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.