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

Abstract

Ocean acidification occurs as a consequence of the absorption of anthropogenic CO2 emissions by the ocean, which lowers sea surface pH and carbonate ion concentrations over time. Ocean acidification has the potential to alter marine biogeochemical cycling and biological processes. Coral reefs are believed to be particularly vulnerable to ocean acidification as the reef framework is built through calcification by marine organisms, where calcification is a key process affected by changing seawater chemistry. Despite the predicted sensitivity of coral reefs to ocean acidification, there are only limited measurements of carbonate chemistry on coral reefs, since the vast majority of carbonate chemistry measurements have been taken in open ocean environments. In this study the diurnal and seasonal carbonate chemistry variability was measured at Lady Elliot Island (LEI), southern Great Barrier Reef, Australia. Seasonal variability was observed in the waters offshore of LEI reef flat, similar to previous observations at subtropical time series locations, and driven primarily by seasonal temperature changes. On the reef flat, diurnal variability dominated, with the daily range of conditions exceeding those that are predicted to occur over the next century as a result of ocean acidification. Reef flat diurnal variability was driven primarily by biological metabolic processes, including community photosynthesis, respiration, calcification and dissolution. At low tide the reef flat was isolated from offshore waters allowing determination of net community calcification rates (Gnet). It was found that Gnet was directly related to the aragonite saturation state (Ωarag), and applying this relationship it was v predicted 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 future carbonate chemistry under a business-as-usual emissions scenario this century, showing that a decrease in seawater buffer capacity will lead to amplified natural variability and extreme carbonate chemistry conditions in the future (pCO2 levels up to ~2100 ppm by end-century). Furthermore, corrosive conditions (where Ωarag <1) are likely to begin by end-century, where this was previously unexpected for a sub-tropical coral reef ecosystem. Organisms will be exposed to the most extreme conditions on timescales of ~2hrs of each day. Therefore exposure time and the incorporation of natural variability into perturbation experiments will be an important future consideration in determining the vulnerability of coral reef species and communities to ocean acidification.