Aquatic community response to ZequanoxRTM: A mesocosm experimen

Zebra and quagga mussels (Dreissena spp.) are European freshwater molluscs that have invaded most major inland water systems across North America, resulting in billions of dollars in damages and losses to fisheries, recreational water use, infrastructure, and industry each year. ZequanoxRTM, a biopesticide made from soil bacteria, has been advertised as dreissenid-selective and environmentally safe. Health Canada has approved the use of ZequanoxRTM in hydroelectric facilities, which may indicate movement toward open-water use in Canada, as has occurred in the USA and Ireland. Data from singlespecies assays indicate that exposure to ZequanoxRTM concentrations near or below those recommended for open-water applications can cause mortality (often ≥50%) in fish and invertebrates. However, little is known about the non-target impacts of applying ZequanoxRTM in an open-water setting, given that toxicants can behave differently in natural versus laboratory environments and because single-species tests are unable to characterize indirect effects such as pesticide-mediated changes to inter-species interactions like competition and predation.;Using a six-week-long replicated aquatic mesocosm experiment, I simulated open-water applications of ZequanoxRTM (100 mg/L of the active ingredient) to determine the responses of primary producers, zooplankton, and macroinvertebrates to ZequanoxRTM exposure in a complex aquatic environment. Short-term increases occurred in phytoplankton and periphyton biomass (250--350% of controls), abundance of large cladoceran grazers (700% of controls), and insect emergence (490% of controls). Large declines initially occurred among small cladoceran zooplankton (88--94% reductions in Chydorus sphaericus, Ceriodaphnia lacustris, and Scapheloberis mucronata), but abundances generally rebounded within three weeks. Declines also occurred in amphipods Hyalella azteca (mean abundance 77% less than controls) and gastropods Viviparus georgianus (survival 73 +/-16%), which did not recover during the experiment. Short-term impacts to water quality included a decrease in dissolved oxygen (minimum 1.2 mg/L), despite aeration of the mesocosms.;This research may assist regulators and managers in assessing the ecological risks of using ZequanoxRTM in open-water systems and support informed decision-making about dreissenid control, including for established infestations, rapid response to new invasions, and efforts under the Species at Risk Act to protect and restore native mussel habitats that have been threatened and damaged by dreissenid invasion.