Dissolved Oxygen Isotopes Unravel Mixing and Respiration Impacts on Coastal Oxygen Depletion Dynamics in the Changjiang Estuary

Understanding the complex processes leading to marine oxygen depletion is challenging. The nonlinear relationship between dissolved oxygen (DO) isotopic composition (δ¹⁸O) and its concentration makes δ¹⁸O a unique tracer for studying O₂ cycling in oxygen depletion zones (ODZs). Based on the DO δ¹⁸O compositions from four cruises conducted from summer 2020 to spring 2021 in the Changjiang Estuary, we identified a dynamic northern oxygen depletion zone (NODZ) with seasonally varying DO δ¹⁸O values, and a relatively stable southern oxygen depletion zone (SODZ). To quantify the contributions of low-O₂ water mixing and respiration to oxygen depletion, we conducted Monte-Carlo mixing experiments involving high-salinity and low-oxygen water (HSLO) and mixed layer water (MLW). We found that HSLO mixing contributed to 59%–100% of oxygen deficit (OD) (O₂ concentration deficit between bottom water and MLW) in ODZs during summer, with SODZ exhibiting an HSLO mixing contribution of more than 78%. Respiration contributed to the remaining OD, with water column respiration consuming approximately 2.5 times more oxygen than sediment oxygen consumption. In the NODZ, oxygen depletion was further enhanced by water column stratification, respiration, and seafloor topography. Overall, the influx of HSLO was a key trigger for oxygen depletion in the Changjiang Estuary. The spatial and temporal variations of the two ODZs were closely linked to the intrusion and retreat of HSLO from summer to autumn. Our findings highlight the importance of utilizing DO δ¹⁸O to differentiate and quantify biological and physical processes in coastal ODZs.