Irradiance modulates coral thermal thresholds shaping bleaching dynamics across the Red Sea

The extent and severity of recent global bleaching events have urged the development of advanced models to understand and predict bleaching dynamics. Current models typically rely on empirical thermal thresholds, yet bleaching results from multiple interacting mechanisms influenced by additional environmental factors such as light and water quality. As a result, these models often misidentify or miss key events. Here, we demonstrate the applicability of an existing mechanistic bioenergetic model to simulate and predict coral bleaching across the Red Sea. The model, based on Dynamic Energy Budget (DEB) theory, integrates physiological dynamics between coral hosts and their symbiotic algae under various environmental factors (temperature, light, nutrients, prey). We forced the DEB-based coral bleaching model with realistic environmental data, derived from satellite products and high-resolution ocean bio-physical models, and evaluated its performance against in situ observations. The model successfully reproduced 31 out of 32 bleaching/no-bleaching observations. High light irradiance in the clearer (less turbid), oligotrophic northern Red Sea lowered the thermal threshold for bleaching, whereas corals in the southern basin tolerated longer periods of heat stress due to reduced light stress and higher nutrient availability. These interacting mechanisms explain the contrasting spatiotemporal patterns simulated across the basin, including the 2015 bleaching event, which was confined to the central and southern Red Sea, and the unprecedented 2024 event that extended into the historically resistant northern refugia. By resolving the combined effects of temperature, light, and nutrients, this study establishes a predictive, physiology-based framework for assessing future bleaching risk and informing targeted intervention strategies.