Variation in rates of herbivory may be driven by direct effects of the abiotic environment on grazers, as well as indirect effects mediated by their food. Disentangling these direct and indirect effects is of fundamental importance for ecological forecasts of changing climate on species interactions and their influence on biogenic habitat. Whilst elevated atmospheric CO 2 may have direct effects on grazers with calcareous structures via 'ocean acidification', it may also have indirect effects via changes caused to their food. In our study we initially tested, and confirmed, that enriched CO2 altered per capita rates of grazing before assessing the relative importance of indirect and direct effects in driving this response. Our results eliminated the model of a direct effect of CO2 enrichment on the grazers themselves and supported the model of an indirect effect driven by a change in the food (i.e. turf algae). We suggest that this indirect effect manifested as grazers responded to the increased nitrogen content (i.e. %N) of algal tissue that resulted under CO2 enrichment. Understanding such indirect effects of modified environmental conditions provide important mechanistic links between climate conditions and the ecological processes they influence. © 2013 Inter-Research. ; Link_to_subscribed_fulltext