Oxygen decline in a temperate marginal sea: Contribution of warming and eutrophication

Dissolved oxygen (DO) decline (i.e., deoxygenation) is an ongoing process in parts of the coastal and open oceans as a result of increased greenhouse gas emissions and nutrient discharges. Yet its controlling mechanisms remain unclear and patchy. Based on continuous observational data collected in a temperate margin, the southern Yellow Sea (SYS), we quantitatively evaluate how deoxygenation responds to warming and eutrophication in different seasons by using an evaluation method that allows us to distinguish the effects of temperature, salinity and biological activities. Results show that during winter, when the water column is vertically well-mixed, and in summer surface waters, deoxygenation is dominated by warming-induced decreases of O₂ solubility due to a quick exchange of O₂ between the ocean and atmosphere. Moreover, we find a regionally accelerated deoxygenation with enhanced warming along the pathway of the YSCC (Yellow Sea Coastal Current) in winter. In contrast, for bottom waters in summer when O₂ exchange is inhibited due to high stratification, deoxygenation appears to be dominated by biological respiration associated with eutrophication. Also, we find the summer bottom deoxygenation can be accelerated by warming, indicating that the bottom waters or the hypolimnion may be vulnerable to deoxygenation in the future. Our study further demonstrates that the deoxygenation mechanisms in shallow coastal oceans are associated with water column structures, i.e., well-mixed vs. stratified water column. Information is assembled into a conceptual model to provide an overview of deoxygenation in temperate marginal systems.