Chlorination Cessation Alters Greenhouse Gas Dynamics in Artificial Urban Ponds

Cities are facing an ecological challenge, and international policies are increasingly focused on implementing nature-based solutions to support this transition. In this context, the naturalization of artificial urban ponds (AUP) is a promising approach with proved benefits for biodiversity and human well-being. However, the naturalization of AUP may be accompanied by increased greenhouse gas (GHG) emissions. Here, we evaluated the effect of chlorination cessation, an essential step in the naturalization process, on GHG dynamics in AUP. Partial pressures of CO2 (pCO2), CH4 (pCH4), and N2O (pN2O) were measured in 41 artificial urban ponds (28 non-chlorinated and 13 chlorinated) in the city of Barcelona during winter and summer to assess: (a) the effect of chlorination treatment, (b) the effect of seasonality, and (c) the main drivers behind the partial pressures of these GHGs. Results show that although chlorination cessation increased pCH4, it reduced pN2O and had no significant effect on pCO2. The main drivers of these patterns were naturalization, with factors related to primary production playing a major role; seasonality, with temperature as a key environmental variable; and groundwater legacy. Importantly, the net global warming potential (GWP), expressed as CO2 equivalents, was not significantly higher in non-chlorinated ponds. These findings suggest that the naturalization of artificial water bodies could be a viable strategy to create more resilient cities without significantly increasing GHG emissions.