Water mass structure and ecological implications in the East China Sea: Insights from rare earth elements-based analysis in autumn

The East China Sea (ECS) is one of the world's largest marginal seas and has a complex water mass structure. However, quantitative knowledge of its water mass structure in autumn remains limited. In this study, dissolved rare earth elements (REEs) from the GEOTRACES GP06-CN cruise (October 2015) were used as tracers to resolve water masses. REEs showed strong correlations with salinity ( P < 0.0001) at salinities >25, indicating semi-conservative behavior. A multi-endmember mixing model incorporating potential temperature, salinity, and heavy REEs revealed that the Taiwan Warm Current (TWC) and Kuroshio Surface Water dominated in autumn, contributing 50 ± 30% and 20 ± 23%, respectively. Kuroshio Subsurface Water was identified, characterized by upwelling at the shelf edge (∼28.7°N) and weak northwestward intrusion onto the shelf. Above the euphotic layer, Changjiang Diluted Water (CDW) and TWC were the major nutrient sources, with CDW supplying 24 ± 33% of dissolved inorganic nitrogen (DIN) and TWC supplying 45 ± 27% of dissolved inorganic phosphorus (DIP). Based on ΔDIP (the difference between observed values and predicted from water mass contributions) and the Redfield C/P ratio, autumn net community production was estimated at 0.18 ± 0.13 g C/m²/day (range: 0.02–0.43 g C/m²/day). A sensitivity analysis suggests that TWC intrusion reduced phosphorus limitation by 47 ± 24% (range: 9–61%). Low-oxygen waters were observed in bottom waters on the inner shelf (CDW: 39 ± 5%) and middle shelf (TWC: 45 ± 1%), with oxygen consumption along the TWC pathway estimated at 2.39 ± 2.89 μmol/kg/day. This study highlights the structure and mixing of autumn water masses in the ECS, demonstrating the key roles of TWC and CDW in nutrient transport and the development of low-oxygen conditions.