Secondary calcification and dissolution respond differently to future ocean conditions

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.气候变化威胁着维持珊瑚礁的礁体加积与侵蚀过程。次级钙化（secondary calcification）、生物侵蚀以及礁体溶解是支撑珊瑚礁结构复杂性与长期存续的核心过程，但相较于珊瑚的造礁加积作用，这些过程受到的研究关注相对较少。本研究采用典型浓度路径8.5（RCP 8.5）下的气候情景，借助流水式水族箱系统，探究海洋酸化加剧与海表温度（SST）升高的复合效应对自然珊瑚碎砾群落的次级钙化速率与溶解速率的共同影响。研究发现，珊瑚礁次级钙化与溶解过程对二氧化碳分压（pCO2）与温度的复合效应呈现出不同的响应特征：钙化速率对二者的复合效应呈非线性响应，最高钙化速率略高于环境基准条件，最低钙化速率则出现在最高温度-pCO2组合情景中；与之相反，溶解速率随温度-pCO2升高呈线性增长趋势。该碎砾群落的净代谢从净钙化转向净溶解的临界点为pCO2较环境基准升高271 µatm、温度较环境基准升高0.75 °C，这表明珊瑚碎礁可能在本世纪末之前就从净钙化状态转向净溶解状态。本研究结果显示：其一，溶解过程对气候变化的敏感性可能高于钙化过程；其二，钙化与溶解过程对气候胁迫因子的功能响应存在差异，这凸显了同时探究气候胁迫对钙化与溶解过程的影响以预测珊瑚礁未来变化的必要性。