The responses of eight coral reef calcifiers to increasing partial pressure of CO₂ do not exhibit a tipping point

• Published in: Limnology and oceanography 2013-01, Vol.58 (1), p.388-398
• Main Authors: Comeau, S., Edmunds, P. J., Spindel, N. B., Carpenter, R. C.
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Fundamental and applied biological sciences. Psychology; General aspects; Synecology
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.2013.58.1.0388

The objective of this study was to investigate whether a tipping point exists in the calcification responses of coral reef calcifiers to CO₂. We compared the effects of six partial pressures of CO₂ (PCO₂) from 28 Pa to 210 Pa on the net calcification of four corals (Acropora pulchra, Porites rus, Pocillopora damicornis, and Pavona cactus), and four calcified algae (Hydrolithon onkodes, Lithophyllum flavescens, Halimeda macroloba, and Halimeda minima). After 2 weeks of acclimation in a common environment, organisms were incubated in 12 aquaria for 2 weeks at the targeted PCO₂ levels and net calcification was quantified. All eight species calcified at the highest PCO₂ in which the calcium carbonate aragonite saturation state was ~ 1. Calcification decreased linearly as a function of increasing partial PCO₂ in three corals and three algae. Overall, the decrease in net calcification as a function of decreasing pH was ~ 10% when ambient PCO₂ (39 Pa) was doubled. The calcification responses of P. damicornis and H. macroloba were unaffected by increasing PCO₂. These results are inconsistent with the notion that coral reefs will be affected by rising PCO₂ in a response characterized by a tipping point. Instead, our findings combined among taxa suggest a gradual decline in calcification will occur, but this general response includes specific cases of complete resistance to rising PCO₂. Together our results suggest that the overall response of coral reef communities to ocean acidification will be monotonic and inversely proportional to PCO₂, with reef-wide responses dependent on the species composition of calcifying taxa.