Regional and genus-specific factors underpin bleaching variation across India’s corals reefs during the fourth global coral bleaching event

On April 15th 2024 the National Oceanic and Atmospheric Association of the US announced the onset of the fourth global coral bleaching event, coinciding with the 2023-24 El Niño Southern Oscillation. Through a coordinated, nationwide collaboration of scientists, dive centers and citizen observers, we assessed bleaching responses across six reef regions in India, spanning most of the subcontinent’s major reef areas, from oceanic atolls and island slopes to mainland fringing and patchy reef formations. While bleaching intensity increased with accumulated heat stress (Degree Heating Weeks or DHW) as expected, it showed distinct regional variability, revealing the limits of DHW as a universal predictor. Among the regions, Lakshadweep was the most affected with severe bleaching even at intermediate DHW. In contrast, Mahatma Gandhi Marine National Park bleached less than other regions, likely reflecting cooling from large amplitude internal waves that could create mesoscale thermal refugia. Across sites, Acropora , Pocillopora , Galaxea , and Montipora , were the most affected genera while Pavona, Platygyra, Goniastrea and Favites were among the least affected. Palk Bay displayed a reversal of canonical genus-level susceptibilities, with only modest bleaching in the normally susceptible Acropora colonies, but high mortality in usually resistant Porites colonies. These contrasting regional and taxonomic responses highlight how context-specific resistance patterns, and local oceanography mediate thermal stress and underscore the need to integrate these local geographical contexts in our understanding of bleaching impacts on tropical reefs.