Effect of weir impoundments on methane dynamics in a river.

We measured CH concentration, CH oxidation in the water column and total CH 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 (CH) dynamics. Sediment samples were also collected to determine CH consumption and production potentials together with the contribution of individual methanogenic pathways. The CH surface water concentration increased 7.5 times in the 16km long river stretch. Microbial CH oxidation in the water column reached values ranging from 51 to 403nmolld and substantially contributed to the CH removal from surface water, together with CH emissions. The total CH emissions to the atmosphere varied between 0.8 and 207.1mmolCHmd with the highest values observed upstream of the weirs (mean 68.5±29.9mmolCHmd). Most of the CH 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 CH emissions. Both CH 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 CH sediment production was 36.7-89.4% and prevailed upstream of the weirs. Our findings indicate that weirs might influence river CH dynamics, especially by increased CH production and consumption by sediments, followed by increasing CH emissions to the atmosphere.