The copper-based molluscicide EarthTec QZ is registered for open water application and has been previously used at the maximum allowable concentration to control zebra mussels in Minnesota lakes. The goal of this project is to provide decision-making support for the use of copper in lake management for controlling zebra mussel veligers with a copper-based molluscicide in select, high value habitats while minimizing non-target impacts. Researchers will determine minimal effective treatment concentrations of copper to suppress zebra mussel recruitment and evaluate beneficial and adverse impacts to the ecosystem.
The Phase I study was conducted in Lake Minnetonka, a central Minnesota lake that has an established zebra mussel population. In the first year, researchers compiled historical monitoring data and conducted pre-treatment sampling of the biotic communities and water chemistry in the lake to characterize the status of the lake before copper treatment. Researchers used the biotic ligand model (BLM) to predict the lake-specific minimum toxic copper concentration for zebra mussel veligers. A lake-side toxicity trial was conducted in a mobile laboratory to test the predicted effective concentration for veliger suppression and evaluate toxicity to select sensitive native species. In the second year, researchers conducted an in-lake application of low-dose copper at the minimum effective concentration that was determined in lake-side trials. A before-after comparison was used to assess zebra mussel recruitment and nontarget community responses between treated and control sites in the lake. The results were compared with the 2019 low-dose copper treatment and 2-year post-treatment monitoring in Lake Minnetonka.
The Minnesota Department of Natural Resources, local governmental units, lake associations and others involved in invasive species management can use information from this project to assess the feasibility of low-dose copper treatments for managing zebra mussel populations. A strategy that targets the veliger life stage has economic and ecological advantages over treatments that target adult mussels and may be feasible for bays and small lakes. This project will also inform management decisions on the frequency of copper treatments for managing zebra mussel populations while minimizing the risk to nontarget organisms. An evaluation of the biotic ligand model will determine whether it can be used to refine copper applications specific to individual waterbodies.
In Phase II, further refinement of copper treatment for zebra mussel control may be achieved by tailoring the effective treatment concentration to the characteristics of a specific lake. The Biotic Ligand Model (BLM) estimates metal bioavailability from 11 measured parameters of the water body including temperature, pH, dissolved organic carbon, major cations (Ca, Mg, Na, and K), major anions (SO 4 and Cl - ), and alkalinity and sulfide (EPA 2007). The BLM can be used as a predictive tool for estimating copper toxicity in a specific water body for target species, like zebra mussels, and the potential impacts to non-target species. Phase II of this project will use the BLM as a prescriptive model to estimate minimum effective molluscicidal concentrations (MEC) of copper in a second study lake. We will also predict mortality of nontarget species at the estimated MEC from published copper toxicity data (EPA 2007) and test the prediction in a lake-side toxicity trial.
The goal is to develop lake-specific, low-dose copper treatments for zebra mussel suppression that minimizes impacts to native biota and maximizes ecosystem benefits. Researchers completed two years of post-treatment monitoring of the 2019 low dose copper treatment in St. Alban’s bay, Lake Minnetonka. The results showed that the treatment effectively reduced the population for at least one year after treatment; mussels reestablished in the treated bay in 2021. One year after treatment, native biotic community composition was similar to pretreatment levels; analysis of the 2021 data on native communities in in progress.
Researchers conducted a lake-side trial at Pelican Lake to evaluate the predictive value of the biotic ligand model (BLM) to estimate a minimum lethal concentration of copper to veligers. They evaluated the effects of three copper concentrations (less than, equal to, and greater than the predicted lethal concentration) to veligers and seven nontarget organisms in a 10-d exposure. Data were summarized and dose response analyses are underway. Veliger data from the lake-side trial and later lab trial were inconclusive and prevented estimation of a minimum lethal copper concentration. However, toxicity data for Daphnia were robust and will be used to verify predictions of the BLM. Together, data from the lake-side trial will guide decisions on a concentration and application method for open water treatment in 2022.
Researchers completed pretreatment sampling to characterize zebra mussel settlement, biotic community composition and water chemistry of selected study bays in Lake Minnetonka. They found abundant zebra mussel settlement in both bays. Water chemistry parameters will be input to the BLM to estimate an effective copper treatment in 2022. One bay was eliminated from further study because of a planned herbicide treatment in 2022. Researchers propose using a bay (Robinsons) that they have monitored for three years to serve as a control.
This project will develop population-level management techniques to reduce the impacts of zebra mussels. The treatments are designed to target the early life stages of zebra mussels and prevent successful settlement. The project includes three main objectives:
- Evaluate the efficacy of low-dose copper treatments to control zebra mussel populations by targeted suppression of the early life stages
- Evaluate the impacts that low-dose copper treatments have on algal biomass and native zooplankton, benthic invertebrates, fish, and mussels
- Provide transferable results by collecting the input parameters required for EPA’s Biotic Ligand Model for copper toxicity
Once complete, these treatments may be useful for population level management or eradication in small water bodies.