Methane distribution patterns along a transect of Lake Fuxian, a deep oligotrophic lake in China

Freshwater ecosystems are recognized as one of the important natural methane (CH₄) sources, but little is known about the emission hotspots and the effects of algal blooms on CH₄ production in deep lakes. In this study, carried out from the littoral (S1), pelagic (S2-S4), and the deepest site (S5), water samples from different depths and sediment cores were collected along the transect of Lake Fuxian, a deep monomictic lake to investigate the spatial–temporal variations of CH₄. Dissolved methane concentration observed at the oxic metalimnion was 37.5% and 19.5% higher than that those observed at the epilimnion and at the layer between 80 and 100 m depth, respectively. During the overturn period, the vertical distribution of CH₄ in the water column was uniform, with an average concentration of 0.031 ± 0.007 μM in S2–S5. Statistical analysis indicated that the CH₄ concentration in the water column was significantly higher in S1 than other sites along the transect during both sampling periods. Sediment CH₄ concentration and methane production potential (MPP) were also significantly higher in S1 than in other sites. Along the sediment depth, the maximum MPP was observed at 6–8 cm in S1, but it moved up to the surface layer in S2–S5 in both sampling periods. In addition, stable carbon isotope analysis indicated that the surface sediments in the pelagic zone (S2–S5) mainly comprised autochthonous organic matters. In this zone, MPP had a significantly positive correlation with sediment total organic carbon (TOC) (R² = 0.401, p < 0.01). In summary, we described the spatial and temporal distributions of CH₄ in deep Lake Fuxian, littoral zones are CH₄ emission hotspots that can contribute to the CH₄ accumulation in the oxic metalimnion layer during the stratification period. In the pelagic zone, autochthonous organic matter was transported into the surface sediment after a massive algal bloom, representing another hotspot for CH₄ production.