Long-Term Spatiotemporal Dynamics of Water Quality Along the River–Brackish Lake Continuum in an Artificial Brackish Lake System

The Saemangeum Reservoir is an artificial reservoir created following completion of a 33.9-km-long seawall in 2010, with seawater inflow regulated through two sluice gates. After construction of the seawall, reduced seawater exchange, increased freshwater residence time, and weakened tidal influence were associated with deteriorating water-quality conditions, including increases in chlorophyll-a and dissolved inorganic nutrients. Since then, land reclamation and road and port construction have continued, while sluice-gate operation and seawater-exchange conditions have also been adjusted. As a result, the hydraulic environment and water-quality characteristics have continued to change. We examined the spatial water-quality structure along the river–brackish lake continuum and its interannual and seasonal dynamics using long-term seasonal water-quality data collected from 2014 to 2025. . Principal component analysis (PCA) of the water-quality variables showed that the first principal component (PC1) primarily reflected a salinity–nutrient/organic gradient that separated riverine from brackish-lake sites. Interannual and seasonal trajectories clustered largely by spatial position, with a common long-term shift toward higher salinity and lower total nitrogen (TN), total phosphorus (TP), and COD; trajectory-based groups were all eutrophic on mean Korean trophic state index (TSIKO) but differed in level and variability, showing that the expected spatial structure alone does not fully capture long-term and seasonal water-quality dynamics. Thus, monitoring and management in artificial brackish systems should track spatially distinct temporal trajectories rather than rely on long-term averages, applying year-round nutrient reduction where high trophic conditions persist and intensified seasonal/interannual monitoring where variability is large, while considering hydrological and hydraulic conditions and potential trade-offs associated with sluice-gate operation.