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https://doi.org/10.5194/bg-2020-188
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-188
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  08 Jun 2020

08 Jun 2020

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This discussion paper is a preprint. It has been under review for the journal Biogeosciences (BG). The manuscript was not accepted for further review after discussion.

An interrupting mechanism to prevent the formation of coastal hypoxiaby winds

Juan Yao1,2, Juying Wang3, Hongbin Liu2,4, and Kedong Yin1,2 Juan Yao et al.
  • 1School of Marine Sciences/Guangdong Key Laboratory of Marine Resources and Coastal Engineering, Sun Yat-Sen University, Guangzhou 510275, Guangdong, China
  • 2Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519082, Guangdong, China
  • 3National Marine Environmental Monitoring Center, Ministry of Ecology and Environment, Dalian 116023, Liaoning, China
  • 4Department of Ocean Sciences, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong SAR, China

Abstract. Enrichment of nutrients is believed to lead to coastal hypoxia which have become a seasonal phenomenon over large river estuarine areas such as the Mississippi River-Northern Gulf of Mexico and Changjiang-East China Sea. There is a similar nutrient enrichment process in the Pearl River. However, hypoxia occurs only as episodic events over a relatively small area. We hypothesize that frequent wind events play the interruptive mechanism in preventing the seasonal formation of bottom hypoxia. We used 29 years time series data of dissolved oxygen (DO) and winds in the Hong Kong coastal waters to test the hypothesis. Our results show that bottom DO at 3 stations in southern waters of Hong Kong occasionally drops below the hypoxic level (2 mg/L), lasting only for less than one month in summer. Episodic hypoxia events appear to occur more frequently in recent years, but bottom DO does not show a significantly decreasing trend. The wind speed of 6 m/s appears to be a threshold, above which a wind event could destroy water column stratification and interrupt the formation of low-oxygen (DO < 3 mg/L) water mass. The wind events above the threshold occur 14.3 times in June, 14.2 times in July and 10.0 times in August during 1990–2018. This explains why episodic events of hypoxia hardly occur in June and July, and only occasionally in August. The frequency of such the above-threshold events appears to show a decreasing trend during 1990–2018, which coincides with an increasing occurrences of episodic hypoxia events in recent years.

Juan Yao et al.

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Juan Yao et al.

Juan Yao et al.

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Short summary
Our study finds that winds appear to be a dominant regulating force on the formation of hypoxia in the Pearl River estuarine waters. The wind speed > 6 m/s appears to a threshold above which the water column stratification is destroyed and the formation of bottom hypoxia is interrupted. The frequency of such the above-threshold wind events decreases significantly, implying that climate change induced variability in wind speeds can exasperate the coastal formation of bottom hypoxia in the region.
Our study finds that winds appear to be a dominant regulating force on the formation of hypoxia...
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