Three-layer trap: Congener-specific PCBs accumulation driven by the biological pump in the Sea of Japan

A high-resolution, three-dimensional hydrodynamic–ecosystem–PCB model was applied to the Sea of Japan to investigate the roles of volatility and particle affinity in shaping the seasonal and spatial variations of four PCB congeners (CB28, CB101, CB153, and CB180). After a 21-year climatological simulation, their concentrations reveal a persistent three-layer vertical structure across all seasons, with dissolved PCBs maxima in the intermediate water (100–600 m). Although CB28 attains the highest absolute concentrations and CB180 the lowest, CB153 and CB101 exhibit the strongest subsurface enrichment, achieving summer accumulation factors of 2–6 compared with 1–4 for CB28 and CB180. Full-basin flux budgets reveal that relatively heavier congeners are more effectively scavenged onto particles during spring blooms and their subsequent sinking as well as remineralization sustain elevated intermediate layer dissolved pool. These findings demonstrate that, in strongly stratified, semi-enclosed basins with restricted exchange and long residence times, the oceanic biological pump preferentially sequesters high-chlorinated PCBs at intermediate water. This mechanism helps explain in situ observations of three-layer PCBs profiles and congener-specific depth trends. The modeling framework is readily transferable to other semi-enclosed seas and offers a predictive tool for assessing how intensified stratification under climate change may alter vertical pollutant fluxes and intermediate contaminant sinks.