THESIS
2022
1 online resource (xix, 128 pages) : illustrations (chiefly color), color maps
Abstract
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...[
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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.
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