Seasonal oxygen depletion has been increasingly reported on the East China Sea (ECS) shelf since the early 2000s; however, its long-term evolution and underlying drivers remain poorly understood. Using historical observations from the World Ocean Database (1951–2006), we reconstruct basin-scale variability of dissolved oxygen (DO) across the ECS shelf and analyze both seasonal patterns and multi-decadal trends. Our results show that oxygen depletion (DO ≤ 4 mg L⁻ 1 ) on the mid–outer ECS shelf has occurred regularly since the 1950s, substantially earlier than previously reported observations, which mainly documented its occurrence after the early 2000s. Oxygen depletion recurs from summer to autumn, with oxygen stress shifting from the inner shelf in late summer toward the mid–outer shelf along the 50–100 m isobaths in autumn. A significant decline in bottom DO is detected over the mid–outer shelf during 1960–1999 and is accompanied by increasing apparent oxygen utilization (AOU). By combining AOU diagnostics with a three-endmember mixing model, oxygen deficits are partitioned into locally generated consumption and remotely advected components. The results indicate that local remineralization dominates oxygen consumption, while the intrusion of oxygen-poor Kuroshio Subsurface Water reduces the background oxygen inventory and modulates the intensity of seasonal depletion. These findings reveal that oxygen depletion on the ECS shelf has a longer history and broader spatial extent than previously recognized.
Li et al. (2026) studied this question.