While research on microplastics contamination in the Atlantic and Pacific Oceans have expanded rapidly, the Western Indian Ocean (WIO) region remains comparatively under-documented, despite its ecological importance and vulnerability to marine pollution. This study aimed to investigate the abundance, characteristics and composition of microplastics in surface seawater in the pelagic, neritic, lagoon, outer-reef and beach sediment across four island states at a regional scale linking Mauritius, Madagascar, Seychelles and Comoros. Surface water sampling was conducted at 21 offshore stations (pelagic and neritic) using the manta trawling technique (315 µm mesh size net and a frame opening of 60 cm × 25 cm; towed at a constant speed of 5 knots for two hours) over a three-month oceanographic campaign, complemented by sampling in lagoons and outer reefs zones across each island state to capture the coastal variability. Sediment samples were collected to evaluate microplastics density in sandy shoreline habitats. Microplastic concentrations in offshore (pelagic) surface waters ranged from 0.00043 - 0.235 items/m³ while coastal (neritic) surface waters concentrations varied between 0.0020 - 0.047 items/m³. Results indicated a lower concentration (0.02 item/m³) of microplastics in pelagic surface water compared to higher concentrations in neritic surface water reaching 0.275 item/m³ at approx. 125 km from the coastline, indicating a significant decrease in microplastics concentration with increasing distance from the coast. Microplastic in beach sediment ranged from 160 – 7120 items/m³ with a mean density of 1321.6 ± 1101.8 items/m³ indicating higher accumulation in this environmental compartment. Microplastic concentrations in lagoons (0.016 items/m³) were significantly lower than those observed in the outer reef (0.038 items/m³). Fragments and fibres were most dominant microplastic particle type across all environmental matrixes, accounting for up to 60% and 34%, respectively, with other shapes (foam, filament, pellet, film) occurring in minor proportions. Transparent-, white-, and blue-coloured particles were most common. Polymer analysis using the fourier transform infrared spectroscopy (FTIR) technique, indicated polyethylene (PE) and polypropylene (PP) dominance, followed by polyethylene terephthalate (PET). The spatial variability among sampling sites likely reflects differences in anthropogenic pressure and hydrodynamic condition highlighting the widespread occurrence of microplastics contamination across studied environmental compartment and WIO region. These findings provide new evidence for a better understanding of plastics debris circulation in the WIO region.

Keywords:  Indian Ocean, Mascarene islands, microplastics, pollution, polymers.