Chromophoric and fluorescent dissolved organic matter in the Red River (Northern Vietnam): Hydrological-period contrasts and spatial variability

Dissolved organic matter (DOM) dynamics in large tropical rivers play a key role in aquatic carbon cycling, yet remain poorly documented in Vietnamese systems, particularly for chromophoric and fluorescent DOM (CDOM and FDOM). In this study, 68 surface water samples were collected along the main channel of the Red River (northern Vietnam), from upstream regions to the estuary, during the dry (March) and wet (July) periods of 2019. CDOM and FDOM characteristics were investigated using UV-visible absorbance and excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC). A three-component PARAFAC model identified two humic-like components (C1 and C3) and one tryptophan-like component (C2). Significant differences between the two sampling periods, representing contrasting hydrological conditions, and marked spatial variability were observed. CDOM and FDOM concentrations were significantly higher during the wet period, with CDOM absorption coefficient at 350 nm [a(350)] of 4.8±2.2 m-1 (wet) and 1.9±0.8 m-1 (dry), specific ultraviolet absorbance (SUVA254) of 10.0±2.6 L mg C-¹ m-¹ (wet) and 4.3±0.8 L mg C-¹ m-¹ (dry), C1 of 118±25 QSU (wet) and 66±29 QSU (dry), C3 of 74±10 QSU (wet) and 47±23 QSU (dry), and C2 of 418±85 QSU (wet) and 119±63 QSU (dry). These patterns reflected enhanced terrestrial inputs driven by monsoon runoff and soil leaching, resulting in more aromatic, higher-molecular-weight DOM. Along the river continuum, upstream waters were dominated by humic, terrestrially derived material (higher values of DOC, a(350), C1, C3). In contrast, the Red River Delta exhibited lower DOM concentrations and greater material processing, consistent with dilution and in-river processing. The higher relative contribution of tryptophan-like compounds (higher %C2, C2/C1, and C2/C3 ratios) in the delta during the dry period may reflect enhanced biological activity and/or possible anthropogenic inputs. These findings highlight the combined influence of monsoon hydrology, in-stream processes, and human pressures on DOM dynamics in a major Southeast Asian river.