Description: Robust estimates of marine species vulnerability to ongoing climate change require realistic stressor experiments. Here, we subjected an important coastal predator, the sea star Asterias rubens, to projected warming and ocean acidification over an annual seasonal cycle. Warming and, less so, acidification, had strongly season-specific impacts on animal energy budgets. Specifically, simulated future summer temperatures caused 〉95% sea star mortality, reduced feeding rate and body mass loss. Additional acute experiments demonstrated that respiratory oxygen flux was preferentially directed to support high summer metabolism at the expense of feeding-related processes. Using 15 years of field temperature data and end of century warming projections, we estimate that potentially lethal summer heat waves will occur in 20% of future years. Our study demonstrates the importance of assessing stress responses along seasonal thermal cycles and the high selective force that future summer heat waves likely can exert on coastal marine animal populations.

Description: The plea for using more “realistic,” community‐level, investigations to assess the ecological impacts of global change has recently intensified. Such experiments are typically more complex, longer, more expensive, and harder to interpret than simple organism‐level benchtop experiments. Are they worth the extra effort? Using outdoor mesocosms, we investigated the effects of ocean warming (OW) and acidification (OA), their combination (OAW), and their natural fluctuations on coastal communities of the western Baltic Sea during all four seasons. These communities are dominated by the perennial and canopy‐forming macrophyte Fucus vesiculosus—an important ecosystem engineer Baltic‐wide. We, additionally, assessed the direct response of organisms to temperature and pH in benchtop experiments, and examined how well organism‐level responses can predict community‐level responses to the dominant driver, OW. OW affected the mesocosm communities substantially stronger than acidification. OW provoked structural and functional shifts in the community that differed in strength and direction among seasons. The organism‐level response to OW matched well the community‐level response of a given species only under warm and cold thermal stress, that is, in summer and winter. In other seasons, shifts in biotic interactions masked the direct OW effects. The combination of direct OW effects and OW‐driven shifts of biotic interactions is likely to jeopardize the future of the habitat‐forming macroalga F. vesiculosus in the Baltic Sea. Furthermore, we conclude that seasonal mesocosm experiments are essential for our understanding of global change impact because they take into account the important fluctuations of abiotic and biotic pressures.