Carbonate chemistry variability in the southern Great Barrier Reef: implications for future ocean acidification

Ocean acidification occurs as a consequence of the absorption of anthropogenicCO2 emissions by the ocean, which lowers sea surface pH and carbonate ionconcentrations over time. Ocean acidification has the potential to alter marinebiogeochemical cycling and biological processes. Coral reefs are believed to beparticularly vulnerable to ocean acidification as the reef framework is builtthrough calcification by marine organisms, where calcification is a key processaffected by changing seawater chemistry. Despite the predicted sensitivity ofcoral reefs to ocean acidification, there are only limited measurements ofcarbonate chemistry on coral reefs, since the vast majority of carbonatechemistry measurements have been taken in open ocean environments. In thisstudy the diurnal and seasonal carbonate chemistry variability was measured atLady Elliot Island (LEI), southern Great Barrier Reef, Australia. Seasonalvariability was observed in the waters offshore of LEI reef flat, similar toprevious observations at subtropical time series locations, and driven primarilyby seasonal temperature changes. On the reef flat, diurnal variability dominated,with the daily range of conditions exceeding those that are predicted to occurover the next century as a result of ocean acidification. Reef flat diurnalvariability was driven primarily by biological metabolic processes, includingcommunity photosynthesis, respiration, calcification and dissolution. At low tidethe reef flat was isolated from offshore waters allowing determination of netcommunity calcification rates (Gnet). It was found that Gnet was directly related tothe aragonite saturation state (Ωarag), and applying this relationship it wasvpredicted that end-century Gnet will be ~55% lower than the preindustrial value.For the first time, coral reef flat natural variability was used to predict futurecarbonate chemistry under a business-as-usual emissions scenario this century,showing that a decrease in seawater buffer capacity will lead to amplified naturalvariability and extreme carbonate chemistry conditions in the future (pCO2 levelsup to ~2100 ppm by end-century). Furthermore, corrosive conditions (whereΩarag