Behavior of Dissolved Organic Carbon such as Humic Substances and its Effect on the Water and Soil Ecosystems

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The increase of COD in Lake Biwa may be attributed to the contribution of not only humic substances but also hydrophilic acids.Three kinds of phytoplankton were cultivated, and the contribution of dissolved organic matter (DOM) released from the phytoplankton was examined to clarify the cause of the organic pollution of Lake Biwa. Microcystis aeruginosa, Staurastrum dorcidentiferum, and Cryptomonas ovata were evaluated with regard to cultivation and degradation. Two significant peaks (2 and 3) were detected in the algal DOM released from the plankton during cultivation by gel chromatography with a fluorescence detector (Ex=340 nm, Em=435 nm). As these peaks correspond with the peaks detected in the surface water of Lake Biwa, it can be concluded that the algal DOM released from the plankton during cultivation makes a considerable contribution to the refractory organic matter in Lake Biwa. Three fluorescence maxima were observed in the cultivation of three kinds of phytoplankton: two fulvic-like fluorescence peaks (A and B) and a protein-like fluorescence peak (C). These peaks became larger as their cell counts of plankton increased. The ratio of hydrophilic DOM is fairly high in DOM produced by three kinds of phytoplankton, and algal DOM from Microcystis aeruginosa exhibits somewhat hydrophobic characteristics in comparison with those from other two plankton. The order of the amount of algal DOM per cell volume during cultivation was Cryptomonas ovata > Microcystis aeruginosa > Staurastrum dorcidentiferum.