Marine Heatwaves Driven by El Niño and their impacts on coral reefs of the Andaman and Nicobar Islands

Marine Heatwaves Driven by El Niño and their impacts on coral reefs of the Andaman and Nicobar Islands Subash Kisku a*, Dr Nibedita Behera a,Department of Marine SciencesBerhampur University, Bhanja Bihar, 760007E-mail: subashkisku@gmail.com Abstract Marine heatwaves (MHWs)—prolonged periods of unusually high sea surface temperatures are intensifying with global climate change and pose a growing threat to marine ecosystems. Coral reefs in the Indian region of the North Indian Ocean are particularly vulnerable, with major bleaching episodes observed in recent decades. This study investigates the link between extreme MHWs and coral bleaching events in 1998, 2010, 2016, 2020, and 2024, focusing specifically on coral reef regions in the Andaman and Nicobar Islands from 1991 to 2024. Our objective was to identify a combination of thermal stress criteria that would capture the most severe MHWs and those more elevated and associated with El Niño in this region and encompass all known bleaching-associated events. Using satellite-derived sea surface temperature anomalies, we analysed the intensity, correlation with the Multivariate El Niño/Southern Oscillation Index and ecological consequences of these heatwaves. The results reveal a strong relationship between high-intensity MHWs and widespread coral bleaching, influenced by this region during the years of strong El Niño events. During this event, coral species such as Acropora cerealis, A. humilis, Montipora sp., Favia pallida, Diploastrea sp., and Goniopora sp. Fungia concinna, Gardineroseries sp., Porites sp., Favites abdita and Lobophyllia robusta were severely affected. The coral bleaching data also illustrated that the seasonal peaks from April to July correlated with the documented bleaching episodes and that DHW values exceeding 4°C-weeks were the ones that predicted severe bleaching stress the most. This trend was exacerbated during the El Niño years, which occurred in 1998, 2010, 2016, and 2024. This research provides critical insights into the vulnerability of Indian coral reef ecosystems and underscores the urgent need for region-specific conservation strategies and climate adaptation measures to enhance reef resilience and ensure the survival of these ecosystems in a rapidly warming ocean.