Hydrodynamic-driven microbial community assembly and coalescence shape nitrogen transformation in river confluence

Abstract

River confluences are characterized by complex hydrodynamics, which can significantly influence mass transport and microbial community, and further make a difference to biogeochemical processes. However, mechanistic links between confluence hydrodynamics, microbial community processes, and nitrogen transformation remain poorly understood. Here, a self-circulating confluence flume was employed to analyse how hydrodynamic characteristic influence microbial community assembly, coalescence, and nitrogen transformation processes. Results revealed that the microbial community succession was primarily governed by deterministic processes, especially homogeneous selection, with its contributions were higher than 50%, causing the similarity of community composition at the earlier stage. With time going by, influence of ecological drift increased, which contributed to the divergence in community composition at the later stage. Community coalescence, assessed by SourceTracker and ASV overlap, was proved to exist in the confluence area, and significantly enhanced in low-velocity zones due to increased hydraulic retention time (p < 0.01). Partial least squares path modelling identified that low flow velocity directly promoted ammonia accumulation, which in turn stimulated ammonia oxidation and ultimately enhanced nitrogen removal. Conversely, strong homogenizing selection and community coalescence in low velocity zones suppressed nitrogen removal by reducing niche diversity. This suggests an optimal design of flow velocity to balance the redox environment and microbial community dynamics is necessary for the maximization of the nitrogen removal capacity. Our study provided mechanistic insights into how hydrodynamic heterogeneity at river confluences regulated nitrogen transformation through mediating microbial community assembly and coalescence, highlighting the critical role of confluence morphology in river network nutrient management.

Graphical abstract

Keywords

River confluence / Hydrodynamic characteristics / Microbial community assembly / Community coalescence / Nitrogen cycling

Highlight

	● Hydrodynamic heterogeneity in confluence governs microbial assembly and coalescence.
	● Varied community coalescence events cause microbial divergence in confluence.
	● Low velocity zones act as nitrogen removal hotspots via enhanced ammonia oxidation.
	● Redox environment and community dynamics decide nitrogen removal in confluences.

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