Evolution of the Dissolved Organic Matter Composition along the Upper Mekong (Lancang) River

The Mekong River is the tenth largest river in the world in terms of water discharge, representing a substantial source of organic matter to the South China Sea. However, the composition and variation of dissolved organic matter (DOM) remain poorly studied in this international river flowing through contrasting climates and severely affected by cascade reservoirs. Herein, using biomarkers, optical spectroscopy, and ultrahigh-resolution mass spectrometry analyses, we present a benchmark investigation of DOM evolution in the upper Mekong (Lancang) River. We show that terrestrial plant-derived DOM accumulates in the wet season but degrades downstream in the dry season in the upper natural reach of Lancang. In contrast, terrestrial DOM decreases consistently in both wet and dry seasons along the lower reach affected by reservoirs, accompanied by decreases (instead of increases) in lignin oxidation indices (i.e., acid-to-aldehyde ratios) and high-oxygen highly unsaturated compounds. While photo- and microbial oxidation is considered to partly contribute to terrestrial DOM removal, we employ a series of batch sorption experiments to further show that sorption to fine-sized mineral particles may explain the decreasing lignin acid-to-aldehyde ratios in DOM. Given the high abundance of fine-sized suspended particles and long residence time in the Lancang reservoirs, removal via sorption (and possibly flocculation) is a potential important pathway for terrestrial DOM in the Lancang River. Cascade reservoirs hence are a hot spot of DOM processing, and an in situ quantitative assessment of sorptive removal of DOM therein warrants closer attention.