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

Freshwater reservoirs are regarded as an important anthropogenic source of methane (CH) emissions. The temporal and spatial variability of CH emissions from different reservoirs results in uncertainty in the estimation of the global CH budget. In this study, surface water CH concentrations were measured and diffusive CH 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 CH 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 CH flux across the water-air interface ranged from 0.66 to 3.61 μmol/(m•hr), with an annual average of 1.67 ± 0.75 μmol/(m•hr). During the study period, the dissolved CH concentration was supersaturated and was a net source of atmospheric CH. Notably, CH concentration and diffusive flux portrayed large temporal and spatial heterogeneity. The river inflow zone was determined to be a hotspot for CH emissions, and its flux was significantly higher than that of the tributary and main basin; the CH flux in autumn was greater than that in other seasons. We also deduced that the CH 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 CH flux from temperate reservoirs to estimate the CH budget at regional and global scales.