Physical and biogeochemical processes in controlling the seasonal variation of dissolved oxygen in the Bohai and Yellow Seas

Deoxygenation in coastal seas has significant implications to the marine ecosystems. In this study, the processes influencing the seasonal variation of dissolved oxygen (DO) in the Bohai and Yellow Seas (BS and YS) are investigated using a coupled physical-biogeochemical model. The seasonal variation of the DO concentration ([DO]) in the surface water is lower in summer and autumn and higher in winter and spring, which is mainly controlled by temperature. Moreover, in this layer, photosynthesis, to some extent, regulates the [DO] and leads to the supersaturation in summer, whereas downward diffusion in autumn is mainly responsible for the gradually decreased saturation degree. The seasonal distribution of the [DO] in the bottom water is similar with that in the surface water, with much lower [DO] and significant undersaturation in summer in the BS and in summer and autumn in the YS. Vertical diffusion is overall the predominant source of DO in the bottom layer, whereas biological oxygen consumption (BOC) is the main sink, especially during stratification. Relative to the BS, the YS experiences a moderate but longer deoxygenation duration, due to the thicker bottom water and persistent stratification. In addition, it reveals that apparent oxygen utilization (AOU) prevalently underestimates BOC in coastal waters, and true oxygen utilization (TOU) is proposed as a more accurate proxy. Notably, the TOU-AOU discrepancy is strongly correlated with temperature variation during this period, suggesting continuous temperature monitoring as a useful pathway in investigating BOC in shallow estuarine and coastal waters.