The influence of human activity and morphological characteristics of beaches on plastic debris distribution along the Caspian Sea as a closed water body.

The Caspian Sea is the largest land-locked lake in the world that includes numerous endemic species. Because of its enclosed nature, the pollutants entering this water body become entrapped. The present paper examines the influence of human activity and beach morphology on the abundance and distribution of stranded plastic debris along the Caspian Sea coastlines. It would be possible by relating some characteristics of these beaches (population, urbanization, substrate type, and slope of beaches) with the abundance of plastic debris through a PCo analysis. The results showed that in our study area, the most effective factors on plastic debris distribution were urbanization and population followed by the slope and substrate of beaches. Three size classes of plastic debris were defined as "large microplastic" (1 to < 5 mm), "mesoplastic" (5 to < 25 mm), and "macroplastic" (> 25 mm). The density of the large microplastic, mesoplastic, and macroplastic materials on the Caspian Sea coastline was 8.43 ± 0.54, 8.74 ± 0.42, and 7.53 ± 0.30 particles/m, respectively. Foam was the most abundant microplastic debris along the study area (47.58%), followed by resin pellets (33.93%) and fragments (16.30%), respectively. We also selected the Boujagh National Park with limited accessibility as an appropriate area for the study of marine debris as one sampling station. Cigarette butts and film pieces displayed a different distribution pattern compared to other debris types in the Boujagh National Park station likely due to their distinct buoyancy specification.