Aquatic community response to Zequanox®: a mesocosm experiment
Abstract
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. Zequanox®, a
biopesticide made from soil bacteria, has been advertised as dreissenid-selective and environmentally
safe. Health Canada has approved the use of Zequanox® 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 Zequanox® 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 Zequanox® 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 Zequanox® (100 mg/L of the active ingredient) to determine the responses of primary
producers, zooplankton, and macroinvertebrates to Zequanox® 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
Zequanox® 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.