Low-Dose Copper Treatment Against Zebra Mussels Shows Limited Harm to Lake Life

Zebra mussels have been quietly reshaping North American lakes since they arrived in the Great Lakes in the mid-1980s, filtering away phytoplankton, altering food webs, fouling water intake pipes, and imposing billions of dollars in economic costs. As the invasive mollusk has spread to hundreds of inland waterbodies, resource managers have grown increasingly willing to fight back with chemical tools, and copper-based pesticides have emerged as one of the leading options. Yet a persistent question has shadowed every open-water application: what happens to everything else living in the lake? A new study published in Environmental Management offers one of the most detailed field-based answers to date, tracking the fate of fish, native mussels, zooplankton, benthic invertebrates, and phytoplankton before and after a large-scale, low-dose copper treatment in a Minnesota bay.

The research, led by Angelique D. Dahlberg of the Minnesota Aquatic Invasive Species Research Center at the University of Minnesota together with colleagues from the U.S. Geological Survey’s Upper Midwest Environmental Sciences Center and Earth Science Laboratories, focused on St. Albans Bay, a 66.3-hectare embayment of Lake Minnetonka, a heavily used recreational lake near Minneapolis-St. Paul. Zebra mussels were first confirmed in the lake in 2010 and have since become well established. In July 2019, the team applied EarthTec QZ, an acid-stabilized ionic copper product registered by the U.S. Environmental Protection Agency specifically for dreissenid mussel control, at a target concentration of 60 micrograms of copper per liter maintained over ten days, with an initial pulse of 100 micrograms per liter. Robinson Bay, an untreated embayment of similar size on the other side of the lake, served as the reference site for comparison.

The researchers then monitored the ecosystem for four consecutive field seasons, from 2019 through 2022, measuring chlorophyll-a as a proxy for phytoplankton abundance, sampling zooplankton and benthic invertebrate communities, and deploying cages of test organisms at five sites in each bay. The caged animals included young-of-year yellow perch, fathead minnows, bluegill, and largemouth bass, along with juvenile fatmucket mussels, a native unionid species, and adult zebra mussels collected from the lake. After the treatment period, tissues from these animals were analyzed for copper concentrations using inductively coupled plasma methods at Iowa State University’s Veterinary Diagnostics Laboratory, allowing the team to compare bioaccumulation across species and between treated and reference bays.

One of the study’s most counterintuitive findings involved phytoplankton. Chlorophyll-a concentrations rose in the treated bay one day after treatment began, accompanied by a drop in water clarity measured by Secchi disk transparency, a pattern indicating an algal bloom rather than an algal kill. The authors suggest this may have occurred because the treatment coincided with a bloom already underway, or because the copper dose was simply too low to broadly suppress algae, which previous research indicates typically require concentrations above 0.318 milligrams per liter to show measurable effects. Chlorophyll-a and clarity returned to near pretreatment levels within two weeks, and elevated chlorophyll in the treated bay in 2020 may have reflected the reduced filtering capacity of a diminished zebra mussel population.

Zooplankton, long known to be among the most copper-sensitive freshwater organisms, showed the clearest nontarget response. Abundance in the treated bay dropped significantly within one day of treatment, and diversity, measured with the Shannon index, fell sharply and remained low through two weeks posttreatment. The declines were not uniform across taxa: daphniids and diaptomids, groups with documented copper sensitivity, decreased most, while cyclopoid copepods tracked the reference bay almost exactly, with mean densities deviating by less than two individuals per liter. By August 2020, one year after treatment, zooplankton abundance in the treated bay had rebounded to levels exceeding the reference bay, and diversity had returned to reference-bay levels, indicating a substantial community recovery within a single year.

The statistical analysis also revealed how noisy lake ecosystems can be. Permutational analysis of variance showed that zooplankton community composition differed at nearly every sampling date in both bays, treated and untreated alike, underscoring the naturally chaotic dynamics of plankton communities documented in long-term experiments. This natural variability complicates attribution: some of the posttreatment shifts likely stemmed from the copper, but others reflected seasonal succession, nutrient dynamics, and predator-prey interactions operating independently of the treatment. The authors note that abundance may be a more responsive and practical metric than community structure for managers evaluating short-term treatment effects over a few years.

Benthic invertebrates, by contrast, showed no clear treatment effect. Abundance and diversity in the treated bay remained statistically similar across nearly all sampling occasions, and community structure was dominated by the same 13 families, including chironomids, hyalellid amphipods, and asellid isopods, throughout the study. The one notable signal was a decline in Hyalellidae immediately after treatment, which the authors attribute to copper exposure, given that the amphipod Hyalella azteca has a reported 48-hour lethal concentration 50 of 87 micrograms per liter in similar water chemistry. Because hyalellids are an important fish food source, such declines could ripple through the food web, although the family appeared to recover in subsequent years. Differences in substrate between the two bays also limited direct cross-bay comparisons of benthic communities.

Among the caged animals, the picture was mixed and, in some respects, sobering. Adult zebra mussels in the treated bay suffered 32 percent mortality and accumulated the highest tissue copper concentrations of any species tested, averaging 40.85 micrograms per gram, consistent with laboratory toxicokinetic studies. Native fatmuckets accumulated substantial copper as well, averaging 26.41 micrograms per gram, yet remarkably, no fatmucket mortality occurred in the treated bay; the only deaths were four individuals in the untreated reference bay. The authors caution, however, that laboratory studies report juvenile fatmucket toxicity thresholds well below the treatment concentration, so delayed or sublethal effects on native mussels remain a concern worth further investigation. Among fish, fathead minnows were the only species showing treatment-related mortality, with survival falling from 84 percent in the reference bay to 38 percent in the treated bay, and their tissue copper concentrations averaged an order of magnitude above reference levels. The authors note that caged fish cannot escape the plume, whereas free-ranging fish typically avoid contaminated water, so the caged results likely represent a worst-case exposure scenario.

The findings carry practical weight for the growing number of managers contemplating open-water copper treatments. The study suggests that a low-dose copper approach can suppress zebra mussels while causing limited and largely reversible collateral damage: zooplankton recovered within a year, benthic communities within two, and phytoplankton effects were transient. But the work also highlights trade-offs that managers must weigh deliberately. Timing treatments to avoid periods when larval fish depend heavily on zooplankton, allowing recovery intervals of one to three years between applications, and clarifying whether the management goal is eradication or suppression below a threshold of ecological impact are all decisions the data can now inform. Complete eradication of an established population may not be achievable at low doses, but reducing mussels below damaging levels while preserving native community function appears within reach. As zebra and quagga mussels continue their spread across North America, this four-year experiment provides a rare, ecosystem-scale evidence base for choosing when, where, and how to deploy copper against one of the continent’s most consequential aquatic invaders.

Subject of Research: Field evaluation of low-dose copper molluscicide effects on nontarget freshwater organisms during zebra mussel suppression in Lake Minnetonka.

Article Title: Assessing Low-Dose Copper Treatment for Dreissenid Mussels: Effects on Nontarget Organisms

Article References: Assessing Low-Dose Copper Treatment for Dreissenid Mussels: Effects on Nontarget Organisms. (n.d.). https://doi.org/10.1007/s00267-025-02354-4

Image Credits: AI Generated

DOI: 10.1007/s00267-025-02354-4

Keywords: zebra mussels, copper molluscicide, invasive species, Lake Minnetonka, nontarget effects, zooplankton, benthic invertebrates, bioaccumulation, ecotoxicology, aquatic invasive species, dreissenid control, lake management