Variability of the Bio-Labile Fraction of Chromophoric Dissolved Organic Matter in Lake Qiandao, a Large Drinking Water Reservoir

Lake Qiandao has low primary productivity and high Secchi disk depth. It is of great importance to explore the bio-availability of chromophoric dissolved organic matter (CDOM) in Lake Qiandao to unravel the carbon cycling in the lake. By comparing the changes of CDOM absorption and fluorescence pre-and post-28 days of bio-incubation, we aimed to reveal the bio-availability characteristics of CDOM in Lake Qiandao. The results showed that the mean values of a254 and S275 295 decreased, and the mean value of humification index (HIX) increased after 28 days of biodegradation. This indicated that laboratory biodegradation resulted in a decreased CDOM concentration and increased aromaticity of CDOM molecules. CDOM absorption a254 decreased by a mean of 14. 3% 4. 8% and with a range from 4. 3 %a to 23. 6% after 28 days of microbial degradation across the sixty sampling sites. Three fluorescent components were obtained by coupling excitation-emission matrices (EEMs) and parallel factor (PARAFAC) analysis, including a terrestrial humic-like Cl, and tryptophan-like C2 and C3. Tryptophan-like C2 and C3 decreased by 54. 1%0 18. 2% and 53. 2% 14. 3% followed by Cl (28. 2% 9. 1%) suggesting a relatively high CDOM bio-availability in Lake Qiandao. After 28 days of biodegradation, and the major fluorescence peak changed from tryptophan-like C2 to terrestrial humic-like Cl, indicating that the bio-availability of the tryptophan-like component was higher than that of humic-like component, and the T peak was degraded and the A peak was retained during the 28 days of laboratory bio-incubation. High values of the difference of absorption coefficient between pre- and post-incubation, representing the bio-availability of CDOM, i. e. Aa254 were found in the downstream southeastern lake regions, and is similar to that of tryptophan-like C2 prior to incubation. This suggested that CDOM bio-availability was the highest in the southeastern lake areas. High values of C2 and C3 were found in the northwestern inflowing lake regions post 28 days of incubation, similar to that of Cl and a254 post 28 days of incubation, suggesting that there might be freshly production of protein-like substances during the bio-incubation. There were close correlations between Cl and a254 pre- and post-bio-incubation, indicating that terrestrial humic-like substances were bio-stable. After 28 days of bio-incubation, the high values of tryptophan-like components disappeared from the lake center and southeastern lake areas, indicating that microbial metabolism has an influence on the application of tryptophanlike fluorophores in point-source-pollution identification due to the longer water residence time of the lake area compared to the remaining lake regions.