Evaluating Coral Proxy Fidelity at Monthly Resolution in Urban Reefs: A 20‐Year δ 18 O and δ 13 C Comparison of Psammocora and Porites

Urbanized reef environments pose challenges for coral‐based paleoclimate reconstructions. Here, we test the hypothesis that Psammocora digitata preserves reproducible δ 18 O and δ 13 C signals comparable to those of conventional coral archives (e.g., massive Porites ) under urban conditions. We present monthly‐resolved δ 18 O and δ 13 C records spanning 20 years from two Psammocora digitata coral records (SG1, SG2), sampled from different branches of the same colony, and one Porites core (SG3) collected from a turbid nearshore reef in Singapore. All three records show clear seasonal cycles, with the Psammocora exhibiting higher intra‐coral reproducibility. SG1 and SG2 share identical δ 18 O and δ 13 C means, variances, and seasonal amplitudes, consistent with their similar hydrographic conditions at ∼3 m depth. The shallower Porites core (SG3, ∼2 m) is offset toward lower δ 18 O and δ 13 C values and displays reduced δ 18 O seasonal amplitude, reflecting species‐specific baselines. Seasonal δ 18 O variability is primarily driven by salinity (∼60%), with temperature exerting a smaller influence (∼40%). SG1 and SG2 show δ 18 O amplitudes of ∼1.70‰–1.86‰, whereas Porites coral SG3 records a smaller amplitude (∼1.24‰) consistent with its lower variance. δ 13 C exhibits larger overall variability (standard deviations 0.65‰–0.71‰; seasonal amplitudes ∼3.02‰–3.94‰), responding to changes in underwater light, turbidity, colored dissolved organic matter (CDOM), ambient δ 13 C of dissolved inorganic carbon ( δ 13 C DIC ), and minor colony‐level physiological effects. In each coral, δ 18 O and δ 13 C show significant positive correlations ( r = 0.59–0.68), reflecting shared environmental drivers, including freshwater input or seasonal hydrological variability, rather than colony‐specific metabolic effects. Taken together, these results show that Psammocora can provide robust, monthly‐resolution isotopic records suitable for reconstructing coastal hydroclimate and biogeochemical variability.