Interannual variabilities, long-term trends, and regulating factors of low-oxygen conditions in the coastal waters off Hong Kong

[摘要] The summertime low-oxygen conditions in the Pearl RiverEstuary (PRE) have experienced a significant spatial expansion associated with notable deoxygenation in recent decades. Nevertheless, thereis still a lack of quantitative data on the long-term trends andinterannual variabilities in oxygen conditions in the PRE as well as on thedriving factors. Therefore, the long-term deoxygenation in a subregion ofthe PRE (the coastal waters off Hong Kong) was comprehensively investigatedin this study using monthly observations during 1994–2018. To evaluate thechanges in scope and intensity of oxygen conditions, an indicator (definedas the low-oxygen index, LOI) that integrates several metrics related tolow-oxygen conditions was introduced as the result of a principal componentanalysis (PCA). Moreover, primary physical and biogeochemical factorscontrolling the interannual variabilities and long-term trends in oxygenconditions were discerned, and their relative contributions were quantifiedby multiple regression analysis. Results showed that the regressionmodels explained over 60 % of the interannual variations in LOI. Both thewind speeds and concentrations of dissolved inorganic nitrogen (DIN) playeda significant role in determining the interannual variations (by 39 % and49 %, respectively) and long-term trends (by 39 % and 56 %,respectively) in LOI. Due to the increasing nutrient loads and alterationsin physical conditions (e.g., the long-term decreasing trend in wind speeds),coastal eutrophication was exaggerated and massive marine-sourced organicmatter was subsequently produced, thereby resulting in an expansion ofintensified low-oxygen conditions. The deteriorating eutrophication has alsodriven a shift in the dominant source of organic matter from terrestrialinputs to in situ primary production, which has probably led to an earlieronset of hypoxia in summer. In summary, the Hong Kong waters have undergoneconsiderable deterioration of low-oxygen conditions driven by substantialchanges in anthropogenic eutrophication and external physical factors.

[学科分类] 大气科学