Vortex and Biogeochemical Dynamics for the Hypoxia Formation Within the Coastal Transition Zone off the Pearl River Estuary

Abstract

AbstractSatellite‐derived chlorophyll data, in situ measurements, and time series buoy monitoring revealed the persistent and deteriorating seasonal eutrophication and hypoxia with two distinct centers in the coastal transition zone (CTZ) between the Pearl River Estuary and the adjacent continental shelf off Hong Kong. We used a process‐oriented coupled physical‐biogeochemical numerical model to investigate the intrinsic physical and biogeochemical dynamics under the extrinsic forcing of wind, river discharge, and tides for the hypoxia formation in the CTZ. We found that the convergence induced by cyclonic vortices in the CTZ, which were formed by the buoyancy‐driven currents and wind‐driven shelf currents, created the stable water column with weak mixing and long residence time and accumulated nutrients and organic matter for the eutrophication and hypoxia development. The biophysical responses to spring‐neap cycle further demonstrated the coherent linkages of oceanic currents, vortex formation, and convergence of organic matter within the CTZ and indicated the critical role of mixing in the dissolved oxygen (DO) budgets during the spring‐neap tidal cycle. The study found that biogeochemical substances and processes are necessary conditions for eutrophication and hypoxia formation, and when these necessary conditions are combined with the favorable hydrodynamic conditions, they form a sufficient condition for eutrophication and hypoxia formation in the CTZ.