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 (<em>G</em>_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 (<em>P</em>_net), <em>G</em>_net, and carbonate chemistry of a reef flat at One Tree Island, Great Barrier Reef. Diurnal <em>p</em>CO₂ 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. <em>G</em>_net was strongly related to <em>P</em>_net, which drove a hysteresis pattern in the relationship between <em>G</em>_net and aragonite saturation state (Ω_ar). Although <em>P</em>_net was the main driver of <em>G</em>_net, Ω_ar was still an important factor, where 95% of the variance in <em>G</em>_net could be described by <em>P</em>_net and Ω_ar. Based on the observed in situ relationship, <em>G</em>_net would be expected to reach zero when Ω_ar is ∼ 2.5. It is unknown what proportion of a decline in <em>G</em>_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.