Spectral characteristics of dissolved organic matter (DOM) in the middle reaches of the Huai River in a dry season†

The present study aims to investigate the characteristics of dissolved organic matter (DOM) in the Huai River in the winter dry season using UV-visible absorption spectroscopy (UV-vis), three-dimensional fluorescence excitation–emission matrix spectroscopy-parallel factor analysis (EEM-PARAFAC), and liquid chromatography-organic carbon detection (LC-OCD). The PARAFAC model results revealed three DOM chemical components, namely: UV-type humic substances (C1), humic acid-like substances (C2), and protein-like substances (C3). However, humic substance components (C1 + C2) were the major fluorescent DOM components, accounting for 61.88 ± 6.45%. In this study, the reduced external inputs in the winter dry season resulted in a significantly higher fluorescence intensity of the C3 component than that of C2 (P < 0.01). On the other hand, the LC-OCD results indicated significant differences (P < 0.01) between different water body types of the Huai River due to the strong influences of human activities and sewage discharge. The polysaccharide, humic substance, low molecular acid, and nitrogenous compound concentrations in the river water exhibited decreasing trends from upstream to downstream of the river. In contrast, the concentrations of amino acid derivatives exhibited a significant increasing trend from upstream to downstream of the river. The concentrations of nitrogenous compounds were accumulated in the confluence zone of the river tributaries and mainstream. The DOM concentrations in the river water were influenced by multiple factors. However, the decrease in the concentrations of proteins and polysaccharides enhanced the autochthonous process in the river water body, gradually increasing the concentrations of humic substances.