Variability of hypoxia in coastal transition zone off Pearl River Estuary: observational and modeling studies

Hypoxia and its spatiotemporal variability in the coastal transition zone (CTZ) neighboring Pearl River Estuary (PRE) and adjacent shelf have been frequently observed from repeated in-situ mappings. However, the underlying mechanism of the variability in response to variable physical forcing of winds-tides-freshwater, which is crucial to holistic understanding of eutrophication/hypoxia formation, remains unclear. In this study, datasets from six cruises and time-series buoy measurement were used to examine the spatial and temporal variability of hypoxia in the CTZ. Observations showed that there existed two characteristic hypoxic centers at the western and eastern side of the CTZ, and they were highly variable in response to intra-synoptic wind forcing, spring-neap tidal cycle and river discharge. A processes-oriented 3D coupled physical-biogeochemical numerical model was used to investigate the intrinsic biogeochemical dynamics in response to the extrinsic forcing, especially winds and tides. We found that water column stability, flow field, transport of nutrients and evolution of hypoxia were largely modulated by wind-driven coastal circulation. The convergence zone in the western and eastern CTZ, which is characterized by positive vorticity, negative divergence, large Richardson number and long resident time are favorable for the formation of hypoxia during upwelling favorable wind. As upwelling favorable wind relaxes and shifts towards downwelling favorable wind, the reverse current further enhances the local convergence of nutrient and detritus, allowing sufficient resident time for the remineralization of organic matter (OM) and intensification of hypoxia in both western and eastern CTZ. During downwelling favorable winds, western bottom hypoxic zone migrates offshore, while eastern hypoxia dissipates due to westward advection of OM and enhanced mixing. Spring and neap tidal cycle influences hypoxia mainly by changing OM concentration through hydrodynamic convergence and by modulating stratification through tidal mixing. Dissipating hypoxia can be quickly regenerated once both the favorable hydrodynamics and sufficient nutrients are rebuilt.