[1] Ocean acidification leads to changes in marine carbonate chemistry that are predicted to cause a decline in future coral reef calcification. Several laboratory and mesocosm experiments have described calcification responses of species and communities to increasing CO 2 . The few in situ studies on natural coral reefs that have been carried out to date have shown a direct relationship between aragonite saturation state (W arag ) and net community calcification (G net ). However, these studies have been performed over a limited range of W arag values, where extrapolation outside the observational range is required to predict future changes in coral reef calcification. We measured extreme diurnal variability in carbonate chemistry within a reef flat in the southern Great Barrier Reef, Australia. W arag varied between 1.1 and 6.5, thus exceeding the magnitude of change expected this century in open ocean subtropical/tropical waters. The observed variability comes about through biological activity on the reef, where changes to the carbonate chemistry are enhanced at low tide when reef flat waters are isolated from open ocean water. We define a relationship between net community calcification and W arag , using our in situ measurements. We find net community calcification to be linearly related to W arag , while temperature and nutrients had no significant effect on G net . Using our relationship between G net and W arag , we predict that net community calcification will decline by 55% of its preindustrial value by the end of the century. It is not known at this stage whether exposure to large variability in carbonate chemistry will make reef flat organisms more or less vulnerable to the non-calcifying physiological effects of increasing ocean CO 2 and future laboratory studies will need to incorporate this natural variability to address this question.

Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems.

Ocean acidification, via an anthropogenic increase in seawater carbon dioxide (CO2 ), is potentially a major threat to coral reefs and other marine ecosystems. However, our understanding of how natural short-term diurnal CO2 variability in coral reefs influences longer term anthropogenic ocean acidification remains unclear. Here, we combine observed natural carbonate chemistry variability with future carbonate chemistry predictions for a coral reef flat in the Great Barrier Reef based on the RCP8.5 CO2 emissions scenario. Rather than observing a linear increase in reef flat partial pressure of CO2 (pCO2 ) in concert with rising atmospheric concentrations, the inclusion of in situ diurnal variability results in a highly nonlinear threefold amplification of the pCO2 signal by the end of the century. This significant nonlinear amplification of diurnal pCO2 variability occurs as a result of combining natural diurnal biological CO2 metabolism with long-term decreases in seawater buffer capacity, which occurs via increasing anthropogenic CO2 absorption by the ocean. Under the same benthic community composition, the amplification in the variability in pCO2 is likely to lead to exposure to mean maximum daily pCO2 levels of ca. 2100 μatm, with corrosive conditions with respect to aragonite by end-century at our study site. Minimum pCO2 levels will become lower relative to the mean offshore value (ca. threefold increase in the difference between offshore and minimum reef flat pCO2 ) by end-century, leading to a further increase in the pCO2 range that organisms are exposed to. The biological consequences of short-term exposure to these extreme CO2 conditions, coupled with elevated long-term mean CO2 conditions are currently unknown and future laboratory experiments will need to incorporate natural variability to test this. The amplification of pCO2 that we describe here is not unique to our study location, but will occur in all shallow coastal environments where high biological productivity drives large natural variability in carbonate chemistry.

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 latm was driven primarily by photosynthesis and respiration. The reef flat was found to be net autotrophic, with daily production of 35 mmol C m 22 d 21 and net calcification of 33 mmol C m 22 d 21 . G net was strongly related to P net , which drove a hysteresis pattern in the relationship between G net and aragonite saturation state (X ar ). Although P net was the main driver of G net , X ar was still an important factor, where 95% of the variance in G net could be described by P net and X ar . Based on the observed in situ relationship, G net would be expected to reach zero when X 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.

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