From surface water to marine fauna: Assessing microplastic pollution in the northeastern Indian Ocean ecosystem

Microplastic pollution, stemming from plastic degradation or direct release, presents a persistent threat to marine environments. While global concern grows, data from the Indian Ocean, especially on uptake by mid-upper trophic organisms, remain scarce. This study characterized microplastic pollution in surface waters (30 stations) and two mid-upper trophic species-frigate tuna (Auxis thazard) and purpleback flying squid (Symplectoteuthis oualaniensis)-in the northeastern Indian Ocean. We quantified abundance, morphology, and polymer composition via manta net sampling, tissue digestion, density separation, and μ-FTIR analysis. Microplastics were detected in 96.7% of surface water samples, averaging 0.216 ± 0.208 items/m (mean ± SD). Over 96% were fibers, primarily black, red, and blue. Rayon (66.5%) and polyethylene terephthalate (26.3%) were dominant, pointing to textile laundering and everyday plastics as major sources. In biota, 66.7% of individuals contained microplastics, with the highest burden in muscle tissue (71.0%), followed by gills and digestive tract. S. oualaniensis exhibited higher loads than A. thazard. Notably, particles retained in organisms were significantly larger than those in ambient water, indicating selective ingestion/retention. These results reveal a fiber-dominated contamination pattern and confirm microplastic transfer into mid-upper trophic levels. The substantial accumulation in edible muscle raises concerns for seafood safety and ecosystem health, highlighting the urgent need for source control, better management of synthetic textile emissions, and long-term monitoring.