Deoxygenation and Its Controls in a Semienclosed Shelf Ecosystem, Northern Yellow Sea

Deoxygenation is a global problem in open oceans and coastal regions and has led to the expansion of oxygen minimum zones and coastal hypoxia. Here, using time series observations in the northern Yellow Sea, we illustrate the seasonality of deoxygenation and associated mechanisms in this semienclosed shelf ecosystem and explore the linkage of dissolved oxygen (DO) declines with seawater warming and anthropogenic eutrophication. Our results show that the DO concentration displayed a decreasing trend in winter, with rates of similar to 0.76 and similar to 0.78 mu mol kg(-1) yr(-1) in the surface and bottom waters, respectively. The rapid decline in DO was also observed in the bottom water in summer, with a decreasing rate of similar to 1.43 mu mol kg(-1) yr(-1). In winter, when the water column is vertically homogeneous, the near saturation of DO indicates that physical mixing overwhelmed biological activities, and the estimated results and a linear correlation between DO concentration and temperature suggest that ocean warming is the most plausible driver of deoxygenation in this region in winter. In the stratified water column during summer, increased nutrient availability at the surface and consequently enhanced productivity are responsible for the drawdown of DO in the bottom layer, and the stoichiometric pattern between DO and nitrate also suggests a cascading linkage between the exacerbation of eutrophic conditions and bottom deoxygenation. Our results provide strong evidence that a rapid, semienclosed shelf-scale decline in DO is underway and highlight the necessity of nutrient reduction strategies in the future. Plain Language Summary Studying the deoxygenation and its drivers is important to understand the changing marine ecosystem. As an important semienclosed marginal shelf sea in the Northwest Pacific, the Yellow Sea (YS) is one of the world's 64 Large Marine Ecosystems. Due to the natural and anthropogenic stressors, the YS ecosystem has undergone drastic changes over the past decades. In this study, using the long-term monitoring data set in the northern YS (NYS), the deoxygenation and associated mechanisms in this semienclosed shelf ecosystem are examined in seasonality, and the linkage of dissolved oxygen declines with seawater warming and anthropogenic eutrophication is explored. The results imply that a rapid, semienclosed shelf-scale declining of dissolved oxygen is ongoing in the continental margin area, reflecting an alarming symptom of coastal environment degradation in the NYS. This work also offers a viewpoint of deoxygenation mechanisms in terms of seasonality in the multiple pressures-influenced margins, improving our understanding of the marine environmental responses to anthropogenic and natural perturbations.