Photodegradation of fluorescent dissolved organic matter in river waters

Photodegradation of dissolved organic matter (DOM), collected from two upstream and one downstream sites in the Lake Biwa watershed in Japan, was investigated using fluorescence properties of three-dimensional excitation emission matrix (3-D EEM). Solar irradiation caused a decrease in the dissolved organic carbon (DOC) concentration and in the fluorescence peak intensity (FI) of fulvic acid (FA)- and protein-like substances in DOM. Mineralization of DOC during an irradiation period (13 days) was observed to be higher in upstream samples (32-36%) compared to that in downstream sample (16%). DOC concentration in samples stored in the dark significantly decreased in the downstream sample (16%), while those in upstream samples hardly decreased (1-8%). The FI of FA-like substance decreased by 72-84% during 13-days irradiation (integrated solar intensity = 176 MJ m(-2)). The protein-like fluorescence was only observed in the downstream sample and the net decrease in the FI was 59% in the irradiated sample. The first-order reaction kinetics was applied to analyze the photo degradation of DOM. Two-step photodegradation was suggested for the fluorescent DOM (FDOM), while not for the DOC. Results obtained here suggested that FA-like FDOM is more susceptible to photodegradation compared with those of protein-like substances and DOC.