1. Cyanobacterial toxin production in response to direct and indirect zooplankton feeding activity was examined using four strains of Microcystis aeruginosa, of which three were previously reported to be toxic to zooplankton and one non-toxic. Direct (Microcystis cultured with zooplankton) and indirect effects (Microcystis cultured with filtered zooplankton culture media, ZCMF) were tested for the zooplankton species, Moina macrocopa, Daphnia magna or D. pulex.
2. With direct exposure to zooplankton, increased mass-specific microcystin productions occurred in all Microcystis strains, with mean microcystin concentrations up to five times greater (61.5–177.3 μg g−1 dry cell) than the controls.
3. With indirect exposure, mass-specific microcystin production increased over controls in three strains of M. aeruginosa. Mean maximum concentrations of microcystin during the experiment were 92.6–125.7 μg g−1 dry cell.
4. These results suggest that several strains of Microcystis aeruginosa increased toxin production in response to direct and indirect exposure to herbivorous zooplankton of several species, and support the hypothesis that this response is an induced defence mediated by the release of info-chemicals from zooplankton.