The mechanisms of hypoxia formation in a major tributary of the Three Gorges Reservoir, China

Abstract

Hypoxia is a persistent global environmental crisis that has garnered significant attention from both the scientific community and the general public. Recent monitoring in tributaries of the Three Gorges Reservoir (TGR) has identified hypoxic zones, yet the mechanisms underlying hypoxia in these systems remain poorly understood. This study investigates the drivers of hypoxia in the Pengxi River (PXR), a major tributary of the TGR. Field measurements reveal a significant hypoxic zone extending approximately 20 km, with dissolved oxygen concentrations nearing 0 μmol/L. The unique inflow dynamics of the PXR, characterized by warm, slow-moving waters that exacerbate stratification, provide new insights into hypoxia mechanisms in reservoir systems. Further analysis, including the ΔCO₂/ΔO₂ ratio, δ¹³C, and C/N values, indicates that microbial respiration of organic matter—primarily due to algal blooms—is a major contributor to hypoxia in the PXR. The sediment oxygen demand (SOD) was measured at an average of 14.90 ± 4.12 mmol O₂ m⁻² d⁻¹, while the vertical diffusive oxygen flux across the thermocline was 1.59 ± 1.61 mmol O₂ m⁻² d⁻¹, accounting for approximately 10 % of the oxygen consumed by sediments. By quantifying SOD and vertical oxygen flux, this study provides a comprehensive understanding of oxygen dynamics. Furthermore, this study explores potential strategies for mitigating hypoxia through optimizing upstream reservoir management in cascade systems, particularly by adjusting outlet elevations to enhance oxygen supply to the hypolimnion. These strategies offer a scalable approach to alleviating downstream hypoxia. The findings offer insights into hypoxia mechanisms in reservoir systems and provide valuable guidance for management strategies aimed at hypoxia mitigation.