Defu Liu

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 x hr) and 104.43 mg/(m2 x 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.

Temporal variation of methane flux from Xiangxi Bay of the Three Gorges Reservoir

Three diel field campaigns and one monthly sampling campaign during June 2010-May 2011 were carried out to investigate the CH4 flux across the water-gas interface in Xiangxi Bay of the Three Gorges Reservoir, China. The average CH4 flux was much less than that reported from reservoirs in tropic and temperate regions. The photosynthesis of phytoplankton dominated the diel gas fluxes during alga bloom in spring and summer. The maximum monthly flux occurred in June 2010 and corresponded to the lowest water level. Water temperature, sediment temperature, and TOC did not have significant correlation with the monthly CH4 fluxes. Continuously decreasing hydrostatic pressure and the low water level resulted in more CH4 emission at the sediment-water during the discharging period, and thus increases the CH4 effluxes because the diffusion time through a thin water column is shorter and less CH4 may be oxidized compared with that in a long water column.

Gas transfer velocities of methane and carbon dioxide in a subtropical shallow pond

Two diel field campaigns under different weather patterns were carried out in the summer and autumn of 2013 to measure CO2 and CH4 fluxes and to probe the rates of gas exchange across the air–water interface in a subtropical eutrophic pond in China. Bubble emissions of CH4 accounted for 99.7 and 91.67% of the total CH4 emission measured at two sites in the summer; however, no bubble was observed in the autumn. The pond was supersaturated with CO2 and CH4 during the monitoring period, and the saturation ratios (i.e. observed concentration/equilibrium concentration) of CH4 were much higher than that of CO2. Although the concentration of dissolved CO2 in the surface water collected in the autumn was 1.24 times of that in the summer, the mean diffusive CO2 flux across the water–air interface measured in the summer is almost twice compared with that in the autumn. The mean concentration of dissolved CH4 in the surface water in the autumn was around half of that in the summer, but the mean diffusive CH4 flux in the summer is 4–5 times of that in the autumn. Our data showed that the variation in gas exchange rate was dominated by differences in weather patterns and primary production. Averaged k 600-CO2 and k 600-CH4 (the gas transfer velocity normalised to a Schmidt number of 600) were 0.65 and 0.55 cm/h in the autumn, and 2.83 and 1.64 cm/h in the summer, respectively. No statistically significant correlation was found between k 600 and U 10 (wind speed at 10 m height) in the summer at low wind speeds in clear weather. Diffusive gas fluxes increased during the nights, which resulted from the nighttime cooling effect of water surface and stronger turbulent mixing in the water column. The chemical enhancements for CO2 were estimated up to 1.94-fold in the hot and clear summer with low wind speeds, which might have been resulted from the increasing hydration reactions in water due to the high water temperature and active metabolism in planktonic algae. However, both the air and surface water temperatures decreased continually, and relatively lower temperature and overcast weather with occasionally light rain dominated the second campaign in the autumn. The concentration of dissolved oxygen in the surface water and U 10 controlled gas transfer velocities of CO2 and CH4, respectively, in the cool autumn. When the surface water temperature was higher than the air temperature, higher CO2 flux was observed because the water body was unstable and overturned quickly, inducing quick CO2 emitted from plankton algae in surface water to the atmosphere.