Unique characteristics and spatiotemporal variations of dissolved organic matter along the Yeongsan River estuary impacted by an estuary dam

Artificial barriers such as dams and weirs profoundly affect the hydrological conditions of aquatic ecosystems and organic carbon dynamics. This study examines the sources, characteristics, and spatial and seasonal variations in dissolved organic matter (DOM) in the Yeongsan River estuary, South Korea, impacted by an estuary dam. We explore how these barriers influence DOM concentrations and compositions seasonally. Observations from five sites (YR1 to YR5) revealed high levels of fluorescence index (FI, 1.7-1.9), biopolymer fraction (2-16%), and percentage of dissolved organic sulfur (DOS%, up to 42%), indicating a dominant algal contribution. The dam's installation disrupted the DOM continuum, distinguishing between upstream sites with terrestrial DOM and downstream sites influenced by marine sources. Compared to downstream sites, upstream sites consistently showed higher dissolved organic carbon (DOC) and absorption at 254 nm (a254), with depleted delta 13C-DOC across all seasons. Seasonal differences included higher concentrations of humic substances and biopolymers in upstream sites during spring, with pronounced protein-like fluorescence. During the monsoon, these sites had enhanced humic substances, biopolymers, low molecular weight neutrals, and peak N fluorescence associated with chlorophyll intensity. The dam controlled freshwater discharge to downstream sites, especially during peak monsoon. Stable isotopic values (delta 13C-DOC) confirmed distinct DOM sources between upstream and downstream sites. Notably, only the middle site (YR3) displayed unique characteristics in the pre-monsoon season, indicating the monsoon's residual impact on DOM. The lower estuary likely experiences extended water residence times post-monsoon. This study underscores the significant influence of phytoplankton blooms and distinct spatiotemporal variations in DOM characteristics in an estuarine system disrupted by artificial barriers.