Projecting Impacts of Climate Warming on Methane Production and Emissions from Urban Ponds

Urban ponds are significant emitters of methane (CH4) to the atmosphere. Methane cycling in these ecosystems is influenced by a multitude of factors, including redox conditions, organic carbon and nutrient supplies, pollutant loads, as well physical environmental factors such as temperature. However, the quantitative relationships between CH4 production and temperature remain insufficiently known. Our aim in the present study was to quantify the impacts of CH4 production in urban pond sediments as a critical prerequisite to projected impacts of global warming on CH4 emissions from freshwaters. We collected intact sediment cores from eight ponds located in the city of Berlin, Germany, and incubated them over a broad range of temperatures (2 to 44 °C) to determine the thermal dependencies of CH4 production. The selected ponds represent a variety of urban land-use types, including residential areas, industrial areas, protected forest areas, and recreational green spaces. Our results indicate a clear dependency of CH4 production on temperature, showing that methanogenesis was consistently driven by mesophilic microorganisms, with optimal temperatures ranging between 28 and 36 °C, despite sediment temperatures that are mostly much lower throughout the year. Our findings indicate that methanogenesis in sediments of urban ponds occurs through both the heterotrophic and hydrogenotrophic pathway, with the prevailing temperatures in these environments being conducive to producing the essential precursors needed for syntrophic hydrogen production. We also integrated these temperature relationships into global warming scenarios, specifically within the Representative Concentration Pathway (RCP) framework, to make projections for the year 2100. The results of the analysis spanning a range of scenarios, from the very stringent RCP 2.6 to the continually rising RCP 8.5 indicate that even with moderate global warming, CH4 production and thus emissions to the atmosphere from urban ponds will markedly increase in the future.