Factors Influencing the Three-dimensional Distribution of Microplastics on Sandy Beaches: A Case Study from the Turkish Coast of the Black Sea

Microplastic pollution stands as an emerging threat to sandy beach ecosystems, globally. However, beaches are three-dimensional systems, and only a limited number of studies investigated the vertical and horizontal distribution of microplastics in these systems. Furthermore, the causative drivers behind the three-dimensional distribution of microplastics on sandy beaches have not been well understood. Therefore, 7 potential factors including total organic content, sand grain size, beach length, and width, the proximity of the study site to the closest city center (a proxy for the tourism influence), cleaning frequency of the beaches, and road type next to the beach on nine sandy beaches of the Turkish Coast of the Black Sea were collectively investigated as causative drivers. Microplastic abundance, size, and compositions were examined in sand samples collected at different depths between 0 and 105 cm. While microplastic abundance was evenly distributed horizontally, it showed a gradual decline with increasing depth. The abundance of microplastics varied between 21.18 +/- 0.98 item/kg-1 (at the beach surface) and 2.78 +/- 0.93 item/kg-1 (at the deepest sampling point). Potential factors examined here explained 84.7% of the variation in microplastic abundance with the highest relative influence by wave actions. Microplastic size showed a seaward decline on the beach surface with 1045.11 +/- 274.36 mu m, but it seemed similar between depths. Other characteristics (color, shape, and polymer type) significantly differed between depths and tidal heights. The majority of the microplastics were fragments (38.4%) and foams (37.8%). White was the most available microplastic color with 30.23%. Microplastics detected on these sites were dominated by polystyrenes. The factors examined here explained their variations of microplastic characteristics between 84.25% and 89.14%. This study provides important insights into the current literature by examining multiple causative drivers for the three-dimensional microplastic distribution on sandy beaches, which should be useful for management strategies to reduce the impact of these contaminants on organisms.