Season- and phase-associated variation in the molecular composition and redox-photochemical reactivity of dissolved organic matter at paired river and lake sites.

Dissolved organic matter (DOM) couples electron-transfer and photochemical processes, yet how molecular composition relates to this coupling across hydrological states and water-sediment phases remains poorly resolved. We characterized water and surface-sediment DOM from dry- and wet-season campaigns at paired sites in a sluice-regulated Huai River reach and an urban lake. Optical spectroscopy, ¹H NMR, HPSEC, and FT-ICR MS were integrated with measurements of electron-donating and electron-accepting capacities (EDC and EAC), photochemically produced reactive intermediates (PPRIs), and sulfonamide phototransformation. Aquatic DOM showed stronger aromatic and lignin-like signatures and generally higher steady-state concentrations of 3DOM* and 1O2, whereas sediment DOM contained more assigned formulas, was CHOS-enriched, and exhibited higher EDC and EAC. Steady-state PPRI concentrations and apparent production efficiencies were partly decoupled: wet-season aquatic DOM had more chromophores but lower fTMP and Φ¹O₂, with EDC negatively associated with both metrics. Polyphenol-rich signatures coincided with stronger light absorption but lower PPRI production efficiency, whereas microbial/aliphatic signatures correlated with higher Φ·OH. After light-screening correction, indirect photolysis accounted for 56-69% of sulfamethazine transformation but contributed less to sulfamonomethoxine and sulfamethoxazole; qualitative quenching indicated the strongest triplet-related influence. Overall, DOM redox-photochemical reactivity reflected a phase- and season-associated balance between photosensitization and deactivation/scavenging.