How Are Greenhouse Gases Coupled Across Seasons in a Large Temperate River with Differential Land Use?

Rivers are known to emit large amounts of green-house gases globally, however, few studies have evaluated the interacting influence of land use, within river features (for example, sites of major confluence, changes in shape), and hydrology on the coupled dynamics of carbon dioxide ( CO₂), methane (CH₄) and nitrous oxide (N₂O). Here, we measured CO₂, CH₄ and N₂O concentrations and fluxes at 15 sites along a 146.6 km stretch of the main stem of a large north temperate river: the Rivière du Nord, which experiences an abrupt change in geology and land use and winter ice cover. Each site along the river was sampled once per season, at targeted low flow and high flow moments, for three consecutive years. Overall, highest concentrations of all gases were found in the lower reaches where urban and agricultural activity are most intensive. CO₂ and N₂O were highest overall under ice during winter, whereas CH₄ was highest during summer. The river was always supersaturated in CH₄ whereas surface water was occasionally undersaturated in N₂O and CO₂, particularly during the summer low flow period in pristine upper reaches. Spatial variability was, however, minimized during periods of high flow. Emissions varied along the continuum, with peaks occurring at turbulent sites of major confluence. Although land use influenced the spatial variability in concentrations along the river, seasonal changes in temperature influenced the relative importance of the different gases to global warming potential, and hydrology mediated where they were produced as well as their overall concentrations and emissions.