Seasonal variation and landscape regulation of dissolved organic carbon concentrations and character in Swedish boreal streams

Sammanfattning: The seasonal variation and landscape regulation of dissolved organic carbon (DOC) concentrations in streams have been studied in two watersheds in the boreal zone. The seasonal variation was found to be highly correlated to variations in runoff. An increase in runoff was always accompanied with an increase in DOC concentration. However, there were indications that the TOC concentration was restricted by the soil TOC pool during snowmelt.The main factors affecting DOC exports varied between seasons. During winter baseflow the spatial variation in DOC exports was strongly influenced by wetland coverage, during snowmelt the exports were correlated to factors describing the size and location of the catchment, and during the snow-free season they were heavily affected by the proportions of wetlands and forests in the catchments. Small headwaters had the highest terrestrial DOC export, per unit area.The properties of the DOC changed during spring flood, towards lower molecular weight and more aliphatic compounds. These changes affected the bioavailability of the DOC, which increased during spring flood. There were also differences in the DOC properties between wetlands and forest soils; the forested soils yielded DOC with lower molecular weight (measured as 254 nm/365 nm light absorbance ratios), largely from superficial layers that were activated during high flow events, while wetland soils generally provided a more constant carbon source with higher molecular weight. The majority of the DOC was exported by wetlands, but most of the short-term bioavailable DOC (BP7) was derived from the forests, during the spring flood period, indicating that bacterial production in streams and lakes is likely to be almost entirely based on DOC exported from forested areas during, and some time after, the spring flood event.