Dissolved methane dynamics and isotopic source analysis in the Chongqing section of Yangtze River, Three Gorges Reservoir

Abstract

As the largest reservoir in China, the Three Gorges Reservoir is an important source of methane (CH4) emissions. In order to further understand the natural carbon cycle of CH4 in reservoir ecosystem, in June, September 2020 and January 2021, the Chongqing main section of the Yangtze River in the Three Gorges Reservoir area was selected as the research object to study the sources and spatio-temporal variations of dissolved CH4 concentration and CH4 isotope (δ13C-CH4) in the water. Compared with previous concentration-based surveys, this study emphasizes isotope-constrained pathway identification and site-specific interpretation of the GJT backwater-bay hotspot. This study provides theoretical support and scientific basis for a clearer understanding of the carbon cycle process. The results show that the dissolved CH4 concentration in the Chongqing main section of the Yangtze River in the Three Gorges Reservoir is 0.039μmol L⁻¹~0.232μmol L⁻¹. The δ13C-CH4 is -59.27‰ ~ -38.38‰. The most abundant CH4 in water is at Guojiatuo. At GJT, the vertical stratification of CH4 was strongest in summer and weaker in autumn and winter. Acetic acid fermentation dominated CH4 production at all observation sites in summer and autumn. In summer, the concentration of CH4 was negatively correlated with the concentration of CO2 and DO. There was a significant positive correlation between δ13C-CH4 and temperature T in both summer and autumn.