Michigan test kills over 80% of invasive zebra mussels in five days

Zebra mussels are small, but their impact on freshwater ecosystems can be enormous. Once established, the invasive shellfish can blanket lakebeds, crowd out native species and alter the communities that depend on them. Scientists in Michigan are now testing a surprisingly simple idea: cover the lakebed and turn up the carbon dioxide underneath. A 2024 experiment at Loon Lake in Sleeping Bear Dunes National Lakeshore tested whether carbon dioxide could make benthic mats—a kind of heavy, flexible cover placed over the lake bottom—more effective at killing zebra mussels. The results suggest that the combination can work considerably faster than mats alone. Testing the mats in Loon Lake Researchers from the U.S. Geological Survey divided three areas of Loon Lake into treatment and reference plots. Each treatment area measured 4 by 4 meters, while the mats themselves were slightly larger to ensure that the lakebed was covered. The scientists tested two approaches. One used a polyethylene mat the experimental plots before treatment began. They then with a vinyl coating, anchored to the bottom with sandbags and weights. The second used an identical mat, but with carbon dioxide-infused water pumped underneath it. To measure the effects, researchers placed roughly 360 zebra mussels in cages across the experimental plots before treatment began. They then left the mats in place for five days before removing them and assessing mussel survival and the surrounding aquatic community. Carbon dioxide and pH measurements The ordinary mat did have an effect, but adding carbon dioxide produced a much stronger result. Under the carbon dioxide mats, concentrations reached levels close to 200 milligrams per liter, with pH measurements indicating sustained exposure during the treatment period. After five days, more than 80 percent of the caged zebra mussels in the carbon dioxide plots had died. By comparison, average mortality was 20.6 percent beneath the ordinary benthic mats and 12.7 percent in untreated reference plots. Researchers also found very few zebra mussels in the carbon dioxide and ordinary mat plots after treatment, with the lowest survival occurring where carbon dioxide had been introduced. Why carbon dioxide? The approach is based partly on earlier laboratory research suggesting that carbon dioxide can be more harmful to zebra mussels than to some native freshwater mussels. That selectivity matters. Eliminating an invasive species is of limited value if the treatment causes comparable damage to native wildlife. Previous experiments found that certain native unionid mussels could survive carbon dioxide exposures that were lethal to zebra mussels, although prolonged exposure could affect growth and tissue condition. The Michigan study therefore explored carbon dioxide as a potential selective toxin, using the mat to keep the treatment concentrated near the lakebed rather than dispersing it throughout the surrounding water. Native life still needs watching The results were encouraging, but the treatment was not without ecological effects. Researchers recorded lower total numbers of benthic macroinvertebrates—the small, tying up equipment and treatment areas for longer periods. If carbon dioxide can achieve high zebra mussel mortality in just a few days, the same equipment could potentially be moved animals living on or near the lakebed—in both types of mat plots compared with reference areas. Interestingly, however, overall diversity remained comparable among the treatments. That finding points to an important question for future work: whether carbon dioxide treatment can be refined enough to kill zebra mussels while minimizing disruption to other organisms living in the same habitat. A potentially faster control method One advantage of the carbon dioxide approach is speed. Conventional benthic mats can require extended deployment, tying up equipment and treatment areas for longer periods. If carbon dioxide can achieve high zebra mussel mortality in just a few days, the same equipment could potentially be moved and redeployed multiple times. The researchers say improvements to the carbon dioxide delivery system could also expand the area that can be treated at once. For now, the Loon Lake experiment is an early step rather than a universal solution. But it offers an intriguing new tool for managing zebra mussels: instead of relying solely on a physical barrier, scientists can transform the space beneath the mat into a short-lived, hostile environment for one of freshwater's most persistent invaders. Image Courtesy: istock