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 δC 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 δC 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 O 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.