Low-Dose Copper Treatment Suppresses Zebra Mussels for Two Years

Zebra mussels have long been one of the most stubborn invaders in North American lakes, clogging water intake pipes, fouling boats and docks, and reshaping entire food webs. Once a population takes hold in an open lake, resource managers have had few realistic options beyond expensive, high-dose chemical treatments that often fail to stop the spread. Now, a multi-year field study on Lake Minnetonka in Minnesota offers some of the strongest evidence yet that a carefully timed, low-dose copper treatment can knock back an established zebra mussel population for years, using a fraction of the copper concentration that regulations typically allow.

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