Heat-induced mass mortality of invasive zebra mussels (Dreissena polymorpha) at sublethal water temperatures

We observed a massive die-off of zebra mussels (Dreissena polymorpha) on the epilimnetic sediments of Gull Lake (Michigan, USA) during the relatively warm summer of 2010, even though water temperatures were below widely reported lethal temperatures of >= 30 degrees C. We followed up this observation with 4 years of in situ monitoring of caged mussels stocked across a depth-temperature gradient in Gull Lake. Mortality of caged D. polymorpha was largely explained by accumulated degree hours >25 degrees C, a temperature threshold that is considerably lower than laboratory-derived lethal temperatures for D. polymorpha. We also assessed both the acute and chronic thermal tolerance of Gull Lake D. polymorpha with laboratory experiments, which confirmed higher acute tolerance (up to 32 degrees C) under otherwise ideal conditions but high susceptibility to prolonged exposure to sublethal temperatures (exceeding 1700 degree hours >25 degrees C) as occurred in Gull Lake during the die-off. Our results indicate that the thermal tolerance of D. polymorpha under natural conditions may be lower than has been reported from laboratory studies. Lower temperature tolerance may have major implications for the dynamics, impacts, and management of this invasive species given future climate change scenarios.