Temporal and Spatial Variations of Methane Emission of a Large and Shallow Eutrophic Lake in Subtropical Climate

Subtropical lakes are an importance source of atmospheric CH4. The goal of this study was to quantify temporal and spatial variability of the CH4 flux in Lake Taihu, a large (area 2338 km2) and shallow (mean depth 1.9 m) eutrophic lake in eastern China. The lake exhibits high spatial heterogeneity in terms of pollution level, macrophyte vegetation abundance and algal growth. Our research methodology consisted of the flux-gradient method, the eddy-covariance method, daily measurement of the diffusive CH4 flux via the transfer coefficient method at a fixed location, and seasonal survey of the lake water CH4 concentration across the whole lake. Results from three years' measurements indicate high spatial and temporal variabilities in the CH4 flux. The spatial pattern of the CH4 emission resulted mostly from uneven growth of algae and from spatial variability in the distribution of aquatic vegetation, and was also associated with pollutant level, the dissolved oxygen concentration, and water depth. The seasonal pattern of the CH4 flux was mostly regulated by water temperature. In comparison to the eddy covariance and the flux-gradient method, the transfer coefficient method yielded a lake CH4 flux that was 1.7 and 3.6 times lower in eutrophic zones and in the habitats of submerged macrophyte, respectively, implying a 41.2 to 72.2% ebullition contribution to the total CH4 flux. The annual mean CH4 emission flux of the whole lake after correction for the ebullition contribution was 1.8 g CH4 m-2 year-1, with 47.7% and 30.9% originated from eutrophic zones and macrophyte habitats, respectively.