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Concerns have been raised about how coral reefs will be affected by ocean acidification, but projections of future seawater CO2 chemistry have focused solely on changes in the pH and aragonite saturation state (Ωa) of open-ocean surface seawater conditions surrounding coral reefs rather than the reef systems themselves. The seawater CO2 chemistry within heterogeneous reef systems can be significantly different from that of the open ocean depending on the residence time, community composition and the main biogeochemical processes occurring on the reef, that is, net ecosystem production (NEP = gross primary production − autotrophic and heterotrophic respiration) and net ecosystem calcification (NEC = gross calcification − gross CaCO3 dissolution), which combined act to modify seawater chemistry. On the basis of observations from the Bermuda coral reef, we show that a range of projected biogeochemical responses of coral reef communities to ocean acidification by the end of this century could partially offset changes in seawater pH and Ωa by an average of 12-24% and 15-31%, respectively. [ABSTRACT FROM AUTHOR]