Effect of interannual variation in winter vertical mixing on CH_4 dynamics in a subtropical reservoir

Abstract

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 CH4 and its carbon isotope ratio (δ_13C) 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 verticalmixing (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 storagewith the development of reducing conditions. In contrast, in years with strong winter vertical mixing to the bottom of the reservoir, CH_4 productionwas 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, whichmay 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.