Vertical hydrologic exchange flows control methane emissions from riverbed sediments

Abstract

CH4 emissions from inland waters are highly uncertain in the current global CH4 budget, especially for the lotic systems like rivers. Previous studies have attributed the strong spatiotemporal heterogeneity of riverine CH4 to different environmental factors through correlation analysis. However, a mechanistic understanding for such heterogeneity is lacking. Here we combine sediment CH4 data with a biogeochemical-transport model to show that vertical hydrologic exchange flows (VHEFs), driven by the difference between river stage and groundwater level, determine CH4 flux at the sediment-water interface. CH4 fluxes show a nonlinear relationship with the magnitude of VHEFs. In addition, VHEFs lead to the hysteresis of temperature rise and CH4 emissions because high river discharge leads to strong downwelling flow that offsets increasing CH4 production with temperature rise. Our findings reveal how the interplay between hydrologic flux and microbial metabolic pathways that compete with methanogenic pathways can produce complex patterns in CH4 production and emission in riverbed sediments.