Synergistic effects of urban tributary mixing on dissolved organic matter biodegradation in an impounded river system

Dams and wastewater may greatly perturb riverine fluxes of dissolved organic matter (DOM) and CO₂, yet little is known about the relationships between altered DOM quality and CO₂ emission in eutrophic impounded river systems. A basin-wide field survey of surface water CO₂ and dissolved organic carbon (DOC) was combined with laboratory incubations to examine how dams and urban tributaries delivering treated wastewater influence longitudinal patterns in DOM properties and CO₂ along the impounded Han River traversing Seoul metropolitan area. Fluorescent DOM indices including parallel factor analysis (PARAFAC) components were used to characterize DOM in relation to biodegradable DOC (BDOC). Compared with distinct downstream increases in DOC and CO₂, BDOC concentration and its proportion in DOC (%BDOC) were highly variable along the mainstem and peaked at urban tributaries. Longitudinal increases in fluorescence index (FI), biological index (BIX), and two PARAFAC components (C2 and C3) contrasted with general decreases in humification index (HIX) and C1, reflecting increasing downstream inputs of anthropogenic DOM. During a 5-day incubation employing continuous CO₂ measurements, the cumulative production of CO₂ in the mainstem water mixed with urban tributary water was significantly higher than the level expected for conservative mixing of the two samples, indicating a synergistic enhancement of DOM biodegradation. Molecular formulas identified by Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) revealed more consumed molecules in the mainstem water and more newly produced molecules in the tributary water over the 5-day incubation, implying abundant labile components in the mainstem water discharged from the upstream dam and highly processed tributary DOM limited in immediately biodegradable organic materials. Downstream increases in CO₂ and DOC along the Han River, combined with the synergistic effect observed in the mixed water, suggest that mixing wastewater-derived DOM with labile autochthonous DOM can enhance CO₂ production in the river system perturbed by impoundment and wastewater.