ALBERT

Description: Although freshwaters are considered to be substantial natural sources of atmospheric methane (CH 4 ), in situ processes of CH 4 production and consumption in freshwater ecosystems are poorly understood, especially in subtropical areas, leading to uncertainties in the estimation of global CH 4 emissions. To improve our understanding of physical and biogeochemical factors affecting CH 4 dynamics in subtropical lakes, we examined vertical and seasonal profiles of dissolved CH 4 and its carbon isotope ratio (δ 13 C) and conducted incubation experiments to assess CH 4 production and oxidation in the deep subtropical Fei-Tsui Reservoir (FTR; Taiwan). The mixing pattern of the FTR is essentially monomixis, but the intensity of winter vertical mixing changes with climatic conditions. In years with incomplete vertical mixing (does not reach the bottom) and subsequent strong thermal stratification resulting in profundal hypoxia, we observed increases in sedimentary CH 4 production and thus profundal CH 4 storage with the development of reducing conditions. In contrast, in years with strong winter vertical mixing to the bottom of the reservoir, CH 4 production was suppressed under NO 3 − -rich conditions, during which denitrifiers have the competitive advantage over methanogens. Diffusive emission from profundal CH 4 storage appeared to be negligible due to the efficiency of CH 4 oxidation during ascent through methane-oxidizing bacteria (MOB) activity. Most of the profundal CH 4 was rapidly oxidized by MOB in both oxic and anoxic layers, as characterized by its carbon isotope signature. In contrast, aerobic CH 4 production in the subsurface layer, which may be enhanced under high temperatures in summer, may account for a large portion of atmospheric CH 4 emissions from this reservoir. Our CH 4 profiling results provide valuable information for future studies predicting CH 4 emissions from subtropical lakes with the progress of global warming.