Urban ponds, often overlooked amidst the sprawling cityscape, conceal complex ecological dynamics that have profound implications for global climate processes. A recent comprehensive study conducted by researchers from the University of Liège and the Free University of Brussels has unveiled surprising findings about greenhouse gas emissions from these aquatic systems, particularly methane. Contrary to conventional assumptions that turbid, nutrient-rich water bodies might be the primary contributors to methane release, the research reveals that clear water ponds produce significantly higher methane emissions in the form of ebullitive bubbles.
In Brussels, urban ponds are common features within parks and neighborhoods, vital fragments of urban biodiversity and ecological stability. While these ponds may look alike at a glance, they exist in fundamentally different ecological states that define their water quality and biological composition. Clear water ponds support abundant populations of macrophytes, large aquatic plants that visibly flourish underwater, contributing to clarity and oxygenation. Conversely, turbid ponds display a greenish, murky appearance dominated by phytoplankton—microscopic algae that proliferate in nutrient-rich, often eutrophic waters, becoming indicative of ecological distress.
The concept of ‘alternative stable states’ in pond ecosystems underpins the ecological dichotomy observed between clear and turbid urban ponds. Initially, ponds tend to maintain clear water conditions with balanced nutrient levels fostering healthy macrophyte communities. However, human-driven nutrient enrichment, principally from agricultural runoff and urban waste, delivers excessive nitrates and phosphates into these water bodies. This nutrient overload triggers eutrophication, a degradation process that fuels phytoplankton blooms, reduces light penetration, and results in the die-off of bottom-dwelling plants. Often, this shift to a turbid state is practically irreversible, reflecting a profound alteration of the aquatic environment and ecological function.
This ecological backdrop sets the stage for the recent investigation focusing on methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) emissions—three potent greenhouse gases—from four carefully selected urban ponds in Brussels. Two ponds characterized by clear water, Silex and Tenreuken, were compared against two turbid, phytoplankton-dominated ponds, Leybeek and Pêcheries. Spanning a rigorous period of two and a half years, this study aimed to quantify both the diffuse gaseous emissions dissolved across the water surface and the ebullitive emissions—the release of gas via bubbling—from these contrasting pond types.
Methane emissions, particularly through ebullition, emerged as the most striking and unexpected finding. While the diffuse emissions of greenhouse gases such as CO₂, CH₄, and N₂O were relatively consistent across all ponds, ebullitive methane emissions surged dramatically in the clear water ponds. Ebullition refers specifically to methane bubbles forming in sediments that accumulate beneath the pond floor, eventually rising swiftly through the water column to escape into the atmosphere. These methane bubbles are particularly noteworthy because their sudden, concentrated release represents a mechanism often underestimated in greenhouse gas inventories.
The underlying mechanism driving the elevated ebullitive methane in clear ponds ties directly to the presence and activity of macrophytes. These aquatic plants fundamentally alter the sediment environment through the deposition of organic matter and root exudates, enriching the substrates where methane-producing archaea thrive. Moreover, the structure of macrophytes facilitates the direct transport of methane bubbles from sediment to atmosphere, bypassing oxidation processes that typically occur in water layers. In contrast, turbid ponds dominated by phytoplankton exhibit fewer ebullitive events likely because the organic matter is more evenly distributed in the water column and sediments lack the plant-mediated conduits that aid bubble formation and release.
Methane’s significance in climate science cannot be overstated. As a greenhouse gas, methane’s global warming potential is approximately 25 times that of carbon dioxide over a century, making even small, localized emissions impactful at a planetary scale. Despite the relatively modest global surface coverage of ponds compared to oceans or forests, the striking ebullitive methane fluxes revealed by this study suggest that ignoring such emissions could lead to substantial underestimations in global greenhouse gas budgets. Urban ponds, therefore, emerge as nontrivial contributors to atmospheric methane accumulation, underscoring the need to understand and manage these ecosystems carefully.
This investigation also casts light on how climatic variability can influence greenhouse gas dynamics at the urban scale. The year 2023, marked by abnormal rainfall patterns associated with an El Niño event, saw a notable increase in CO₂ emissions from the studied ponds compared to the previous year. This escalation is attributed to soil leaching processes amplified by intense precipitation, which transport additional organic matter and nutrients into aquatic ecosystems, thereby stimulating microbial respiration and carbon dioxide release. These findings highlight the sensitive interplay between climate phenomena and urban aquatic biogeochemistry.
The methodology employed in this research is notable for integrating long-term field measurements with fine-scale gas flux analyses. By deploying sensors and sampling techniques attuned to both dissolved gases and bubble-mediated emissions, the scientists achieved a comprehensive assessment of pond-atmosphere exchange. This dual approach is rarely applied in urban contexts, making the study particularly pioneering. It reinforces the need for similar multidisciplinary measurements in diverse urban and natural aquatic systems worldwide to refine greenhouse gas emission estimates.
The implications of these findings extend beyond Brussels or urban ponds alone. They challenge prevailing conceptions that clear water conditions inherently signify ecological health or climate mitigation benefits. Instead, the ability of macrophyte-rich ponds to emit substantial methane via ebullition demands a nuanced understanding of aquatic plant roles in carbon cycling. Restoration projects aimed at water clarity improvement may inadvertently amplify methane emissions if they promote dense macrophyte populations without addressing sediment methane production pathways.
Furthermore, this research alerts urban planners and environmental managers to the hidden climate costs associated with pond restoration and management strategies. While enhancing urban pond biodiversity and aesthetics remains a valid goal, carbon accounting frameworks must integrate greenhouse gas emission dynamics, especially concerning methane ebullition. Future urban water body designs might consider innovative sediment management or plant species selection to balance ecological benefits with climate mitigation priorities.
In conclusion, urban ponds, serene and often admired for their scenic tranquility, harbor dynamic and potent biogeochemical processes impacting greenhouse gas emissions. The subtle yet significant ebullitive methane release from clear water ponds demonstrates that even small-scale aquatic systems contribute meaningfully to global climate forcing. As cities worldwide seek sustainable ways to integrate blue-green infrastructures, comprehensive assessments like this are vital. They remind us that environmental stewardship requires not only visible ecological improvements but also careful consideration of less apparent biochemical feedbacks that echo far beyond urban boundaries.
Subject of Research: Methane, carbon dioxide, and nitrous oxide emissions from urban ponds with differing ecological states in Brussels.
Article Title: Methane, carbon dioxide, and nitrous oxide emissions from two clear-water and two turbid-water urban ponds in Brussels (Belgium)
Web References:
DOI: 10.5194/bg-22-3785-2025
Image Credits: University of Liège / Thomas Bauduin
Keywords: urban ponds, methane emissions, ebullition, greenhouse gases, macrophytes, eutrophication, biogeochemistry, aquatic ecosystems, climate change, Brussels, diffuse emissions, turbid water
