Contrasting changes in diel variations of net ecosystem calcification support that carbonate dissolution can be more sensitive to ocean acidification than coral calcification

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 ecosystem calcification (NEC) 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 NEC remained unchanged, whereas the nighttime NEC decreased between the control (ambient) and treatment (OA) conditions, suggesting that carbonate dissolution could be subtler than biological calcification to OA. The average sensitivity of the integrated daily NEC to changes in the seawater saturation state (Ω_a) was estimated to be a reduction of 54% in NEC per unit change in Ω_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 NEC.