Isotopes unveil overestimation of nutrient-driven oxygen deficit in the tidal rivers of Pearl River Delta during the wet season

A low dissolved oxygen (DO) concentration in summer has been observed in river-estuary systems worldwide. Many studies have caused our stereotype that biochemical oxygen depletion was higher in summer than in winter; however, there was no direct evidence particularly in the tidal river with complex hydrological and biochemical processes. This study employed natural-abundance and labeled isotopes to quantify seasonal apportionment of biochemical oxygen depletion. In this study, apparent oxygen utilization (AOU) and carbon (C) and nitrogen (N) turnover potentials (nitrification rates and δ¹³C signals) were higher in the wet season than in the dry season. However, calculation results of the nitrification flux demonstrated that actual N turnover was constrained by shorter river residence time in the wet season. Similarly, the δ¹³C end-member mixing and Rayleigh fractionation models revealed that the conservative C behavior was more pronounced than degradation in situ in the river channel. Overall, C- and N-driven oxygen depletion accounted only for ~8% of AOU in the wet season. This substantiated that the hydrological control regulated C and N behaviors to "the conservative transport" to mitigate O₂ depletion in the wet season. In contrast, a good correspondence between C and N turnover and low oxygen was recorded in the dry season. Therefore, the "nutrient- and non-nutrient-constrained DO cold spots" during the dry and wet seasons provided new insights into oxygen deficits in tidal rivers. Our study provided compelling evidence that seasonal apportionment of C- and N-driven oxygen depletion in situ has changed in tidal rivers. Biochemical oxygen depletion was more evident in the dry season than in the wet season; thus, it had been previously overestimated in the wet season, which will provide implications for using different water management strategies in different seasons.