Impacts of zebra mussel (Dreissena polymorpha) invasion in an inland lake with suboptimal water chemistry

Zebra mussel (Dreissena polymorpha) biofouling has caused rapid declines of native unionid mussel populations in North American inland waters with calcium Ca2+ concentrations at or above 25 mg/L. We surveyed zebra mussel fouling and unionid mortality across sites and time in a large invaded lake (Lake Memphrémagog, Québec) and its connected waterbodies. The lake has a mean calcium concentration of 20.3 (range 18.6–23.2) mg/L, considered suboptimal for supporting a zebra mussel population large enough to exert lethal impacts on native mussels. Nevertheless, fouling intensities in the lake are of the same magnitude as those associated with rapid unionid declines elsewhere. Substantial fouling and unionid mortality in the lake began ~7 years after invasion, longer than the typical 3–5 year pattern recorded in calcium-rich waterbodies. We hypothesize that suboptimal water chemistry delayed the onset of lethal fouling but is insufficient to provide a refuge for unionid populations within the lake. Our results desmonstrate that risk assessment must give more nuanced consideration to how the biodiversity impacts of zebra mussel invasion vary with water chemistry.