Seawater carbonate chemistry and net community calcification, production and respiration of coral reef

Previous studies have found that calcification in coral reefs is generally stronger during the day, whereas dissolution is prevalent at night. On the basis of these contrasting patterns, the diel variations of net community calcification (NCC) were monitored to examine the relative sensitivity of CaCO3 production (calcification) and dissolution in coral reefs to ocean acidification (OA), using two mesocosms that replicated a typical subtropical coral reef ecosystem in southern Taiwan. The results revealed that the daytime NCC remained unchanged, whereas the nighttime NCC decreased between the control (ambient) and treatment (OA) conditions, suggesting that carbonate dissolution could be more sensitive to OA than coral calcification. The average sensitivity of the integrated daily NCC to changes in the seawater saturation state (Omega a) was estimated to be a reduction of 54% in NCC per unit change in Omega a, which is consistent with the global average. In summary, our results support the prevailing anticipation that OA would lead to a reduction in the overall accretion of coral reef ecosystems. However, increased CaCO3 dissolution rather than decreased coral calcification could be the dominant driving force responsible for this OA-induced reduction in NCC.

过往研究已证实，珊瑚礁的钙化作用通常在日间更为活跃，而碳酸钙溶解过程则多发生于夜间。基于这一截然相反的昼夜变化模式，本研究依托两套复刻中国台湾南部典型亚热带珊瑚礁生态系统的中型实验生态系统（mesocosms），对群落净钙化作用（net community calcification, NCC）的日动态进行监测，以探究珊瑚礁生态系统中碳酸钙生成（钙化作用）与溶解过程对海洋酸化（ocean acidification, OA）的相对敏感性。
研究结果显示，日间群落净钙化作用未出现显著变化，而对照组（环境海水条件）与处理组（海洋酸化条件）间的夜间群落净钙化作用显著下降，这表明碳酸钙溶解过程对海洋酸化的敏感性或高于珊瑚钙化作用。
经估算，每日累计群落净钙化作用对海水饱和状态（seawater saturation state, Ω_a）变化的平均敏感性为：每单位Ω_a变化对应群落净钙化作用降低54%，该结果与全球平均水平相符。
综上，本研究结果支持学界普遍预期，即海洋酸化会导致珊瑚礁生态系统的整体加积量下降。不过，引发海洋酸化诱导的群落净钙化作用降低的主导驱动因素，或许是碳酸钙溶解过程增强而非珊瑚钙化作用减弱。