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

Abstract

Using field measurements 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 and maintenance of the bottom hypoxia on the western shelf of the estuary during summer.