Spatial variations of methane emission in a large shallow eutrophic lake in subtropical climate

Abstract

Subtropical lakes are an important source of atmospheric methane (CH4). This study aims to investigate spatial variations of the CH4 flux in Lake Taihu, a large (area 2400 km2) and shallow (mean depth 1.9 m) eutrophic lake in Eastern China. The lake exhibited high spatial variations in pollution level, macrophyte vegetation abundance, and algal growth. We measured the diffusion CH4 flux via the transfer coefficient method across the whole lake. In addition, data obtained with the flux-gradient and the eddy-covariance methods were used in conjunction with the data on the diffusion flux to estimate the contribution by ebullition. Results from three years’ measurements indicate high spatial variabilities in the diffusion CH4 flux. The spatial pattern of the diffusion CH4 emission was correlated with water clarity, and dissolved oxygen concentration, and with the spatial distributions of algal and submerged vegetation. In comparison to the transfer coefficient method, the eddy covariance and the flux-gradient method observed a lake CH4 flux that was 3.39 ± 0.58 (mean ± one standard deviation) and 1.95 ± 0.36 times higher in an open-water eutrophic zone and in a habitat of submerged macrophytes, respectively. The result implied an average of 71% and 49% ebullition contribution to the total CH4 flux in the two zones. The annual mean diffusion CH4 flux of the whole lake was 0.54 ± 0.30 g m-2 year-1. Our CH4 emission data suggest that the average CH4 emission reported previously for lakes in Eastern China was overestimated.