High‐resolution coral oxygen and carbon isotope records reveal temperature and autotrophy dynamics in a Mediterranean climate change hotspot

The Mediterranean Sea is warming at a rate exceeding the global average. Long‐term, high‐resolution data are essential for contextualizing changes within broader temporal scales, and coral skeletons provide valuable environmental archives, especially in data‐sparse regions or as supplements to existing records. While coral‐based reconstructions are well established in tropical settings, they remain limited in temperate areas. As the only reef‐building zooxanthellate coral in the Mediterranean, Cladocora caespitosa is particularly important for expanding coral‐based environmental archives in these understudied regions. Here, we present records of δ18O and δ13C in the skeletons of C. caespitosa from a global change sentinel site in NW Mediterranean. This study provides the most accurate temperature–δ18O calibration equations for C. caespitosa, including a traditional linear model and a novel exponential model that better accounts for the region's wide seasonal temperature range. Both calibrations rely on long‐term in situ water temperature data and a multi‐corallite composite approach to reduce non‐climatic variability. Seasonal trends in δ13C reveal, for the first time, variation in the coral's autotrophy–heterotrophy balance, while geochemical anomalies in the skeletons signal thermal stress effects on biomineralization. Our findings establish C. caespitosa skeletons as critical archives for reconstructing anthropogenic warming and its ecological effects in the Mediterranean.