(a) Porites spp. coral calcifying fluid pHcf derived from δ11B systematics (see ‘Methods’ section and Supplementary Data) for colonies D-2 and D-3 from Davies Reef (18.8° S) in the Great Barrier Reef, Queensland. Shading denotes the summer period when pHcf and seawater pHsw values are at a minimum. Dashed line shows pH*cf expected from artificial experimental calibrations (pH*cf=0.32 pHsw+5.2)10,17 with an order of magnitude lower seasonal range than measured pHcf values. (b) Same as previous for Porites colonies from Coral Bay (CB-1 and CB-2) in the Ningaloo Reef of Western Australia (23.2° S) showing seasonal fluctuations in pHcf and seawater pHsw. The blue shading denotes the anomalously cool summer temperatures in 2010. (c) Enrichments in calcifying fluid DICcf (left axis; coloured circles) derived from combined B/Ca and δ11B systematics together with synchronous seasonal variations in reef-water temperatures (right axis; black line) for Porites colonies from Davies Reef (GBR). The strong temperature/light control on DICcf is consistent with enhanced metabolic activity of zooxanthellae symbionts in summer. (d) Same as previous but for Porites from Coral Bay (Ningaloo Reef, Western Australia).