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

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Abstract Understanding the complex processes leading to marine oxygen depletion is challenging. The nonlinear relationship between dissolved oxygen (DO) isotopic composition (δ 18 O) and its concentration makes δ 18 O a unique tracer for studying O 2 cycling in oxygen depletion zones (ODZs). Based on the DO δ 18 O 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 δ 18 O values, and a relatively stable southern oxygen depletion zone (SODZ). To quantify the contributions of low‐O 2 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) (O 2 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 δ 18 O to differentiate and quantify biological and physical processes in coastal ODZs.

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