Methane Dynamics in Braided River Sections of the Free-Flowing Vjosa River, Albania

Abstract

Greenhouse gas emissions give rivers an important role in the global carbon cycle. Since methane (CH₄) has a higher warming potential than carbon dioxide, it is essential to understand how, where, and when CH₄ is produced and emitted. We investigated spatiotemporal variations of CH₄ concentration and outgassing fluxes in free-flowing braided river sections of the Vjosa River and their relation to seasonal discharge shifts, which drive fine sediment deposition, hyporheic isolation, and surface drying. Localized sampling in the hyporheic zone allowed the investigation of potential dependencies of CH₄ concentration on the availability of oxygen, alternative electron acceptors, and organic matter (OM). Compared to other river systems, we found very low CH₄ concentrations (from 1.7 to 1,167.3 nmol L⁻¹) and emission fluxes (from −0.03 to 0.21 mmol m⁻² d⁻¹). Hyporheic CH₄ concentrations diverged between campaigns, depths, and habitats differing in surface water availability. Surface drying led to decreased or even negative CH₄ fluxes. Dissolved OM best explained CH₄ occurrence, yet large uncertainties remained, and patterns along depth, across habitats and campaigns were not always as expected from simplified redox gradients. Our results indicate the importance of small-scale habitat heterogeneity in the hyporheic zone driven by sediment composition and fine sediment deposition. This likely affects the availability of oxygen, other electron acceptors, and OM, that define the habitat suitability for CH₄ production. In spatiotemporally dynamic braided rivers, exchange processes between hyporheic zone, surface water, and atmosphere must be considered simultaneously in order to mechanistically decipher final greenhouse gas emissions to the atmosphere.