Large Spatial Variations in Diffusive CH₄ Fluxes from a Subtropical Coastal Reservoir Affected by Sewage Discharge in Southeast China

Abstract

Coastal reservoirs are potentially CH₄ emission hotspots owing to their biogeochemical role as the sinks of anthropogenic carbon and nutrients. Yet, the fine-scale spatial variations in CH₄ concentrations and fluxes in coastal reservoirs remain poorly understood, hampering an accurate determination of reservoir CH₄ budgets. In this study, we examined the spatial variability of diffusive CH₄ fluxes and their drivers at a subtropical coastal reservoir in southeast China using high spatial resolution measurements of dissolved CH₄ concentrations and physicochemical properties of the surface water. Overall, this reservoir acted as a consistent source of atmospheric CH₄, with a mean diffusive flux of 16.1 μmol m^-2 h^-1. The diffusive CH₄ 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 CH₄ emissions. Moreover, the mean CH₄ 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 CH₄ flux from 73.7 to 3.4%. Our findings highlighted the role of sewage in governing the spatial variations in reservoir CH₄ 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 CH₄ budgets.