Local adaptation mediates direct and indirect effects of multiple stressors on consumer fitness.

Anthropogenic impacts are expected to increase the co-occurrence of stressors that can fundamentally alter ecosystem structure and function. To cope with stress, many organisms locally adapt, but how such adaptations affect the ability of an organism to manage co-occurring stressors is not well understood. In aquatic ecosystems, elevated temperatures and harmful algal blooms are common co-stressors. To better understand the role and potential trade-offs of local adaptations for mitigating the effects of stressors, Daphnia pulicaria genotypes that varied in their ability to consume toxic cyanobacteria prey (i.e., three tolerant and three sensitive) were exposed to five diets that included combinations of toxic cyanobacteria, Microcystis aeruginosa, and a green alga, Ankistrodesmus falcatus, under two temperatures (20 °C vs. 28 °C). A path analysis was conducted to understand how local adaptations affect energy allocation to intermediate life history traits (i.e., somatic growth, fecundity, survival) that maximize Daphnia fitness (i.e., population growth rate). Results from the 10-day study show that tolerant Daphnia genotypes had higher fitness than sensitive genotypes regardless of diet or temperature treatment, suggesting toxic cyanobacteria tolerance did not cause a decrease in fitness in the absence of cyanobacteria or under elevated temperatures. Results from the path analysis demonstrated that toxic cyanobacteria had a stronger effect on life history traits than temperature and that population growth rate was mainly constrained by reduced fecundity. These findings suggest that local adaptations to toxic cyanobacteria and elevated temperatures are synergistic, leading to higher survivorship of cyanobacteria-tolerant genotypes during summer cyanobacterial bloom events.