Nighttime dissolution in a temperate coastal ocean ecosystem increases under acidification

Anthropogenic emissions of carbon dioxide (CO₂) are causing ocean acidification, lowering seawater aragonite (CaCO₃) saturation state (ω_arag), with potentially substantial impacts on marine ecosystems over the 21^st Century. Calcifying organisms have exhibited reduced calcification under lower saturation state conditions in aquaria. However, the in situ sensitivity of calcifying ecosystems to future ocean acidification remains unknown. Here we assess the community level sensitivity of calcification to local CO₂-induced acidification caused by natural respiration in an unperturbed, biodiverse, temperate intertidal ecosystem. We find that on hourly timescales nighttime community calcification is strongly influenced by ω_arag, with greater net calcium carbonate dissolution under more acidic conditions. Daytime calcification however, is not detectably affected by ω_arag. If the short-term sensitivity of community calcification toω_arag is representative of the long-term sensitivity to ocean acidification, nighttime dissolution in these intertidal ecosystems could more than double by 2050, with significant ecological and economic consequences.