Bioaccumulation and trophic transfer of microplastics in oceanic food webs

Bioaccumulation of microplastics (MPs) in oceanic food webs remains poorly characterized. This study quantifies MP ingestion across trophic levels, from zooplankton to top predators, in the Laccadive Sea, Western Indian Ocean. Overall, 95 % of the samples examined contained MPs. Among zooplanktons, fish larvae had the highest concentration of MPs (0.147 ± 0.02 items/ind) compared to copepods and chaetognaths. Predatory fishes also had high MP concentrations, with swordfish (832.25 ± 527.59 items/ind), sailfish (207.50 ± 61.65 items/ind), and tiger shark (179.66 ± 41.47 items/ind) showing particularly elevated concentrations. When grouped by habitat, pelagic fishes had greater levels of MP concentrations (153.11 ± 308.72 items/ind; N = 70), compared to benthopelagic fishes (64.80 ± 99.50 items/ind; N = 65). A significant positive correlation between trophic level and MP abundance in fish guts (R = 0.3382, P < 0.05), and a trophic magnification factor (TMF) of 4.4, both indicate biomagnification of MPs within the oceanic food web. Polymer identification revealed that polypropylene (PP; 28.84 %) and polyethylene (PE; 23.07 %) were dominant in pelagic species, whereas bathypelagic taxa were primarily contaminated by high-density polymers - polyurethane (PU; 50.00 %) and polyethylene terephthalate (PET; 35.71 %). We demonstrate a clear increase in MP concentrations from lower to higher trophic levels, with top predators accumulating the greatest loads. Our study provides critical baseline for MP pollution across marine trophic levels, and help inform the development of targeted mitigation strategies and policy measures to reduce MP contamination in marine ecosystems.