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 pCO₂ 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.