Methane formation and consumption by sediments in a cross-channel profile of a small river impoundment

Methane dynamics in aquatic sediments

Rivers are a natural source of methane (CH₄) into the atmosphere and may contribute significantly to total CH₄ emissions. Even though the details of sources of CH₄ in rivers are not fully understood, weirs have been recognized as a hotspot of CH₄ emissions. In this study, we investigated CH₄ production and consumption in air-exposed river sediments along a cross-channel transect located upstream of a weir. Stable carbon isotopes were used for determination of individual methanogenic pathways. In order to understand the relationship between physicochemical and biological processes, additional parameters such as organic matter, grain median size, and carbon and nitrogen content were characterized as well. Generally, samples from the surface sediment layer (0-10 cm) had higher CH₄ production than sediments from the deeper layer (10-20 cm) during the incubation experiments. Sediments near the bank zones and in the mid-channel were characterized by the highest organic carbon content (6.9 %) as well the highest methanogenic activity (2.5 mmol g^-1 DW d^-1). The CH₄ production was predominated by H₂/CO₂ dependent methanogenesis in the surface sediment layer (0-10 cm), while the proportion of acetoclastic and hydrogenotrophic methanogenesis in the deeper sediment layer (10-20 cm) was balanced. The CH₄ oxidation potential of sediments showed the same spatial pattern as observed for the CH₄ production. Our results showed high spatial variability of sediment CH₄ production and oxidation in the cross-channel profile upstream of the weir, whereas the highest CH₄ dynamics were observed in the littoral zones. This variability was closely linked with the carbon and nitrogen content in the sediment samples.