Diurnal versus spatial variability of greenhouse gas emissions from an anthropogenically modified lowland river in Germany

Greenhouse gas (GHG) emissions from rivers are globally relevant, but quantification of these emissions comes with considerable uncertainty. Quantification of ecosystem-scale emissions is challenged by both spatial and short-term temporal variability. We measured spatio-temporal variability of CO 2 and CH 4 fluxes from a 1 km long reach of the lowland river Elbe in Germany over 3 d to establish which factor is more relevant to be taken into consideration: small-scale spatial variability or short-term temporal variability of CO 2 and CH 4 fluxes.GHG emissions from the river reach studied were dominated by CO 2 , and 90 % of total emissions were from the water surface, while 10 % of emissions were from dry fallen sediment at the side of the river. Aquatic CO 2 fluxes were similar at different habitats, while aquatic CH 4 fluxes were higher at the side of the river. Artificial structures to improve navigability (groynes) created still water areas with elevated CH 4 fluxes and lower CO 2 fluxes. CO 2 fluxes exhibited a clear diurnal pattern, but the exact shape and timing of this pattern differed between habitats. By contrast, CH 4 fluxes did not change diurnally. Our data confirm our hypothesis that spatial variability is especially important for CH 4 , while diurnal variability is more relevant for CO 2 emissions from our study reach of the Elbe in summer. Continuous measurements or at least sampling at different times of the day is most likely necessary for reliable quantification of river GHG emissions.