Abstract
Marine calcifying organisms, such as stony corals, are under threat by rapid ocean acidification (OA) arising from the oceanic uptake of anthropogenic CO2. To better understand how organisms and ecosystems will adapt to or be damaged by the resulting environmental changes, field observations are crucial. Here, we show clear evidence, based on boron isotopic ratio (δ11B) measurements, that OA is affecting the pH of the calcification fluid (pHCF) in Porites corals within the western North Pacific Subtropical Gyre at two separate locations, Chichijima Island (Ogasawara Archipelago) and Kikaijima Island. Corals from each location have displayed a rapid decline in δ11B since 1960. A comparison with the pH of the ambient seawater (pHSW) near these islands, estimated from a large number of shipboard measurements of seawater CO2 chemistry and atmospheric CO2, indicates that pHCF is sensitive to changes in pHSW. This suggests that the calcification fluid of corals will become less supersaturated with respect to aragonite by the middle of this century (pHCF = ~8.3 when pHSW = ~8.0 in 2050), earlier than previously expected, despite the pHCF-upregulating mechanism of corals.
Highlights
The pH of the surface seawater is considered to have declined by ~0.1 since the beginning of the industrial era, and an additional decline of ~0.3 is projected by the end of this century[1,2,3]
To better understand the response of corals to ocean acidification, we reconstructed past pH changes using skeletal δ11B (Methods; Fig. 1b) as an indicator of the pH of the calcification fluid in long-living massive Porites corals obtained from the islands of Chichijima (27.1°N, 142.2°E)[10] and Kikaijima (28.3°N, 130.0°E)[11]
This trend correlates with the trend in pH of the ambient seawater (pHSW) evaluated from the time series record of the atmospheric CO2 concentration, and the correlation is derived from a long-term decreasing trend, not inter-annual variability (Fig. 3a,b)
Summary
Insignificant variations in δ11B were recorded for the period before 1960, whereas a rapid decline in δ11B occurred after 1960 (−0.18 ± 0.04‰/decade for the Chichijima coral, p < 0.001; −0.29 ± 0.07‰/decade for the Kikaijima coral, p < 0.01). A recent study found that the pHCF of the branching coral Porites cylindrica cultured on Heron Island, on the Great Barrier Reef, was unaffected by a reduction in pHSW28, which contradicts our observations on Chichijima and Kikaijima Islands. The more sensitive declines in pHCF seen in the Chichijima and Kikaijima corals may be attributable to seawater in which the CO2 chemistry is less variable than that affecting large coral reefs such as the Great Barrier Reef. This suggests that colonies in similar environments may be more susceptible to OA. More field-based studies with the boron technique are required to understand how corals have adapted to or are threatened by OA, before corals disappear under the impact of physical, chemical, and biological erosion[4, 31]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
