Spatial avoidance of Microcystis aeruginosa by Daphnia: Fitness consequences and evolutionary implications

We tested the hypothesis that species (clones) of Daphnia, originating from lakes with very different cyanobacterial abundances, use strategies to optimize their performance in the presence of toxic Microcystis aeruginosa by distributing differently in vertical gradients of valuable food, toxigenic cyanobacteria, and temperature. A laboratory tube system with different combinations of food items in the temperature gradient was used to determine the vertical distribution and performance (growth and lipid index) of Daphnia from three contrasting environments: (1) Daphnia carinata from a eutrophic lake with dense populations of cyanobacteria; (2) Daphnia galeata adapted to low cyanobacteria densities in a deep, mesotrophic lake; and (3) arctic Daphnia pulex assumed to be naive with few adaptations against pelagic cyanobacteria. When confronted with toxic Microcystis in the epilimnion, Daphnia can respond by avoidance behavior (i.e., suffer metabolic costs from low temperature), reduction of their overall feeding rate in order to avoid the ingestion of toxic cells, metabolizing the toxin biochemically, or not responding if they were never confronted with toxic cyanobacteria. The experiments suggest that D. carinata was sensitive to toxigenic Microcystis and responded by avoidance, D. galeata was less sensitive and preferred to stay in the warm epilimnion, and D. pulex was naive as expected. Thus, the behavioral strategies of the three Daphnia species appear to reflect interplay between evolutionary history, sensitivity to Microcystis, and the environmental conditions.