Dissolved organic carbon (DOC), humic substances (HS), polysaccharides (PS) and low molecular weight acids (LMWA) were characterized in water from the dystrophic Lake Große Fuchskuhle over a period of seven months. In addition, porewater from an adjacent fen was investigated in order to obtain information about the DOC in the catchment area. Size- exclusion-chromatography combined with UV- and organic carbon (IR)-detection was used to quantify DOC and its fractions. The lake had previously been divided into four separate sections by large sheets of plastic, and the DOC composition differed markedly between the four compartments. Spatial variations in HS and PS concentrations were greater than seasonal variations. The high amounts of HS (up to 58%) in the western sections of the lake, indicated influence by subsurface water from the fen, whereas the eastern sections were dominated by PS (up to 35%) of algal origin. These differences could be explained by hydrological condi- tions, indicating that completely different catchment areas influenced the water chemistry in the separate compartments. By characterizing the HS by their average molecular weight and their aromaticity, three different groups of HS could be distinguished depending on their origin and fate. Microbial degradation of DOC and its fractions differed between two of the compartments during incubation studies over a period of six weeks.
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