Diel and seasonal variation of methane and carbon dioxide fluxes at Site Guojiaba, the Three Gorges Reservoir

In order to investigate the CH₄ and CO₂ fluxes across the water-gas interface and identify their controlling factors, four diel field campaigns and one monthly sampling campaign during June 2010-May 2011 were carried out at a site near the Three Gorges Dam, China. The averaged CH₄ and CO₂ fluxes across the air-water interface from the site were much less than those reported from reservoirs in tropic and temperate regions, and from the natural river channels of the Yangtze River. CH₄ Fluxes at the site were very low compared to most other reservoirs or natural lakes. One of the most important reasons may be due to the oxidation of CH₄ in the water column owing to the great water depth and high DO in water in the Three Gorges Reservoir. The averaged monthly CH₄ and CO₂ fluxes at the site during the observation year were 0.05 mg/(m²·hr) and 104.43 mg/(m²·hr) respectively with the maximum occurred in July 2010. The monthly CO₂ fluxes during the observation year were positively correlated to the surface water temperature, and negatively correlated to the air pressure and the surface water pH. The CO₂ flux showed a positive correlation with DOC to some extent, although not significantly, which indicated that allochthonous organic C was a major source of CO₂ and biogeochemical processes in this reservoir were C-limited. The significantly positive correlation between the reservoir outflow and the seasonal gas flux indicate the disturbance condition of the water body dominated the seasonal gas emission.