Seasonal and diel variation in greenhouse gas emissions from an urban pond and its major drivers

Small water systems are important hotspots of greenhouse gas (GHG) emission, but estimates are poorly constrained as data are scarce. Small ponds are often constructed in urban areas, where they receive large amountsof nutrients and therefore tend to be highly productive. Here, we investigated GHG emissions, seasonal and dielvariation, and net ecosystem production (NEP) from an urban pond. In monthly 24-h field campaigns during11 months, diffusive water–atmosphere methane (CH4) and carbon dioxide (CO2) fluxes and CH4 ebullitionand oxidation were quantified. With oxygen (O2) measurements, NEP was assessed. The pond was a net GHGsource the entire year, with an emission of 3.4 kg CO2 eq m−2 yr−1. The dominant GHG emission pathway wasCH4 ebullition (bubble flux, 50%), followed by diffusive emissions of CO2 (38%) and CH4 (12%). Sediment CH4release was primarily driven by temperature and especially ebullition increased exponentially above a temperature threshold of 15C. The pond’s atmospheric CO2 exchange was not related to NEP or temperature but likelyto a high allochthonous carbon (C) input via runoff and anaerobic mineralization of C. We expect urban pondsto show a large increase in GHG emission with increasing temperature, which should be considered carefullywhen constructing ponds in urban areas. Emissions may partly be counteracted by pond management focusingon a reduction of nutrient and organic matter input