The distributions of dissolved organic carbon (DOC) in the warm season were elucidated in ten lakes of different trophic types in Japan, Russia, and China. DOC showed similar vertical distributions in all the lakes in summer when thermal stratification occurred. DOC in the epilimnion was higher than the value of 0.8 mg C l Ϫ1 found in the hypolimnion. In three Japanese lakes, hypolimnion DOC was negatively correlated with apparent oxygen utilization (AOU), reflecting the net oxidation of DOC using the dissolved oxygen in lake water. The DOC : O 2 ratios (0.115-0.179), calculated by the slopes of the regression lines of DOC versus AOU in hypolimnion water, were as low as those of deep-sea water, which indicates low bioavailability of lake water DOC for heterotrophic bacteria. DOC and conductivity did not correlate well except in two Japanese lakes: one showed a positive correlation and the other a negative correlation, indicating DOC loading from the inflowing rivers. Eutrophic lakes tended to have higher DOC values than meso-and oligotrophic lakes, and DOC values in the surface water negatively correlated with Secchi depths.

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The dissolved organic carbon (DOC) concentrations in mesotrophic Lake Biwa were determined by a total organic carbon (TOC) analyzer, and DOC molecular size distributions were determined by size exclusion chromatography (SEC) using a fluorescence detector at excitation/ emission (Ex/Em) levels of 300/425 nm with the eluent at pH 9.7. The fluorescence wavelengths for detection were chosen from the result of excitation-emission matrix spectrometry (EEM) analysis for dissolved fulvic acid (DFA) extracted from Ado River (peak A, Ex/Em = 260-270/430-440 nm; peak B, Ex/Em = 300-310/420-430 nm). Ado River DFA was eluted with a retention time (RT) of 7.4-8.9 min and the apparent molecular weight was estimated at 22-87 kDa based on the elution curve for the spherical protein molecular weight standard. A DFA peak eluted at the same retention time as Ado River DFA also appeared in all the samples of Lake Biwa water. From the linear relationship between the peak areas with an RT of 7.4-8.9 min by SEC analysis and DOC values of DFA by TOC analysis of a series of DFA samples (r 2 = 0.9995), the concentrations of DFA in the lake water were roughly calculated. DFA was distributed within the range 0.25-0.43 mg C l -1 and accounted for 15%-41% of DOC, with the highest ratios observed at a depth of 70 m in August and the lowest at 2.5 m in May.

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