Seawater carbonate chemistry and calcification in Caribbean reef-building corals

Projected increases in ocean pCO2 levels are anticipated to affect calcifying organisms more rapidly and to a greater extent than other marine organisms. The effects of ocean acidification (OA) have been documented in numerous species of corals in laboratory studies, largely tested using flow-through exposure systems. We developed a recirculating ocean acidification exposure system that allows precise pCO2 control using a combination of off-gassing measures including aeration, water retention devices, venturi injectors, and CO2 scrubbing. We evaluated the recirculating system performance in off-gassing effectiveness and maintenance of target pCO2 levels over an 84-day experiment. The system was used to identify changes in calcification and tissue growth in response to elevated pCO2 (1000 μatm) in three reef-building corals of the Caribbean: Pseudodiploria clivosa, Montastraea cavernosa, and Orbicella faveolata. All three species displayed an overall increase in net calcification over the 84-day exposure period regardless of pCO2 level (control + 0.28–1.12 g, elevated pCO2 + 0.18–1.16 g), and the system was effective at both off-gassing acidified water to ambient pCO2 levels, and maintaining target elevated pCO2 levels over the 3-month experiment.