The reef-building coral Siderastrea siderea exhibits parabolic responses to ocean acidification and warming

Abstract

Anthropogenic increases in atmospheric CO2 over this century are predicted to cause global average surface ocean pH to decline by 0.1-0.3 pH units and sea surface temperature to increase by 1-4°C. We conducted controlled laboratory experiments to investigate the impacts of CO2-induced ocean acidification (pCO2 = 324, 477, 604, 2553 µatm) and warming (25, 28, 32°C) on the calcification rate of the zooxanthellate scleractinian coral Siderastrea siderea, a widespread, abundant and keystone reef-builder in the Caribbean Sea. We show that both acidification and warming cause a parabolic response in the calcification rate within this coral species. Moderate increases in pCO2 and warming, relative to near-present-day values, enhanced coral calcification, with calcification rates declining under the highest pCO2 and thermal conditions. Equivalent responses to acidification and warming were exhibited by colonies across reef zones and the parabolic nature of the corals' response to these stressors was evident across all three of the experiment's 30-day observational intervals. Furthermore, the warming projected by the Intergovernmental Panel on Climate Change for the end of the twenty-first century caused a fivefold decrease in the rate of coral calcification, while the acidification projected for the same interval had no statistically significant impact on the calcification rate-suggesting that ocean warming poses a more immediate threat than acidification for this important coral species.

Keywords: Caribbean; Siderastrea siderea; calcification; ocean warming, ocean acidification; tropical scleractinian coral.

Figure 1.

Parabolic calcification responses of the coral S. siderea to elevated pCO₂ and temperature across the 95-day experiments. (a) Calcification rates for corals at mean pCO₂ (s.d.) of 324 (89), 477 (83), 604 (107) and 2553 (506) µatm and at mean temperature (s.d.) of 28.10 (0.28)°C. (b) Calcification rates at mean temperatures (s.d.) of 25.01 (0.14), 28.16 (0.24) and 32.01 (0.17)°C and at mean pCO₂ (s.d.) of 488 (88) µatm. Ninety-five per cent (thin bars) and 83.5% (thick bars) confidence intervals of the means are shown.

Figure 2.

Effects of exposure duration on S. siderea coral calcification response to pCO₂ and temperature. (a) Calcification rates at three monthly observational intervals for S. siderea corals reared at mean pCO₂ (s.d.) of 324 (89), 477 (83), 604 (107) and 2553 (506) µatm and maintained at mean temperature (s.d.) of 28.10 (0.28)°C. (b) Calcification rates at three monthly observational intervals for S. siderea corals reared at temperatures (s.d.) of 25.01 (0.14), 28.16 (0.24) and 32.01 (0.17)°C and at mean pCO₂ (s.d.) of 488 (88). Ninety-five per cent confidence intervals (black bars) show precision of estimated calcification rates. Eighty-three and one-half per cent confidence intervals (pink bars) are for across-panel (i.e. across treatment) comparison. Forty-two and one-half per cent confidence intervals (blue bars) are for within-panel (i.e. within treatment) comparison.

Figure 3.

Conceptual diagram (constrained by study results) illustrating how photosynthesis (green curve; estimated from measured F_v/F_m using empirical F_v/F_m–ETR_max relationship from Frade et al. [58] (see the electronic supplementary material, figure S6)) and aragonite saturation state (Ω_A) (red curve; data from this study) interact to generate the corals' parabolic calcification response (blue curve; data from this study) to rising atmospheric pCO₂.