Beyond the Surface: Vertical Distribution of Plastics in Coastal Areas of the Gulf of Cádiz

Marine plastic pollution research has traditionally focused on the abundance and impacts in surface waters, resulting in substantial knowledge gaps regarding their vertical distribution and ultimate fate. This study addresses these limitations by providing a comprehensive assessment of plastic transport dynamics across multiple compartments. Plastic concentrations and composition (shape, size, and polymer type) were quantified from the sea surface to the sediment using high-resolution sampling instruments.

Our results demonstrate an exponential decrease in plastic abundance with depth, while revealing particle retention within the mixed layer, immediately above the pycnocline. Notably, sediments constitute the dominant sink, containing the vast majority of the plastic load (98% of the total measured abundance). Furthermore, sediments accumulated the highest-density polymers and smallest particle sizes, suggesting that density-driven sedimentation and fragmentation processes play a key role in vertical transport.

These findings highlight the importance of understanding vertical transport processes as a critical step in identifying the ultimate sinks of plastic and assessing its potential long-term environmental impacts, thereby supporting the development of more effective mitigation strategies.