Timing of calcification and environmental variability determine <scp>pH</scp> proxy fidelity in coastal calcifying macroalgae

Abstract Long‐lived calcifying marine biota are increasingly used as paleo‐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 (&lt; 0.02 pH units). This bias resulted in pH offsets up to 0.043 pH units over the period 1860–2020, representing up to 34% of the projected pH change from 1860 in the tropics and up to 1.8% in temperate latitudes. Therefore, when proxy records are used to extend modern instrumental records of pH, we find that this may lead to bias, indicating daytime, nighttime, and full diel pH records should be assessed separately. We suggest that temporal pH cycles should be characterized at a local scale to enable incorporation of potential biases in the application of calcifying marine macroalgae to reconstruct pH change.