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

Abstract

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.