Higher emissions of carbon dioxide, nitrous oxide, and methane during the daytime in two reservoirs

Greenhouse gas (GHG) emissions from reservoirs are quantitatively relevant for atmospheric climatic forcing. The magnitude of these fluxes depends on the mechanisms promoting the production of carbon dioxide (CO 2 ), nitrous oxide (N 2 O), and methane (CH 4 ), and on the physical forces determining their emission. GHG emissions exhibit large temporal variability, with diel changes accounting for a substantial part of this variability. However, most GHG flux estimations rely on upscaling discrete measurements taken at daytime and typically overlook nighttime emissions. This study explored the diel patterns of CO 2 , N 2 O, and both diffusive and ebullitive CH 4 fluxes in two eutrophic reservoirs with different morphometries, using hourly GHG flux measurements over a summer day in two different years. Daytime emissions of CO 2 , N 2 O, and diffusive CH 4 were on average 159, 267, and 194% higher than nighttime emissions, respectively. Despite the different production pathways, the diffusive fluxes showed strong daily synchrony, suggesting an external common driver for the three of them. Daily emissions of CO 2 , N 2 O, and diffusive CH 4 were positive and significantly related to wind speed and solar time. In contrast, ebullitive CH 4 fluxes showed no consistent daily pattern, and were influenced by reservoir management (i.e., water level drawdown) in the shallowest system. Ebullitive CH 4 fluxes represented an average of 51% of the total CH 4 emitted. Our study suggests that diel variability in GHG emissions may be as relevant as spatial or inter-system differences and should be integrated into future GHG budgets to improve their accuracy.