Consequences of acclimation to Microcystis on the selective feeding behavior of the calanoid copepod Eudiaptomus gracilis

We tested the hypothesis that calanoid copepods would adapt to extended periods of Microcystis exposure by increasing selective feeding on alternative food. Copepod (Eudiaptomus gracilis) clearance rates were compared before and after a 5-d acclimation to Microcystis aeruginosa using paired food mixtures containing a microcystin-producing (MC+) or -lacking (MC-) strain and the green alga Chlamydomonas reinhardtii. Acclimation reduced the ingestion of Microcystis, increased ingestion of Chlamydomonas, and subsequently increased feeding selectivity. The effect of acclimation was more pronounced for food mixtures with MC+ Microcystis, suggesting that E. gracilis uses a strain-specific cue related to microcystin or microcystin itself to avoid harmful food. The results indicate that exposure to sublethal abundances of Microcystis may increase copepod tolerance to blooms, given sufficient alternative food. Zooplankton grazing behavior should, therefore, be viewed as flexible and adaptive to extended periods of cyanobacteria blooms.