Fluorescence as an Indicator of Dissolved Organic Matter Sources in 31 Subtropical Urban Lakes at the Lower Yangtze River

To explore the relationship between the fluorescence properties of dissolved organic matter (DOM) and trophic states in aquatic systems, this study analyzed 488 water samples from 31 subtropical shallow lakes in central China over a cool-dry and a warm-wet season using excitation–emission matrix parallel factor analysis (EEM-PARAFAC) and machine learning. The lakes had variable trophic states ranging from oligotrophic to hypereutrophic. The results show that autochthonous DOM dominated in these lakes and that its contribution was significantly lower in the warm-wet season than in the cool-dry season. Tryptophan-like C1 was more abundant in the cool-dry season (21.72%) than in the warm-wet season (19.96%), while tyrosine-like C2 and terrestrial humic-like C4 were more abundant in the warm-wet season (29.30% and 18.10%) than in the cool-dry season (27.69% and 16.67%). Trophic states were strongly associated with the presence of microbial humic-like and tryptophan-like DOM components, as well as the Freshness index (β:α). Based on these findings, we conclude that the DOM fluorescence characteristics are reflective of trophic states in these subtropical shallow lakes. More studies on the linkage of DOM characteristics─trophic states are needed in other ecoregions and deep lakes.