Characteristics, sources, and photobleaching of chromophoric dissolved organic matter (CDOM) in large and shallow Hongze Lake, China

Hongze Lake, the fourth largest freshwater lake in China, is located on the eastern route of the South-to-North Water Diversion Project. The current study provides a first look at the source and composition of chromophoric dissolved organic matter (CDOM) in Hongze Lake through the combination of excitation emission matrix fluorescence spectra and parallel factor (PARAFAC) analysis. Six fluorescence components were identified by the PARAFAC model: three humic-like components (C1, C2, and C3), two autochthonous protein-like components (C4 and C6), and one photoproduced humic-like component (C5). The results of fluorescence indices and principal component analyses of fluorescence components demonstrate that CDOM is primarily of autochthonous origin, specifically comprising C4 and C5 in surface and bottom waters and C6 in sediment pore water. Sediment pore water contained CDOM of relatively lower aromaticity and molecular weight than that of surface and bottom waters, suggesting various biogeochemical processes involving CDOM in the lake. Moreover, the decrease in fluorescence intensities (primarily humic-like), aromaticity, and the molecular weight suggested photobleaching of CDOM. However, the hydrological conditions in Hongze Lake were strongly affected by human activities due to the urgent demands for water resources. Therefore, the hydrological controls may affect the CDOM characteristics and biogeochemical processes (e.g., photochemical), which should be investigated in further studies.