The study, conducted by researchers with the U.S. Geological Survey, the University of Minnesota, and partner institutions, treated St. Albans Bay, a 66.3-hectare embayment of Lake Minnetonka, with EarthTec QZ, an acid-stabilized copper formulation registered for open-water application. Over a 10-day period in late July 2019, the team applied the product five times on alternating days, maintaining a mean concentration of 83.0 micrograms per liter as copper. That figure sits far below the maximum allowable concentration of 1 milligram per liter on the product label, representing roughly 6 percent of the permitted dose. In total, the team applied 7,286 liters of the molluscicide across an estimated treated water volume of more than 2.1 million cubic meters.

The key to the low-dose strategy lies in the peculiar life cycle of the zebra mussel. The animals spawn when water temperatures exceed about 12 degrees Celsius, releasing gametes into the water column for external fertilization. The resulting larvae, called veligers, drift planktonically for two to three weeks before settling onto hard surfaces and attaching permanently. Laboratory work has shown that veligers are roughly an order of magnitude more sensitive to copper than adults, with reported lethal concentrations far lower than those needed to kill mature mussels. By timing the treatment to coincide with peak veliger production, the researchers reasoned, they could target the most vulnerable life stage while minimizing the amount of copper entering the ecosystem.

Timing the application to a spawning event required careful limnological groundwork. The team exploited thermal stratification in the bay, confining the treatment to the warm epilimnion where veligers concentrate. Using a thermocline sensor, they measured temperature and depth at multiple points, calculated relative thermal resistance to define the thermocline, and combined those measurements with bathymetric data to estimate the water volume above the mean thermocline depth. This approach allowed them to dose only the layer of water where the planktonic larvae actually lived, further reducing the total copper required and limiting exposure of deeper waters and benthic communities.

The results were striking. Before treatment, veliger densities in the treated bay were statistically indistinguishable from those in Robinson Bay, an untreated reference embayment. One day after the final application, mean veliger abundance in St. Albans Bay had plummeted from 6.0 veligers per liter to 0.3 veligers per liter, and it fell further to 0.1 veligers per liter two weeks later. Statistical comparisons confirmed the reduction was significant and persisted through 2021, nearly two full years after the treatment. Meanwhile, veliger densities in the untreated reference bay remained consistent with typical seasonal patterns, strongly suggesting the decline in the treated bay was a direct effect of the copper application rather than a natural fluctuation.

Settlement data told an even more dramatic story. Plate samplers deployed in both bays collected juvenile mussels as they attached to hard surfaces. In October 2019, shortly after the treatment, mean settlement in the treated bay was 50.8 mussels per square meter, compared with 107,838 mussels per square meter in the reference bay, a reduction of roughly 1,900-fold despite similar pretreatment veliger densities. Settlement remained significantly lower in the treated bay through 2021 and only returned to reference-bay levels by 2022, when mean settlement in St. Albans Bay reached 89,133 mussels per square meter. The prolonged suppression surprised the researchers, who had expected the surviving adult population, roughly 68 percent of stocked caged mussels, to resume reproduction at a proportionally reduced but still substantial rate.

That unexpected longevity of effect raises intriguing biological questions. The authors suggest that delayed mortality among resident adults, or sublethal effects of copper on adult reproduction, could explain why veliger production remained depressed long after copper concentrations had declined. Caged adult mussels showed about 30 percent treatment-related mortality, with survival of 68.0 percent in the treated bay versus 96.0 percent in the reference bay, an odds ratio indicating significantly reduced survival. That mortality level is consistent with laboratory toxicological endpoints; prior work reported a 14-day LC50 of 125 micrograms per liter as copper at 22 degrees Celsius, close to the realized treatment concentration under warmer field conditions. Copper toxicity in zebra mussels is known to be temperature-dependent, linked to respiratory demand and metabolic rate, which may have amplified effects during the warm July treatment window.

Environmental fate of the applied copper also featured prominently in the monitoring program. Dissolved copper concentrations, verified with inductively coupled plasma-optical emission spectroscopy, returned to near pretreatment levels within 90 days of the final application, declining from 82.7 micrograms per liter on August 1 to 2.92 micrograms per liter by late October 2019. Background concentrations in both bays had been below the analytical limit of quantification before treatment. No unusual trends appeared in monitored water chemistry parameters, including dissolved oxygen, pH, temperature, specific conductance, hardness, and alkalinity. The authors note, however, that questions remain about the long-term environmental fate of applied copper and its potential mobility through the food web.

Beyond the technical results, the study carries broader implications for how invasive species are managed. The authors frame their work within emerging frameworks such as functional eradication, which aims to reduce the ecological impact of an invasive population without necessarily eliminating every individual, and regional propagule suppression, which seeks to reduce the pressure of invasive larvae and adults spreading to uninfested waters. Previous rapid-response treatments in Minnesota lakes, applied at the full labeled concentration of up to 1 milligram per liter, largely failed to prevent population establishment, often because mussels were later found outside the treated areas. Earlier low-dose applications, including a quarry lake treatment in Pennsylvania that achieved complete caged-mussel mortality at 0.2 milligrams per liter and an Illinois lake treatment at 0.24 milligrams per liter, hinted that lower doses could work. The Lake Minnetonka study now provides three years of population-level evidence supporting that approach for long-term suppression rather than all-out eradication.

The authors are candid about the limitations of their work. Surveys of the resident adult population using petite ponar samplers and SCUBA transects returned highly variable results, limiting their ability to detect treatment effects on the established population, and the ponar method failed to detect a single zebra mussel in the treated bay until two years after treatment. More frequent veliger sampling through the summer months and a Before-After Control-Impact sampling design would strengthen future assessments. Still, the core finding stands: a 10-day, low-dose copper treatment timed to a spawning event corresponded with sustained declines in veliger abundance and settlement lasting into 2021, while an untreated bay showed no comparable change. As zebra mussels continue to spread across North American waterbodies, threatening recreation, infrastructure, and property values, the study suggests that precision-timed, low-dose molluscicide treatments could become a practical tool for managing established populations, provided managers continue to investigate sublethal effects on adults, impacts on nontarget organisms such as phytoplankton, and the optimal frequency of repeat applications.

Subject of Research: Low-dose copper molluscicide treatment for suppressing invasive zebra mussel populations in lakes

Article Title: Assessing a low-dose copper treatment for dreissenid mussels: Effects on zebra mussel (Dreissena polymorpha) population

Article References: Barbour, M. T., Luoma, J. A., Dahlburg, A., Severson, T. J., Wise, J. K., Meulemans, M. J., Bennie, B., Hammond, D., & Waller, D. (2026). Assessing a low-dose copper treatment for dreissenid mussels: Effects on zebra mussel (Dreissena polymorpha) population. Environmental Management, 76(10), Article 322. https://doi.org/10.1007/s00267-025-02355-3

Image Credits: AI Generated

DOI: 10.1007/s00267-025-02355-3

Keywords: zebra mussels, Dreissena polymorpha, invasive species, copper molluscicide, EarthTec QZ, veligers, Lake Minnetonka, aquatic invasive species control, water treatment, limnology, ecotoxicology, population suppression

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