Hydrodynamics, Diagenesis and Hypoxia Variably Drive Benthic Oxygen Flux in a River-Reservoir System

Benthic oxygen flux with complex spatiotemporal variations is essential for the global budget of carbon dioxide and the regional security of water quality and ecology, but its dominant driver under different circumstances has yet to be identified. In this study, a parametric scheme of oxygen flux was proposed and validated with aquatic eddy correlation measurements and then coupled with a diagenesis model and a water environment model. The coupled model was applied to a river-reservoir with significant environmental gradients in hydrodynamics, diagenesis, and hypoxia, which are three factors that competitively drive the variation in benthic oxygen flux. The results indicate that hydrodynamics dominate the flux in the riverine and thalweg areas, diagenesis is the dominant driver of the lacustrine and bank areas, and hypoxia shows dominance only in the hypolimnetic anoxic area. In general, diagenesis is the dominant driver of oxygen flux in river-reservoirs, followed by hydrodynamics, both of which are more prominent than hypoxia. If the operated reservoir experiences a wet year, the dominance of hydrodynamics tends to increase, while that of diagenesis and hypoxia decreases. The three divers exhibit similar but more stable dominance in riverine systems than in reservoirs, while diagenesis becomes the exclusive driver of oxygen fluxes in lacustrine systems. The consumption of dissolved oxygen in sediments regulates the types of substances released from them. When the oxygen supply to the sediments is insufficient, they release substances with a stronger greenhouse effect (such as methane) and a more significant threat to water quality (such as heavy metals). Flow conditions, sediment features, and water properties collectively drive the variation in sediment oxygen consumption, but their relative importance within complex aquatic environments remains to be investigated. This study quantifies the dominance of these three drivers on sediment oxygen consumption in a river-reservoir system and then establishes their relationship with spatiotemporal variations in the aquatic environment. Sediment features are the primary drivers of sediment oxygen consumption in the studied river-reservoir, followed by flow conditions and water properties represented by oxygen concentrations. The results also suggest that the relative dominance of these three drivers in river-type systems is similar to that in reservoirs, but the dominance of sediment features could further increase in lake-type systems. This study highlights the variable role of factors driving sediment oxygen consumption, which is critical for quantifying the release of deoxygenation products in complex aquatic ecosystems. A practical parametric scheme for determining benthic oxygen fluxes was proposed and validatedRiver-reservoirs have large environmental gradients in hydrodynamics, sedimentation, and stratificationDiagenesis is the primary driver of benthic oxygen fluxes in river-reservoirs, followed by hydrodynamics and hypoxia