Large Spatial Variations in Diffusive CH 4 Fluxes from a Subtropical Coastal Reservoir Affected by Sewage Discharge in Southeast China.

Coastal reservoirs are potentially CH ₄ emission hotspots owing to their biogeochemical role as the sinks of anthropogenic carbon and nutrients. Yet, the fine-scale spatial variations in CH ₄ concentrations and fluxes in coastal reservoirs remain poorly understood, hampering an accurate determination of reservoir CH ₄ budgets. In this study, we examined the spatial variability of diffusive CH ₄ fluxes and their drivers at a subtropical coastal reservoir in southeast China using high spatial resolution measurements of dissolved CH ₄ concentrations and physicochemical properties of the surface water. Overall, this reservoir acted as a consistent source of atmospheric CH ₄ , with a mean diffusive flux of 16.1 μmol m ^-2 h ^-1 . The diffusive CH ₄ flux at the reservoir demonstrated considerable spatial variations, with the coefficients of variation ranging between 199 and 426% over the three seasons. The shallow water zone (comprising 23% of the reservoir area) had a disproportionately high contribution (56%) to the whole-reservoir diffusive CH ₄ emissions. Moreover, the mean CH ₄ flux in the sewage-affected sectors was significantly higher than that in the nonsewage-affected sectors. The results of bootstrap analysis further showed that increasing the sample size from 10 to 100 significantly reduced the relative standard deviation of mean diffusive CH ₄ flux from 73.7 to 3.4%. Our findings highlighted the role of sewage in governing the spatial variations in reservoir CH ₄ emissions and the importance of high spatial resolution data to improve the reliability of flux estimates for assessing the contribution of reservoirs to the regional and global CH ₄ budgets.