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. 2025 Jul 1;15(1):22364.
doi: 10.1038/s41598-025-02893-0.

Concentration gradient of plastic debris larger than 500 μm detected across the Southwest Indian ocean

Margot Thibault  1   2   3   4 , Adrian Fajeau  5 , Aina Ramanampananjy  6 , Sarah-Jeanne Royer  7 , Gwennaïs Fustemberg  8 , Vyctoria Marillac  8 , Julie Gindrey  9 , Anjara Saloma  6 , Manon Condet  10 , Perrine Mangion  10 , April Burt  11 , Matthias Egger  7   12 , Maxime Amy  13 , Sébastien Jaquemet  9 , Thierry Mulochau  14 , Philippe Jourand  15 , Alexandra Ter Halle  16 , Matthieu Le Corre  9 , Laurent Lebreton  7   17
Affiliations

Concentration gradient of plastic debris larger than 500 μm detected across the Southwest Indian ocean

Margot Thibault et al. Sci Rep. .

Abstract

Marine plastic pollution is increasing in the world's ocean, with the Indian Ocean understudied compared to the Pacific and Atlantic Oceans. This study investigates plastic pollution in the Southwest Indian Ocean, focusing on a size range from large debris to microplastics (> 500 μm). Using visual surveys and manta trawling, we assessed plastic concentrations, compositions, and polymer types across 19 oceanographic campaigns. A total of 11,438 litter items were identified, with over 70% consisting of plastics. Larger plastic debris was predominantly observed near Glorieuses Islands during visual surveys, while microplastics were more prevalent offshore, collected through manta trawling. We observed a gradient of increasing plastic concentrations along the 30°/33°S latitudes, from 40°E (macroplastics: 10 items/km²; microplastics: 103 items/km²) to 65°E (macroplastics: 102 items/km²; microplastics: 105 items/km²). The majority of plastic debris consisted of hard fragments, primarily polyethylene (45.7%) and polypropylene (26.7%). Our findings provide new insights into microplastic concentrations in offshore regions, highlight the significant degradation of plastic debris, and emphasize the need for further research to identify and map the Indian Ocean's garbage patch along these latitudes. Keys words: Indian Ocean, Marine litter, Visual survey, Manta trawling, Microplastics.

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Conflict of interest statement

Declarations. Competing interests: M.T., L.L., M.E., S.J.R. are employed by The Ocean Cleanup, a non-for-profit developing and scaling technologies to rid the oceans of plastics, headquartered in the Netherlands. A.F., A.R., A.S., G.F., M.C., P.M.,A.B., M.A., S.J., T.M., P.J., A.T.H., M.L.C declare no competing interests. All the remaining authors declare no competing interests.

Figures

Fig. 1
Fig. 1
Map of the Southwest Indian Ocean showing the different field trips for plastic collection at sea and on beaches. Circles correspond to manta trawl samples (N = 204). The blue line corresponds to visual surveys (N = 1,884 h effort). All information about oceanographic campaigns and programs are included in the Supplementary information Tables S1 and Figure S1.
Fig. 2
Fig. 2
Composition of marine litter observed from visual survey (A) Percentage abundance of marine litter by category, (B) percentage abundance of hard plastic subcategory.
Fig. 3
Fig. 3
Percentage abundance (%) of items collecte by manta net by (A) polymers, (B) types and (C) size classes.
Fig. 4
Fig. 4
The concentration of plastic debris by abundance recorded from (A) manta trawl sampling (size class: 500 μm − 5 mm) and (B) visual surveys (size class: 2.5–100 cm). *The South Indian Subtropical Gyre boundary is adapted from Lebreton.
See this image and copyright information in PMC

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