Secondary calcification and dissolution respond differently to future ocean conditions

Abstract. Climate change threatens both the accretion and erosion processes that sustain coral reefs. Secondary calcification, bioerosion, and reef dissolution are integral to the structural complexity and long-term persistence of coral reefs, yet these processes have received less research attention than reef accretion by corals. In this study, we use climate scenarios from RCP 8.5 to examine the combined effects of rising ocean acidity and sea surface temperature (SST) on both secondary calcification and dissolution rates of a natural coral rubble community using a flow-through aquarium system. We found that secondary reef calcification and dissolution responded differently to the combined effect of pCO₂ and temperature. Calcification had a non-linear response to the combined effect of pCO₂ and temperature: the highest calcification rate occurred slightly above ambient conditions and the lowest calcification rate was in the highest temperature–pCO₂ condition. In contrast, dissolution increased linearly with temperature–pCO₂ . The rubble community switched from net calcification to net dissolution at +271 μatm pCO₂ and 0.75 °C above ambient conditions, suggesting that rubble reefs may shift from net calcification to net dissolution before the end of the century. Our results indicate that (i) dissolution may be more sensitive to climate change than calcification and (ii) that calcification and dissolution have different functional responses to climate stressors; this highlights the need to study the effects of climate stressors on both calcification and dissolution to predict future changes in coral reefs.