Spatial variability of methane fluxes driven by lacustrine groundwater discharge and its influence on lake methane...

Methane (CH₄) input into lakes via groundwater discharge (LGD-derived CH₄) has recently been identified as a significant contributor to the lake methane budget. The high spatial variability of LGD-derived CH₄ can substantially influence the delineation of methane emission hotspots and thus cause failure to accurately estimate and mitigate aquatic methane emissions. However, systematic studies on the spatial variability of LGD-derived CH₄, its controlling factors, and its influence on lake methane emissions remain limited. Using a combination of ²²²Rn mass balance modeling, hydrological analysis, and isotopic techniques, we here tested a systematic approach to quantify the contribution of LGD-derived CH₄, characterize its spatial variability, and identify the underlying factors regulating lake methane emissions to the atmosphere, based on a case study in an oxbow lake which is naturally cut off and separated by evolution of original river channels creating a free-standing body of water within the central Yangtze River plain. The results indicated that the LGD-derived CH₄ fluxes (20.24 mmol/m²/d) were significantly higher than those from lakebed sediment diffusion (0.15 mmol/m²/d), can be an main contributor to the lake methane budget. The LGD-derived CH₄ exhibited the substantial spatial variability (0.01-137.13 mmol/m²/d), primarily controlled by water depth, the hydraulic conductivity of lakebed sediment, and groundwater methane concentration. Along the lakeshore, LGD-derived CH₄ played a more potentially important role in lake methane emissions due to less loss of methane oxidation in the transport pathway. In the interior of the lake, despite higher LGD-derived CH₄ inputs, obvious methane oxidation caused lowering of the methane emissions before entering the atmosphere. Consequently, the lakeshore emerged as a potential hotspot for subsequent lake methane emissions, driven by negligible methane loss from oxidation. These findings provide new insights into methane cycling and emissions in global lake ecosystems, particularly in regions with strong groundwater-lake interactions.