Beyond PAM: Digital photography and RGB color analysis as a cost-effective method for coral bleaching assessment

Coral reefs are experiencing increased frequency and severity of thermal stress events, underscoring the urgent need for rapid and scalable methods to assess coral bleaching susceptibility. Quantifying bleaching responses typically relies on pulse-amplitude modulated (PAM) fluorometry, an expensive tool that limits widespread implementation, particularly in standardized acute heat stress assays such as the Coral Bleaching Automated Stress System (CBASS). Digital photography, by contrast, is low-cost, accessible, and easily standardized, yet its quantitative potential remains underutilized. Here, we evaluated whether RGB image analysis can serve as an accessible proxy for physiological bleaching metrics using two coral species (Acropora hemprichii and Stylophora pistillata) in a CBASS experiment. RGB values were extracted from coral fragments using a semi-automated segmentation pipeline, and 63 color indices, including indices commonly used in plant phenotyping, were correlated with PAM-derived Fv/Fm, symbiont density, chlorophyll content, and host protein content. We additionally calculated D_toWhite, the Euclidean distance in CIELAB color space between each coral fragment and a co-imaged white reference patch, extracted directly from uncorrected photographs. Across both species, D_toWhite performed nearly identically to the best color-corrected index (DeltaE76) and outperformed PAM-derived Fv/Fm for predicting symbiont density and total chlorophyll content. In contrast, green-weighted indices best approximated Fv/Fm and host protein content, highlighting complementary optical signals for photophysiology versus bleaching severity. These results demonstrate that a simple whitening metric derived from uncorrected photographs provides a reliable, scalable, and inexpensive proxy for bleaching severity, supporting higher-throughput thermal tolerance screening in resource-limited settings.