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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 3 clone interactions indicated that phenotypic plasticity in response to toxic cyanobacteria had a genetic basis in two grazers. The significant food 3 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.

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Many aquatic organisms respond phenotypically, through morphological, behavioral, and physiological plasticity, to environmental changes. The small-size cladoceran Bosmina longirostris , a dominant zooplankter in eutrophic waters, displayed reduced growth rates in response to the presence of a toxic cyanobacterium, Microcystis aeruginosa , in their diets. The magnitude of growth reduction differed among 15 clones recently isolated from a single population. A significant interaction between clone and food type indicated a genetic basis for the difference in growth plasticity. The variation in phenotypic plasticity was visualized by plotting reaction norms with two diets. The resistance of each clone to dietary cyanobacteria was measured as the relative change in growth rates on the “poor” diet compared with the “good” diet. The enhanced resistance to M . aeruginosa in B . longirostris was derived from both the reduced slope of reaction norms and the increased mean growth rates with two diets. The large clonal variation within a B . longirostris population may contribute to local adaptation to toxic cyanobacteria and influence ecosystem function via clonal succession.

Evolutionary adaptation could assist organisms to cope with environmental changes, yet few experimental systems allow us to directly track evolutionary trajectory. Using experimental evolution, evolutionary tolerance to Microcystis aeruginosa was investigated in two cladocerans (Daphnia pulex and Simocephalus vetulus) to test the hypothesis that cladoceran grazers rapidly adapt to toxic cyanobacteria. After exposure for either three or six months, both grazers evolved a higher tolerance. The intrinsic rate of population increases in S. vetulus feeding on cyanobacteria was negatively correlated with that on green algae, which suggests that evolutionary adaptation in tolerance would carry a cost in the absence of cyanobacteria. However, the cyanobacterial selection resulted in a general increase in D. pulex when fed both cyanobacteria and green algae. Following a three-month relaxation of selection, S. vetulus in the selection line exhibited reverse evolution back to their original state when their diets were switched back to pure green algae. The present experimental evolution, both forwards and reverse, not only demonstrates the evolutionary responses of cladoceran grazers to toxic cyanobacterial cells in the laboratory, but also indicates that the grazer-cyanobacteria interaction would be an effective system to empirically study rapid evolution to environmental changes.