Processes affecting the spatial and temporal variability of methane in a temperate dammed river system

Research on the role of rivers as a significant methane source to coastal waters and the atmosphere has previously focused on tropical regions. This study attempted to understand the spatial and temporal variability of methane within a dammed temperate river system in northern Germany, and the river's methane contribution to the atmosphere and coastal waters. Dissolved methane and dissolved oxygen were routinely sampled from the dam to 3.2km upstream. The sediment pore water methane was measured, as well as the atmospheric methane flux from the river. A 22 hour study was performed to understand the diurnal variability of methane in the river. To determine the spatial variability of methane in the Brackish Discharge Zone, the water column and pore water were analyzed for methane concentration. Methane concentrations in the river were supersaturated with respect to the atmosphere, reaching concentrations between 311-2257 ± 51nM and saturation ratios (R) from 62-451. It appeared that methane concentrations increased downstream as a result of methane accumulating from runoff. Precipitation-induced runoff and dam opening were found to be the dominant factors affecting the overall river temporal methane variability. The dam outlet caused a significant degassing methane flux, from 5-25% of the methane concentration immediately before the dam. In coastal waters near the dam, the river was the dominant methane source, contributing 2.1- 4.5kg of methane per day. This study suggests temperate rivers, especially those which are dammed, may need to be considered when constraining coastal and atmospheric methane budget