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 p CO 2 variability of 289–724 μatm was driven primarily by photosynthesis and respiration. The reef flat was found to be net autotrophic, with daily production of ∼ 35 mmol C m −2 d −1 and net calcification of ∼ 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 (Ω ar ). Although P net was the main driver of G net , Ω ar was still an important factor, where 95% of the variance in G net could be described by P net and Ω ar . Based on the observed in situ relationship, G net would be expected to reach zero when Ω ar is ∼ 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.