Seasonal and photochemical changes of DOM in an acidified forest lake and its tributaries

Abstract

The concentration, composition, and photochemical changes of dissolved organic matter (DOM) were evaluated in surface and bottom layers, and surface and subsurface tributaries of Plesne Lake, Bohemian Forest, Czech Republic, during one hydrological year. DOM was fractionated using the Amberlite XAD-8 and ion exchange resins into five fractions: hydrophilic neutrals, hydrophilic acids, hydrophilic and hydrophobic bases, hydrophobic acids, and hydrophobic neutrals. Photochemical transformations of DOM by photosynthetically active radiation (PAR) were tested with in-situ experiments using water from the surface tributary. The fraction composition of DOM was dominated by hydrophobic acids in all samples, with annual averages of 54–58% of DOM. The surface tributary differed from the subsurface one in higher and lower percentages of hydrophobic neutrals and hydrophilic neutrals, respectively. The surface and bottom water of the lake did not significantly differ in the composition of DOM fractions despite significantly higher DOM concentrations at the bottom. The results of in-situ photochemical experiments showed the significance of PAR in photochemical as well as biological transformations of allochthonous DOM in this lake. The net rates of photo-production of DIC and photochemical plus biological degradation of DOC ranged from 0.3 to 0.6 and 0.6 to 4.3 μmol L–1 day–1, respectively, with maximum values in late spring. The hydrophobic acids fraction and the remaining DOM fractions were decomposed biologically (in dark bottles) and by combined photochemical and biological degradation (in transparent bottles) at similar rates, resulting in unchanged fraction composition of DOM in the experiments.