19 Jan 2022
19 Jan 2022
Status: this preprint is currently under review for the journal BG.

Interannual variabilities, long-term trends, and regulating factors of low-oxygen conditions in the eastern Pearl River Estuary

Zheng Chen1, Bin Wang4, Chuang Xu5, Zhongren Zhang1,6, Shiyu Li1, and Jiatang Hu1,2,3 Zheng Chen et al.
  • 1School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China
  • 2Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou, 510275, China
  • 3Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, 519000, China
  • 4Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada
  • 5Center for Water Resources and Environment, Sun Yat-sen University, Guangzhou, 510275, China
  • 6Guangdong Zhihuan Innovative Environmental Technology Co., Ltd, Guangzhou, 510030, China

Abstract. The summertime low-oxygen conditions in the Pearl River Estuary (PRE) have experienced a significant expansion in spatial extent associated with notable deoxygenation in recent decades. Nevertheless, there is still a lack of quantitative understanding of the long-term trends and interannual variabilities in oxygen conditions in the PRE as well as the driving factors, which was comprehensively investigated in this study using monthly observations in the eastern PRE during 1994–2018. To evaluate the changes in scope and intensity of oxygen conditions, an indicator (defined as the Low-oxygen Index, LOI) that integrates several metrics related to low-oxygen conditions was introduced through the principal component analysis (PCA). Moreover, primary physical and biogeochemical factors controlling the interannual variabilities and long-term trends in oxygen conditions were discerned, and their relative contributions were quantified by the multiple regression analysis. Results showed that the regression models explained over 60 % of the interannual variations in LOI. Both the wind speeds and concentrations of dissolved inorganic nitrogen (DIN) played a significant role in determining the interannual variations (by 39 % and 49 %, respectively) and long-term trends (by 39 % and 56 %, respectively) in LOI. Due to the increasing nutrient loads and alterations in physical conditions (e.g. the long-term decreasing trend in wind speeds), coastal eutrophication was exaggerated and massive marine-sourced organic matter was subsequently produced, thereby resulting in an expansion of intensified low-oxygen conditions. It has also driven a shift in the dominant source of organic matter from terrestrial inputs to in situ primary production, which has probably led to an earlier onset of hypoxia in summer. In summary, the eastern PRE has undergone considerable deterioration of low-oxygen conditions in the context of substantial changes in anthropogenic eutrophication and external physical factors.

Zheng Chen et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-358', Anonymous Referee #1, 04 Feb 2022
  • RC2: 'Comment on bg-2021-358', Anonymous Referee #2, 22 Feb 2022

Zheng Chen et al.

Zheng Chen et al.


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Short summary
Deterioration of low-oxygen conditions in eastern Pearl River Estuary (PRE) was revealed by monitoring data over 25 years. The declining wind forcing and increasing nutrient input contributed significantly to areal expansion and intensity deterioration of low oxygen. Besides, the exacerbated eutrophication drove a shift in the dominant source of organic matter from terrestrial inputs to in situ primary production, which has probably led to an earlier onset of hypoxia in summer.