(a) Calcifying fluid pHcf and Ωcf values for Porites coral (D-2) from Davies Reef (GBR), where Ωcf=[Ca2+]cf [CO32−]cf/Karag. Dashed line shows the Ω*cf calculated using fixed experimental10,17 pH*cf values (see Fig. 2a,b). (b) Same as previous for Coral Bay (Ningaloo Reef, Western Australia) Porites (CB-2). (c) Calcification rates calculated using the inorganic rate equation28 G=k(Ω−1)n, where k and n are the temperature-dependent constant and order of the reaction, respectively28. Because of opposing changes in pHcf relative to DICcf (Fig. 1), Ωcf and hence coral growth rates are strongly modulated reducing seasonal variations by twofold compared to those estimated from fixed condition experiments (G*). (d) Same as previous for Porites from Coral Bay (Ningaloo Reef, Western Australia).