Effect of weir impoundments on methane dynamics in a river

- CH4 emissions were higher upstream of the weirs compared to river reaches.
- Sediments upstream of the weirs resemble many previous observations for lake systems.
- Small impoundments significantly affect the CH4 cycle in a river.

We measured CH4 concentration, CH4 oxidation in the water column and total CH4 emissions to the atmosphere (diffusion and ebullition) in three weir impoundments and river reaches between them, in order to understand their role in river methane (CH4) dynamics. Sediment samples were also collected to determine CH4 consumption and production potentials together with the contribution of individual methanogenic pathways. The CH4 surface water concentration increased 7.5 times in the 16 km long river stretch. Microbial CH4 oxidation in the water column reached values ranging from 51 to 403 nmol l− 1 d− 1 and substantially contributed to the CH4 removal from surface water, together with CH4 emissions. The total CH4 emissions to the atmosphere varied between 0.8 and 207.1 mmol CH4 m− 2 d− 1 with the highest values observed upstream of the weirs (mean 68.5 ± 29.9 mmol CH4 m− 2 d− 1). Most of the CH4 was transported through the air-water interface by ebullition upstream of the weirs, while the ebullition accounted for 95.8 ± 2.0% of the total CH4 emissions. Both CH4 production and oxidation potential of sediments were higher upstream of the weirs compared to downstream of the weirs. The contribution of hydrogenotrophic methanogenesis to total CH4 sediment production was 36.7-89.4% and prevailed upstream of the weirs. Our findings indicate that weirs might influence river CH4 dynamics, especially by increased CH4 production and consumption by sediments, followed by increasing CH4 emissions to the atmosphere.

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