One-year methane ebullition measurements over a cross section of a small stream

Ebullition is a major transport pathway of methane from aquatic ecosystems to the atmosphere. Several studies have highlighted that methane ebullition plays an important role in reservoirs, but not much data is available from non-impounded stream sections. Quantifying river methane emissions on a global scale is a challenge due to the high spatiotemporal heterogeneity in these dynamic and variable systems. A better conceptual understanding of riverine methane ebullition is needed for sound carbon budgets, both in terms of flux volumes and predictive factors for hot-spot emission zones.

This study targeted the knowledge gap in riverine methane ebullition with a time-resolved observation-based analysis of the ebullitive transportation pathway in a river cross section. We installed four bubble traps in a small stream in southern Germany and monitored volumes and concentrations of the two greenhouse gases CO₂ and CH₄ along with carbon stable isotopes (δ¹³C) of CH₄ over the course of a year. The bubble traps were evenly distributed over one cross-section in a curve to represent different sediment-compositions and flow regimes between the undercut slope and slip-off slope. Sediment characterization at each site included grain-size distribution curves, porosity measurements and determination of organic carbon content.

Ebullitive gas fluxes were extremely high at two of the locations centrally in the river: up to 1000 ml m^-2 d^-1 during summer and autumn, and 100-400 ml m^-2 d^-1 in December. CH₄ concentrations of up to 65% were measured in the gas samples and CH₄ exceeded CO₂ concentrations (<5%) by far. Each site in the cross section showed relatively stable gas volumes and concentrations during the summer period. As expected, at the undercut slope, only very small gas volumes were detected year-round. Contrary to prior expectations, gas volumes and greenhouse gas concentrations were highest in the central section of the river and not above the fine-grained deposits of the slip-off slope. CH₄ isotopes were generally <-60‰, indicating a major contribution of hydrogenotrophic methanogenesis. During July however, δ¹³C of CH₄ at the slip-off slope showed higher values between -60‰ and -45‰, potentially due to a shift in the main methanogenic pathway or connected to increased macrophyte growth observed simultaneously. Overall, this study underlines the importance of ebullitive greenhouse gas fluxes from rivers for the global climate and provides novel insights into the role of different streambed sections for CH₄ ebullition during all seasons of the year.