Timing of calcification and environmental variability determine pH proxy fidelity in coastal calcifying macroalgae

Long-lived calcifying marine biota are increasingly used as palaeo-archives for reconstructing ocean pH. They enable exploration of the rate and magnitude of ocean acidification in shallow-water ecosystems serving as proxies for environmental pH reconstruction. However, shallow water systems often have highly variable carbonate chemistry and the impact of this on the accuracy of pH reconstructions from long-lived marine calcifiers is not known. In particular, a better understanding of the timing of calcification with respect to environmental pH cyclicity is needed. To test the fidelity of coastal environmental pH proxies, we assessed the synchronicity between calcification and in-situ diel carbonate chemistry in a tropical (One Tree Island, Great Barrier Reef, Australia) and a temperate (Loch Sween, Scotland) location using calcifying macroalgae (rhodolith forming coralline algae) as a model system. Calcification occurred primarily during daylight hours meaning a recording bias was introduced when compared to the full diel pH range (