The temporal-spatial distribution and changes of dissolved oxygen in the Changjiang Estuary and its adjacent waters for the last 50 a

The Changjiang Estuary and its adjacent waters form one of the most important estuarine and coastal areas in China. Multi source and long-term data are assembled to examine the temporal-spatial distribution features of dissolved oxygen (DO) in the Changjiang Estuary and its adjacent waters for the past 50 a. The results show that the DO concentration in the surface of different seasons generally stays stable, while the DO concentration in winter displays a slight increase for the last 50 a. The DO average concentration in winter and spring varies from 7 to 11 mg/L, and in summer and autumn from 6 to 8 mg/L. Hypoxic values first appear in May, and low DO value plume can be observed on the bottom in spring along coastal areas of Zhejiang and Fujian Provinces, China. In summer, the plume advances northward, and the hypoxic intensity of northern transects is much higher than southern transects. Until autumn, hypoxia areas fade away little by little, and completely disappear in winter. Within last 50 a, hypoxia in the Changjiang Estuary and its adjacent waters starts to appear in the 1980s. Since 2000, the degree of hypoxia has increased seriously and the distribution depth has become smaller. It is performed based on a large amount of historical data, and the research results will be of great significance to further study on the dynamic development of hypoxia around the Changjiang Estuary.