Daily variation in net primary production and net calcification in coral reef communities exposed to elevated pCO₂

Abstract. The threat represented by ocean acidification (OA) for coral reefs has received considerable attention because of the sensitivity of calcifiers to changing seawater carbonate chemistry. However, most studies have focused on the organismic response of calcification to OA, and only a few have addressed community-level effects, or investigated parameters other than calcification, such as photosynthesis. Light (photosynthetically active radiation, PAR) is a driver of biological processes on coral reefs, and the possibility that these processes might be perturbed by OA has important implications for community function. Here we investigate how CO₂ enrichment affects the relationships between PAR and community net O₂ production (P_net), and between PAR and community net calcification (G_net), using experiments on three coral communities constructed to match (i) the back reef of Mo'orea, French Polynesia, (ii) the fore reef of Mo'orea, and (iii) the back reef of O'ahu, Hawaii. The results were used to test the hypothesis that OA affects the relationship between P_net and G_net. For the three communities tested, pCO₂ did not affect the P_net–PAR relationship, but it affected the intercept of the hyperbolic tangent curve fitting the G_net–PAR relationship for both reef communities in Mo'orea (but not in O'ahu). For the three communities, the slopes of the linear relationships between P_net and G_net were not affected by OA, although the intercepts were depressed by the inhibitory effect of high pCO₂ on G_net. Our result indicates that OA can modify the balance between net calcification and net photosynthesis of reef communities by depressing community calcification, but without affecting community photosynthesis.