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Abstract

Marine heatwaves cause massive, but often patchy coral bleaching. Why neighboring colonies often respond differently remains unresolved. Here, we show that interactions between metabolic demand and flow-mediated resupply generate heterogeneous oxygen microenvironments inside branching corals under heat stress. During the 2023 and 2024 Red Sea heatwaves, high-frequency in situ measurements revealed hypoxic to near-anoxic interbranch conditions despite well-oxygenated surrounding seawater. In 2023, neighboring colonies developed divergent oxygen regimes preceding paling and bleaching; in 2024, stronger and earlier thermal stress was associated with recurrent nighttime near-anoxia before tissue disintegration and mortality. A physics-based three-dimensional fluid-metabolism model reproduced these patterns. Oxygen depletion and coral impacts emerged when metabolic oxygen demand exceeded flow-mediated resupply. Organism-scale variability in oxygen regimes may explain divergent coral fate under shared heatwave exposure.

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