Oxic urban rivers as a potential source of atmospheric methane.

Urban rivers play a vital role in global methane (CH4) emissions. Previous studies have mainly focused on CH4 concentrations in urban rivers with a large amount of organic sediment. However, to date, the CH4 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-1) with a stony riverbed containing very little organic sediment. Dissolved CH4 concentrations were measured using a membrane inlet mass spectrometer to investigate whether such rivers potentially act as an important source of atmospheric CH4 and the corresponding potential drivers. The results showed that CH4 was supersaturated at all sampling sites in the five sampling months. The mean CH4 saturation ratio (ratio of river dissolved CH4 concentration to the corresponding CH4 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 CH4 emission potential. The CH4 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 CH4 level. Statistical analyses and incubation experiments indicated that algae can produce CH4 under oxic conditions, which may contribute to the significantly higher CH4 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 CH4 concentration. Our study enhances the understanding of dissolved CH4 dynamics in oxic urban rivers with very little organic sediment and further proposes feasible measures to control the CH4 concentration in urban rivers.