Peatland drainage - a missing link behind increasing TOC concentrations in waters from high latitude forest catchments?

Total Organic Carbon (TOC) concentrations in stream waters from peat-covered catchments have increased over the last 15-25 years, resulting in large-scale brownification of lakes and rivers in high latitudes. While this increase has primarily been attributed to decreased acid deposition and climatic warming in most regions, we studied whether peatland drainage in forested catchments has contributed to the increasing TOC concentrations. We analysed the spatial variability of average TOC concentrations from a total of 133 peatland dominated catchments in Sweden and Finland, of which 62 were pristine and 71 were drained during the last century. In addition, we performed a trend analysis on 37 catchments for which long-term data were available. We found about 14 mg l-1 higher TOC concentrations in streams discharging from drained than undrained sites in southern latitudes, and about 8 mg l-1 higher concentrations from drained sites in northern latitudes. Trend analysis did not indicate significant differences in TOC concentration trends between drained and undrained catchments but indicated that tree stand volume correlated with increasing trends. This supports earlier findings in that the general increase in forest cover and biomass that has occurred in high latitudes during the last decades is another factor that has contributed to brownification.