Single and mixed diets of the toxic Cyanobacteria Microcystis aeruginosa and Raphidiopsis raciborskii differently affect Daphnia feeding behavior

Cyanobacteria usually dominate phytoplankton in eutrophic water bodies, where they form dense monospecific or mixed blooms. Moreover, these organisms are an important energy source for aquatic food webs in these environments, establishing a direct link with zooplankton. In contrast, cyanobacteria can weaken that link by limiting zooplankton grazing pressure due to morphological and chemical traits such as the production of toxic peptides and alkaloids. Nevertheless, some zooplankton survive under CyanoHABs (cyanobacterial harmful algal blooms) and it has been related to both physiological tolerance and behavioral mechanisms, the last being less explored. Thus, we aimed to investigate the impact of a single and mixed diet of different toxin-producing cyanobacteria on the feeding behavior of two species of Daphnia. Adult individuals of the species Daphnia similis (large) and D. laevis (small) were fed with single and mixed cell suspensions of the toxic cyanobacteria Microcystis aeruginosa and Raphidiopsis raciborskii at gradually increased proportions (0–90%) relative to the edible green algae Monoraphidium capricornutum. Here, we found a marked decrease in the filtration (measured as clearance rate) of D. similis but not for D. laevis as cyanobacteria increased in suspension. At the same condition, both daphnids displayed a nonselective behavior despite a reduced ingestion rate food of edible algae. Our study shows that cyanobacteria may directly and indirectly impact Daphnia filtration rate and ability to forage for edible food.