Seasonal variation in greenhouse gas concentrations and diffusive fluxes in three river-reservoir systems in the Seine Basin (France).

River and reservoir ecosystems have been considered as hot spots for GHG (greenhouse gas) emissions while their specific hydrological and biogeochemical processes affect GHG concentrations; however, few studies integrated river-reservoir systems to identify the dominant drivers of GHG concentrations and flux changes associated with these systems. In the present study, we examined the seasonal variations in GHG concentrations in the surface water of three river-reservoir systems in the Seine Basin. The levels and seasonal variations of GHG concentrations exhibited distinct patterns among reservoirs, upstream, and downstream rivers. The concentrations of CH (methane) in the reservoirs were notably higher than those observed in both upstream and downstream rivers and showed higher values in summer and autumn, which contrasted with CO (carbon dioxide) concentrations, while NO (nitrous oxide) concentrations did not show an obvious seasonal pattern. A high mole ratio of CH/CO was found in these reservoirs, with a value of 0.03 and was more than 30 and 10 times higher than that in the upstream and downstream rivers, respectively. The three river-reservoir systems were oversaturated with GHG during the study period, with the average diffusive fluxes (expressed as CO: CO equivalent) of 810 ± 1098 mg CO m d, 9920 ± 2413 mg CO m d, and 7065 ± 2704 mg CO m d in the reservoirs, upstream and downstream rivers, respectively. CO and CH-CO were respectively the dominant contributors to GHG diffusive fluxes in river and reservoir sections, while NO contributed negligibly to GHG diffusive fluxes in the three river-reservoir systems. Our results showed that GHG concentrations and gas transfer coefficient have varying importance in driving GHG diffusive fluxes among different sections of the river-reservoir systems. In addition, our results also show the combined effect of reservoirs and upstream rivers on the water quality variables and hydrological characteristics of downstream rivers, highlighting the future need for additional investigations of GHG processes in the river-reservoir systems.