Greenhouse gas emissions from urban ponds are driven by nutrient status and hydrology

Inland waters emit significant quantities of greenhouse gases (GHGs) such as methane (CH4) and carbon dioxide (CO2) to the atmosphere. On a global scale, these emissions are large enough that their contribution to climate change is now recognized by the Intergovernmental Panel on Climate Change. Much of the past focus on GHG emissions from inland waters has focused on lakes, reservoirs, and rivers, and the role of small, artificial waterbodies such as ponds has been overlooked. To investigate the spatial variation in GHG fluxes from artificial ponds, we conducted a synoptic survey of forty urban ponds in a Swedish city. We measured dissolved concentrations of CH4 and CO2, and made complementary measurements of water chemistry. We found that CH4 concentrations were greatest in high‐nutrient ponds (measured as total phosphorus and total organic carbon). For CO2, higher concentrations were associated with silicon and calcium, suggesting that groundwater inputs lead to elevated CO2. When converted to diffusive GHG fluxes, mean emissions were 30.3 mg CH4·m？2·d？1 and 752 mg CO2·m？2·d？1. Although these fluxes are moderately high on an areal basis, upscaling them to all Swedish urban ponds gives an emission of 8336 t CO2eq/yr (±1689) equivalent to 0.1% of Swedish agricultural GHG emissions. Artificial ponds could be important GHG sources in countries with larger proportions of urban land. The global importance of inland waters in the cycling of carbon and greenhouse gases (GHGs) is now well recognized (Cole et al. 2007). Lakes, rivers, streams, reservoirs, and ponds emit large amounts of carbon dioxide (CO2) and methane (CH4; Bastviken et al. 2004, Holgerson and Raymond 2016, DelSontro et al. 2018). The emissions of these GHGs can occur diffusively from the water surface, and also by ebullition, which is the pathway by which CH4 is emitted directly to the atmosphere via the sporadic release of CH4‐containing bubbles from the sediment. Inland waters also contain dissolved and particulate organic carbon, which can be degraded to release GHGs (Tranvik et al. 2009, Evans et al. 2017), or buried in sediments, thus acting as a carbon (C) sink (Sobek et al. 2009). On a global scale, the area of small artificial ponds is similar to that of large reservoirs, due to the extremely large number of small ponds (Downing 2010). Additionally, it has been suggested that C and GHG processing in small waterbodies is particularly intense due to the specific physical properties of ponds, such as shallow water and frequent mixing (Downing 2010, Holgerson and Raymond 2016). In