasive-species biologist, says that treating an entire lake with this stuff wouldn’t be that effective. Instead, he says we need something that gets into every nook and cranny of the Great Lakes—and something that doesn’t cost a fortune to use repeatedly. (Applying the pesticide to 3,000 square feet of Minnesota’s Christmas Lake cost $6,800. Lake Erie, by comparison, has a surface area of 9,990 square miles.)

“You need something that will work lakewide, and this requires a next-generation control agent, a live one,” says Molloy. A parasite. Molloy’s convinced he can find one—ideally one that’s safe for all except zebras and quaggas—but it could be a decade before he does. Scientists are also looking into ways to disrupt mussel spawning and biobullets, which would deliver a musselcide on the microparticles bivalves ingest. Those methods, however, are largely untested, and environmental groups, such as NRDC (disclosure), remain skeptical. In other words, solving this quagga quagmire is a long ways away.

Sorry, Bunnell. More summers of hauling in bivalves by the boatful are ahead of you. 


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