When the zebra mussel settlement sampler emerged from Lake Minnetonka’s Robinson Bay in October of 2019, it looked as researchers had expected.
“We’re talking tens of thousands of zebra mussels per square meter [on the sampler],” explained Angelique Dahlberg, a PhD student at the University of Minnesota involved in the research project. “Typical numbers.”
This settlement plate – used to measure the density of juvenile zebra mussels in an area – had been placed in the bay’s warm waters a few months earlier, when the invasive species is typically reproducing. This was the research team’s control bay, left to its own devices so any zebra mussel growth could be compared with findings in a treatment bay – St. Alban’s, a few miles south.
While Robinson Bay ran its course unimpeded, St. Alban’s received an application of a copper sulfate product known as EarthTec QZ. The goal was to see what effect a low dose of this EarthTec might have on zebra mussels across multiple life stages.
The settlement plate retrieved from St. Alban’s provided an encouraging sight. In stark contrast to the mussel-encrusted sampler from Robinson Bay, this one had just “a few specks of juveniles,” said Diane Waller, Research Fishery Biologist with U.S. Geological Survey (USGS) and principal investigator. Samples from St. Alban’s also turned up a lower density of both adult zebra mussels and larvae, known as veligers.
These preliminary findings, recently announced by the U of M’s Minnesota Aquatic Invasive Species Research Center, left Waller “pleasantly surprised.” They suggest that even a relatively light dose of copper sulfate, applied strategically, could significantly impact the number of zebra mussels that are able to reach adulthood and wreak havoc on a lake, Waller said.
The small shellfish, which is native to the Caspian Sea area, was first discovered in Minnesota – in Lake Superior – in 1988, likely entering North America’s unique freshwater ecosystem via unwitting international ships. Since that time, it has quickly spread through the state’s treasured waters.
Nearly 500 waterways infested with zebra mussels
The Minnesota Department of Natural Resources has confirmed the presence of zebra mussels in 214 lakes and wetlands, but considers many more to be infested. As of June 25, 2020, the official Infested Waters List has 463 entries for zebra mussels. Most of these water bodies have been added over the past decade, including Lake Minnetonka in 2010.
Zebra mussels are not just prolific spreaders. They are also destructive.
They dismantle the natural ecosystem of Minnesota lakes by gobbling up important food sources and nutrients, such as certain algae, from the base of the food web, explained Vickie Schleuning, executive director of the Lake Minnetonka Conservation District. This harms other animals, including native mussels and fish. And the impacts are not just ecological.
“The economic harm is significant,” Schleuning said.
Zebra mussels’ glue-like secretions allow them to attach to underwater surfaces in seemingly endless sums, causing damage to boats, docks and other aquatic equipment. Their sharp shells can slice open the feet of swimmers and others trying to enjoy time on the lake. Dead, rotting zebra mussels wash up on beaches in vast quantities. And they can clog important pipes.
The species can, as Schleuning put it, “change overall enjoyment of the lake for all lake users.”
Zebra mussels’ hardiness and astounding reproduction rate (a mature female can release more than 500,000 eggs in a single year) make them tough foes. Researchers have tried many management strategies: chlorine and other chemicals, pesticides, ultraviolet light, filters, electrical fields and more.
Killing a zebra mussel is not necessarily hard. But doing so in a live lake environment with multiple uses, without negatively impacting other plants and animals (non-target species) is a more complex proposition.
This was one of the issues top of mind for Dahlberg during the Lake Minnetonka project. Previous applications of copper sulfate in Minnesota lakes have generally been about swift eradication of zebra mussels. In St. Alban’s Bay, they wanted to see not just how zebra mussels responded to low amounts of copper sulfate, but how non-target species fared as well.
It’s “the reason we’re trying to find the lowest possible concentration [of copper sulfate] to use,” Dahlberg said, “because we want the fewest impacts possible.”
In St. Alban’s Bay, they noted lower levels of zooplankton and benthic invertebrates compared to the control bay, as well as a higher mortality rate among the fathead minnows intentionally kept nearby.
While these results weren’t surprising, Waller said, impacts on non-target impacts are always a concern. It is one of the main things they’ll be monitoring when they take follow-up samples from both bays, this summer and again in 2021.
“Hopefully it’s a resilient system and things will be closer to what they were last year,” Dahlberg said of the non-target species levels.
And, of course, they want to see how the zebra mussels are faring.
“What we want to know is, how long the zebra mussel population was knocked down or suppressed from this one treatment,” Waller said. “If we find, this year, that it’s back to its normal number, then we know that this is a type of treatment that may have to reoccur.”
Schleuning said she is “encouraged” with the preliminary results, noting the conservation district “will continue to support and encourage research that will further the advancement of lake management tools and provide ecological benefits to all water bodies.”
Eventually, Dahlberg and Waller hope to have an answer about whether low doses of copper sulfate might be a viable, low-impact tool to manage zebra mussels in lakes across Minnesota.
“Everyone hopes that, oh we’re going to get rid of them, it’ll be a silver bullet,” Dahlberg said, describing such an outcome as “almost impossible in many scenarios.” More realistically, she said, “we want to bring the population down to below impact level, so that it’s not having those harmful [economic and ecological] effects.”