Selected water quality parameters and spectroscopic characteristics of dissolved organic matter (DOM) were examined during two different seasons for an artificial coastal lake (Shiwha Lake in South Korea), which are affected by seawater exchange due to the operation of a tidal power plant and external organic loadings from the upstream catchments. The coastal lake exhibited much lower concentrations of organic matter and nutrients than the upstream sources. The spectroscopic properties of the lake DOM were easily distinguished from those of the catchment sources as revealed by a lower absorption coefficient, lower degree of humification, and higher spectral slopes. The observed DOM properties suggest that the lake DOM may be dominated by smaller molecular size and less condensed structures. For the lake and the upper streams, higher absorption coefficients and fluorescence peak intensities but lower spectral slopes and humification index were found for the premonsoon versus the monsoon season. However, such seasonal differences were less pronounced for the industrial channels in the upper catchments. Three distinctive fluorophore groups including microbial humic-like, tryptophan-like, and terrestrial humic-like fluorescence were decomposed from the fluorescence excitation-emission matrix (EEM) of the DOM samples by parallel factor analysis (PARAFAC) modeling. The microbial humic-like component was the most abundant for the industrial channels, suggesting that the component may be associated with anthropogenic organic pollution. The terrestrial humic-like component was predominant for the upper streams, and its relative abundance was higher for the rainy season. Our principal component analysis (PCA) results demonstrated that exchange of seawater and seasonally variable input of allochthonous DOM plays important roles in determining the characteristics of DOM in the lake.