Seasonal Dynamics of Methane Fluxes from Groundwater to Lakes:Hydrological and Biogeochemical Controls.

Methane (CH) inputs to lakes through lacustrine groundwater discharge (LGD-derived CH) represent a potentially important but often overlooked source of lake methane emissions. Although great efforts have been made to quantify LGD-derived CH fluxes and their spatial variablity, the underlying mechanisms controlling seasonal LGD-derived CH fluxes and their influence on lake CH emissions remain poorly understood, particularly in humid inland areas. To address this gap, we applied the Rn mass balance model, as well as hydrological, isotopic and microbial methods to assess seasonal LGD-derived CH fluxes and their influence on the seasonal variability of lake methane emissions in a typical oxbow lake, central Yangtze River. The results revealed wide seasonal differences in LGD-derived CH fluxes, which were controlled by hydrological and biogeochemical processes. During the dry season, although more intense methane oxidation and weaker methanogenesis occurred in groundwater, the much higher LGD rate (51.71 mm/d) produced a higher LGD-derived CH flux (16.41 mmol/m/d). During the wet season, methanogenesis was more active and methane oxidation was weaker, but a lower LGD rate (12.16 mm/d) led to a lower LGD-derived CH flux (5.33 mmol/m/d). Furthermore, higher LGD-derived CH flux in the dry season resulted in higher CH emissions from the lake and diminished the extent of methane oxidation in the lake. In comparison to other regions, the differences in LGD-derived CH fluxes and their seasonal variations were found to be controlled by climatic conditions and lake types in different global regions. Higher LGD-derived CH fluxes and more pronounced seasonal variations could be associated with higher temperature, larger water depth and more intense water level fluctuations. This study provides a novel perspective and broader implications for the comprehension and evaluation of seasonal methane emissions and understanding the carbon cycle in global lake ecosystems in humid areas with intense water level fluctuations.