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Spatial–vertical distribution, polymer degradation, and ecotoxicological risk of microplastics in the aquatic matrices of Punnakayal Estuary and upstream Thamirabarani River, Southeast India
Summary
Scientists found tiny plastic particles (microplastics) throughout a river and estuary system in India, in the water near the surface, deeper water, and the sediment below, with the highest concentrations floating at the top and worsening pollution levels as the river meets the sea. Common plastics like polyethylene (used in bags and bottles) and polyester-type fibers showed up most, and many particles were breaking down further from sun and water exposure, which can make them easier for fish and other wildlife to absorb. Since these estuaries connect to fisheries and coastal food sources, this pollution buildup matters for anyone who eats seafood from aff
This study presents the first integrated assessment of spatial-vertical MPs distribution, polymer degradation, and ecotoxicological risk in the upstream stretches of Thamirabarani River and Punnakayal Estuary, a biodiversity-sensitive system adjacent to the Gulf of Mannar. Triplicate samples were collected from five sites (three riverine, two estuarine) across three environmental matrices: surface water (0-20 cm), subsurface water (4 m depth), and surface sediments. MPs were detected in all samples, with highest abundance in surface water (28 ± 2 items L⁻¹), followed by subsurface water (19 ± 4 items L⁻¹) and sediments (19 ± 1 items kg⁻¹). One-way ANOVA confirmed significant vertical stratification (p < 0.05), while spatial variation across sites was not statistically significant. Fibres predominated (> 53%), and ATR-FTIR identified six polymer types, polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene: polypropylene (PE: PP), polyamide (PA), and polyurethane (PU). PE and PP dominant in water and PU and PA enriched in sediments, reflecting density-driven partitioning. Weathering indices indicated progressive oxidative degradation: carbonyl index (CI) ranged 0.07-1.05 (PE), 0.32-1.03 (PP), and 0.75-1.29 (PU), while vinyl index (VI) ranged 0.75-1.40. Multi-index risk assessment revealed moderate contamination in riverine zones (PRI: 14.8-16.2; PLI-MPs: 1.34-1.45; PERI-MPs: 61.2-72.4) escalating to higher ecological risk in the estuary (PRI: 23.9-26.4; PLI-MPs: 1.61-1.67; PERI-MPs: 118.7-131.6). Size-based risk indices further confirmed elevated bioavailability risks in estuarine zones. These findings demonstrate that hydrodynamics, polymer properties, and weathering collectively govern MPs fate, establishing estuaries as critical accumulation hotspots with implications for coastal ecosystem management.