Spatial patterns and drivers of riverine methane (CH4) emissions in highly urbanised areas: A case study in Yangtze River Delta

CH₄ is the one of the most important greenhouse gases (GHGs) with a higher global warming potential than carbon dioxide. Increasing evidence suggests that riverine networks surrounding urban landscapes are considered important hotspots for CH₄ emissions. However, the factors influencing the spatial pattern of riverine CH₄ emissions in heavily urbanised areas remains unclear. Here, we investigated the spatial variability of diffusive CH₄ fluxes across the water-air interface (fCH₄) and dissolved CH₄ concentrations in the water column (dCH₄) in river reaches that drain multiple land covers (i.e., urban, agricultural and mixed landscapes) in a major urban river in the Yangtze River Delta, eastern China. fCH₄ were measured using a portable infrared gas analyser combined with a floating chamber, dCH₄ were determined by the headspace equilibration technique, and various water quality parameters were analysed in the laboratory. Our results showed that almost all sampling sites in the river were oversaturated with dissolved CH₄. Rivers in urbanised areas were identified as CH₄ emission hotspots, with mean fluxes of 3.76±4.58 mmol·m^-2·d^-1 and mean concentrations 6.91±6.95 μmol·L^-1, corresponding to 6.4 and 3.2 times of those from river reaches in non-urban areas, respectively. Factors related to the high CH₄ emissions in urban rivers included nutrient supply (e.g., NO₃-N, NH₃-N, TP), carbon input and hypoxia. Overall, these findings highlight the need for greater awareness regarding the role of urban river networks in contributing to global warming, especially given ongoing urban expansion.