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While it was already known that Brussels ponds were sources of greenhouse gases to the atmosphere, a scientific study by researchers from the University of Liège (ULiège) and the Free University of Brussels reveals that clear water ponds emit more methane than those with turbid water.

The research is in the journal Biogeosciences.

In the Belgian capital, urban ponds dot parks and neighborhoods. Although they may appear similar, they differ in one fundamental way: their ecological status. Some have clear water, rich in visible aquatic plants called macrophytes. Others are turbid, their greenish waters filled with phytoplankton, microscopic algae that thrive in nutrient-rich environments.

These two states, known as "alternative states," reflect the ecological history of each pond. Originally, a pond tends towards a clear state. But when too many nutrients (nitrates, phosphates) from agricultural soils or urban runoff are discharged into it, it undergoes eutrophication. This process promotes the proliferation of microalgae (phytoplankton), blocks light and causes the plants at the bottom to die. The pond then shifts to a turbid state, which is often irreversible, as it is difficult to restore the clear state even after the polluting inputs have been stopped.

Methane bubbles

It is in this context that Thomas Bauduin, a Ph.D. student at the University of Liège and the Free University of Brussels, launched a large-scale study of four ponds in Brussels over a period of two and a half years. Two of these ponds have clear water (Silex and Tenreuken) and two have turbid water (Leybeek and Pêcheries). The aim is to measure their greenhouse gas emissions: CO₂ (carbon dioxide), CH₄ (methane) and N₂O (nitrous oxide).

"The results are surprising," explains the researcher. "While the diffuse emissions of these gases are similar from one pond to another, methane emissions in the form of bubbles, known as ebullitive emissions, literally explode in clear water."

"Diffuse" emissions refer to gases dissolved in water that slowly escape to the surface through molecular diffusion, much like steam escaping from a cup of hot tea. This applies to all gases measured (CO₂, CH_4,, N₂O).

"Ebullitive" emissions, on the other hand, only involve methane. Less soluble than other gases, it accumulates in sediments at the bottom, forming small bubbles that suddenly rise to the surface and escape directly into the atmosphere. These can sometimes be seen on summer days when the water is very warm. In clear ponds, the presence of macrophytes seems to promote bubble production.

"These plants alter the composition of the bottom, enrich the sediments, and can facilitate the escape of methane into the air," the researcher explains. "In comparison, turbid ponds, dominated by phytoplankton, produce fewer of these methane bubbles."

An underestimated role in climate change

Methane (CH₄) is a greenhouse gas 25 times more powerful than carbon dioxide (CO₂) over a 100-year period. Although ponds cover only a small area of Earth's surface, their role in the global climate could be underestimated, especially if we neglect ebullitive emissions.

"This study is one of the few to have quantified both diffuse and ebullitive emissions of several gases in an urban environment," concludes Alberto Borges, director of the Chemical Oceanography Laboratory. "It has also highlighted the impact of specific climatic conditions. In 2023, an exceptionally rainy year linked to an El Niño episode, CO₂ emissions increased compared to 2022, probably due to soil leaching, which led to more organic matter in the ponds."

Beneath their calm surface, urban ponds hold surprises. Clear waters, often perceived as an indicator of good ecological health, can also be significant sources of methane when they harbor aquatic plants that promote ebullition. This study highlights the complexity of aquatic ecosystems and their discrete but significant role in global climate dynamics.