Seasonal variation of zebra mussel susceptibility to molluscicidal agents

1. The zebra mussel Dreissena polymorpha is a well-known biofouler, having potentially serious impacts on freshwater-dependent industries. Chemical treatment is the most popular approach to control this pest. The identification of potential peaks of increased susceptibility of the species to molluscicidal agents may provide environmental and economic benefits, allowing control to be achieved through lower toxin load. 2. In this study, the seasonal pattern of the tolerance of zebra mussels to three reference toxicants with distinct modes of action was recorded by conducting static bioassays. The annual profiles of dry tissue weight of a reference individual and mussels' filtration rate were also followed. 3. The tolerance of the species to toxicants was found to follow a pronounced annual cycle, increasing up to 22 times between early summer and winter. 4. The role of temperature and filtration rate as predictors of the mussels' susceptibility was confirmed, but there was no relation between this and the animals' body condition. Indeed, this study contradicts the common belief that physiologically exhausted adult zebra mussels are more susceptible to chemical control measures. 5. Synthesis and applications. The results presented here enlarge the body of knowledge of D. polymorpha biology and may have practical implications at three levels. First, they reinforce the importance of judicious sampling while testing molluscicides against zebra mussels as well as the specification of the specimens' collection date in toxicological studies. Secondly, they provide information that may be useful to the design of reactive chemical control strategies. In particular, they suggest that the dosage scheme in periodic treatment may be optimized to take advantage of the significant variation of a mussel's susceptibility between late spring and autumn. Finally, the outcome of this study highlights the need of taking into account the seasonality of bivalve responses in ecological risk assessment and the regulation of the discharge of chemicals into surface waters.