Minnesota’s first-ever, whole-lake application of copper sulfate in the battle against invasive zebra mussels is taking place this summer on Lake Riley, situated on the border of Chanhassen and Eden Prairie.
Monday, July 13, marked the beginning of the latest phase of the Minnesota Aquatic Invasive Species Research Center’s (MAISRC) groundbreaking Low-Dose Copper Application Project.
Over a three-week period, contractor PLM Lake & Land Management Corporation are spreading hundreds of gallons of an EPA-approved copper sulfate solution throughout the 296-acre lake. PLM uses computers and GPS to guide its boats through the water to evenly distribute the chemical.
“We’re applying an incredibly low dose … only 10 to 11% of the maximum dose allowed,” said Olivia Nyffeler, a PhD student working on the project. “We’re trying to target only the zebra mussels and not have any harmful effects to anything else in the lake … the fish community, plant community and snails.”
The dose is so low that it does not impact recreational uses of the lake, like swimming or skiing. “It’s very safe,” she assured.
The copper sulfate being distributed in Lake Riley is specifically targeting zebra mussel veliger (the larval stage) and juvenile zebra mussels.
Once the copper treatment is completed, the UofM, USGS Upper Mississippi Environmental Science Center, Riley-Purgatory-Bluff Creek Watershed District and the Lake Riley Improvement Association will partner to monitor the lake’s water quality and zebra mussel populations to see if the experimental effort works.
“We plan to monitor all aspects of the lake’s ecosystem for at least two to three years post-treatment,” Nyffeler said.
The Solution?
Copper sulfate has frequently been used to control algae growth and invasive vegetation in lakes, but only recently has it been used on zebra mussels.
“It’s not a new technology,” Nyffeler said. “But we started doing lab experiments to see what it takes to kill zebra mussels in a jar.”
Those experiments led to larger scale testing within St. Alban’s Bay on Lake Minnetonka from 2019 through 2021.
“We saw some success with that,” she reported. “The next step was to scale this to something larger than a bay. Can we do this in a real lake that is closed and measure the effects?”
Researchers identified Lake Riley as a strong candidate for the copper sulfate project. The lake was first infected with zebra mussels around 2018 and populations exploded in just a few short years, Nyffeler said. It now has a strong, reproducing zebra mussel population.
The Lake Riley Improvement Association supports the local project and hopes to see positive results. But it’ll be years before any future treatment option is approved for wide use, Nyffeler warned.
“I don’t think we’ll ever be able to fully get rid of zebra mussels in a lake, but maybe we will be able to tamp down that population,” she said. “But now we’re kind of starting to offer a path toward a potential solution of at least suppressing the population, so homeowners can see the health of their lake improve.”
The Problem
According to researchers, non-native mussels were first introduced in the United States in the 1980s when ships unknowingly infected the Great Lakes with zebra mussels. They have since spread to lakes throughout the U.S. and Canada.
According to the Minnesota Department of Resources, about 5% of the state’s lakes — a total of 378 lakes as of June 2025 — are infested with the invasive species. Several rivers in the state also have zebra mussel populations.
“Zebra mussels are ecosystem engineers,” Nyffeler explained. “They essentially change the lake just by being in it.”
Zebra mussels drive out native mussel species, and cling to boats and infrastructure causing damage. The invasive species also can impact water quality and promote harmful algae blooms. In addition, they can form dense colonies on the bottom of a lake, often injuring swimmers who step on the sharp shells.
