Sub-daily variability of carbon dioxide, methane, and nitrous oxide emissions from two urban ponds in Brussels (Belgium)

Sub-daily variations might significantly impact the estimates of GHG emissions from lakes and ponds. The objectives of this study are (i) to quantify sub-daily variations of the emissions of CO2, CH4, and N2O from two urban ponds (Silex and Pêcheries) in the city of Brussels, (ii) to quantify if the sub-daily variations of GHG emissions were significant compared to their seasonal variations and to inter-pond variations among 20 other ponds in the city of Brussels. The partial pressure of CO2 (pCO2), CH4 concentration, and N2O saturation level (%N2O) were measured hourly from dawn to dusk in the Pêcheries turbid-water pond and in the Silex clear-water pond during the four seasons in 2023–2024. pCO2 followed the day-night cycle of photosynthesis in spring and summer but was more erratic in winter and fall. The variations of CH4 concentration and %N2O were on most occasions erratic and difficult to attribute systematically to specific biogeochemical processes. The sub-daily variations of computed GHG emissions were mostly driven by variability in wind speed that usually peaked around mid-day. The comparison with previously acquired seasonal and inter-pond data (n = 22) showed that sub-daily variations of GHG fluxes were lower than seasonal variations, which were in turn lower than inter-pond variations. Consequently, to design sampling strategies to reduce the uncertainty on the estimate of CO2, CH4, and N2O emissions a priority should be given to describe inter-system variability, followed by seasonal variability, and lastly sub-daily variability, in the context of the environmental management of inland waters, including urban ponds.