Spatiotemporal characteristics of summer hypoxia in Mirs Bay and adjacent coastal waters, South China

Mirs Bay is a semi-enclosed bay neighboring the Zhujiang (Pearl) River estuary, one of the largest estuarine systems in the world. The long-term historical observational data (1994-2017) of temperature, salinity, dissolved oxygen (DO), and biochemical parameters were used to examine the spatiotemporal distribution of hypoxia in Mirs Bay and adjacent coastal waters. Results show that bottom hypoxia varied seasonally and interannually. Hypoxia mainly occurred from June to September in Mirs Bay and the transition zone in the southern waters of Hong Kong, and the recorded hypoxia events have increased from 2007. The density difference between the bottom and surface layers was positively related to the bottom apparent oxygen utilization (AOU) (R=0.620, P<0.001) and negatively related to the bottom DO (R=0.616, P<0.001), indicating that water column stratification was an essential prerequisite for the formation of bottom hypoxia in summer. The bottom oxygen consumption and hypoxia had higher positive correlation with the seasonal thermocline (R=0.683, P<0.001) than the halocline (R=0.540, P< 0.001), including in the area was affected by freshwater plume. The insignificant relationship between AOU and nutrients indicated that local eutrophication was not the only important factor in the formation of the hypoxic zone during summer. The decrease in phosphorous owing to the pollutant reduction policy and the increase in nitrate may have led to an increase in hypoxia events in the bay where waters therein are characterized by nitrogen-limitation. The increase in chemical oxygen demand in wastewater also promoted oxygen consumption. Compared to the adjacent coastal waters influenced by Zhujiang River plume water, the Mirs Bay experienced more hypoxia events. The high concentrations of ammonium and total Kjeldahl nitrogen in the sediment of Mirs Bay increased the oxygen depletion in the bottom water. The long residence time of the near-bottom water in Mirs Bay increased the risk of bottom hypoxia events, although the nutrient concentrations were lower than those in the transition zone. These factors lead to differences in hypoxia occurrence in Mirs Bay and adjacent coastal waters.