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

Abstract

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 CH4fluxes in two eutrophic reservoirs with contrasted morphometries in two different years. We found a daily pattern for CO2, N2O, and diffusive CH4fluxes 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 CH4were also positive and significantly related to oxygen saturation. In contrast, we did not find a consistent daily pattern for the ebullitive CH4fluxes, 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.

Abiotic nitrous oxide formation driven by sunlight in fresh and marine waters

“…For instance, Liu et al 27 found that the seasonal pattern in N2O fluxes in Antarctic lakes was positively related to daily radiation, but they suggested that the increase of daily radiation may favor photosynthesis and thus stimulate N2O production from the decomposition of the algae. In our previous study, we consistently reported significant diurnal variation in the N2O emissions in two reservoirs in two different years, with N2O emissions up to one order of magnitude greater during the daytime than during the nighttime 17 . The experiments presented in this work indicate that solar radiation causes the production of N2O in abiotic conditions, and it may represent a significant source of N2O to the atmosphere which has not been reported previously o included in greenhouse gas budgets.…”

“…In a previous study we found recurrently higher N2O emissions during the daytime than during the nighttime during 24 hour flux measurement campaigns conducted in two different years in two reservoirs in southeast Spain with high inputs of N from catchments (i.e., Cubillas and Iznájar). Despite the differences in magnitude among years and reservoirs, this pattern in N2O emissions was consistent, and significantly correlated to the solar cycle 17 .…”

“…N2O photoproduction represented an important contribution to the N2O pool, adding up to 39 % of the initial N2O pool per day (i.e., production of N2O per volume in relation to the initial concentration). Previous studies reported higher N2O emissions or concentrations during the daytime than during the nighttime in rivers [21][22][23][24] , and reservoirs 17 , although the opposite has also been shown in rivers 25,26 . These patterns have been attributed to changes in N cycling by biological activity and water temperature [21][22][23][24][25] .…”