Seasonal and spatial variations of greenhouse gas (CO2, CH4 and N2O) emissions from urban ponds in Brussels

Abstract

Freshwaters have been recognized as important sources of greenhouse gases (GHG) to the atmosphere. However, urban ponds have received little attention even though their number is increasing due to expanding urbanisation globally. Ponds are frequently associated to urban green spaces that provide several ecosystemic services such as cooling local climate, regulating the water cycle, and acting as small carbon sinks This study aims to identify and understand the processes producing GHGs (CO₂, CH₄, and N₂O) in the urban ponds of the temperate European city of Brussels in Belgium. 22 relatively small ponds (0.1-4.6 ha) surrounded by contrasted landscape (strictly urban, bordered by cropland or by forest), were sampled during four seasons in 2021-2022. The mean ± standard deviation was 3,667 ± 2,904 ppm for the partial pressure of CO₂ (pCO₂), 2,833 ± 4,178 nmol L^-1 for CH₄, and 273 ± 662% for N₂O saturation level (%N₂O). Relationships of GHGs with oxygen and water temperature suggest that biological processes controlled pCO₂, CH₄ concentration and%N₂O. However, pCO₂ was also controlled by external inputs as indicated by the higher values of pCO₂ in the smaller ponds, more subject to external inputs than larger ones. The opposite was observed for CH₄ concentration that was higher in larger ponds, closer to the forest in the city periphery, and with higher macrophyte cover. N₂O concentrations, as well as dissolved inorganic nitrogen, were higher closer to the city center, where atmospheric nitrogen deposition was potentially higher. The total GHG emissions from the Brussels ponds were estimated to 1kT CO₂-eq per year and were equivalent to the carbon sink of urban green spaces.

Keywords: Brussels; Carbon dioxide; Macrophytes; Methane; Nitrous oxide; Urban ecology; Urban ponds.