Hydrological seasonality largely contributes to riverine dissolved organic matter chemical composition: Insights from EEM-PARAFAC and optical indicators

Dissolved organic matter (DOM) chemodiversity shows spatial and temporal variations in aquatic systems due to different biogeochemical process and complex sources. However, there remains a large gap on DOM compositions and source proportion, as well as factors driving DOM chemical compositions and source variations in monsoonal rivers of China. Here, 126 water samples were collected from monsoonal rivers in both dry and wet seasons in the Three Gorges Reservoir (TGR) region, an area characterized by monsoonal climate. We therefore consequently investigated the temporal and spatial variations of DOM compositions and sources using UV-Visible absorption and fluorescence excitation emission matrix (EEM)-parallel factor analysis (PARAFAC) as well as principal component analysis (PCA). There was higher degree of humification, aromaticity and lignin substances of DOM in the wet season than in the dry season. EEM-PARAFAC analysis identified four fluorescent components of DOM in the dry season and three components in the wet season. More protein-like component (tryptophan-like) was identified in the dry season when compared to the wet season, demonstrating that biological and autochthonous sources were the primary source of DOM in the dry season. PCA and the percentage of components also demonstrated more autochthonous sources and weaker terrestrial sources of DOM in the dry season. The spatial distribution of DOM optical properties demonstrated the higher protein abundance and lower molecular weight in urban land use and farmland area along the river bank. The findings demonstrated that hydrological seasonality and anthropogenic activities were key drivers for the DOM compositions and sources variations, which will improve our understanding on the crucial role of DOM in biogeochemical cycle, as well as help to guide water quality protection in the monsoonal rivers.