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The unprecedented rate of CO 2  increase in our atmosphere and subsequent ocean acidification (OA) threatens coastal ecosystems. To forecast the functioning of coastal seagrass ecosystems in acidified oceans, more knowledge on the long-term adaptive capacities of seagrass species and their epibionts is needed. Therefore we studied morphological characteristics of  Posidonia oceanica  and the structure of its epibiont communities at a Mediterranean volcanic CO 2  vent off Panarea Island (Italy) and performed a laboratory experiment to test the effect of OA on  P .  oceanica  photosynthesis and its potential buffering capacity. At the study site east of Basiluzzo Islet, venting of CO 2  gas was controlled by tides, resulting in an average pH difference of 0.1 between the vent and reference site.  P .  oceanica  shoot and leaf density was unaffected by these levels of OA, although shorter leaves at the vent site suggest increased susceptibility to erosion, potentially by herbivores. The community of sessile epibionts differed in composition and was characterized by a higher species richness at the vent site, though net epiphytic calcium carbonate concentration was similar. These findings suggest a higher ecosystem complexity at the vent site, which may have facilitated the higher diversity of copepods in the otherwise unaffected motile epibiont community. In the laboratory experiment,  P .  oceanica  photosynthesis increased with decreasing pH T  (7.6, 6.6, 5.5), which induced an elevated pH at the leaf surfaces of up to 0.5 units compared to the ambient seawater pH T  of 6.6. This suggests a temporary pH buffering in the diffusive boundary layer of leaves, which could be favorable for epibiont organisms. The results of this multispecies study contribute to understanding community-level responses and underlying processes in long-term acidified conditions. Increased replication and monitoring of physico-chemical parameters on an annual scale are, however, recommended to assure that the biological responses observed during a short period reflect long-term dynamics of these parameters.

Response of Posidonia oceanica seagrass and its epibiont communities to ocean acidification