Stable carbon and nitrogen isotope analysis (SIA) was used to examine trophic interactions in a naturally acidified shallow coastal CO 2 area in the Mediterranean Sea. SIA was helpful to determine the potential effects of ocean acidification (OA) on the interactions of organic matter sources and polychaete consumers, which appear to be tolerant to high pCO 2 . Mesoherbivore consumers, represented by three polychaete species, Platynereis dumerilii (Audouin and Milne Edwards, 1834) (Nereididae), Polyophthalmus pictus (Dujardin, 1839) (Opheliidae) and Syllis prolifera Krohn, 1852 (Syllidae), are abundant in the low pH conditions of the CO 2 vents. Samples of consumers and potential organic matter sources (macroalgae, seagrass and epiphytes) were collected in two periods (May-June and Oct-Nov 2012) in two acidified areas (Castello Aragonese CO 2 vents, off Ischia, Italy) and in two control areas. Results suggest a substantial effect of high pCO 2 on isotopic composition of both organic matter sources and consumers. The recurring 13 C depletion observed in the acidified sites is probably due to an increase in carbon availability and to the exploitation of volcanic-derived dissolved inorganic carbon (DIC) present in the area. The nutritional quality of organic matter sources was higher in low pH stations, where indeed C:N ratios decreased. Despite the occurrence of these effects, the trophic habit of high pCO 2 tolerant polychaete consumers seems not to be dramatically affected. Our findings shed the light on how some benthic organisms may respond well to near-future OA conditions, exhibiting high plasticity also in their feeding habits, without being subject to dramatic ecological changes.
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