Seasonally variable thermal performance curves prevent adverse effects of heatwaves

Abstract The increasing frequency and intensity of heatwaves may represent a significant challenge for predicting vulnerability of populations in a warming ocean. The direct impacts of heatwaves on populations depend on the relative position of environmental temperatures to the thermal performance curve optima. If thermal performance curves are static, the effects of heatwaves may therefore change seasonally over the annual temperature cycle. However, these seasonal changes in the effects of heatwaves may be dampened by corresponding variation in thermal performance curves which, in organisms with relatively short generation times, may be driven by phenotypic plasticity as well as genetic differentiation. Here we investigate the effects of seasonal timing and duration on the impacts of heatwaves in the ecologically important copepod congeners Acartia tonsa and Acartia hudsonica , and test the hypotheses that 1) seasonal variation in thermal performance curves will reduce overall population vulnerability to heatwaves, and 2) that seasonal variation in TPCs will prevent negative transgenerational effects of heatwave. We characterized seasonal variation in thermal performance curves for several fitness-related traits. These experiments uncovered strong seasonal variation in the thermal performance curves of Acartia tonsa , and indicate that this variation buffers against negative effects of simulated heatwaves. We also quantified both direct and trans-generational effects of different duration heatwaves on copepods collected at various times throughout the season using simulated heatwave experiments. There was no consistent pattern in the transgenerational effects of parental exposure to heatwaves, which may indicate that seasonal variation in thermal performance curves reduces the effects of parental stress on offspring performance. Our results show that seasonal variation in thermal performance curves will likely play an important role in limiting the adverse effects of heatwaves on populations.