Microplastic Concentration and Morphotype Composition in Beach Sediments: Contrasting Patterns Between a Semi-Enclosed Gulf (Pagasitikos) and the Open NW Aegean Coast, Greece

Direct comparisons of microplastic (MP) contamination between semi-enclosed and open Mediterranean coasts remain scarce. We quantified MP concentration and morphotype composition at ten sandy beaches in Thessaly, Greece (five in the semi-enclosed Pagasitikos Gulf and five on the open NW Aegean coast), sampling supralittoral and active-swash sediments (n = 100): (~800 g composite per 1 m2 quadrat). Particles were extracted by NaCl density separation and screened using Nile Red fluorescence. We recorded 10,506 Nile Red-positive putative MPs (1–1297 items m−2; mean 105.1 ± 175.1; median 53.5 items m−2). Pellets/beads dominated (51.7%), followed by fragments (37.2%) and fibres (10.6%). Mean concentration was higher in the gulf than on the open coast (147.6 ± 226.0 vs. 62.5 ± 73.6 items m−2), but the area effect was non-significant (GLMM, p = 0.110), while among-beach heterogeneity was high (CV = 91.4%). A localized pellet hotspot occurred in the Kala Nera active swash (525.8 pellets m−2), producing a significant pellet-specific Area × Zone interaction (p = 0.033). Pellets contributed 42.9% of between-area dissimilarity (SIMPER, p = 0.013). These results indicate that localized, source-specific inputs may outweigh broad hydrodynamic differences in shaping regional contamination patterns.