Cascade Reservoirs Regulate Molecular Signatures and Biogeochemical Pathways of Dissolved Organic Matters in the Lancang River

Dissolved organic matter (DOM) serves as a critical biogeochemical nexus in river systems, yet its compositional restructuring under reservoirs remains poorly constrained. Through comparative Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis of the Lancang River's natural and impounded reaches, we identified 9,410 molecular formulas that reveal systematic reservoir-induced transformations in DOM architecture. Notably, 531 compounds were reservoir-specific, exhibiting distinct molecular signatures with reduced double bond equivalence (DBE), elevated H/C ratios, and enrichment in peptide-/lipid-like components, indicative of enhanced autochthonous production. Phytoplankton community analysis revealed the reservoir-specific dominance of Chlorophyta and Cyanophyta, whose metabolic activities promoted the biosynthesis of aliphatic components and unsaturated compounds. Reaction networks uncover intensified anaerobic processing in reservoirs, where demethylation and dehydrogenation pathways generate methane precursors, potentially amplifying CH₄ flux. Carbon oxidation state (NOSC) emerges as a key molecular selector, with redox-selected preservation of low-energy CHO compounds (NOSC < 0) forming refractory carbon pools, while bioavailable CHON species demonstrate significant correlations of sediment organic carbon with biological index and peptide-like content. These molecular-level insights suggest that cascade reservoirs simultaneously process organic matter through microbial carbon pumps and sequester carbon via selective molecular preservation, benefiting from optimizing hydropower management in riverine ecosystems.