Vertical profiling of microplastic contamination in coastal sediments of Tamil Nadu, Southeast India.

Plastic pollution presents escalating threats to coastal ecosystems, with limited insights available from Southeast India. This study investigated vertical and horizontal stratification of macroplastics (>25 mm), mesoplastics (5-25 mm), and microplastics (<5 mm) in sediments of Rameswaram, Tuticorin, and Pichavaram. Stratified cores (15, 50, 75 cm) were analyzed for plastic morphology, polymer composition (FTIR), surface degradation (SEM-EDX), granulometry, and organic matter. Rameswaram exhibited the highest loads (up to 2800 ± 125 particles/kg dry sediment), driven by coarse, permeable sediments and dynamic hydrodynamics. Fibers dominated (59.1 %), with red, blue, and white indicating fishing gear, textile, and packaging origins. High-density polymers such as PET and PA were disproportionately enriched in deeper layers, reflecting density-driven sinking and long-term burial, while particles <1 mm were most abundant due to fragmentation. Environmental risk assessments (PLI, PERI, PHI) highlighted site-specific patterns, with Rameswaram showing the greatest ecological risk. NMDS ordination and ANOVA confirmed distinct inter-site clustering (p < 0.001). SEM imaging revealed advanced degradation and mineral accretion in buried plastics. This integrated framework benchmarks contamination, demonstrating how geomorphology, sediment characteristics, and polymer traits regulate plastic distribution and persistence, thereby offering a scientific basis for prioritised coastal management and targeted mitigation strategies.