Spatial and Temporal Distribution of Dissolved Oxygen in Zhejiang Coastal Area

Dissolved oxygen is an important biogenic element in marine ecosystem, and the variation of its content and distribution could affect marine life activities directly or indirectly. In this paper, the spatial and temporal variaton characteristics were systematically analyzed and the occurrence of hypoxia was preliminarily discussed based on the seawater field monitoring data in Zhejiang coastal area from 2014 to 2018. The results showed that the spatial and temporal variation characteristics of dissolved oxygen and its saturation were obvious in Zhejiang coastal area. The spatial span analysis showed that the horizontal distribution of dissolved oxygen and its saturation presented different characteristics in different seasons, and the vertical distribution showed that dissolved oxygen and its saturation in the surface layer was higher than that in the bottom layer, and the difference between the surface layer and the bottom layer was larger in spring and summer, but smaller in autumn and winter, which was related to surface photosynthesis and seasonal thermohalocline. The time span analysis showed that the highest dissolved oxygen content in surface layer appeared in winter and the lowest in summer, which were affected by water temperature and surface photosynthesis, and the highest dissolved oxygen in surface layer in summer appeared in 2018 and the lowest in 2015, which was mainly caused by sea surface temperature. In addition, hypoxia was also found in some areas outside the coastal area, and its potential risks are being further tracked and monitored.