Deconvolving the long-term impacts of ocean acidification and warming on coral biomineralisation

Identifying the long-term effects of ocean acidification (OA) and global warming on coral calcification has proven elusive yet has major implications for the continuing viability of coral reefs in the face of climate change. Here we address this question using seasonally and annually resolved boron proxies (B-11/B-10 and B/Ca) of calcifying fluid (cf) pH(cf) and carbonate ion concentrations ((CO32-](cf)) preserved in a long-lived Porites coral from the Great Barrier Reef (GBR). From 1939 to 2013 we find that the coral pH(cf) closely followed the decline in seawater pH of similar to 0.1 units, but at a reduced rate of similar to 60%, indicative of biological buffering. Of the decline in pH(cf) similar to 82% is attributed to OA and similar to 17% to the similar to 0.5 degrees C long-term warming observed over this period. This long-term warming induced change in pH(cf) is consistent with the much larger seasonally modulated changes in pH(cf) where similar to 4 to 6 degrees C seasonal changes in temperatures are accompanied by relatively large antithetic similar to 0.1 changes in pH(cf). Furthermore, we find that although the supply of dissolved inorganic carbon (DIC) of the coral cf has remained at constant elevated levels of similar to 1.5 x seawater, there has been a significant long-term decline (4 to 11%) in the [CO32-](cf), due primarily to the OA-induced change in pHcf. This decline in [CO32-](cf), a critical parameter controlling calcification, is thus likely responsible for the similar to 15% decline in coral calcification observed since 1939 and across the GBR generally. (C) 2019 The Author(s). Published by Elsevier B.V.