Seasonal TOC export from seven boreal catchments in northern Sweden

Total organic carbon (TOC) concentrations from seven boreal catchments in northern Sweden were monitored between June 1996 and May 1998 to examine spatial and temporal variations in streamwater TOC export and its relationship with catchment characteristics. The annual average export of TOC ranged between 36 and 76 kg ha(-1) yr(-1) and correlated positively with the areal extent of wetlands (r(2) = 0.72; p = 0.03). The daily output of TOC was 5-11 times higher during the spring than during any other season. In total, the four week long spring period contributed between 50% and 68% of the annual TOC export from the seven catchments. The relative importance of the spring snow melt period for the annual TOC export, however, correlated negatively with the percentage of wetlands (r(2) = 0.83; p<0.01). We suggest that the smaller relative importance of the spring runoff period for the annual TOC export from wetland dominated catchments is a result of the hydrological flow paths associated with the snow melt period. While a large fraction of the spring runoff from forested areas reaches the stream via subsurface flow paths across riparian soils rich in TOC, the flow paths through wetland dominated systems include a much larger component of low-TOC snow melt water via surface flow over ice and frozen peat.