Natural in situ relationships suggest coral reef calcium carbonate production will decline with ocean acidification

There are few in situ studies showing how net community calcification (G(net)) of coral reefs is related to carbonate chemistry, and the studies to date have demonstrated different predicted rates of change. In this study, we measured net community production (P-net), G(net), and carbonate chemistry of a reef flat at One Tree Island, Great Barrier Reef. Diurnal pCO(2) variability of 289-724 atm was driven primarily by photosynthesis and respiration. The reef flat was found to be net autotrophic, with daily production of approximate to 35 mmol C m(-2) d(-1) and net calcification of approximate to 33 mmol C m(-2) d(-1). G(net) was strongly related to P-net, which drove a hysteresis pattern in the relationship between G(net) and aragonite saturation state ((ar)). Although P-net was the main driver of G(net), (ar) was still an important factor, where 95% of the variance in G(net) could be described by P-net and (ar). Based on the observed in situ relationship, G(net) would be expected to reach zero when (ar) is approximate to 2.5. It is unknown what proportion of a decline in G(net) would be through reduced calcification and what would occur through increased dissolution, but the results here support predictions that overall calcium carbonate production will decline in coral reefs as a result of ocean acidification.