Origins of Low-Oxygen Bottom Water Influenced by Tide and ENSO on the Outer-Edge Shelf of East China Sea: Multi-Chemical Tracer Approaches

The East China Sea (ECS) is one of the largest marginal seas in the world with high primary productivity and a large area of low dissolved oxygen. This study observed the low-oxygen bottom water on the outer-edge shelf of ECS in summer seasons of 2018–2020. The contributions of various water masses to low-oxygen waters were quantified, and the interaction between low-oxygen water and Kuroshio water was validated by a mixing model using heavy rare earth elements (HREEs) together with potential temperature, and salinity. For further reliability verification, tracers such as δ³⁴S and ²²⁶Ra were additionally utilized to avoid the non-conservative processes on the shelf area. The low-oxygen water on the shelf was clarified to be linked to the inner/middle shelf water, and dominated by Kuroshio Subsurface Water (KSSW, 81% ± 3%) and influenced by a non-negligible extent of pore water (3.0% ± 0.5%). Our findings revealed the transport of low-oxygen water from the outer-shelf edge to the Kuroshio area, whose dissolved inorganic nitrogen and dissolved inorganic phosphorus contributions to Kuroshio were 34%–82% and 35%–83%, respectively. The nutrients of low-oxygen water were regulated primarily by the origins of water and secondarily by organic matter remineralization (DIN: ∼17% and DIP: ∼24%). As control factors, the tide, especially spring tide largely enhanced the water contribution of pore water to low-oxygen water by 67%, and the El Niño-Southern Oscillation potentially affected the contributions of KSSW and mid-shelf water to low-oxygen water.