Patterns and drivers of CH₄ concentration and diffusive flux from a temperate river-reservoir system in North China

Freshwater reservoirs are regarded as an important anthropogenic source of methane (CH₄) emissions. The temporal and spatial variability of CH₄ emissions from different reservoirs results in uncertainty in the estimation of the global CH₄ budget. In this study, surface water CH₄ concentrations were measured and diffusive CH₄ fluxes were estimated via a thin boundary layer model in a temperate river-reservoir system in North China, using spatial (33 sites) and temporal (four seasons) monitoring; the system has experienced intensive aquaculture disturbance. Our results indicated that the dissolved CH₄ concentration in the reservoir ranged from 0.07 to 0.58 µmol/L, with an annual average of 0.13 ± 0.10 µmol/L, and the diffusive CH₄ flux across the water-air interface ranged from 0.66 to 3.61 μmol/(m²•hr), with an annual average of 1.67 ± 0.75 μmol/(m²•hr). During the study period, the dissolved CH₄ concentration was supersaturated and was a net source of atmospheric CH₄. Notably, CH₄ concentration and diffusive flux portrayed large temporal and spatial heterogeneity. The river inflow zone was determined to be a hotspot for CH₄ emissions, and its flux was significantly higher than that of the tributary and main basin; the CH₄ flux in autumn was greater than that in other seasons. We also deduced that the CH₄ concentration/diffusive flux was co-regulated mainly by water temperature, water depth, and water productivity (Chla, trophic status). Our results highlight the importance of considering the spatiotemporal variability of diffusive CH₄ flux from temperate reservoirs to estimate the CH₄ budget at regional and global scales.