Deciphering methane dynamics in the urban ponds of the city of Brussels using carbon stable isotope ratios

Methane (CH4) emissions from lakes contribute significantly to atmospheric CH4 concentrations. However, there is a notable lack of understanding regarding the dynamics that regulate the sources and sinks of CH4 in freshwater aquatic environments, especially in urban freshwater environments. Methanotrophy is described as a major sink for CH4 in freshwater and it is important to better understand the mechanisms that regulate it. The dynamics of CH4 can be studied by examining the stable carbon isotopic ratios. The carbon stable isotopic composition of CH4 (δ13C-CH4) produced by methanogenesis in sediments is typically more negative than the usual values for organic matter because methanogens prefer to produce CH4 with lighter isotopes. In the water column, CH4 undergoes methanotrophy, leading to an increase in δ13C-CH4 values because methanotrophs preferentially oxidise the lighter CH4 pool and the heavier isotopes remain in the water. By comparing the δ13C-CH4 of methanogenesis with the values measured in the water column, it is possible to quantify the methanotrophy. Four urban ponds in Brussels were monitored 48 times at regular intervals from June 2021 to December 2023 to assess dissolved CH4 concentrations, δ13C-CH4, and other environmental variables. δ13C-CH4 was measured by headspace equilibration using a Picarro G2201-i 13C/12C CO2 and CH4 analyzer. The study, conducted within the Woluwe Basin in Brussels, included two clear ponds dominated by macrophytes and two turbid ponds dominated by phytoplankton. Concentrations of CH4 and δ13C-CH4 in surface waters ranged from 194 to 48380 nmol L-1 and from -64.59 to -1.55 ‰ respectively. CH4 concentrations was not significantly different between turbid ponds (2719 ± 1668 nmol L-1) and clear ponds (3264 ± 3004 nmol L-1). The δ13C-CH4 showed a significant difference between turbid ponds (-36.4 ‰ ± 14.1 ‰) and clear ponds (-55.2 ‰ ± 12.2 ‰), indicating that oxidation was higher in turbid ponds. Our study found that the oxidizing fraction of methane was positively correlated with both total suspended matter (TSM) concentration and the light extinction coefficient in the water column. This suggests that higher TSM concentrations could enhance methanotrophy by mitigating light inhibition and providing a fixation substrate for methanotrophic bacteria in the water column. As a result, turbid ponds exhibit higher methanotrophic activity.