Seasonal preferences of nitrogen-associated oxygen depletion strategies in the river-estuary continuum: Implications for water quality management.

Dissolved oxygen (DO) depletion in the river-estuary continuum is a growing global concern, yet whether nitrogen (N)-associated oxygen depletion exhibits distinct seasonal strategies remains poorly constrained by field observations. To address this gap, we integrated high-frequency monitoring, multi-isotope tracing, 15N-labeled nitrification assays, and hydrodynamic modeling. Our findings supported the hypothesis of two distinct, seasonally-preferred oxygen depletion strategies. In the dry season, the dominant point sources (e.g., domestic outfalls, contributing 47.8 ± 8.3%) were discharged directly into the river and mixed rapidly with river water. Intense saltwater intrusion prolonged river residence times, allowing N to undergo in-stream nitrification (upper-bound flux of 2.90 ± 0.66 μmol N/L) even at low potential rates. This favored nitrate production and oxygen depletion concurrently within the channel and supported a direct, in-situ oxygen depletion. In contrast, non-point sources (e.g., agricultural fertilizers and soil N, contributing >54.9 ± 8.7%) dominated the riverine N pool and entered the river indirectly via runoff in the wet season. These N may have been pre-converted to nitrate through mineralization-nitrification occurring outside the channel. Short river residence times limited potential in-stream nitrification (upper-bound flux of 1.84 ± 0.30 μmol N/L), and convergent evidence pointed to enhanced subsurface flow that potentially exported "pre-formed" nitrate and hypoxic water into the river, strongly supporting the indirect, transport-constrained, ex-situ oxygen depletion hypothesis. Our study challenges the concentration- or rate-dependent paradigm and establishes process-based cascade relationships to reshape the seasonal dynamics of N-associated oxygen depletion in the river-estuary continuum. We propose seasonally adaptive management for mitigating DO depletion: point-source control and flow regulation (e.g. upstream reservoir optimal operation) in the dry season and watershed-scale interventions in the wet season.