Preprints
https://doi.org/10.5194/bg-2020-346
https://doi.org/10.5194/bg-2020-346

  12 Oct 2020

12 Oct 2020

Review status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Destruction and reinstatement of coastal hypoxia in the South China Sea off the Pearl River Estuary

Yangyang Zhao1,2, Khanittha Uthaipan1, Zhongming Lu3, Yan Li1, Jing Liu1,4, Hongbin Liu5,6, Jianping Gan3,6,7, Feifei Meng1, and Minhan Dai1 Yangyang Zhao et al.
  • 1State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, 361102, China
  • 2Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, 8092, Switzerland
  • 3Division of Environment and Sustainability, Hong Kong University of Science and Technology, Kowloon, Hong Kong SAR, China
  • 4Marine Biogeochemistry Division, GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstraße 1–3, Kiel, 24148, Germany
  • 5Division of Life Science, Hong Kong University of Science and Technology, Kowloon, Hong Kong SAR, China
  • 6Department of Ocean Science, Hong Kong University of Science and Technology, Kowloon, Hong Kong SAR, China
  • 7Department of Mathematics, Hong Kong University of Science and Technology, Kowloon, Hong Kong SAR, China

Abstract. We examined the evolution of intermittent hypoxia off the Pearl River Estuary during three cruise legs conducted in July 2018: one during severe hypoxic conditions before the passage of a typhoon and two post-typhoon legs showing destruction of the hypoxia and its reinstatement. The lowest ever regional dissolved oxygen (DO) concentration of 3.5 μmol kg−1 (~ 0.1 mg L−1) was observed in bottom waters during Leg 1, with a ~ 660 km2 area experiencing hypoxic conditions (DO < 63 μmol kg−1). Hypoxia was completely destroyed by the typhoon passage but was quickly restored ~ 6 days later, resulting primarily from high biochemical oxygen consumption in bottom waters that averaged −14.6 ± 4.8 μmol O2 kg−1 d−1. The shoreward intrusion of subsurface shelf waters contributed to an additional 8.6 ± 1.7 % of oxygen loss during the reinstatement of hypoxia. Freshwater input-induced stratification, suppressing turbulent mixing induced by wind stress and/or tidal forcing, stabilized the water column and facilitated the hypoxia formation. The rapid reinstatement of summer hypoxia has a comparable timescale with water residence time and that of its initial disturbance from frequent tropical cyclones or high-wind events throughout the season. This has important implications towards better understanding the intermittent nature of hypoxia, and predicting coastal hypoxia in a changing climate.

Yangyang Zhao et al.

 
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Yangyang Zhao et al.

Yangyang Zhao et al.

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Short summary
We examined the evolution of intermittent hypoxia off the Pearl River Estuary as disturbed by the typhoon. In-situ oxygen consumption rate and associated time scales for the hypoxia formation was estimated for the first time at large-scales over the destruction and reinstatement of hypoxia. The comparable time scales for hypoxia formation, water residence time and disturbances from tropical cyclones have important implications towards better predicting coastal hypoxia in a changing climate.
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