Consistent higher emissions of carbon dioxide, nitrous oxide, and methane during the daytime in reservoirs

Greenhouse gas (GHG) emissions from reservoirs are quantitatively relevant for atmospheric climatic forcing. These emissions have large temporal variability, with daily changes accounting for a substantial part of this variability. However, most estimations for GHG fluxes are based on the upscaling of discrete measurements performed during the daytime and, in general, they do not account for nighttime emissions. Here, we explored the daily patterns of CO2, N2O, and diffusive and ebullitive CH4 fluxes in two eutrophic reservoirs with contrasted morphometries in two different years. We found a daily pattern for CO2, N2O, and diffusive CH4 fluxes with consistent higher emissions during the daytime than during the nighttime, irrespectively of reservoir morphometry. These three diffusive fluxes showed evident daily synchrony suggesting a common driver. The emissions were coupled with the daily solar cycle, wind speed, and water temperature. The daily emissions of the CO2, N2O, and CH4 were also positive and significantly related to oxygen saturation. In contrast, we did not find a consistent daily pattern for the ebullitive CH4 fluxes, although they represented a significant fraction of the total CH4 emitted in these reservoirs. Our study suggests that the daily variability in GHG emissions may be as relevant as the variability at spatial scale or inter-system variability. Therefore, daily ranges should be considered in future GHG budgets to refine temporal trends of GHG emissions from reservoirs.