A publication in Water Research

Brussels' ponds emit greenhouse gases



A study conducted by Thomas Bauduin, a doctoral student at ULiège and ULB, sampled twenty-two ponds in Brussels, showing that the greenhouse gas emissions from these Brussels ponds are numerically equivalent to the carbon sink of the city's green spaces. The study is published in the journal Water Research. 

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reen spaces embellish cities and are places of recreation that contribute to many city dwellers' physical and mental well-being. Urban green areas provide numerous services, such as reducing heat (cooling islands), managing rainwater (stormwater basins), reducing air pollution (aerosol sinks), reducing noise pollution, acting as biodiversity refuge, and acting as carbon sinks (a necessary process that absorbs and stores carbon dioxide from the atmosphere), thereby helping to reduce the concentrations of greenhouse gases responsible for climate change.

Ponds - often part of urban green spaces - should, in theory, emit greenhouse gases to the atmosphere like most natural lakes. However, urban ponds are artificial and different from natural lakes because of the drainage basin (city vs. forest or countryside) and a larger deposition of atmospheric pollution (particularly nitrogen). We can, therefore, assume that emissions from urban ponds are higher than those from natural lakes.

Thomas Baudouin, a doctoral student at the Chemical Oceanography Laboratory of the FOCUS research unit at ULiège who is carrying out his doctoral thesis under joint supervision with ULB (EBB), took samples of dissolved concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in twenty-two urban ponds in Brussels over four seasons. “The study shows that concentrations of CO2 and CH4 in the Brussels ponds were higher than in natural lakes of equivalent size, confirming the initial hypothesis," explains the researcher. What's more, our study highlights variations between the different ponds. As observed in natural lakes, CO2 emissions were higher in the smaller ponds. Emissions of N2O were higher in ponds close to the city centre due to atmospheric deposition of nitrogen, as demonstrated by cross-checking with nitrous oxide data collected as part of the CurieuzenAir citizens' initiative". Unexpectedly, Thomas Baudouin measured lower methane emissions in the smaller ponds close to the city centre, and higher emissions in the larger ponds on the outskirts of Brussels. "This is due to submerged algae in the larger ponds on the outskirts and the input of plant biomass from the Forêt de Soignes. These organic matter inputs to the ponds fuel fermentation and methane production at the bottom of the ponds.

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Diagram of the processes controlling greenhouse gas emissions from urban ponds

Cities host a large proportion of global greenhouse gas emissions produced by traffic, housing, and industrial production. Unsurprisingly, greenhouse gas emissions from Brussels ponds are negligible compared with the total emissions of the city of Brussels (0.04%). These emissions are primarily dominated by CO2. If only methane emissions are compared, then the Brussels ponds contribute an additional 4% to the total methane emissions of the City of Brussels, which have so far been accounted for by the Region.

“The most significant result of this study is that greenhouse gas emissions from ponds in Brussels are numerically equivalent to the carbon sinks created by the green spaces in the city of Brussels that have so far been accounted for by the Region," concludes Alberto Borges, F.R.S.-FNRS Research Director at ULiège. “This study in an urban setting confirms what we have also shown in natural settings, particularly in Africa: greenhouse gas emissions from bodies of water (rivers and lakes) are significant compared with the carbon sink attributed to terrestrial vegetation such as forests”.

Scientific reference

Thomas Bauduin, Nathalie Gypens, Alberto V Borges (2024) Seasonal and spatial variations of greenhouse gas (CO2, CH4 and N2O) emissions from urban ponds in Brussels, Water Research, 2 February 2024. https://doi.org/10.1016/j.watres.2024.121257

Thomas Bauduin is doing his doctoral thesis at the Chemical Oceanography Laboratory (Alberto Borges, UR FOCUS), under joint supervision with ULB (Nathalie Gypens, EBB).

Your contacts at ULiège

Thomas Bauduin

Alberto Borges

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