Vortex and Biogeochemical Dynamics for the Hypoxia Formation Within the Coastal Transition Zone off the Pearl River Estuary. Issue 8 (14th August 2020)

• Record Type: Journal Article
• Authors: Li, Dou; Gan, Jianping; Hui, Rex; Liu, Zhiqiang; Yu, Liuqian; Lu, Zhongming; Dai, Minhan
• Is Part Of: Journal of geophysical research. Volume 125:Issue 8(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-08-14
• Subjects: Eutrophication and hypoxia -- Coastal transition zone -- Coupled physical and biogeochemical dynamics -- Pearl River Estuary -- Numerical Modeling -- Observation; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC016178
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Satellite‐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. Plain Language Summary: Observations show that eutrophication and persistent hypoxia (dissolved oxygen less than 2 mg L −1 ) existed at the bottom of the coastal transition zone (CTZ) between the Pearl River Estuary and adjacent continental shelf. We found that rich nutrients from river discharge and sufficient light stimulated the primary production, which was the necessary condition for hypoxia to form in the CTZ. Cyclonic vortices and centers of convergence in the CTZ were favorable for the accumulation of nutrients and organic matter and were characterized by longer residence time, strong stratification, and weak mixing. Combined with the necessary condition, they provided a sufficient condition for the eutrophication/hypoxia to form in the CTZ. Key Points: Seasonal eutrophication/hypoxia is persistently found in the coastal transition zone (CTZ) off the Pearl River Estuary The vortex converges nutrients and organic matter, produces a longer residence time, and stabilizes waters for the hypoxia in the CTZ Combinations of biogeochemical processes and unique hydrodynamics in the CTZ provide a sufficient condition for the eutrophication/hypoxia