Abstract.: Elemental carbon (C) and nitrogen (N) concentrations, C:N ratios, and stable C and N isotopic compositions (δ13C and δ15N) were used to determine the sources of riverine solid phase extracted and ultrafiltered dissolved organic matter (DOM) and suspended fine particulate organic matter (FPOM). DOM and FPOM as well as potential source materials were seasonally collected in downstream direction along the last large semi-natural river draining the European Alps, the Tagliamento River (Italy). These investigations suggest that the major sources of FPOM and solid phase extracted and ultrafiltered DOM in this river are a mixture of river-borne bacterial and microalgal biomass and soil-derived organic matter. Low C:N ratios of FPOM and bulk DOM, compared to DOM isolates, further indicate that FPOM is highly reactive and that its degradation releases bioavailable DOM compounds. Downstream abundance and distribution of organic C and N were also compared to inorganic nutrient concentrations and indicate that FPOM remineralization is a major pathway for the formation of inorganic nutrients in Tagliamento surface water
