Intertidal predator-prey interactions within the context of a changing coastal ocean

Environmental change in the ocean is known to induce mechanical, physiological, and behavioral costs for many marine organisms. While such alterations have the potential to influence the outcome of trophic interactions, the nature and strength of these shifts will depend on the differential effects of such combined stressors on key trophic levels. Here we investigated the interactive impacts of CO₂, temperature and nutrients on a rocky intertidal three-level food web. We found that our intermediate consumer, blue mussels, grew largest under warming and ambient pH conditions when not in the presence of predators. However, overall mussel feeding, and growth was reduced in the presence of predators and the positive effect of warming on shell growth was reversed. This led to predators consuming the largest fraction of mussels under elevated temperature and pH conditions. Furthermore, although low pH seawater impaired mussel growth without the addition of nutrients, this effect was reduced under elevated nutrient and food conditions. This demonstrates that the influence of predators and basal resources can mediate the effects of warming and elevated CO₂ on the behavior and physiology of an intermediate consumer and highlights the importance of considering the vulnerability of species to changing ocean conditions within the context of the food web in which they are embedded.