Greenhouse gas emissions from urban ponds in Denmark

Ponds are increasingly recognized as significant sources of greenhouse gases (GHGs) emitted to the atmosphere. Concomitant with increasing urbanization, more urban ponds are created, many with the aim of buffering peak runoff and improving water quality in downstream waterbodies. However, the impact of urban ponds on GHG emissions is poorly elucidated. In this study, we measured the dissolved concentrations of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) 4 times over a year in 37 ponds located in the city of Silkeborg, Denmark. The results show that the ponds generally acted as a source of GHG with CO2-C concentrations (standard deviation) of 1938 (2208) µg L−1, CH4-C of 44 (198) µg L−1, and N2O-N of 0.8 (1.8) µg L−1. Boosted regression tree models show that vegetation cover, water temperature, and nitrate concentration were the main drivers of CO2, CH4, and N2O concentrations, respectively. Upscaling of the results to the Danish national level showed that urban ponds emit about 38 × 109 g CO2-equivalents per year, suggesting that urban ponds are significant sources of GHG in urban landscapes.