Rapid decline in pH of coral calcification fluid due to incorporation of anthropogenic CO(2)

Marine calcifying organisms, such as stony corals, are under threat by rapid ocean acidification (OA) arising from the oceanic uptake of anthropogenic CO(2). 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 (δ(11)B) measurements, that OA is affecting the pH of the calcification fluid (pH(CF)) 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 δ(11)B since 1960. A comparison with the pH of the ambient seawater (pH(SW)) near these islands, estimated from a large number of shipboard measurements of seawater CO(2) chemistry and atmospheric CO(2), indicates that pH(CF) is sensitive to changes in pH(SW.) This suggests that the calcification fluid of corals will become less supersaturated with respect to aragonite by the middle of this century (pH(CF) = ~8.3 when pH(SW) = ~8.0 in 2050), earlier than previously expected, despite the pH(CF)-upregulating mechanism of corals.