Recruits of the temperate coral Oculina arbuscula mimic adults in their resilience to ocean acidification

Ocean acidification has been investigated extensively in scleractinian corals, but studies on different life stages of the same species are lacking. We investigated the response of recruits of the temperate coral Oculina arbuscula to increased CO2 concentrations, a species whose adults show significant tolerance to elevated concentrations of CO2. Specifically, we exposed small colonies (5-12 mm diameter) to 475, 710, and 1261 ppm CO2 for 75 d in the laboratory to address the hypothesis that, like adults, the health of O. arbuscula recruits is not affected by increased CO2 concentrations. Calcification rates were monitored regularly during the experiment, while mortality, respiration rates, photosynthetic rates, algal symbiont densities, and soluble protein were quantified at the end. As predicted, CO2 concentration did not impact survival, algal densities, or soluble protein concentrations in O. arbuscula recruits. In contrast, both calcification rates and photosynthesis:respiration ratios tended to be lower at higher CO2. Comparing the results of this study on recruits with published studies on adults suggested that both life stages exhibit a similar non-linear response to CO2 concentration, whereby recruits may be unable to counter the increased energetic cost of calcification that occurs at the highest CO2. Based on these results and environmental monitoring showing that mean pCO(2) is increasing by similar to 2.4 % yr(-1) in the waters off Georgia, USA, we conclude that O. arbuscula recruits may begin to exhibit depressed calcification rates within the current century if CO2 emissions are not reduced.