Understanding the ecological implications of global climate change requires investigations of not only the direct effects of environmental change on species performance but also indirect effects that arise from altered species interactions. We performed CO 2 perturbation experiments to investigate the effects of ocean acidification on the trophic interaction between the brown seaweed Fucus vesiculosus and the herbivorous isopod Idotea baltica. We predicted faster growth of F. vesiculosus at elevated CO 2 -concentrations and higher carbon content of the algal tissue. We expected that I. baltica has different consumption rates on algae that have been grown at different CO 2 levels and that the isopods remove surplus carbon metabolically by enhanced respiration. Surprisingly, growth of F. vesiculosus as well as the C:N-ratio of the algal tissue were reduced at high CO 2 -levels. The changes in the elemental composition had no effect on the consumption rates and the respiration of the herbivores. An additional experiment showed that consumption of F. vesiculosus by the isopod Idotea emarginata was independent of ocean acidification and temperature. Our results could not reveal any effects of ocean acidification on the per capita strength of the trophic interaction between F. vesiculosus and its consumers. However, reduced growth of the algae at high CO 2 -concentrations might reduce the capability of the seaweed to compensate losses due to intense herbivory.

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