Observational data revealed the persistent and deteriorating seasonal eutrophication/hypoxia with high spatiotemporal variability in the coastal transition zone (CTZ) between the Pearl River Estuary (PRE) and the adjacent continental shelf off Hong Kong, while the underlying physical and biogeochemical dynamics are not well investigated. Based on observations and a coupled physical-biogeochemical numerical model, we explored the role of hydrodynamics (e.g., estuarine circulations and submesoscale processes) and biogeochemical processes (e.g., eutrophication and subsurface Chlorophyll maximum (SCM)) in the hypoxia formation and variability off an estuary-shelf system driven by the complex forcing of wind, river discharge, and tides. The results show that convergent circulations induced...[ Read more ]

Observational data revealed the persistent and deteriorating seasonal eutrophication/hypoxia with high spatiotemporal variability in the coastal transition zone (CTZ) between the Pearl River Estuary (PRE) and the adjacent continental shelf off Hong Kong, while the underlying physical and biogeochemical dynamics are not well investigated. Based on observations and a coupled physical-biogeochemical numerical model, we explored the role of hydrodynamics (e.g., estuarine circulations and submesoscale processes) and biogeochemical processes (e.g., eutrophication and subsurface Chlorophyll maximum (SCM)) in the hypoxia formation and variability off an estuary-shelf system driven by the complex forcing of wind, river discharge, and tides. The results show that convergent circulations induced by cyclonic vortices in the CTZ create a stable water column with weak mixing and long residence time and accumulate nutrients and organic matter. In conjunction with the favorable hydrodynamics, biogeochemical processes form sufficient conditions for hypoxia formation in the CTZ. This study, for the first time, revealed that the oxygen consumption contributed by the onshore-transported SCM and associated organic matter accounted for an average of 26% of the DO depletion under the pycnocline when there was a low river discharge and persistent upwelling circulations. The varying wind-driven currents and river plume jointly regulate the nutrient and detritus transport, water vertical mixing, and residence time, and consequently, the hypoxia variability, which can be quantified by the hypoxia generation index. Additionally, the nonlinear interactions between the tidally-fluctuated plume and wind-driven shelf currents induce vigorous submesoscale processes and trigger heterogenetic frontogenesis and instabilities at the offshore and nearshore plume fronts where the mechanisms generating and sustaining the plume fronts have similarities but also differences. This systematic study advances the understanding of biophysical dynamics of hypoxia formation and variability and provides implications for designing strategies for hypoxia abatement in the PRE and other hypoxic systems.