Strong Convergence of Floating Microplastics at a Small-Scale Estuarine Surface Front

Estuarine surface fronts, where freshwater outflow and saline tidal inflow converge, may play a critical yet understudied role in microplastic (MP) accumulation. This study presents compelling evidence of strong convergence of MP at a small-scale surface front near the Eastern Johor Strait (EJS), a typical urbanized coast in Southeast Asia, revealing an estuary tidal intrusion front as a recurrent hotspot that concentrates MP at levels comparable to known estuarine frontal systems. Significantly elevated levels of floating microplastics (150.5 ± 19.8 MP pieces/L) were detected at the surface front compared to other surface water samples along the coast (p < 0.001; 9.2 ± 4.8 MP pieces/L). Hydrodynamic simulations demonstrate that the dynamic interplay between freshwater and seawater are the key mechanisms governing this process. The resulting convergence zones effectively trap buoyant plastics, forming quasi-persistent accumulation zones. Specifically, during ebb tide, parallel outflows from the Johor River and inner EJS amplify frontal convergence, actively trapping floating plastics at the salinity boundary. These findings highlight estuarine surface fronts as influential and previously underquantified systemic pathways influencing MP circulation and retention at marginal seas. Recognizing these traceable accumulation hotspots at ocean boundaries is an urgent priority, offering opportunities for targeted mitigation, particularly in high-leakage regions.