Dissolved organic carbon is the main component of dissolved carbon in water

and contributes to the control of the chemical states and the residence times of the other elements or materials existing there. Chromatic dissolved organic carbon（CDOC） is the fraction of the dissolved organic carbon（DOC） pool that absorbs light in both the ultra violet and visible ranges. CDOC is of particular interest to remote sensing because it absorbs blue light in the same region of the spectrum as chlorophyll a. The significant absorption of light in the blue wavelengths by CDOC can result in overestimation of chlorophyll a by satellite sensors in open ocean waters. CDOC can also emit fluorescence when being excited by light and fluorescence measurements have been used in oceanography since the early sixties to detect chromophoric constituents of dissolved organic matter. Nowadays these measurements are widely used in water mass analysis as well as for phytoplankton biomass determinations. The absorbance and fluorescence of chromatic organic carbon in Lake Tai

Lake Chao and Lake Longan at the lower reach of Changjiang River were studied for the period from July 2001 to December 2002. The results show as follows. S value was 0.0046nm -1（SD=0.0015

n=284）

far less than 0.014nm -1 in the literature

maybe due to intense fluorescence emitted by high concentration of DOC when being excited; the relationship between florescence intensity and DOC content was uncertain; there was a good linear fitness of DOC content and a（350）

relativity value reached 0.673（n=284）; but there is weak relationship between florescence intensity and a（350）

relativity value only at 0.497（n=214）; The results showed that it had higher accuracy of DOC value by using absorbance than that of fluorescence.