Spatial variations in diffusive methane fluxes and the role of eutrophication in a subtropical shallow lake.

Shallow lakes account for most of the diffusive CH4 emissions from global lakes, and they also suffer from eutrophication worldwide. Determining the effect of eutrophication on diffusive CH4 fluxes is fundamental to understanding CH4 emissions in shallow lakes. This study aimed to investigate the spatial variations in diffusive CH4 fluxes and explore the role of eutrophication in Lake Chaohu, a large and shallow eutrophic lake in the lower reaches of the Yangtze River. A one-year field observation was carried out to examine CH4 concentrations in the sediment and water and the diffusive fluxes of CH4 across the sediment-water interface (Fs-w) and water-air interface (Fw-a). Both Fs-w (0.306-1.56 mmol m-2 d-1) and Fw-a (0.097-0.529 mmol m-2 d-1) were upward and showed significant spatial heterogeneity and were significantly positively correlated. Parameters related to eutrophication had significant positive relationships with Fw-a, and the total phosphorus distribution in the water explained the greatest proportion of the spatial variation in Fw-a. Distance to shore and water depth were inversely correlated with Fw-a and modified the effects of eutrophication. Overall, the results provide direct evidence of the key role of eutrophication in shaping the spatial distribution of diffusive CH4 fluxes and a scientific basis for predicting changes in CH4 emissions with future eutrophication changes in shallow lakes in subtropical zones.