Thermal Tolerances of the Predatory Cladocerans Bythotrephes cederstroemi and Leptodora kindti: Relationship to Seasonal Abundance in Western Lake Erie

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Abstract
Abundances of two predatory cladocerans, recently introduced Bythotrephes cederstroemi (Schoedler) and native Leptodora kindti Focke, in western Lake Erie were negatively correlated in 1987, with the invader abundant in late autumn and the native more abundant in summer. Laboratory experiments were conducted in summer and autumn of 1988 to determine if differences in thermal tolerance could explain species replacement in these two ecologically similar species. Body mass, sex ratio, fecundity, and population abundance were also measured in summer and autumn populations. Mortality of Bythotrephes increased with temperature, while mortality of Leptodora increased both above and below ambient lake temperatures. Leptodora was less tolerant of low temperature (5–15 °C), and both species were equally tolerant of high temperature (20–30 °C). Seasonal acclimation to temperature was absent in Bythotrephes, while Leptodora showed seasonal shifts in thermal tolerance. Reduced fecundity, greater percentage of males, and presence of sexual eggs were evidence that Bythotrephes reached an environmental limit during summer. Decreased abundance of Leptodora in autumn can be explained by sensitivity to declining water temperature, but warm water temperature alone cannot explain numerical dominance of Leptodora in summer.