Dissolved organic carbon in wet deposition around the Danjiangkou Reservoir, China: Temporal patterns, sources, and ecological implications in different sites

Despite the recognized significance of wet deposition of atmospheric dissolved organic carbon (DOC), its dynamics within large reservoir catchments remain poorly understood. This study investigated the spatiotemporal variations in rainwater DOC concentrations and deposition fluxes across urban, agricultural, and reservoir areas around the Danjiangkou Reservoir in China. Volume-weighted mean (VWM) DOC concentrations varied significantly, with the highest levels observed in urban areas (7.52 mg C L⁻¹) compared to agricultural (4.94 mg C L⁻¹) and reservoir areas (4.66 mg C L⁻¹). Elevated DOC concentrations occurred seasonally in spring and winter, likely due to reduced rainfall scavenging, increased heating emissions, and stable atmospheric conditions favoring pollutant accumulation. Source apportionment indicated dominant terrestrial contributions to rainwater DOC. Agricultural activities, mineral dust, and mobile emissions were associated with DOC in agricultural and reservoir areas, whereas stationary sources like coal combustion and industrial emissions contributed more to urban areas. Stepwise multiple regression analysis identified rainfall amount and electrical conductivity (EC) as significant DOC concentration predictors, reflecting atmospheric washout and terrestrial influences. Monthly DOC fluxes ranged from 0.24 to 14.12 kg C ha⁻¹ in different sites, highlighting the temporal variability driven by precipitation. Summer witnessed the highest regional DOC deposition fluxes across all regions (16.43–25.88 kg C ha⁻¹), over twofold higher than spring and winter. The urban area had the highest annual DOC deposition flux (56.07 kg C ha⁻¹ yr⁻¹), followed by the reservoir (34.77 kg C ha⁻¹ yr⁻¹) and agricultural areas (30.29 kg C ha⁻¹ yr⁻¹). Annually, atmospheric wet deposition contributes an estimated 1898 t of DOC to the reservoir, potentially enriching the upper two meters by 1.74 mg C L⁻¹. This study enhances the understanding of DOC wet deposition dynamics in a large reservoir catchment, emphasizing the importance of considering atmospheric inputs in reservoir water quality management.