Mapping marine debris hotspots on Boa Vista Island, Cabo Verde.

Abstract

Coastal ecosystems are under increasing threat, with the accumulation of marine debris-particularly plastics-posing significant ecological risks. Oceanic islands are especially vulnerable due to ocean currents depositing marine debris on their exposed shores. This study presents the first assessment of marine debris accumulation on sandy beaches of Boa Vista Island, Cabo Verde. Using a combination of drone-based aerial imagery and sand sampling, we quantified micro-, meso-, and macro-debris densities across 29 beaches. North- and east-facing beaches of the island showed the highest accumulation of marine debris (>85 % plastics), driven by ocean currents. Mean drone-based densities varied between 5 and 2371 macro-debris items per 100 m of beach length, totalling 23,085 items. As for sand samples, mean densities ranged from 0 to 1639 items/m², totalling 4272 large microplastics, 1221 mesoplastics and 350 macroplastics. The easternmost beach, Ponta de Roque, alone accounted for 31 % of the total debris recorded across all locations, with sand samples averaging 1639 items/m² (1453 microplastics larger than 1 mm), and drone-based surveys averaging 68 macro-debris items/100 m². Fishing-related items comprised ∼24 % of drone-surveyed debris, suggesting input from the Northwest African coast. Plastic fragments predominated, with significant correlations between drone-surveyed macro-plastics and sand-sampled large microplastic densities. Drone surveys effectively identified marine debris hotspots, aligning with ground-based data. This study provides important baseline data for long-term monitoring in the archipelago and offers a transferable methodology for assessing plastic pollution in other island systems.