Temporal dynamics and tidal influences on plastic deposition, remobilization, and accumulation in an estuarine mixing...

Plastic pollution represents a critical threat to estuarine environments, with the complex dynamics of plastic deposition, remobilization, and accumulation remaining understudied. This study investigated these three processes in a natural French estuary during semi-diurnal tidal cycles, focusing on the influence of these cycles, wind patterns, and substrate types. Repeated samplings were conducted at regular intervals, with six consecutive observations during low tides each month over a period of 3 months. OSPAR protocol along a 100 m² transect was used, and plastic deposition rates ranged from 1.5 ± 0.54 plastics/100 m² to 31.16 ± 8.06 plastics/100 m² per semi-diurnal tide cycle. Remobilization rates varied from 50 to 100%, with substrates such as mud and gravel showing rapid remobilization and low accumulation, while dense vegetation retained up to nine times more plastic, acting as hotspots for accumulation. In addition, landward winds amplified this plastic deposition, increasing rates by a factor of 5 to 20, particularly during spring tides. Macroplastics dominated the various sizes of plastic deposited, and polymer analysis identified polyethylene and polypropylene as the most prevalent polymers. Long-term monitoring underestimated plastic retention by a factor of up to 20, highlighting the value of short-term, high-frequency observations to accurately study plastic dynamics. These results reveal the complex interaction of substrate characteristics, environmental factors, and tidal cycles in the regulation of plastic dynamics in estuarine environments and the need for targeted mitigation strategies to protect these ecosystems.