SOURCES, CONCENTRATIONS AND CHARACTERISTICS OF ORGANIC-MATTER IN SOFTWATER LAKES AND STREAMS OF THE SWEDISH FOREST REGION

18 Swedish forest lakes covering a wide range of dystrophy were studied in order to quantify and characterize the organic matter in the water with respect to origin (allochthonous or autochthonous), physical state (particulate or dissolved) and phosphorus content. Samples were collected repeatedly during a two-year period with unusually variable hydrological conditions. Water from three different depths and from tributaries was analysed with standard monitoring methods, including water colour, Secchi disk transparency, total organic carbon (TOC), COD(Cr), COD(Mn), total phosphorus and molybdate reactive phosphorus. Interrelationships were used to compare different methods and to assess the concentration and composition of organic matter. It is estimated that in remote softwater lakes of the Swedish forest region, autochthonous carbon is typically < 5 g m-3. Most lakes in this region receive significant amounts of humic matter originating from coniferous forest soils or peatland in the catchment area. In most humic lakes with a water colour of greater-than-or-equal-to 50 g Pt m-3, more than half of the organic carbon in the surface water is of allochthonous origin, and in polyhumic lakes (> 200 g Pt m-3) the proportion can exceed 90%. Secchi depth readings were related similarly to organic matter from both sources and provided good estimates of TOC with a single optical measurement. Water colour was used to distinguish allochthonous and autochthonous matter. High concentrations of phosphorus were found in humic waters, most of it being molybdate reactive, and probably associated with humic matter rather than as dissolved free inorganic forms. COD(Mn) yielded only 25-60% of TOC and appears to include mainly truly dissolved substances of low molecular weight.