Oxic urban rivers as a potential source of atmospheric methane.

Urban rivers play a vital role in global methane (CH) emissions. Previous studies have mainly focused on CH concentrations in urban rivers with a large amount of organic sediment. However, to date, the CH concentration in gravel-bed urban rivers with very little organic sediment has not been well documented. Here, we collected water samples from an oxic urban river (Xin'an River, China; annual mean dissolved oxygen concentration was 9.91 ± 1.99 mg L) with a stony riverbed containing very little organic sediment. Dissolved CH concentrations were measured using a membrane inlet mass spectrometer to investigate whether such rivers potentially act as an important source of atmospheric CH and the corresponding potential drivers. The results showed that CH was supersaturated at all sampling sites in the five sampling months. The mean CH saturation ratio (ratio of river dissolved CH concentration to the corresponding CH concentration that is in equilibrium with the atmosphere) across all sampling sites in the five sampling months was 204 ± 257, suggesting that the Xin'an River had a large CH emission potential. The CH concentration was significantly higher in the downstream river than in the upstream river (p < 0.05), which suggested that human activities along the river greatly impacted the CH level. Statistical analyses and incubation experiments indicated that algae can produce CH under oxic conditions, which may contribute to the significantly higher CH concentration in August 2020 (p < 0.001) when a severe algal bloom occurred. Furthermore, other factors, such as heavy rainfall events, dissolved organic carbon concentration, and water temperature, may also be vital factors affecting CH concentration. Our study enhances the understanding of dissolved CH dynamics in oxic urban rivers with very little organic sediment and further proposes feasible measures to control the CH concentration in urban rivers.