Infl uence of conductivity on life history traits of exotic and native amphipods in the St. Lawrence River

We compared the survival, growth and fecundity of the Ponto-Caspian amphipod Echinogammarus ischnus and the North American amphipod Gammarus fasciatus across a range of conductivities in a fl uvial lake at the confl uence of the St. Lawrence and Ottawa rivers (Quebec, Canada). Previous work has found that water chemistry (conductivity) regulates the direction and intensity of intraguild predation between these two species. Because E. ischnus evolved in ion-rich waters, we hypothesized that low conductivity has a negative effect on its life history traits, whereas G. fasciatus would be more tolerant. Consistent with this hypothesis, E. ischnus is abundant along the south shore of the lake in water of high conductivity (300 μS/cm), but rare along the north shore where conductivity is subject to strong fl uctuations (85–230 μS/cm). In laboratory experiments using water with low (108 μS/ cm), high (300 μS/cm) and intermediate (50:50 mix) conductivities, both species showed reduced growth at low conductivity, but E. ischnus also suffered higher mortality. E. ischnus collected from both shores did not differ in size-specifi c fecundity, size at reproduction, or size of adults. On the north shore, E. ischnus was present in low abundance at one site and absent from another site where it had been recorded in previous years. We conclude that low conductivity negatively affects the growth rate and mortality of E. ischnus, which is apparently eliminated from the north shore during periods of strong infl ux of ion-poor water from the Ottawa River, but re-establishes populations temporarily when higher conductivity is restored.