Retention of floating macroplastics in semi-enclosed coastal systems: Insights from modeling and drifter experiments

Coastal and estuarine systems can retain floating macroplastics before they reach the open sea, yet these environments are often underrepresented in marine litter monitoring and assessments. We investigated floating macroplastic retention under six wind conditions and over a year in two semi-enclosed Baltic Sea systems (the Warnow Estuary and Greifswald Bay). We used high-resolution (20 m and 200 m) 2D hydrodynamic models with Lagrangian particle tracking, adding retention based on entrapment observed in drifter experiments in the reed belts. Model results were compared to in situ surface drifter trajectories in the Warnow Estuary. Drifter trajectories under two contrasting wind conditions closely matched the modeled accumulation patterns and travel times. Annual retention of simulated particles exceeded 80% in both systems, while short-term (72 h) retention reached up to 100% in the Warnow Estuary. Reed belts were the primary retention zones, whereas artificial structures also contributed to retention. Extrapolating our results to the Baltic Sea, macroplastic retention in coastal and estuarine systems may be at least 50% lower than previous estimations (38 million items compared to the 76 million items per year reported from previous studies)