Coastal Hypoxia in the Indian Ocean: Unraveling Drivers of Spatio-Temporal Variability.

Coastal hypoxia in the densely populated Indian Ocean regions has dramatic consequences for ecosystems and ecosystem services such as fisheries and coastal protection. However, inadequate understanding of the spatio-temporal variability of coastal hypoxia in the region, and its physical and biological drivers, poses a major challenge for anticipating these risks. Here we use in-situ observations and a high-resolution (1/12 degree) biophysical model of the Indian Ocean to map areas with heightened vulnerability to coastal hypoxia and identify the drivers of coastal oxygen dynamics. We find strong regional disparities in the mechanisms and temporal scales that govern coastal hypoxia: seasonal upwelling in the eastern Arabian Sea, interannual upwelling/downwelling linked to Indian Ocean Dipole events in the eastern Bay of Bengal, intraseasonal fluctuations introduced by coastal currents, Kelvin waves, and eddies along the western Bay of Bengal, and biologically driven oxygen variations at the mouth of major rivers in the northern Bay of Bengal (e.g., Ganges-Brahmaputra and Irrawaddy-Sittang Deltas). This basin-scale mapping identifies regions where intraseasonal hypoxia (e.g., western Bay of Bengal, river deltas) makes prediction of these events challenging and calls for intense monitoring, and regions where seasonal and interannual hypoxia (e.g., eastern Arabian Sea, eastern Bay of Bengal) facilitates the prevention of the adverse impacts on coastal ecosystems and their services.