Impacts of ocean acidification in naturally variable coral reef flat ecosystems

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 CO₂. 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 (Ω_arag) and net community calcification (G_net). However, these studies have been performed over a limited range of Ω_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. Ω_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 Ω_arag, using our in situ measurements. We find net community calcification to be linearly related to Ω_arag, while temperature and nutrients had no significant effect on G_net. Using our relationship between G_net and Ω_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 CO₂ and future laboratory studies will need to incorporate this natural variability to address this question