Effekter av kalavverkning på mindre boreala sjöars vattenkvalitet
Forestry is an important industry in Sweden, with clear-cut as the most commonly used method for harvesting. Here the long-term impact of clear-cut on lake water quality (dissolved organic carbon (DOC), nitrogen, phosphorus and pH) has been investigated, based on 23 lakes sampled during autumn. Data for additional 15 lakes sampled for the Swedish monitoring program were also used. Lakes were classified as 5, 25, 40 and 60 year based on data of the forest age in the catchment, gathered from the Swedish national forest inventory. Clear-cut lakes were classified as 5 years (>20% of the catchment clear-cut during the last 10 years). The results showed that DOC and total nitrogen (TN) concentrations decreased with increasing age of the forests. In clear-cut lakes (5 year) average DOC- (20.5 mg/L) and TN concentrations (477 µg/L) were higher than the average concentration in lakes with older forests (25, 40 and 60 year) (i.e. DOC 13.5 mg/L and TN 351 µg/L). Also the quality of DOC were changing (measured as changes in absorbance at 254) with forest age, with more bioavailable (low molecular weight) DOC in clear-cut lakes. A greater clear-cut area compared to the catchment area did not significantly result in higher concentrations of DOC and TN in clear-cut lakes. In conclusion, the concentration of TN, and the concentration and quality of DOC, in small boreal lakes are affected by clear-cutting and are changing with the age of the forest stand. Thus, highest concentrations and more bioavailable carbon compounds are found in clear-cut lakes where forests recently has been removed (<10 year).