2018 Volume 9 Issue 2 Pages 23-32
The effects of photoirradiation on changes in the fluorescence intensities, dissolved organic carbon (DOC) concentrations, and molecular weights of dissolved organic matter (DOM) in Lake Biwa and its surrounding rivers were evaluated and compared with the results of humic substances and algal DOM. During the completely stratified period (summer), the fluorescence intensities of fulvic-like fluorophores in the surface water in the northern basin of Lake Biwa decreased and were lower than those in other months and in bottom water. The fluorescence quenching and degradation of high-molecular substances by further solar irradiation were hardly observed in the surface water samples but were significantly observed in bottom water samples. On the other hand, changes in the DOC concentrations in all samples were relatively small with solar irradiation. These results suggest that in the northern basin of Lake Biwa, the susceptibility of fulvic-like fluorophores to degradation by further solar irradiation is dependent on the water depth collected during the stratified period (summer), but the rest of fulvic-like fluorophores might be resistant to further photochemical degradation regardless of the water depth.
Furthermore, the effects of the wavelength region on the characteristics of DOM and fluorophores in Lake Biwa and its surrounding rivers were examined by Xe lamp irradiation using two kinds of wavelength cut filters. From these results, it is considered that wavelengths between 290 and 495 nm and below 290 nm might largely affect the characteristics of fulvic-like fluorophores and protein-like fluorophores, respectively.