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Joint Effects of Extrinsic Biophysical Fluxes and Intrinsic Hydrodynamics on the Formation of Hypoxia West off the Pearl River Estuary

Using fieldmeasurements and a process-oriented three-dimensional coupled physical-biogeochemical numerical model, we investigated the physical and biogeochemical processes governing the bottom hypoxic zone west off the Pearl River estuary. The intensity and area of the hypoxia grew with increasing total nutrient input from the Pearl River that has increased continuously in recent decades. The hypoxic zone was formed and maintained largely associated with the stable water column where the stability was provided simultaneously by wind stress and freshwater discharge, favorable local hydrodynamics for flow convergence, and westward organic matter transport. Wind stress altered the stratification, while freshwater discharge changed the stratification and baroclinic velocity shear simultaneously. Two-layered flow with a cyclonically rotating current around a coastal salient edge of the western shelf off the estuary hydrodynamically enhanced the local convergence, allowing sufficient residence time in the bottom for the remineralization of organic matter produced in the hypoxic zone and organic matter transported into the region. Our results suggest that a combination of unique local hydrodynamic feature and decomposition of organic matter in water column (and possibly in the sediment) are the cause of the formation andmaintenance of the bottom hypoxia on the western shelf of the estuary during summer. Plain Language Summary Hypoxia results in dead zones in the ocean. It often occurs in the bottom waters below surface eutrophication due to decomposition of organic matter in the water column and also possibly in sediment. Both physical and biogeochemical processes in the ocean control the formation of the eutrophication and hypoxia. This study investigates these processes for an observed strong hypoxia zone off Pearl River estuary using a numerical model and field measurement data. Through comprehensive analyses, we found that the intensity and area of the hypoxia grew with increasing total nutrient input from Pearl River. The hydrodynamic conditions of ocean flow field such as stability of the water column associated with wind forcing and freshwater buoyancy discharge, local flow convergence, and external organic matter input provide favorable conditions for the formation of hypoxia.