From Biogeochemical Nexus to Static Conduit: Large-Scale Water Diversion Reshapes Dissolved Organic Matter Molecular Networks

Large-scale hydraulic infrastructure has profoundly affected global aquatic ecosystems. However, its influence on the molecular-level transformations of dissolved organic matter (DOM) is not well understood. Therefore, this study used ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), coupled with paired mass distance (PMD) network analysis, to compare the DOM molecular composition and potential transformation connectivity between a source-water-linked natural river and the artificial canal of the Middle Route of the South-to-North Water Diversion Project. The natural river displayed a substantially higher DOM molecular diversity and a larger PMD-inferred molecular network than the canal, whereas the canal displayed a more centralized and compositionally constrained network dominated by CHO- and CHON-related molecular connections. The PMD-inferred reaction classes further indicated stronger river-associated signals for redox- and phosphorylation-related molecular relationships, whereas the canal exhibited greater relative contributions to hydration, glycosylation, carbonylation, and acetylation patterns. Selected water quality gradients—particularly total phosphorus, dissolved organic carbon, turbidity, and heteroatom-bearing molecular groups—were significantly associated with specific molecular network features. A multi-method exploratory driver-prioritization analysis integrating Linear Discriminant Analysis (LDA), Random Forest, and Similarity Percentage Analysis (SIMPER) further highlighted the reaction network size and CHOS/CHONS-related molecular features as candidate variables associated with river canal differentiation. These findings support the interpretation that large-scale canalization is associated with a shift from a chemically diverse, highly connected riverine DOM network to a more constrained conduit-like molecular network configuration.