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The effect of elevated pC[O.sub.2]/low pH on marine invertebrate benthic biodiversity, community structure and selected functional responses which underpin ecosystem services (such as community production and calcification) was tested in a medium-term (30 days) mesocosm experiment in June 2010. Standardised intertidal macrobenthic communities, collected (50.3567°N, 4.1277°W) using artificial substrate units (ASUs), were exposed to one of seven pH treatments (8.05, 7.8. 7.6, 7.4, 7.2, 6.8 and 6.0). Community net calcification/dissolution rates, as well as changes in biomass, community structure and diversity, were measured at the end of the experimental period. Communities showed significant changes in structure and reduced diversity in response to reduced pH: shifting from a community dominated by calcareous organisms to one dominated by non-calcareous organisms around either pH 7.2 (number of individuals and species) or pH 7.8 (biomass). These results were supported by a reduced total weight of CaC[O.sub.3] structures in all major taxa at lowered pH and a switch from net calcification to net dissolution around pH 7.4 ([Ω.sub.calc] = 0.78, [Ω.sub.ara] = 0.5). Overall community soft tissue biomass did not change with pH and high mortality was observed only at pH 6.0, although molluscs and arthropods showed significant decreases in soft tissue. This study supports and refines previous findings on how elevated pC[O.sub.2] can induce changes in marine biodiversity, underlined by differential vulnerability of different phyla. In addition, it shows significant elevated pC[O.sub.2-]/low pH-dependent changes in fundamental community functional responses underpinning changes in ecosystem services., Introduction Since the start of the industrial revolution (circa 1750), atmospheric carbon dioxide (C[O.sub.2]) levels have risen from 280 to 380 ppm (Gollety et al. 2008), mainly caused by anthropogenic [...]