Low environmental microcystin concentrations affected sublethal population-level responses but not survival among freshwater keystone species

Microcystin is an emerging global environmental threat, widely associated with the increasing incidence of freshwater cyanobacterial harmful algal blooms. However, the ecological effects of environmental microcystin concentrations on survival and fitness-related traits that underpin ecosystem functions among key freshwater populations remains poorly understood. Using a suite of sublethal and chronic studies, we tested the population-level effects of purified MC-LR and crude extract of Microcystis aeruginosa on individual fitness-related endpoints among three ecologically important freshwater species, Daphnia magna, Gammarus pulex and Dikerogammarus villosus. Low microcystin concentrations had no effects on survival but altered a range of ecologically relevant sublethal responses across the three test species studied. Purified MC-LR increased the feeding rate and stimulated parameters of reproduction in D. magna. Non-monotonic responses were observed on the mean number of broods produced per female, mean number of neonates produced per female and the intrinsic rate of natural increase (r), resulting in the stimulation of the population growth rate. However, the somatic growth rate of daphnids was unaffected by purified MC-LR concentrations. Both treatments affected feeding and growth rates of amphipods differently, suggesting low environmental microcystin concentrations can alter energy acquisition and biological fitness, thereby influencing population size, community structure and ecosystem functioning.