Dissolved organic carbon characteristics in boreal streams in a forest-wetland gradient during the transition between winter and summer

The character and quantity of dissolved organic carbon (DOC) were studied in nine small boreal streams and adjacent soils during two years, with focus on the spring snowmelt period. The streams cover a forest-wetland gradient, spanning from 0% to 69% wetland coverage. Lower values of the absorbance ratio measured at 254 nm and 365 nm (A(254)/A(365)), in both soil plots and streams, indicated that wetland-derived DOC had higher average molecular weight than forest DOC. Higher SUVA(254) ( DOC specific ultraviolet absorption at 254 nm) in wetland runoff indicated more aromatic DOC from wetlands than forests. During low flow, the stream DOC character was sensitive to the forest-wetland proportion of the catchment, and when wetland coverage exceeded 10%, the streams appeared to be dominated by wetland-derived DOC. During the spring snowmelt period, the character changed to lower molecular weight and more aliphatic DOC, particularly in streams with a high proportion of forest in the catchment. The forested soil solutions had higher A(254)/A(365) in the surface horizons that were hydrologically activated during the high flow events, while wetland soil solution had relatively low A(254)/A(365) at all depths. Consequently forest soils contributed more to stream DOC concentration during snowmelt that during winter low flow.