Regional variations in the concentrations of DOM were studied in running waters in central-northern Sweden. The highest concentrations occurred in the coastal areas, and there was a marked trend with decreasing concentrations towards the inland and the mountain range further west. Explanation of these differences are regional differences in forestland productivity, the occurrence and type of peatlands, annual runoff patterns and soil drainage conditions. Anthropogenic factors, such as atmospheric deposition of acids and forestry, have probably not contributed to the regional differences.

The impact of runoff on allochthonous organic carbon was studied in the River Öre, Northern Sweden, using extensive TOC (total organic carbon) and runoff measurements. No relationship existed between TOC concentration and runoff on an annual basis. However, positive correlations between TOC concentration and runoff were found when observations were divided into three different seasons (winter, spring and summer/autumn). During these seasons runoff explained 62 -70 % of the TOC variation. Differences in these seasonal relationships indicated that the TOC concentration was restricted by the soil TOC pool during snowmelt, while the pool of TOC in the soil or its availability never limited the TOC export during the rest of the year. Two sets of data were used, a detailed study over 2 years and a long-term study over 14 years. Both showed similar results which indicated that the seasonal variation in the relationship between TOC and runoff is similar from year to year. The chemical variation usually decreases downstream in large rivers due to mixing of water from different sources. Our study, however, showed a strong correlation between TOC and runoff even in a large river like the River Öre. This result indicated that the general pattern of the TOC concentrations was to a large extent determined by the hydrology and climate conditions.

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