Investigating photosynthetic efficiency & performance of major reef-building corals across different turbid environments in the Gulf of Thailand

Thermal stress is known as the primary driver of coral bleaching, yet bleaching outcomes depend on interacting environmental factors such as light, turbidity, dissolved oxygen, and pH. Turbidity can mitigate irradiance and photoinhibition, potentially buffering heat stress and creating natural climate-refugial conditions. The Gulf of Thailand provides a key setting to evaluate this mechanism, as its reefs maintain high coral cover despite chronic turbidity, elevated temperatures, and repeated bleaching events. This study examined how environmental variation along a nearshore-offshore turbidity gradient influences photosynthetic performance (ETR) and efficiency (Y(II)) of three major reef building corals, Porites, Acropora, and Montipora, at three reefs within the Samui Archipelago using environmental measurements, benthic surveys, and in situ PAM fluorometry. Distinct abiotic niches emerged, with moderately turbid conditions offering stress-buffering environments that supported the highest coral cover and diversity. Coral assemblages aligned with these regimes, while photophysiological patterns showed consistently high Y(II) across genera. Porites reached the highest ETR under turbid conditions, and Acropora and Montipora maintained stable ETR through effective acclimation. Combined ETR, Y(II), and PAR responses demonstrated strong photophysiological resilience across all genera, especially under moderate turbidity where reduced irradiance mitigated light and thermal stress without limiting photosynthetic capacity. These findings suggest that intermediate turbidity can sustain robust photosynthetic function and support coral persistence under future warming, underscoring the value of moderately turbid reefs for targeted monitoring and conservation.