Food quality mediates responses of Daphnia magna life history traits and heat tolerance to elevated temperature

Surface waters are warming due to climate change, potentially pushing aquatic organisms closer to their thermal tolerance limits. However, cyanobacterial blooms are expected to occur more often with rising temperature, increasing the likelihood of poor-quality food available for herbivorous zooplankton. Zooplankton can adapt locally by genetic differentiation or via adaptive phenotypic plasticity to increasing temperatures, but there is limited knowledge on how these processes may be affected by food quality limitation imposed by cyanobacteria. To test the effects of cyanobacteria-mediated food quality on local temperature adaptation, we measured juvenile somatic growth and reproduction of five Daphnia magna clones from different latitudinal origin grown on three food qualities at 20, 24, and 28 degrees C. Additionally we estimated short-term heat tolerance, measured as knockout time (time to immobility) at lethally high temperature, of two clones acclimated to the three temperatures and two food quality levels to test for the effects of food quality on adaptive plastic responses. As expected, clones from lower latitudes showed on average better somatic growth and reproduction than clones from higher latitudes at higher temperatures. However, the difference in somatic growth diminished with increasing cyanobacteria abundance in the diet, suggesting constraints on local genetic adaptation under predicted decreases in food quality. As expected, short-term heat tolerance of the clones generally increased with increasing acclimation temperature. However, heat tolerance of animals acclimated to the highest temperature was larger when grown at medium than at good food quality, whereas the opposite response was observed for animals acclimated to the lowest temperature. This suggests a better adaptive phenotypic response of animals to elevated temperatures under higher cyanobacteria abundance, and thus shows an opposite pattern to the results for somatic growth. Overall, we demonstrate that food quality limitation can mediate responses of D. magna life history traits and heat tolerance to increasing temperatures, and that the effects differ depending on the time scale studied, that is, mid-term (somatic growth) versus short-term (tolerance to acute heat stress). These aspects will need further attention to accurately predict of how organisms will cope with future global warming by local adaptation and adaptive phenotypic plasticity.