Main factors dominating the development, formation and dissipation of hypoxia off the Changjiang Estuary (CE) and its adjacent waters, China

Hypoxia off the Changjiang Estuary (CE) and its adjacent waters is purported to be the most severe in China, attracting considerable concern from both the scientific community and the general public. Currently, continuous observations of dissolved oxygen (DO) levels covering hypoxia from its appearance to disappearance are lacking. In this study, twelve consecutive monthly cruises (from February 2015 to January 2016) were conducted. The consecutive spatiotemporal variations in hypoxia throughout the annual cycle were elucidated in detail, and the responses of annual variations in hypoxia to the different influential factors were explored. Overall, hypoxia experienced a consecutive process of expanding from south to north, then disappearing from north to south. The annual variations in hypoxia were mainly contingent on stratification variations. Among different stages, there was significant heterogeneity in the dominant factors. Specifically, low-DO waters initially appeared from the intrusion of nearshore Kuroshio branch current (NKBC), as NKBC intrusion provided a low-DO background and triggered stratification. Thereafter, stratification was enhanced and gradually expanded northward, which promoted the extension of low-DO areas. The formation of hypoxia was regionally selective, and more intense organic matter decomposition at local regions facilitated the occurrence and discontinuous distribution of hypoxia. Hypoxic zones were observed at the Changjiang bank and Zhejiang coastal region from August (most extensively at 14,800 km2) to October. Thereafter, increased vertical mixing facilitated the dissipation of hypoxia from north to south.