Flow regime specific regulation shapes microbial-mediated nitrogen cycling of plain tidal river network

Inter-basin water diversion projects are critical for mitigating regional water scarcity yet impose complex ecological pressures on recipient river networks. Understanding their microbial impacts is essential to optimize sluice operations and minimize ecosystem disruption. As pivotal regulators of biogeochemical cycles and ecological health, microbial communities in plain tidal networks remain poorly characterized under diversion-induced hydrodynamic shifts. This study integrated intensive field sampling across water and sediment sites in the lower tidal plain river network with a calibrated one-dimensional MIKE 11 hydrodynamic model, stratifying sampling points into low, medium, and high flow-velocity regimes. Results indicate a positive correlation between hydrological regime stability and microbial community stability. While community composition reorganizes along the flow gradient, microbial diversity and core taxa abundance remain resilient. Co-occurrence network analysis reveals that intermediate flow variability maximizes network connectivity and modular cohesion, whereas extreme hydrological conditions fragment network structures. Landscape modeling further identifies high-discharge variability zones as distinct "hotspots" for denitrification and organic matter processing, while hydrologically stable reaches act as "functional shadows" (coldspots). Structural equation modeling confirms that hydrological regulation operates not merely through direct physical forcing but via a "resource-diversity-function" cascade, indirectly driving biogeochemical cycles by modulating nutrient fluxes and reshaping microbial diversity. Consequently, this study recommends shifting management strategies toward maintaining intermediate flow variability to reinforce the robustness and self-purification capacity of riverine ecological networks.