Colony-specific calcification and mortality under ocean acidification in the branching coral Montipora digitata

• Colony-specific calcification and mortality of Montipora digitata under acidification were investigated.
• Elevated pCO2 exposure on average reduced calcification.
• The calcification rates of individual colonies were highly variable.
• Significantly higher mortality rate was observed in elevated pCO2 compared to ambient conditions.
• The mortality data also reflected colony-specific responses.

Ocean acidification (OA) threatens calcifying marine organisms including reef-building corals. In this study, we examined the OA responses of individual colonies of the branching scleractinian coral Montipora digitata. We exposed nubbins of unique colonies (n = 15) to ambient or elevated pCO2 under natural light and temperature regimes for 110 days. Although elevated pCO2 exposure on average reduced calcification, individual colonies showed unique responses ranging from declines in positive calcification to negative calcification (decalcification) to no change. Similarly, mortality was greater on average in elevated pCO2, but also showed colony-specific patterns. High variation in colony responses suggests the possibility that ongoing OA may lead to natural selection of OA-tolerant colonies within a coral population.