Characteristics, Sources, and Seasonal Variability of Dissolved Organic Matter (DOM) in the Ganges River, Bangladesh

Abstract

This study investigated seasonal variability of sources and fluorescence characteristics of dissolved organic matter (DOM) in the Ganges River using three-dimensional excitation-emission matrix (3DEEM) spectroscopy and parallel factor analysis (PARAFAC). 3DEEM and PARAFAC identified five fluorophores (Peak: A, C, M, T, W) and four fluorescent components, i.e., detergent, fulvic acid (M-type), fulvic acid (C-type), and protein-like, respectively, showing high fluorescence characteristics in the pre-monsoon (March-June) and monsoon (July-October) compared to the post-monsoon (November-February) period. Dissolved organic carbon (DOC) and Specific ultraviolet absorbance (SUVA254) were in the range 1.85-4.7 mg/L and 1.23-5.51 L/mg-m, respectively. DOM components were both fresh and microbial-derived throughout the year. Local hydrological and physicochemical conditions supported the autochthonous and aromatic DOM production in post-monsoon, and richness of terrestrial-derived natural and anthropogenic DOM in the pre-monsoon and monsoon periods. The molecular weight of DOM decreased from the mid of pre-monsoon and continued till monsoon with the increase of terrestrial-derived DOM and photo-irradiation as well as microbial activity. Tryptophan-like fluorescence (TLF) showed high intensity throughout the peak-flow pre-monsoon and monsoon periods, demonstrating a high to intermediate risk of microbial contamination of the Ganges River. The seasonal variability of DOM characteristics suggests that the source and biogeochemistry of DOM in the Ganges Rivers might be sensitive to local hydrology and climate. • DOC concentrations and DOM fluorescence intensity attained their highest values at the beginning of monsoon and showed high seasonal variability. • The compositions of DOM carried by the Ganges River varied at different times of the year, and fresh, autochthonous and less aromatic DOM dominated in the late pre-monsoon and monsoon. • PARAFAC model identified four fluorescent DOM components (detergent-, fulvic- (M like), fulvic- (C like) and protein-like) which showed high seasonal variability with rainfall and river discharge.