Variations in dissolved organic matter (DOM) quality has far-reaching implications that affect, e.g., aquatic productivity, food web structures, trace element and pollutant transport. In this study, a total of 186 water samples were collected at 62 sites (three points in time within one year) in Nansi Lake. UV-Vis spectra, Synchronous fluorescence (SF) spectra, the excitation-emission matrix and parallel factor analysis (EEM-PARAFAC) were applied to indicate the source and quality of DOM. Water transferring of the eastern route of China’s South-to-North Water Diversion Project had a great influence on the water level of Nansi Lake. Results of SF spectra, EEM-PARAFAC and principal component analysis (PCA) suggested that protein-like substances played a more important role in DOM properties in April and July than October. This result is related to a high fluorescence intensity occurred in April (Fmax=0.72±0.03 in the upper lake and 1.84±0.13 in the lower lake) and July (Fmax=1.10±0.05 in the upper lake and 1.49±0.04 in the lower lake), which might be caused by water transferring from other lakes to Nansi Lake, death and decomposition of submerged plants. At the same time, relatively good correlations were found between humic-like substances, DOC and a254 in April, July and October, which indicated the important contribution of humic-like substances to Nansi Lake. With the completion of the water diversion, the ratio of the fluorescence intensity of component to the total fluorescence intensity (%Fmax) suggested that the proportion of humic-like substances started to increase. And when it came to October, humic-like substances become the main substance in DOM collected from Nansi Lake (%Fmax=66.56%±0.58% in the upper lake and 61.98%±0.99% in the lower lake). Moreover, among the two areas in Nansi Lake, the upper lake always had a higher degree of humification (HIX=2.23±0.06, 2.38±0.11 and 3.10±0.05 in April, July and October, respectively) than the lower lake (HIX=1.06±0.05, 1.68±0.05 and 2.62±0.08 in April, July and October, respectively), which implied extraneous contaminants might have a more important impact on DOM properties in the upper lake.
