Abstract
Coral calcification is dependent on both the supply of dissolved inorganic carbon (DIC) and the up-regulation of pH in the calcifying fluid (cf). Using geochemical proxies (δ11B, B/Ca, Sr/Ca, Li/Mg), we show seasonal changes in the pHcf and DICcf for Acropora yongei and Pocillopora damicornis growing in-situ at Rottnest Island (32°S) in Western Australia. Changes in pHcf range from 8.38 in summer to 8.60 in winter, while DICcf is 25 to 30% higher during summer compared to winter (×1.5 to ×2 seawater). Thus, both variables are up-regulated well above seawater values and are seasonally out of phase with one another. The net effect of this counter-cyclical behaviour between DICcf and pHcf is that the aragonite saturation state of the calcifying fluid (Ωcf) is elevated ~4 times above seawater values and is ~25 to 40% higher during winter compared to summer. Thus, these corals control the chemical composition of the calcifying fluid to help sustain near-constant year-round calcification rates, despite a seasonal seawater temperature range from just ~19° to 24 °C. The ability of corals to up-regulate Ωcf is a key mechanism to optimise biomineralization, and is thus critical for the future of coral calcification under high CO2 conditions.
Highlights
Coral reefs face an uncertain future due to increasing seawater temperatures and ocean acidification resulting from CO2-driven climate change[1,2]
Results from Ross et al.[36] demonstrated that calcification rates generally deviated from their long-term (>1 year) average growth rates of 1.6 mg cm−2 d−1 for A. yongei and 0.67 mg cm−2 d−1 for P. damicornis by just ± 20% to ± 30% over the 18-month period, respectively (Table 2)[36]
These calcification rates were either negatively correlated with temperature for P. damicornis (r2 = 0.45) or showed little or no seasonal coherency for A. yongei[36]
Summary
Coral reefs face an uncertain future due to increasing seawater temperatures and ocean acidification resulting from CO2-driven climate change[1,2]. Previous studies have shown that estimates of internal coral pHcf derived from geochemical tracers[12,13,40,45] are consistent with more direct measurements[14,19,44], affirming that boron isotopes are providing unbiased measurements of pH at the site of calcification With these new developments[12,21,40,45], we can determine how the carbonate chemistry of the calcifying fluid (pHcf, DICcf, Ωcf) responds to natural and seasonally varying changes in light, temperature and seawater pH. We show that quantifying these relationships is critical to understanding and predicting how coral growth will respond to man-made climate change under real-world conditions
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