Methane Dynamics in Braided River Sections of the Free‐Flowing Vjosa River, Albania.
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| Title: | Methane Dynamics in Braided River Sections of the Free‐Flowing Vjosa River, Albania. |
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| Authors: | Jechsmayr, Barbara1 (AUTHOR) barbara.jechsmayr@uibk.ac.at, Estévez Cano, Edurne1 (AUTHOR), Singer, Gabriel1 (AUTHOR) |
| Source: | Journal of Geophysical Research. Biogeosciences. Jun2025, Vol. 130 Issue 6, p1-21. 21p. |
| Subject Terms: | *Greenhouse gases, *Sedimentation & deposition, *Carbon cycle, Braided rivers, Composition of sediments, Electrophiles, River channels |
| 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. Plain Language Summary: Large amounts of methane (CH4) leave from rivers to the atmosphere. Since methane has a higher warming potential than CO2, it is important to understand how rivers contribute to global CH4 emissions. We investigated CH4 concentrations in the gravel bed of a free‐flowing braided river and emission fluxes to the atmosphere, focusing on eventual differences emerging with seasonal flow changes. Compared to other rivers, CH4 concentrations and emission fluxes were very low. We found differences in CH4 concentrations between sampling campaigns, among various sediment depths, and between habitats differing in surface water coverage. Surface‐dry areas showed to be potentially tended to take up atmospheric CH4. The environmental variable best explaining CH4 concentrations in the sediment was dissolved organic matter. Patterns along depth, across habitats, and between campaigns did not always turn out as expected. Our results suggest a high habitat diversity at a small scale with strong differences in environmental conditions and CH4 production. However, remaining uncertainties emphasize our knowledge gaps in understanding CH4 dynamics in braided rivers, where numerous interacting variables seem to influence CH4 production and final emissions to the atmosphere. Key Points: CH4 concentrations in the Vjosa River are above atmospheric equilibrium but lower than other riversFine sediment deposition on the riverbed curbs emissions of CH4 by constraining exchange processes with the riverbedSeasonal drying turns exposed riverbed sediments into a sink for CH4 [ABSTRACT FROM AUTHOR] |
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| Database: | GreenFILE |
| 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. Plain Language Summary: Large amounts of methane (CH4) leave from rivers to the atmosphere. Since methane has a higher warming potential than CO2, it is important to understand how rivers contribute to global CH4 emissions. We investigated CH4 concentrations in the gravel bed of a free‐flowing braided river and emission fluxes to the atmosphere, focusing on eventual differences emerging with seasonal flow changes. Compared to other rivers, CH4 concentrations and emission fluxes were very low. We found differences in CH4 concentrations between sampling campaigns, among various sediment depths, and between habitats differing in surface water coverage. Surface‐dry areas showed to be potentially tended to take up atmospheric CH4. The environmental variable best explaining CH4 concentrations in the sediment was dissolved organic matter. Patterns along depth, across habitats, and between campaigns did not always turn out as expected. Our results suggest a high habitat diversity at a small scale with strong differences in environmental conditions and CH4 production. However, remaining uncertainties emphasize our knowledge gaps in understanding CH4 dynamics in braided rivers, where numerous interacting variables seem to influence CH4 production and final emissions to the atmosphere. Key Points: CH4 concentrations in the Vjosa River are above atmospheric equilibrium but lower than other riversFine sediment deposition on the riverbed curbs emissions of CH4 by constraining exchange processes with the riverbedSeasonal drying turns exposed riverbed sediments into a sink for CH4 [ABSTRACT FROM AUTHOR] |
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| ISSN: | 21698953 |
| DOI: | 10.1029/2025JG008856 |
