Indirect effects may buffer negative responses of seagrass invertebrate communities to ocean acidification

Ocean acidification has been shown to have highly variable effects, with many negative and some positive responses from individual species, while community level effects are largely unknown. Although an overall loss of biodiversity is expected, predicting the effects of ocean acidification on whole assemblages can be problematic as both direct and indirect effects of acidification must be taken into consideration. This study demonstrates how invertebrate assemblages associated with the highly productive seagrass, Posidonia oceanica, respond to natural acidification that occurs at CO2 vents off the coast of Italy. We examined seasonal differences in invertebrate community structure between two distinct pH zones: control (pH 8.1) and acidified (pH 7.8) and show that many groups of invertebrate taxa were robust to acidification effects. Differences in community struc- ture appeared to be driven by the indirect effects of acidification, such as changes to canopy structure and food availability, rather than physiological intolerance to low pH. The number of invertebrates collected in acidified stations was almost double that of control stations during the study and many heavily calcified species appeared to thrive. These results highlight how positive indirect effects may buffer the ecological impacts of acidification, and provide evidence that this highly productive, nearshore habitat may provide refuge to its associated commu- nities from future ocean acidification.