UV absorbance: DOC ratios were determined for the dissolved component of the water samples obtained from several specific sources, influent rivers and in-lake points of the three lakes as well as the evaluation of their changes during long-term incubations. There were significant differences in the ratios between influent rivers and in-lake points, which were consistently due to the difference between allochthonous sources mainly composed of pedogenic humic substances and autochthonous sources e.g. substances excreted through primary production. The nearly constancy of the ratio during the biodegradation process was confirmed for both allochthonous and autochthonous components; hence a two-component model could be applied to these lakes in order to calculate the proportion of DOC in each of the two components. Because of its usefulness to the model, easiness of measurement, and the close relation to trihalomethane productivity, this ratio could be one of the important indices for lake environmental management.
