Quantifying environmental controls on estuarine dissolved organic matter transformation: Integrated insights from...

Estuaries represent dynamic interfaces where dissolved organic matter (DOM) undergoes significant transformation through intertwined biogeochemical processes. However, disentangling and quantifying the relative influence of key environmental drivers remains a persistent challenge due to their concurrent and often synergistic nature. This study applied an integrated approach combining seasonal field observations along a dam-affected estuary (the Yeongsan River estuary, South Korea), controlled laboratory simulations, and optical-tracer-based end-member mixing analysis (EMMA) to assess the respective roles of salinity, biodegradation, and photodegradation in shaping DOM composition. Surface water samples were collected from five stations spanning a 26.7 km estuarine gradient under both monsoon and non-monsoon regimes. Laboratory experiments simulating salinity-induced flocculation, microbial degradation, and photochemical transformation were used to derive process-specific end-members. Among various optical indices, specific UV absorbance (SUVA) and dissolved organic carbon-normalized fluorescence regional integration in region 5 emerged as the most responsive and discriminating tracers. Quantitative EMMA results revealed shifting dominance of environmental drivers across space and season: photodegradation accounted for 60.4-91.8% of DOM transformation attributable to the three processes at mid-estuarine sites (YSR2-YSR3) during the non-monsoon period, while biodegradation became influential under monsoonal conditions (up to 47.2% at YSR2), and salinity influence increased to 28.2-33.9% at marine-influenced sites (YSR4-YSR5). Collectively, these results extend the application of EMMA beyond conventional source apportionment to quantitatively resolve process-level controls on DOM transformation. This framework provides a robust basis for improving DOM monitoring, modeling, and ecosystem management in estuarine systems subject to hydrological regulation and climate-driven variability.