Not just a corridor: Hydrodynamic traps and fiber risk in the Kuroshio extension

Microplastics (MPs) are widespread in marine systems, yet their dynamics and risks remain poorly understood in open-ocean regions. This study presents the first high-resolution assessment of MPs in surface waters of the Kuroshio Extension (KE), a dynamic current system in the Northwest Pacific. Across 26 sites (35°-45°N, 148°-162°E), MP concentrations ranged from 0.32 to 23.68 items/m³ (mean 3.04 ± 5.54 items/m³), with fiber Polyethylene Terephthalate (PET) dominating (>79%). We identified two key mechanisms shaping pollution patterns: mesoscale eddies enhance local accumulation, while strong current systems drive long-range transport. Fisheries and shipping emerged as major MP sources based on polymer type, morphology, and color signatures. A partial least squares (PLS) regression model showed that surface salinity and dissolved oxygen promote MP retention, while deeper chlorophyll maxima and warmer Sea Surface Temperature (SST) reduce it. These dynamics were captured by a newly developed Eco-Hydrological Co-regulation Index (ECI), which strongly correlated with MP abundance (r = 0.84, p < 0.05). Ecological risks were evaluated using the Pollution Load Index (PLI), Polymer Hazard Index (PHI), and Ecological Risk Index (ERI), supported by Monte Carlo simulations. While most sites posed low to moderate risk, two sites exceeded regional thresholds due to high-toxicity polymers. Overall, the KE acts as both a transboundary conduit and a sorting hub for MPs, emphasizing the need for microfiber-specific regulation and multidimensional risk assessment frameworks in offshore environments.