Data summary for Dissolved oxygen isotopes reveal the seasonal and spatial variations of the oxygen depletion in the Changjiang Estuary

The Changjiang Estuary and its adjacent sea experience increasingly seasonal hypoxia in bottom waters. Understanding the complex processes leading to hypoxia, including water column respiration (WCR), sediment oxygen consumption (SOC), and physical mixing, is challenging based solely on dissolved oxygen (DO) and nutrient concentrations. The nonlinear relationship between oxygen isotopic composition of DO (δ¹⁸O) and DO concentrations ([O₂]) make it a unique tracer for studying and quantifying O₂ cycling in oxygen depletion zones. We conducted four cruises to investigate the DO d¹⁸O in the Changjiang Estuary in July, August, October 2020, and March 2021. Our observations revealed two oxygen depletion zones (ODZs) in the northern and southern region, exhibiting distinct seasonal behaviors. Oxygen depletions were prominent in July, peaked in August for both regions, but retreated to the south in October, and totally disappeared by March. Depth profiles from ODZ stations in both regions revealed that most oxygen consumption and d¹⁸O increase occurred primarily in the middle of the water column, where stratification was greatest. Through seawater incubations, we derived a typical Changjiang Estuary WCR oxygen isotopic fractionation factor of 0.9791. Subsequently, we quantified the relative oxygen consumptions by WCR, which accounted for more than 76% in the total oxygen consumption, underscoring its importance in shaping oxygen dynamics in this region. However, relative contributions of WCR in the northern region varied seasonally, peaking in August and decreasing in October, whereas they remain relatively stable in the south. Community oxygen isotopic fractionation factors () observed from samples also exhibited more variations in the northern region than in the south.