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Anthropogenic CO2is decreasing oceanic pH and contributing to seawater warming. We tested the effects of low pH and high temperature at levels predicted for 2100 on an ecologically important predator–prey system (sea stars,Asterias rubens, and mussels,Mytilus edulis) from the NW Atlantic coast. Mussels are dominant competitors for space and important ecosystem engineers, while sea stars control mussel populations and thus local community structure. We found sea stars to be negatively affected in growth rate by low pH, with growth further reduced by a high temperature. In contrast, mussel growth rate was positively affected by low pH, with no response to temperature within the tested range. Predation of sea stars on mussels, measured as per-capita consumption rate, decreased in acidified conditions by 50%. Our study suggests that mussels may not be negatively affected by pH at the levels predicted for the end of this century and that mussels may be subjected to a reduced predation from sea stars under future conditions. [ABSTRACT FROM PUBLISHER]