Comprehensive assessment of anthropogenic debris pollution on beaches of the oceanic island São Vicente, Cabo Verde

Plastic debris is a major threat to coastal ecosystems. Although, numerous studies have assessed the extent of this pollution in various regions of the world, our knowledge about the situation on the African continent and its oceanic islands is still limited. To address this gap, repeated beach debris surveys were conducted on six beaches on the island of São Vicente, Cabo Verde, over the course of six weeks from May to July 2023, with surveys conducted every four days. This was done to compare debris accumulation rates as well as debris composition between beaches that differ in their orientation, exposure and level of human use. Furthermore, we assessed the density of mesoplastic particles (1-10 mm) and correlated it with the amount of macroplastic debris (>10 mm) encountered. A backward ocean current simulation with PARCELS allowed to identify possible origins of the plastic items from ocean-sources. We observed an average macrodebris (> 10 mm) accumulation rate of 0.05 ± 0.06 items/m²/d (0.2 ± 0.4 g/m²/d), while the rates varied strongly between beaches. The number of macroplastic items was positively correlated with the abundance of mesoplastic particles (r² = 0.982, p = 0.0004), for which we found an average density of 121.7 ± 339.1 mesoplastic particles/m². The particle tracking by backward ocean current simulations demonstrated the existence of a transport pathway from the northwest coast of Africa towards the Cabo Verde archipelago. This suggests the northwest African shore as a potential source of the anthropogenic debris found on São Vicente.