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

We conducted and evaluated a low-dose copper treatment (applied as EarthTec QZ) to suppress zebra mussel ( Dreissena polymorpha Pallas 1771) veliger abundance and settlement in a 66.3 ha bay in Lake Minnetonka (Hennepin County, MN) over a 3-y period. We maintained a mean (standard deviation [SD]) concentration of 83.0 (10.3) µg/L as copper over the 10-d treatment period, much lower than the maximum allowable 1 mg/L as copper. Veliger density was reduced from 6.0 veligers/L before treatment to 0.3 veligers/L following the treatment period. Posttreatment zebra mussel settlement was 1900 times lower in the treated bay compared to an untreated bay days after the treatment despite similar pretreatment veliger densities. Veliger density and settlement remained suppressed nearly 2 y following the treatment. Sampling for adult zebra mussels within the treated bay returned variable results but survival of caged adult zebra mussels indicated ~ 30% treatment-related mortality. Copper in surface waters returned to near pretreatment concentrations 90 d after treatment. Our study demonstrates that low-dose applications of a copper molluscicide can effectively reduce zebra mussel veliger densities and settlement.