Microgeographic adaptation to toxic cyanobacteria in two aquatic grazers

The hypothesis that zooplankton display local adaptations to toxic cyanobacteria was tested by investigating the performance of 24 clones from six populations of the large-bodied Daphnia pulex and 13 clones from four populations of the small-bodied Chydorus sphaericus. They were fed a pure nutritious green alga Chlorella pyrenoidosa (good food) and a mixed diet with a toxic cyanobacterium Microcystis aeruginosa (poor food). The grazer performances feeding on the poor food significantly differed among their populations over a small geographic scale (7–10 km for D. pulex and 15–20 km for C. sphaericus). Both the absolute tolerance (performance on poor food) and relative tolerance (performance on poor food relative to good food) to toxic M. aeruginosa were enhanced with increasing microcystin concentrations in the sediments and finally reached the maximum values. Additionally, the significant food × clone interactions indicated that phenotypic plasticity in response to toxic cyanobacteria had a genetic basis in two grazers. The significant food × population interactions and correlations between growth plasticity and microcystin concentrations indicated that this phenotypic plasticity evolved across grazer populations. The reduced plasticity was evidenced by both the reduced slope of reaction norms and the enhanced mean performance for both diets. D. pulex had more evolutionary potential for the reduced plasticity than C. sphaericus. These results suggest that zooplankton populations developed evolutionary tolerance to toxic cyanobacteria on a microgeographic level.