The influence of hydrology on coloured DOC-species - a long term study in a forest lake system in SE Norway

Sammendrag

In the forested catchment Østmarka, SE Norway, a study of 24 pristine lakes shows that the water quality has changed significantly from 1983 to 2005 with decreased concentration of sulphate and increased concentration of dissolved organic carbon (DOC) and especially colour. From comparing these trends with inter annual fluctuations in precipitation and long term trends in sulphate deposition we propose two mechanisms on how precipitation quantity and quality influences coloured dissolved organic matter (DOM) species in Østmarka. First; inter annual changes in concentrations of DOC and also the specific colour of DOM in the investigated surface waters are basically driven and explained by inter annual variations in amount of precipitation. The DOM in lakes is generally more coloured during wet years compared to drier years. Size separation experiments demonstrate that the high molecular weight DOM compounds (nominal molecular weight cut off > 10 kDa) have a higher specific colour than low molecular weight (< 10 kDa) DOM compounds. Increased concentration of especially the high molecular weight DOM from the upper forest floor horizon could therefore conceptually explain the inter annual fluctuations in the colour of lakes. Second; we propose the reduction in sulphur concentration in precipitation as an underlying reason for the observed long term trend of increased lake colour. By using the decline in sulphur as a quality correction factor in a deposition expression we are able to model the colour trend in the study area (r2 = 0.86).