Vertical Distribution and Composition of Plastics in Coastal Areas of the Gulf of Cádiz: Insights into Transport Dynamics.

Plastic distribution in the surface water of the marine environment has been well-documented over the years. However, vertical distribution of plastic within the water column remains poorly understood due to a lack of in situ data. This study investigates the vertical distribution of plastic particles in coastal areas of the Gulf of Cádiz, examining the surface (0-0.2 m), subsurface (0.2-1.2 m), water column (1.2-100 m), and sediment layers. Using a high vertical resolution sampling, we analyzed plastic concentration patterns across different environmental compartments, as well as plastic particle characteristics. Our results show the highest plastic concentrations in the sediments (97.9%). Concentrations along the water column decrease exponentially with depth, retaining the particles in the mixed layer just before reaching the pycnocline. Fragment-type microplastics dominate in all compartments, while film-type and line are most prevalent below the sea surface (>0.2 m). The sediment contains the densest polymers and the smallest particle sizes, likely due to density-driven sedimentation and fragmentation processes. These findings highlight the role of vertical transport in the distribution and potential accumulation of plastics across compartments, which is crucial for understanding their fate and long-term environmental impacts.