Impacts of ocean acidification in naturally variable coral reef flat ecosystems

Ocean acidification leads to changes in carbonate chemistry predicted to cause a decline in coral reef calcification. Several laboratory experiments have described calcification responses to increasing CO2. The few in situ studies on natural coral reefs that have been carried out have shown a direct relationship between aragonite saturation state (Ωarag) and net community calcification (Gnet). These studies have been performed over a limited range of Ωarag values, where extrapolation outside the observational range is required to predict future changes in calcification. We measured extreme diurnal variability in carbonate chemistry within a reef flat in the southern Great Barrier Reef, Australia. Ωarag 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. We define a relationship between Gnet and Ωarag, using our in situ measurements. We find Gnet to be linearly related to Ωarag, while temperature and nutrients had no significant effect on Gnet. Using our relationship between Gnet and Ωarag, we predict that net community calcification will decline by 55% of its preindustrial value by end-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 CO2 and future laboratory studies will need to incorporate this natural variability to address this question.

Shaw E. C., McNeil B. I., & Tilbrook B. D., in press. Impacts of ocean acidification in naturally variable coral reef flat ecosystems. Journal of Geophysical Research doi:10.1029/2011JC007655. Article (subscription required).

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