Spatial and temporal variability of dissolved methane concentrations and diffusive emissions in the Three Gorges...

Methane (CH₄) emissions from freshwater aquatic systems such as rivers and reservoirs are an important component of the global methane budget. However, the estimation can be largely affected by the spatial and temporal resolutions of measurements. Especially, the lack of high-resolution studies in the Three Gorges Reservoir (TGR), one of the largest reservoirs in the world, has led to a longstanding debate on its CH₄ emissions. In this study, the spatial distribution and seasonal variations of dissolved CH₄ concentrations were measured using a fast-response automated gas equilibrator in the TGR. We observed large spatiotemporal variations of dissolved CH₄ (mean ± SD: 0.26 ± 0.19 μM in summer and 0.24 ± 0.17 μM in winter). Higher concentrations with stronger variations were found in the upstream than in the section close to the Three Gorges Dam. The dissolved CH₄ concentration in the TGR was mainly influenced by sewage discharge, sedimentation, topographical conditions, tributaries, and spatial and seasonal variations in hydrodynamics. Regression analyses suggest that the concentration can be characterized by sewage discharge, water depth, and electrical conductivity to a certain extent. Mean diffusive CH₄ fluxes from the TGR in summer and winter were 16.2 mg m^-2 d^-1 and 3.1 mg m^-2 d^-1, respectively. Downsampling simulations show that scaling dissolved CH₄ in the TGR from one site likely involves large errors, and at least ∼38 sites and ∼52-58 sites are needed to achieve an accurate estimate in summer and winter, respectively. Due to the large spatial and temporal heterogeneity, high-resolution measurements are key to improving the reliability of CH₄ estimates and assessing the contribution of the TGR to regional and global CH₄ budgets.