Ocean acidification accelerates dissolution of experimental coral reef communities

This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract Ocean acidification (OA) poses a severe threat to tropical coral reefs, yet much of what is know about these effects comes from individual corals and algae incubated in isolation under high pCO 2. Studies of similar effects on coral reef communities are scarce. To investigate the response of coral reef communities to OA, we used large out-5 door flumes in which communities composed of calcified algae, corals, and sediment were combined to match the percentage cover of benthic communities in the shallow back reef of Moorea, French Polynesia. Reef communities in the flumes were exposed to ambient (∼ 400 µatm) and high pCO 2 (∼ 1300 µatm) for 8 weeks, and calcification rates measured for the constructed communities including the sediments. Community 10 calcification was depressed 59 % under high pCO 2 , with sediment dissolution explaining ∼ 50 % of this decrease; net calcification of corals and calcified algae remained positive, but was reduced 29 % under elevated pCO 2. These results show that despite the capacity of coral reef calcifiers to maintain positive net accretion of calcium car-bonate under OA conditions, reef communities might switch to net dissolution as pCO 2 15 increases, particularly at night, due to enhanced sediment dissolution.