Plastic debris (> 500µm) concentration gradient detected across the Southwest Indian Ocean

Plastic debris (> 500µm) concentration gradient detected across the Southwest Indian Ocean | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Plastic debris (> 500µm) concentration gradient detected across the Southwest Indian Ocean Margot Thibault, Adrian Fajeau, Aina Ramanampananjy, Sarah-Jeanne Royer, and 14 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4982071/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 01 Jul, 2025 Read the published version in Scientific Reports → Version 1 posted 15 You are reading this latest preprint version Abstract Marine plastic pollution is increasing. The Indian Ocean is understudied compared to the Pacific and Atlantic Oceans. This study investigates plastic pollution in the Southwest Indian Ocean using a multi-faceted approach that includes both floating (visual survey and manta trawls) and beach-collected plastics, assessing their concentration, composition, and origin. Through 19 oceanographic campaigns and 153 uninhabited beach surveys, a total of 101,055 pieces of marine litter were identified, with 95% being plastics. Floating macroplastics were predominantly found near remote island waters, particularly at Glorieuses (10 3 items.km -2 ). Meanwhile, an increasing gradient of floating microplastic concentrations was observed from 40°E (10 3 items.km -2 ) to 65°E (10 5 items.km -2 ) along 30°/33°S. High concentration of beached macroplastics where observed on the east coast of Madagascar and Tromelin. Mesoplastics were more abundant than macroplastics, on remote islands. Floating and beached plastic debris were mainly hard fragments, mostly made of polyethylene (floating, beached: 72%, 57%) or polypropylene (26%, 34%). The majority of macroplastics identified in the brand audit, was mainly mineral water food packaging (81%) from Southeast Asian manufacturers. Our results will inform national management and provide evidence to support international plastic treaty negotiations on legacy plastics. Physical sciences/Chemistry/Polymer chemistry/Polymer characterization Earth and environmental sciences/Environmental sciences/Environmental impact Earth and environmental sciences/Ocean sciences/Marine chemistry Indian Ocean plastic debris concentration origin manta trawling beaches Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction There is growing evidence of the accumulation of plastics in marine and coastal ecosystems 1 , 2 , 3 , 4 , 5 . This accumulation is impacting wildlife and humans but is costly to remediate. While a global plastics treaty is currently being negotiated with the aim of ending plastic pollution, the ongoing production of plastics and the resulting amount entering the ocean means that there is an urgent need to locate accumulation zones. These zones can be targeted for ocean and coastal clean-up, maximizing the use of limited resources. Plastic debris in the Indian Ocean, especially the Southwest Indian Ocean has been under-sampled and under-studied compared to other oceans, despite half of the top 10 countries contributing most to ocean plastic pollution being located along the Indian Ocean rim 6 . Locating accumulation zones in this region, will inform national management and provide evidence to support international plastic treaty negotiations on legacy plastics 7 . Floating plastics can be transported over long distances by wind, ocean currents 8 and accumulate along coastlines or in subtropical gyres where they form the so-called ‘garbage patches’. Five ‘garbage patches’ have been identified in the global ocean: two in the Pacific (North 9 – 11 ; South 12 ), two in the Atlantic (North 13 , 14 ; South 15 – 17 ), and one in the Southern Indian Ocean 8 , 18 – 20 . The exact location of the latter is still debated. Some studies place it in the southwestern part of the Indian Ocean 21 – 23 , while others place it on the southeastern side of the basin 4 , 19 , 24 , 25 and one model in the central part of the basin 26 . The Indian Ocean ‘garbage patch’ is predicted by numerical models to be the second largest accumulation of floating plastics in the ocean 22 but direct observations are limited in this region. Only four oceanographic campaigns have been conducted in the Southern Indian Ocean to sample floating plastic debris 18 , 19 , 27 , 28 . The greater sampling effort has focused on beached plastic debris, with data indicating Southeast Asia as potential main origin 5 , 29 – 36 . With only a handful of pelagic field observation campaigns, very little is known about the concentration, location, composition, and distribution of floating plastic pollution accumulation in this part of the ocean. In this study, we conducted an extensive in situ sampling all around the Southwest Indian Ocean. Our aim was to cover a large geographical area in order to complete previous studies and gain a deeper understanding of the concentration, composition, origin of the plastic accumulation zones in the Southwest Indian Ocean. Our specific objectives were (i) to estimate the concentration of plastic debris (macro-meso-micro) using different in situ samplings: visual survey, manta trawling deployment, and beach surveys, (ii) to characterize the composition of plastic debris in terms of shape, size class, polymer, and (iii) to determine the origin of macroplastics beached on uninhabited remote islands through a “brand audit” method. By strengthening our understanding of accumulation zones we can, with more confidence, allocate resources for ocean and coastal clean-up activities. Methods Study site. This research utilizes data collected during 19 oceanographic campaigns that have been conducted in the Southwest Indian Ocean (SWIO, 4°/40°S − 38°/82°E, Fig. 1 ) between 2019 and 2023. During these campaigns, data on the concentration of plastic debris at the ocean surface was collected using visual surveys and manta trawls. In addition, nine beaches were surveyed to record the number of items of beached marine litter collected. Campaign details, such as date, location, type of vessel, are listed in Supplementary Tables S1, S2, and Figures S1 , S2. Sea surface surveys. Before each offshore campaign, we conducted training sessions for each observer on board the vessel, focusing on the identification of floating plastic debris at the sea surface. Observations were made during daylight hours while the vessel was moving at constant speed 27 , 37 – 40 . At least two observers were situated on the bridge to allow a visual survey area of 180 degrees. Observers used both naked eye and binoculars to make observations. There was no standard survey duration or distance but observers recorded start/end GPS coordinates for the starting and ending points of the survey, duration of the survey, vessel speed, platform height, number of observers, and environmental parameters (sea state, wave height, and weather). During these surveys, observers counted plastic debris larger than 2.5 cm up to 1 meter (referred to as macroplastics 41 ) and those exceeding 1 meter (referred to as megaplastics 41 ). Beach surveys. Each of the nine beaches we surveyed met the following criteria: (i) open to the ocean (without coral barrier); (ii) moderate slope; (iii) low granulometry; (iv) with no beach cleanup activities; (v) long distance (> 10 km) from city or harbor, (vi) with a maximum of 100 habitants living around the beach. Macrolitter items were collected along a transect parallel to the coastline on all beaches from the vegetation supralittoral to the sea (length size transect by beach is written in Supplementary Table S2 ). Each macrolitter was counted, measured, and weighted by category used by each program according to Barnardo & Ribbink (2020) 42 (e.i. ceramic, clothing, paper/cartoon, glass, metal, rubber, wood, plastic, fishing items, personal care, other, wood). Mesolitter items (5 mm − 2.5 cm) were sampled on only four beaches i.e. Ampanihy, Ampahiry, Juan de Nova, and Lys. Items were collected on the surface of the sand using a sieve (size mesh: 3 mm) in 50cm wide transects stretching from the sea to the vegetation To ensure optimal information and comparability in this study, we meticulously collected all raw data from each program 10 , 41 , 42 using different list characterization to then create a unified list for comparison for this paper (Supplementary Table S3). Brand audit. The identification of the origin of macrolitter was undertaken by the “brand audit” method 42 for all beaches. When a brand was visible or incrusted, we recorded for each identifiable item: the manufacturer, the type of product (e.g., food packaging, household product, personal care product), the subtype of the product (e.g., drink, mineral water, oil container), the written polymer code (e.g., high-density polyethylene, HDPE; polyethylene terephthalate, PET; polystyrene, PS). We used online investigations 29 , 43 to supplement and fact-check our data. First, we searched by using different search engines (e.g., Google, Yahoo, Ecosia) with different search setting language (e.g: Indonesian, Chinese, Japanese, Korean, Thai …) when it was identifiable 35 . Once the website of the company was found, we noted all information about production and exportation areas. The category “International” was created when the country could not easily be identified, mostly for brands established and sold internationally (e.g: The Coca-Cola Company®). This analysis provided a first-order map for the origin of the macroplastics we collected on these remote beaches. Manta trawl sampling. On board, the manta net was deployed at > 30 meters behind the vessel to avoid the vessel's wake; it collected plastic debris at the sea surface 11 (all information about dimensions of nets and flowmeters are given in Supplementary Table S1 ). At each site, three consecutive 30-minute transects were conducted at a speed of 2 knots using an individual single-use cod-end for each transect. Between transects, the net was rinsed with seawater on the outside of the net to move any missed plastic debris towards the cod-end. The cod-end was then removed, placed into an annotated Ziplock bag (date, mission, cod-end identification number), and stored in a freezer until transferred for analysis in the laboratory ashore. For the next deployment, a new code-end was placed. For each manta trawling, the following environmental parameters were recorded: wave height (m), wind speed (m.s - 1 ), atmospheric pressure (hPa), year, month, season (Dec-Feb: Wet season; Mar-May: Interseason1; Jun-Aug: Dry season; Sept-Nov: Interseason2), surface area of sampling (km - 2 , flowmeters, gps points) and vessels characteristics. The manta net was not deployed if wave height exceeded 2 meters. Once at the laboratory, each manta trawl cod-end was externally rinsed to deposit all the content on a sieve (500 µm). Under light and a magnifying glass, all plastic debris were collected with ultra-fine tweezers (300 µm point diameter) and placed in a Petri dish until analysis and characterization. When all plastic debris were placed on the Petri dish, an image was taken with a camera and used to count the number of items (Nikon D7500 - lens: AF-S MICRO NIKKOR 105 mm). Then we attributed were determined for each item: (i) shape (hard plastic, foam, pellet, fiber); (ii) dry weight (10 − 5 g precision balance); (iii) size (ImageJ software 1.5K 44 ). The concentration of plastic debris was calculated by incorporating the effect of wind mixing into the calculation of the concentration of plastic debris at the sea surface 45 : $$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:{C}_{i}=\:\frac{{c}_{s}}{1-{e}^{-d{W}_{b}(1.5\sqrt{\frac{{\rho\:}_{a}}{{\rho\:}_{w}}{C}_{d}{U}^{2}},\:\:k,\:\:\frac{0.96}{g},\:\:{\sigma\:}^{\frac{3}{2}},\:{C}_{d},\:{U}^{2}{)}^{-1}}}$$ 1 Where C i is the depth-integrated concentration for the upper 5 m of the water column (item/km − 2 ), C s corresponds to the raw concentration of plastic debris type and size class as measured in the laboratory linked with the sampling surface area (km − 2 ), d is the depth of the manta net, W b is the rising velocity by plastic type and size (m/s determined by Lebreton et al. 11 ), ρ a is the air density (kg.m − 3 ), ρ w is the seawater density (kg.m − 3 ), C d is the drag coefficient (0.0012), U is the sea surface wind speed during sampling (m.s − 1 ), k is the Karman constant (0.4), g is the gravitational constant (9.81 m.s − 2 ) and σ is the wave age equal to the constant 35. Infrared spectroscopy ATR-FTIR. For each plastic debris collected from manta sampling and mesolitter from beach surveys, we determined the polymer type of each item using Fourier Transform InfraRed (FTIR) spectroscopy at the UMR Softmat laboratory, in University Paul Sabatier II, France. This was done with a Thermo Nicolet Nexus 6700 instrument equipped with a diamond crystal Attenuated Total Reflection (ATR) mode and a deuterated triglycine sulphate detector. During the analysis, white background and sample spectra were obtained using 16 scans covering the wavelength range of 400 cm⁻¹ to 4,000 cm⁻¹ with a resolution of 4 cm⁻¹. A white background spectrum was taken every 2 hours to ensure accuracy. Each piece of plastic debris was pressed between the diamond crystal and the base. The diamond crystal was cleaned between each measurement to avoid any bias between spectra. The obtained spectra were corrected using the ATR thermo-correction method to obtain transmission-like spectra 46 . The final infrared spectra were observed using the Omnic software (version 9.9.0.473). Only spectra with more than 80% similarity to one of the spectra in the spectra database created by the laboratory SOFTMAT at the University of Toulouse Paul Sabatier, were validated. In the cases where the similarity was less than 80%, the assignment to a specific polymer type was not made to avoid identification errors. To provide additional information on the level of degradation of plastic debris, a carbonyl index was determined for polyethylene (PE) and polypropylene (PP) polymer particles. These particles were matched with a similarity of over 80%. For the PE carbonyl index, the ratio between the integrated absorbance of the carbonyl peak in the range of 1,850 cm - 1 to 1,650 cm - 1 and the methylene scissoring peak in the range of 1,500 cm - 1 to 1,420 cm - 1 was calculated. The Specified Area Under these two bands (SAUB) provided by Almond et al. 47 was calculated using Omnic software with the options band analysis tool. For the PP carbonyl index, the ratio between the peak height at 1,715 cm - 1 and the area under the band in the range of 1,500 cm - 1 to 1,450 cm - 1 was also calculated. The area under the band and peak height were calculated by the same software using the options peak analysis tool. For each measurement, a flat baseline was applied using the data between 4,000 cm - 1 and 2,000 cm - 1 . Statistical analysis. The normality and homoscedasticity of our data were tested by Shapiro and Levene tests, respectively. Linear models (LM) were performed to explain the fluctuation in abundance of plastic debris collected by manta nets (Y variable) by these explicative variables (X n ): latitudes, longitudes, sea states and seasons (Dec-Feb: wet; Mar-May: Interseason1; Jun-Aug: Dry; Sep-Nov: Interseason2). LM were also used to explain the concentration observed by visual surveys by these explicative variables: latitude, longitude, year, season, platform height (m, 0–3; 3–6; 6–9; 9–12), cloud coverage (%, 0, 25, 50, 75, 100), sea state, and effort. When, we had replicated data from beaches by season and year, LM were conducted on plastic debris abundance beached on remote islands and investigated the influence of the following variables: site, year, and season. For each LM, we tested the normality and selected models with Akaïke's Information Criteria adjusted for small sample sizes (AICc). All statistical tests were conducted in the R computing environment (R Core Team, 2023). Results Overall. A total of 101,055 marine debris items were recorded, from manta trawling (n = 2,780 items; > 500µm), sea surface survey (n = 8,655 items; > 2.5 cm), and beach monitoring (n = 89,620 items; > 5 mm). Sea surface plastic concentration. Among the 220 manta trawl samples, 97% contained plastic debris. The highest concentration of sea surface microplastics was 1,194,230 items.km − 2 , recorded northwest of Reunion Island from the IOTA program. The lowest was recorded on Aldabra 9 items.km − 2 . (Fig. 2 A). Overall, the microplastic concentration exhibits a noticeable increasing longitudinal gradient along latitude 30°/33°S, ranging from 10 3 items.km − 2 at 40°E to 10 5 items.km − 2 at 65°E. The season of Interseason2 and sea state calm also had an impact on the high concentration of microplastic (N = 220, LM, p-values < 0.001, Supplementary Table S4, Figure S3.). During the 1,884 hours of sea surface survey from different oceanographic campaigns, 8,655 pieces of marine litter were observed, of which 97% were plastic debris. Macroplastics concentration (mean ± sd) was 90.8 ± 411 items.km − 2 (Fig. 2 B). The maximum concentration, reaching 4,585 items.km − 2 , was observed around Mayotte and Glorieuses Islands. A gradient was also observed from the west (10 0 items.km − 2 ) to the east (10 2 items.km − 2 ) on the latitude 30/33°S for macroplastics. The average abundance at 32°S latitudes was 40 ± 869 items.km − 2 (max: 237 items.km − 2 , 32°S and 60°E). However, LM did not reveal a significant explanatory factor to explain the visual survey concentration (N = 236, LM, p-value > 0.05, Supplementary Table S4, Figure S1 ). Beached plastic concentration. Since 2019, a total of 89,620 pieces of marine litter have been collected, of which 95% were identified as made of plastic. The concentrations of macroplastics were particularly higher for: Tromelin with 11.7 ± 16.8 items.m − 1 , Juan de Nova with 7.8 ± 0.2 items.m − 1 , and Ampanihy with 5.84 ± 5.74 items.m − 1 (Fig. 3 A, Supplementary Table S5). Furthermore, the highest concentrations of mesoplastics were mainly observed on Ampanihy with 140 ± 89 items.m − 1 , Ampahiry 62 ± 59 items.m − 1 , and Juan de Nova with 92.3 ± 13.6 items.m − 1 (Figure.3B). LM indicate no influence of the season on the concentrations of beached plastic debris, however a trend to a high concentration was observed during dry season (N = 153, LM > 0.05 Supplementary Figure S4, Table S4). Composition of marine litter and polymer identification. Hard plastic was the most frequently recorded marine debris category in all surveys with averages of 70%, 77% and 85% for sea surface surveys, beach surveys and manta trawling, respectively (LM, p-value < 0.001; Supplementary Table S4). Fishing gear (line, rope, bucket, drum), foam, and soft materials (film, sheet) constituted the second most common category in terms of floating and beached marine litter, while glass, ceramic, and healthcare-related items were generally less frequently collected (Fig. 4 A, Supplementary Table S3 for list of categories). The category, “Hard Plastics” was predominantly composed of “small fragments under 50 cm”, specifically 82 ± 17% for floating plastic debris and 28 ± 30% for beached plastic debris (Fig. 4 B). The second significant subcategory observed with surveys at sea consisted of “PET bottles”, followed by “large fragments exceeding 50 cm”. However, on beaches, the second most represented subcategory was “bottle caps”. Microplastics collected by manta sampling were composed of hard plastic (77%), then fiber (21%) and a few items were foam (2%) or pellet (1%, (LM, p.value < 0.01, Supplementary Table S4). Out of the total of 101,055 collected and observed items, we specifically selected all the mesoplastics, beached on remote islands (N = 106 beached items), and all microplastics collected with manta trawl nets (N = 1,175 floating items) to analyse using ATR FTIR. In the end, a total of 1,281 items were analyzed using ATR FTIR, and 82% (N = 960 floating; N = 87 beached) were successfully matched in the database. Floating microplastics and beached mesoplastics debris were predominantly composed of PE (72% of floating; 57% of beached) followed by PP (26% of floating; 34% of beached) of various shapes and size classes. Foam-shaped plastics, for both floating and beached debris, exhibited a diverse range of polymers. Plastic debris found floating inshore ( 12 miles) debris was composed of 75% PE, and 23% PP and 2% (PVA, PES, PS). There was no significant difference in polymer composition among different islands and distance from the coastline. PE and PP carbonyl index showed no significant difference between onshore and offshore samples. Nevertheless, there was a noticeable trend indicating increased degradation of PP with distance from the coast towards offshore (Supplementary Figure S5, Table S6). Origins of beached macroplastics. Among 89,620 items, we counted 2,787 macroplastics with identified brands, beached on remote islands in the SWIO. In total, we listed 282 different brands. The most recognized brand was Aqua® from Danone®, accounting for 39% of identified markings, followed by a product of The Coca-Cola Company® in the 2nd place (6%), and Sungreen® from Thailand ranking 3rd (4%, Table 1). The identified brands originated primarily from Southeast Asia, accounting for more than 50% of identified origins on items. Additionally, 21% of brands were involved in international trade for exportation and importation. The East Africa sub-region appeared as the second main origin of beached macroplastics on uninhabited remote islands, accounting for 1–9% of the total (Figure. 5A). Among all brands, 76 (27%) represented “food packaging products” and concerned mainly “mineral water bottle brands”, predominantly identified by caps (N = 1,464 made of HDPE) and bottles (N = 20 made of PET, Figure. 5B, Supplementary Table S7_ List of the 282 brands). Discussion Our results confirm the problem of sea surface and beached plastic pollution in the Southwest Indian Ocean. Our key finding was an increasing concentration gradient of plastic debris, mainly composed of hard fragments, on the sea surface at latitude 30°/33°S from 40°E to 65°E, reaching up to 10 5 items.km − 2 (500 µm − 5 mm). In this gradient, the concentrations are higher than those found in the South Atlantic (10 4 items.km − 2 ), but lower than those in the North Pacific garbage patches (> 10 6 items.km − 2 ) 10 , 17 . This result indicates a large amount of plastic debris is entering the Indian Ocean and circulates towards the subtropical latitudes. Locally, the important amount of microplastics located on the Northwest of Reunion Island reaching 10 6 items.km − 2 , which could be the result of mesoscale anticyclonic eddies created by the island mass effect 48 , 49 . Consequently, high concentrations of microplastics in pelagic or coastal regions of the Southwest Indian Ocean have impact on marine ecosystems. It can increase the likelihood of ingestion by marine fauna, ranging from zooplankton to megafauna species, through bioaccumulation, leading to the development of diseases 35 , 50 , 51 . Further sampling is required, in the middle and eastern parts of the Indian Ocean, to observe the gradient of floating plastic concentration identified in this study, across different seasons and understand the long-term impact on marine fauna. Macroplastics beached on the eastern side of Madagascar (Ampanihy, Ampahiry, Tromelin), mostly came from Southeast Asia. The South equatorial current, monsoons, and inadequate waste management in this region contribute to the influx of plastic debris towards islands of the Southwest Indian Ocean like the Seychelles 29 , Saint Brandon 30 , and the Maldives 52 , 53 . In the Mozambique Channel, our study recorded more plastic debris beached on Juan de Nova than on Europa. However, among the 1,000 top most plastic-emitting rivers, two are located in Mozambique and one in Kenya 54 . In the Mozambique Channel, mesoscale eddies 55 can carry floating plastic debris to remote islands such as Juan de Nova and Europa 56 . In our study, mesoplastics were more abundant than macroplastics on the beaches, however our concentrations may be underestimated, because plastic debris can be buried in the sand 43 , 57 or remain trapped in the coral reefs 58 . As these uninhabited islands are protected areas with biodiversity hotspots, species can directly interact with beached plastic, impacting their life 34 . Identifying the use of plastic pollution on these remote uninhabited islands can be crucial to reducing future marine debris deposition. The “brand audit” in our study, indicated that macroplastics collected on the remote beaches located both in the east and west of Madagascar, mainly originated from Southeast Asia (more than 50%) and were primarily associated with “food packaging”, specifically branded mineral water bottles identifiable by caps or bottles, with Aqua ® from Danone ® being the most prevalent. This brand represents the first mineral water sold by Danone ® in Indonesia 59 , 60 . Similar observations were done in various studies on remote islands such as the Seychelles 29 , 61 , Saint Brandon 30 , and countries including South Africa 62 and Madagascar 35 . While this study sheds further lights on the origin and extent of floating plastic pollution in the Southwest Indian Ocean, its long tem fate remains unclear. Models of plastic debris dispersion predict a residence time of around 50 years, with plastic debris gradually moving towards the South Atlantic and a total disappearance of the predicted accumulation after 100 years 23 , 63 . Systematic collection of plastic debris in both the South Indian and South Atlantic Oceans, along with the identification of their origins and age, could help verify these model predictions and provide a better understanding of the long-term fate and persistence of the South Indian garbage patch. Declarations Acknowledgments We would like to express our gratitude to the donors of The Ocean Cleanup for funding the doctoral project and the Western Indian Ocean Marine Science Association for the Marine Litter Monitoring Project in Madagascar (MALIMO). We also thank the French government (DEAL) for the DEMARRE project, for their support under the 2019-2022 Convergence, Transformation Contract (CCT) and by Région Réunion. We thank the European Regional Development Fund and Region Reunion for OSIRIS campaign precisely for COMBAVA and PRIMO projects. Our sincere appreciation goes to the Sea Sustainable Trust for providing training in South Africa to monitor plastic debris. We extend our thanks to all the volunteers and organizations who have contributed to the expeditions cruises and beach monitoring project: Madagascar, MALIMO project (Cetamada association, Alida Tihelle, Givio Mareva, Jonesse Lydivano, Antonio Fotaka, Angelico Monrose, Aquatic Service ONG); Terres australes et antarctiques françaises (all volunteers between August 2020 and July 2023: Michaël Arlandis, Marie-France Bernard, Sophie Bertrand, Audrey Cartraud, Clément Clasquin, Marine Delmas, Quentin d’Orchymont, Florian Falaise, Antoine Goguelat, Raphaël Gouyet, Nicolas Guillerault, Rémi Joly, Bérenger Laurent, Camille Legrand, Cédric Roy and Robin Thibault); Reunion Island, IOTA, PLAST, DEMARRE, projects (BESTRUN association, Christopher Graziano, Valentin Lauféron, Daniel Rasbash, Loïc Sabadadichetty, Captain Michel Guillemard of the Lys vessels from "Travaux sous Marins de l’Océan Indien", Master students from BESTALI 2022-2023 at the University of Reunion Island, Lisa Rolland and Amanda Lejeune, Ifremer); offshore expeditions: MADCAPS project ( S.A Aghullas II vessel, Exploration de Monaco, April Burt from Seychelles Island Foundation, Chloé Thibault), SIOM1 (Antsiva shooner, Captain Nicolas Tisné), Ecole Bleue Outre Mer 2022 ( Marion Dufresne II, Ifremer); We are also grateful to GLOBICE association and Directive Marine South Indian Ocean (DMSOI) for their commitment to monitoring plastic observation from OSIRIS II vessel patrols and volunteers (Paul Lallement, Alain Dubois (MMCO), Emmanuelle Leroy, Michael Mwang’ombe (Watamu Marine Association), Tristan Simille (Goodbye Plastic), Ina Madler, Vincent Quinquempois, Rémi Trimouille, Lana Barteneva (MMCO), Bernard Rota, Erwan Bailby (Cetamada), and Marine Malen). Thank you very much to Lisa Weiss for her review in improving the paper. Authors contributions M.T. and L.L. designed the study; M.T., A.F., A.R., A.S., G.F., M.C., P.M., M.A., S.J., P.J., S.J.R., M.E., A.T.H., M.L.C., L.L managed expeditions cruises or beach monitoring or directly facilitated the expeditions; M.T., A.F., A.R., G.F., V.M., M.C., T.M., S.J collected the samples; M.T., A.R., M.C., V.M., J.G., analysed the samples at laboratory; M.T. conducted the data analyses and the calculations, and prepared figures and tables; M.T. wrote the manuscript. All authors reviewed and edited the manuscript. Funding Part of this study is funded under Western Indian Ocean Marine Scientific Association (WIOMSA) Marine Litter Monitoring Project, as well as by the donors of The Ocean Cleanup. The DEMARRE project is supported by the French government (DEAL) under the 2019-2022 Convergence and Transformation Contract (CCT) and by Region Reunion. OSIRIS campaigns from 1-5 was funded by European Regional Development Fund and Region Reunion for COMBAVA project. OSIRIS campaigns from 6-9 was funded by European Regional Development Fund and Region Reunion for PRIMO project 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., 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 interest. Additional information Supplementary information: Tables S1, S2, S3, S4, S5, S6, S7 and Figures S1, S2, S3, S4, S5 Data availability The datasets generated during manta trawling are available in the [Raw data of plastic debris collected in the Southwest Indian Ocean by manta trawling] repository, [10.6084/m9.figshare.26828164]. 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A first assessment of marine meso-litter and microplastics on beaches: Where does Mauritius stand? Mar. Pollut. Bull. 173 , 112941 (2021). Mulochau, T., Lelabousse, C. & Séré, M. Estimations of densities of marine litter on the fringing reefs of Mayotte (France – South Western Indian Ocean) - impacts on coral communities. Mar. Pollut. Bull. 160 , 111643 (2020). Andika, A. & Mandang, E. Pengaruh Komuniukasi Pemasaran Dan Promosi Harga Terhadap Variabel Ekuitas Merek Air Minum Kemasan Danone-Aqua. (2004). Achmad, A. et al. Pengaturan kegiantan industri amdk (air minum dalam kemasan) oleh pt aqua danone di kabupaten klaten jawa tengah. Diponegoro law J. 5 , 1–10 (2016). Dunlop, S. W., Dunlop, B. J. & Brown, M. Plastic pollution in paradise: Daily accumulation rates of marine litter on Cousine Island, Seychelles. Mar. Pollut. Bull. 110803 (2019) doi:10.1016/j.marpolbul.2019.110803. Ryan, P. G., Weideman, E. A., Perold, V., Hofmeyr, G. & Connan, M. Message in a bottle: Assessing the sources and origins of beach litter to tackle marine pollution. Environ. Pollut. 288 , (2021). Van Sebille, E., England, M. H. & Froyland, G. Origin, dynamics and evolution of ocean garbage patches from observed surface drifters. Environ. Res. Lett. 7 , (2012). Additional Declarations Competing interest reported. 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., 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. Supplementary Files SupplementaryDataFigureS1S5.docx SupplementaryinformationTableS1S7.xlsx Table1.xlsx Cite Share Download PDF Status: Published Journal Publication published 01 Jul, 2025 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 15 Nov, 2024 Reviews received at journal 21 Oct, 2024 Reviews received at journal 23 Sep, 2024 Reviews received at journal 20 Sep, 2024 Reviews received at journal 19 Sep, 2024 Reviewers agreed at journal 15 Sep, 2024 Reviewers agreed at journal 14 Sep, 2024 Reviewers agreed at journal 12 Sep, 2024 Reviewers agreed at journal 12 Sep, 2024 Reviewers agreed at journal 12 Sep, 2024 Reviewers invited by journal 12 Sep, 2024 Editor assigned by journal 07 Sep, 2024 Editor invited by journal 05 Sep, 2024 Submission checks completed at journal 04 Sep, 2024 First submitted to journal 27 Aug, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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1","display":"","copyAsset":false,"role":"figure","size":320209,"visible":true,"origin":"","legend":"\u003cp\u003eMap 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=220). The blue line corresponds to visual surveys (N=1,884 hours effort). Orange triangles correspond to beach monitoring (N=153 beach surveys). All information about oceanographic campaigns and programs are included in the Supplementary information Tables S1, S2 and Figure S1 and S2.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4982071/v1/59f5ab4fbb9261b65857ace7.png"},{"id":66047915,"identity":"7500b922-8bdf-4059-8710-ba4fb653e236","added_by":"auto","created_at":"2024-10-07 07:29:21","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":158423,"visible":true,"origin":"","legend":"\u003cp\u003eThe 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 cm - 100 cm). *The South Indian Subtropical Gyre boundary is adapted from Lebreton\u003csup\u003e23\u003c/sup\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4982071/v1/455f7895e264b2280b8bfc13.png"},{"id":66048163,"identity":"b36b83ee-c8a8-49b5-9506-4b36757e5d31","added_by":"auto","created_at":"2024-10-07 07:37:21","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":84578,"visible":true,"origin":"","legend":"\u003cp\u003eMean concentration by beaches for A) macroplastics (\u0026gt; 2.5 cm) in items.m\u003csup\u003e-1\u003c/sup\u003e and items.m\u003csup\u003e-2\u003c/sup\u003e and B) mesoplastics (5 mm – 2.5 cm) in items.m\u003csup\u003e-1\u003c/sup\u003e and items.m\u003csup\u003e-2\u003c/sup\u003e. 1: Ampanihy (Ste Marie Island), 2: Ampahiry (Fort Dauphin),), 3: Lys, 4: Juan des Nova, 5 : Tremblet, 6 : Aldabra, 7 : Tromelin, 8 : Europa, 9 : Glorieuses (Supplementary information Figure S2).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4982071/v1/fcb8a94370c09f7b1506591d.png"},{"id":66047914,"identity":"84ed38dc-a408-4b52-aae8-a56b7b8e453e","added_by":"auto","created_at":"2024-10-07 07:29:21","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":186021,"visible":true,"origin":"","legend":"\u003cp\u003eA) Percentage abundance of marine litter by category, B) percentage abundance of hard plastic subcategory. The yellow boxplot corresponds to beach surveys and the green boxplot corresponds to visual surveys.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4982071/v1/57301b0b43f9ae9865865317.png"},{"id":66047916,"identity":"684c823f-c669-4d6e-abb8-8c0550f04c32","added_by":"auto","created_at":"2024-10-07 07:29:21","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":239621,"visible":true,"origin":"","legend":"\u003cp\u003eA) Origin of plastic debris used for the brand audit collected from beach surveys, quantified by proportion; B) Categorization of plastic debris from the brand audit (N=2787 items), a) abundance of three major subcategories in ‘household product’, b) abundance of three major subcategories in ‘personal care’, c) abundance of three major subcategories in ‘food packaging’ and d) abundance of other categories\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4982071/v1/00fbfd943627b06019edddea.png"},{"id":86178917,"identity":"24acfde1-734a-4ed3-ab79-7ee29a8e3141","added_by":"auto","created_at":"2025-07-07 16:11:14","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1806872,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4982071/v1/46fbd86d-6d2e-47b7-b26d-cbab545746cc.pdf"},{"id":66047912,"identity":"c9243cad-ea69-4ae2-96a1-18d410cf316b","added_by":"auto","created_at":"2024-10-07 07:29:21","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1266370,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryDataFigureS1S5.docx","url":"https://assets-eu.researchsquare.com/files/rs-4982071/v1/5b0741854b3530af7faf2b33.docx"},{"id":66047918,"identity":"1292cef4-81e7-45d4-894a-8acd0765795f","added_by":"auto","created_at":"2024-10-07 07:29:21","extension":"xlsx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":286228,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryinformationTableS1S7.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-4982071/v1/da25c3738060d7bff0eb210c.xlsx"},{"id":66047913,"identity":"32a55962-fa93-4159-ac00-4f351ad35ba8","added_by":"auto","created_at":"2024-10-07 07:29:21","extension":"xlsx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":12118,"visible":true,"origin":"","legend":"","description":"","filename":"Table1.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-4982071/v1/3b43db3dc03ef7cb44e46ef6.xlsx"}],"financialInterests":"Competing interest reported. 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., 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.","formattedTitle":"Plastic debris (\u003e 500µm) concentration gradient detected across the Southwest Indian Ocean","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThere is growing evidence of the accumulation of plastics in marine and coastal ecosystems\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. This accumulation is impacting wildlife and humans but is costly to remediate. While a global plastics treaty is currently being negotiated with the aim of ending plastic pollution, the ongoing production of plastics and the resulting amount entering the ocean means that there is an urgent need to locate accumulation zones. These zones can be targeted for ocean and coastal clean-up, maximizing the use of limited resources. Plastic debris in the Indian Ocean, especially the Southwest Indian Ocean has been under-sampled and under-studied compared to other oceans, despite half of the top 10 countries contributing most to ocean plastic pollution being located along the Indian Ocean rim\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. Locating accumulation zones in this region, will inform national management and provide evidence to support international plastic treaty negotiations on legacy plastics\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFloating plastics can be transported over long distances by wind, ocean currents\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e and accumulate along coastlines or in subtropical gyres where they form the so-called \u0026lsquo;garbage patches\u0026rsquo;. Five \u0026lsquo;garbage patches\u0026rsquo; have been identified in the global ocean: two in the Pacific (North\u003csup\u003e\u003cspan additionalcitationids=\"CR10\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e; South\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e), two in the Atlantic (North\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e; South\u003csup\u003e\u003cspan additionalcitationids=\"CR16\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e), and one in the Southern Indian Ocean \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. The exact location of the latter is still debated. Some studies place it in the southwestern part of the Indian Ocean \u003csup\u003e\u003cspan additionalcitationids=\"CR22\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e, while others place it on the southeastern side of the basin \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e and one model in the central part of the basin\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. The Indian Ocean \u0026lsquo;garbage patch\u0026rsquo; is predicted by numerical models to be the second largest accumulation of floating plastics in the ocean\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e but direct observations are limited in this region. Only four oceanographic campaigns have been conducted in the Southern Indian Ocean to sample floating plastic debris \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. The greater sampling effort has focused on beached plastic debris, with data indicating Southeast Asia as potential main origin \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan additionalcitationids=\"CR30 CR31 CR32 CR33 CR34 CR35\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. With only a handful of pelagic field observation campaigns, very little is known about the concentration, location, composition, and distribution of floating plastic pollution accumulation in this part of the ocean.\u003c/p\u003e \u003cp\u003eIn this study, we conducted an extensive \u003cem\u003ein situ\u003c/em\u003e sampling all around the Southwest Indian Ocean. Our aim was to cover a large geographical area in order to complete previous studies and gain a deeper understanding of the concentration, composition, origin of the plastic accumulation zones in the Southwest Indian Ocean. Our specific objectives were (i) to estimate the concentration of plastic debris (macro-meso-micro) using different \u003cem\u003ein situ\u003c/em\u003e samplings: visual survey, manta trawling deployment, and beach surveys, (ii) to characterize the composition of plastic debris in terms of shape, size class, polymer, and (iii) to determine the origin of macroplastics beached on uninhabited remote islands through a \u0026ldquo;brand audit\u0026rdquo; method. By strengthening our understanding of accumulation zones we can, with more confidence, allocate resources for ocean and coastal clean-up activities.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e \u003cb\u003eStudy site.\u003c/b\u003e This research utilizes data collected during 19 oceanographic campaigns that have been conducted in the Southwest Indian Ocean (SWIO, 4\u0026deg;/40\u0026deg;S \u0026minus;\u0026thinsp;38\u0026deg;/82\u0026deg;E, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) between 2019 and 2023. During these campaigns, data on the concentration of plastic debris at the ocean surface was collected using visual surveys and manta trawls. In addition, nine beaches were surveyed to record the number of items of beached marine litter collected. Campaign details, such as date, location, type of vessel, are listed in Supplementary Tables S1, S2, and Figures \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, S2.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eSea surface surveys.\u003c/b\u003e Before each offshore campaign, we conducted training sessions for each observer on board the vessel, focusing on the identification of floating plastic debris at the sea surface. Observations were made during daylight hours while the vessel was moving at constant speed \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan additionalcitationids=\"CR38 CR39\" citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. At least two observers were situated on the bridge to allow a visual survey area of 180 degrees. Observers used both naked eye and binoculars to make observations. There was no standard survey duration or distance but observers recorded start/end GPS coordinates for the starting and ending points of the survey, duration of the survey, vessel speed, platform height, number of observers, and environmental parameters (sea state, wave height, and weather). During these surveys, observers counted plastic debris larger than 2.5 cm up to 1 meter (referred to as macroplastics\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e) and those exceeding 1 meter (referred to as megaplastics\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e).\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003e \u003cb\u003eBeach surveys.\u003c/b\u003e Each of the nine beaches we surveyed met the following criteria: (i) open to the ocean (without coral barrier); (ii) moderate slope; (iii) low granulometry; (iv) with no beach cleanup activities; (v) long distance (\u0026gt;\u0026thinsp;10 km) from city or harbor, (vi) with a maximum of 100 habitants living around the beach. Macrolitter items were collected along a transect parallel to the coastline on all beaches from the vegetation supralittoral to the sea (length size transect by beach is written in Supplementary Table \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003e). Each macrolitter was counted, measured, and weighted by category used by each program according to Barnardo \u0026amp; Ribbink (2020)\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e (e.i. ceramic, clothing, paper/cartoon, glass, metal, rubber, wood, plastic, fishing items, personal care, other, wood). Mesolitter items (5 mm \u0026minus;\u0026thinsp;2.5 cm) were sampled on only four beaches i.e. Ampanihy, Ampahiry, Juan de Nova, and Lys. Items were collected on the surface of the sand using a sieve (size mesh: 3 mm) in 50cm wide transects stretching from the sea to the vegetation\u003c/p\u003e\u003cp\u003eTo ensure optimal information and comparability in this study, we meticulously collected all raw data from each program\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e,\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e using different list characterization to then create a unified list for comparison for this paper (Supplementary Table S3).\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003cb\u003eBrand audit.\u003c/b\u003e The identification of the origin of macrolitter was undertaken by the \u0026ldquo;brand audit\u0026rdquo; method\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e for all beaches. When a brand was visible or incrusted, we recorded for each identifiable item: the manufacturer, the type of product (e.g., food packaging, household product, personal care product), the subtype of the product (e.g., drink, mineral water, oil container), the written polymer code (e.g., high-density polyethylene, HDPE; polyethylene terephthalate, PET; polystyrene, PS). We used online investigations\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e to supplement and fact-check our data. First, we searched by using different search engines (e.g., Google, Yahoo, Ecosia) with different search setting language (e.g: Indonesian, Chinese, Japanese, Korean, Thai \u0026hellip;) when it was identifiable\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. Once the website of the company was found, we noted all information about production and exportation areas. The category \u0026ldquo;International\u0026rdquo; was created when the country could not easily be identified, mostly for brands established and sold internationally (e.g: The Coca-Cola Company\u0026reg;). This analysis provided a first-order map for the origin of the macroplastics we collected on these remote beaches.\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003e \u003cb\u003eManta trawl sampling.\u003c/b\u003e On board, the manta net was deployed at \u0026gt;\u0026thinsp;30 meters behind the vessel to avoid the vessel's wake; it collected plastic debris at the sea surface\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e (all information about dimensions of nets and flowmeters are given in Supplementary Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). At each site, three consecutive 30-minute transects were conducted at a speed of 2 knots using an individual single-use cod-end for each transect. Between transects, the net was rinsed with seawater on the outside of the net to move any missed plastic debris towards the cod-end. The cod-end was then removed, placed into an annotated Ziplock bag (date, mission, cod-end identification number), and stored in a freezer until transferred for analysis in the laboratory ashore. For the next deployment, a new code-end was placed. For each manta trawling, the following environmental parameters were recorded: wave height (m), wind speed (m.s\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e), atmospheric pressure (hPa), year, month, season (Dec-Feb: Wet season; Mar-May: Interseason1; Jun-Aug: Dry season; Sept-Nov: Interseason2), surface area of sampling (km\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, flowmeters, gps points) and vessels characteristics. The manta net was not deployed if wave height exceeded 2 meters. Once at the laboratory, each manta trawl cod-end was externally rinsed to deposit all the content on a sieve (500 \u0026micro;m). Under light and a magnifying glass, all plastic debris were collected with ultra-fine tweezers (300 \u0026micro;m point diameter) and placed in a Petri dish until analysis and characterization. When all plastic debris were placed on the Petri dish, an image was taken with a camera and used to count the number of items (Nikon D7500 - lens: AF-S MICRO NIKKOR 105 mm). Then we attributed were determined for each item: (i) shape (hard plastic, foam, pellet, fiber); (ii) dry weight (10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e g precision balance); (iii) size (ImageJ software 1.5K\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e). The concentration of plastic debris was calculated by incorporating the effect of wind mixing into the calculation of the concentration of plastic debris at the sea surface\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e:\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:{C}_{i}=\\:\\frac{{c}_{s}}{1-{e}^{-d{W}_{b}(1.5\\sqrt{\\frac{{\\rho\\:}_{a}}{{\\rho\\:}_{w}}{C}_{d}{U}^{2}},\\:\\:k,\\:\\:\\frac{0.96}{g},\\:\\:{\\sigma\\:}^{\\frac{3}{2}},\\:{C}_{d},\\:{U}^{2}{)}^{-1}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere C\u003cem\u003ei\u003c/em\u003e is the depth-integrated concentration for the upper 5 m of the water column (item/km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), C\u003cem\u003es\u003c/em\u003e corresponds to the raw concentration of plastic debris type and size class as measured in the laboratory linked with the sampling surface area (km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), d is the depth of the manta net, W\u003csub\u003e\u003cem\u003eb\u003c/em\u003e\u003c/sub\u003e is the rising velocity by plastic type and size (m/s determined by Lebreton et al.\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e), \u003cem\u003eρ\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e is the air density (kg.m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e), \u003cem\u003eρ\u003c/em\u003e\u003csub\u003e\u003cem\u003ew\u003c/em\u003e\u003c/sub\u003e is the seawater density (kg.m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e), C\u003csub\u003e\u003cem\u003ed\u003c/em\u003e\u003c/sub\u003e is the drag coefficient (0.0012), U is the sea surface wind speed during sampling (m.s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), k is the Karman constant (0.4), g is the gravitational constant (9.81 m.s\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) and σ is the wave age equal to the constant 35.\u003c/p\u003e \u003cp\u003e \u003cb\u003eInfrared spectroscopy ATR-FTIR.\u003c/b\u003e For each plastic debris collected from manta sampling and mesolitter from beach surveys, we determined the polymer type of each item using Fourier Transform InfraRed (FTIR) spectroscopy at the UMR Softmat laboratory, in University Paul Sabatier II, France. This was done with a Thermo Nicolet Nexus 6700 instrument equipped with a diamond crystal Attenuated Total Reflection (ATR) mode and a deuterated triglycine sulphate detector. During the analysis, white background and sample spectra were obtained using 16 scans covering the wavelength range of 400 cm⁻\u0026sup1; to 4,000 cm⁻\u0026sup1; with a resolution of 4 cm⁻\u0026sup1;. A white background spectrum was taken every 2 hours to ensure accuracy. Each piece of plastic debris was pressed between the diamond crystal and the base. The diamond crystal was cleaned between each measurement to avoid any bias between spectra. The obtained spectra were corrected using the ATR thermo-correction method to obtain transmission-like spectra\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. The final infrared spectra were observed using the Omnic software (version 9.9.0.473). Only spectra with more than 80% similarity to one of the spectra in the spectra database created by the laboratory SOFTMAT at the University of Toulouse Paul Sabatier, were validated. In the cases where the similarity was less than 80%, the assignment to a specific polymer type was not made to avoid identification errors.\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eTo provide additional information on the level of degradation of plastic debris, a carbonyl index was determined for polyethylene (PE) and polypropylene (PP) polymer particles. These particles were matched with a similarity of over 80%. For the PE carbonyl index, the ratio between the integrated absorbance of the carbonyl peak in the range of 1,850 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e to 1,650 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e and the methylene scissoring peak in the range of 1,500 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e to 1,420 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e was calculated. The Specified Area Under these two bands (SAUB) provided by Almond et al.\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e was calculated using Omnic software with the options band analysis tool. For the PP carbonyl index, the ratio between the peak height at 1,715 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e and the area under the band in the range of 1,500 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e to 1,450 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e was also calculated. The area under the band and peak height were calculated by the same software using the options peak analysis tool. For each measurement, a flat baseline was applied using the data between 4,000 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e and 2,000 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003cb\u003eStatistical analysis.\u003c/b\u003e The normality and homoscedasticity of our data were tested by Shapiro and Levene tests, respectively. Linear models (LM) were performed to explain the fluctuation in abundance of plastic debris collected by manta nets (Y variable) by these explicative variables (X\u003csub\u003en\u003c/sub\u003e): latitudes, longitudes, sea states and seasons (Dec-Feb: wet; Mar-May: Interseason1; Jun-Aug: Dry; Sep-Nov: Interseason2). LM were also used to explain the concentration observed by visual surveys by these explicative variables: latitude, longitude, year, season, platform height (m, 0\u0026ndash;3; 3\u0026ndash;6; 6\u0026ndash;9; 9\u0026ndash;12), cloud coverage (%, 0, 25, 50, 75, 100), sea state, and effort. When, we had replicated data from beaches by season and year, LM were conducted on plastic debris abundance beached on remote islands and investigated the influence of the following variables: site, year, and season. For each LM, we tested the normality and selected models with Aka\u0026iuml;ke's Information Criteria adjusted for small sample sizes (AICc). All statistical tests were conducted in the R computing environment (R Core Team, 2023).\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003e \u003cb\u003eOverall.\u003c/b\u003e A total of 101,055 marine debris items were recorded, from manta trawling (n\u0026thinsp;=\u0026thinsp;2,780 items; \u0026gt; 500\u0026micro;m), sea surface survey (n\u0026thinsp;=\u0026thinsp;8,655 items; \u0026gt; 2.5 cm), and beach monitoring (n\u0026thinsp;=\u0026thinsp;89,620 items; \u0026gt; 5 mm).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eSea surface plastic concentration.\u003c/b\u003e Among the 220 manta trawl samples, 97% contained plastic debris. The highest concentration of sea surface microplastics was 1,194,230 items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, recorded northwest of Reunion Island from the IOTA program. The lowest was recorded on Aldabra 9 items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA). Overall, the microplastic concentration exhibits a noticeable increasing longitudinal gradient along latitude 30\u0026deg;/33\u0026deg;S, ranging from 10\u003csup\u003e3\u003c/sup\u003e items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 40\u0026deg;E to 10\u003csup\u003e5\u003c/sup\u003e items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 65\u0026deg;E. The season of Interseason2 and sea state calm also had an impact on the high concentration of microplastic (N\u0026thinsp;=\u0026thinsp;220, LM, p-values\u0026thinsp;\u0026lt;\u0026thinsp;0.001, Supplementary Table S4, Figure S3.).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDuring the 1,884 hours of sea surface survey from different oceanographic campaigns, 8,655 pieces of marine litter were observed, of which 97% were plastic debris. Macroplastics concentration (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;sd) was 90.8\u0026thinsp;\u0026plusmn;\u0026thinsp;411 items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB). The maximum concentration, reaching 4,585 items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, was observed around Mayotte and Glorieuses Islands. A gradient was also observed from the west (10\u003csup\u003e0\u003c/sup\u003e items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) to the east (10\u003csup\u003e2\u003c/sup\u003e items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) on the latitude 30/33\u0026deg;S for macroplastics. The average abundance at 32\u0026deg;S latitudes was 40\u0026thinsp;\u0026plusmn;\u0026thinsp;869 items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (max: 237 items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, 32\u0026deg;S and 60\u0026deg;E). However, LM did not reveal a significant explanatory factor to explain the visual survey concentration (N\u0026thinsp;=\u0026thinsp;236, LM, p-value\u0026thinsp;\u0026gt;\u0026thinsp;0.05, Supplementary Table S4, Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cb\u003eBeached plastic concentration.\u003c/b\u003e Since 2019, a total of 89,620 pieces of marine litter have been collected, of which 95% were identified as made of plastic. The concentrations of macroplastics were particularly higher for: Tromelin with 11.7\u0026thinsp;\u0026plusmn;\u0026thinsp;16.8 items.m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, Juan de Nova with 7.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 items.m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and Ampanihy with 5.84\u0026thinsp;\u0026plusmn;\u0026thinsp;5.74 items.m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e(Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA, Supplementary Table S5). Furthermore, the highest concentrations of mesoplastics were mainly observed on Ampanihy with 140\u0026thinsp;\u0026plusmn;\u0026thinsp;89 items.m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, Ampahiry 62\u0026thinsp;\u0026plusmn;\u0026thinsp;59 items.m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and Juan de Nova with 92.3\u0026thinsp;\u0026plusmn;\u0026thinsp;13.6 items.m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Figure.3B). LM indicate no influence of the season on the concentrations of beached plastic debris, however a trend to a high concentration was observed during dry season (N\u0026thinsp;=\u0026thinsp;153, LM\u0026thinsp;\u0026gt;\u0026thinsp;0.05 Supplementary Figure S4, Table S4).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eComposition of marine litter and polymer identification.\u003c/b\u003e Hard plastic was the most frequently recorded marine debris category in all surveys with averages of 70%, 77% and 85% for sea surface surveys, beach surveys and manta trawling, respectively (LM, p-value\u0026thinsp;\u0026lt;\u0026thinsp;0.001; Supplementary Table S4). Fishing gear (line, rope, bucket, drum), foam, and soft materials (film, sheet) constituted the second most common category in terms of floating and beached marine litter, while glass, ceramic, and healthcare-related items were generally less frequently collected (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA, Supplementary Table S3 for list of categories). The category, \u0026ldquo;Hard Plastics\u0026rdquo; was predominantly composed of \u0026ldquo;small fragments under 50 cm\u0026rdquo;, specifically 82\u0026thinsp;\u0026plusmn;\u0026thinsp;17% for floating plastic debris and 28\u0026thinsp;\u0026plusmn;\u0026thinsp;30% for beached plastic debris (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB). The second significant subcategory observed with surveys at sea consisted of \u0026ldquo;PET bottles\u0026rdquo;, followed by \u0026ldquo;large fragments exceeding 50 cm\u0026rdquo;. However, on beaches, the second most represented subcategory was \u0026ldquo;bottle caps\u0026rdquo;. Microplastics collected by manta sampling were composed of hard plastic (77%), then fiber (21%) and a few items were foam (2%) or pellet (1%, (LM, p.value\u0026thinsp;\u0026lt;\u0026thinsp;0.01, Supplementary Table S4).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOut of the total of 101,055 collected and observed items, we specifically selected all the mesoplastics, beached on remote islands (N\u0026thinsp;=\u0026thinsp;106 beached items), and all microplastics collected with manta trawl nets (N\u0026thinsp;=\u0026thinsp;1,175 floating items) to analyse using ATR FTIR. In the end, a total of 1,281 items were analyzed using ATR FTIR, and 82% (N\u0026thinsp;=\u0026thinsp;960 floating; N\u0026thinsp;=\u0026thinsp;87 beached) were successfully matched in the database. Floating microplastics and beached mesoplastics debris were predominantly composed of PE (72% of floating; 57% of beached) followed by PP (26% of floating; 34% of beached) of various shapes and size classes. Foam-shaped plastics, for both floating and beached debris, exhibited a diverse range of polymers. Plastic debris found floating inshore (\u0026lt;\u0026thinsp;12 miles) consisted of 25% PE, 44% PP, and 31% (PVA, PES, PS), whereas offshore (\u0026gt;\u0026thinsp;12 miles) debris was composed of 75% PE, and 23% PP and 2% (PVA, PES, PS). There was no significant difference in polymer composition among different islands and distance from the coastline. PE and PP carbonyl index showed no significant difference between onshore and offshore samples. Nevertheless, there was a noticeable trend indicating increased degradation of PP with distance from the coast towards offshore (Supplementary Figure S5, Table S6).\u003c/p\u003e \u003cp\u003e \u003cb\u003eOrigins of beached macroplastics.\u003c/b\u003e Among 89,620 items, we counted 2,787 macroplastics with identified brands, beached on remote islands in the SWIO. In total, we listed 282 different brands. The most recognized brand was Aqua\u0026reg; from Danone\u0026reg;, accounting for 39% of identified markings, followed by a product of The Coca-Cola Company\u0026reg; in the 2nd place (6%), and Sungreen\u0026reg; from Thailand ranking 3rd (4%, Table\u0026nbsp;1). The identified brands originated primarily from Southeast Asia, accounting for more than 50% of identified origins on items. Additionally, 21% of brands were involved in international trade for exportation and importation. The East Africa sub-region appeared as the second main origin of beached macroplastics on uninhabited remote islands, accounting for 1\u0026ndash;9% of the total (Figure. 5A). Among all brands, 76 (27%) represented \u0026ldquo;food packaging products\u0026rdquo; and concerned mainly \u0026ldquo;mineral water bottle brands\u0026rdquo;, predominantly identified by caps (N\u0026thinsp;=\u0026thinsp;1,464 made of HDPE) and bottles (N\u0026thinsp;=\u0026thinsp;20 made of PET, Figure. 5B, Supplementary Table S7_ List of the 282 brands).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eOur results confirm the problem of sea surface and beached plastic pollution in the Southwest Indian Ocean. Our key finding was an increasing concentration gradient of plastic debris, mainly composed of hard fragments, on the sea surface at latitude 30\u0026deg;/33\u0026deg;S from 40\u0026deg;E to 65\u0026deg;E, reaching up to 10\u003csup\u003e5\u003c/sup\u003e items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (500 \u0026micro;m \u0026minus;\u0026thinsp;5 mm). In this gradient, the concentrations are higher than those found in the South Atlantic (10\u003csup\u003e4\u003c/sup\u003e items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), but lower than those in the North Pacific garbage patches (\u0026gt;\u0026thinsp;10\u003csup\u003e6\u003c/sup\u003e items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. This result indicates a large amount of plastic debris is entering the Indian Ocean and circulates towards the subtropical latitudes. Locally, the important amount of microplastics located on the Northwest of Reunion Island reaching 10\u003csup\u003e6\u003c/sup\u003e items.km\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, which could be the result of mesoscale anticyclonic eddies created by the island mass effect \u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e,\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. Consequently, high concentrations of microplastics in pelagic or coastal regions of the Southwest Indian Ocean have impact on marine ecosystems. It can increase the likelihood of ingestion by marine fauna, ranging from zooplankton to megafauna species, through bioaccumulation, leading to the development of diseases\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e,\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. Further sampling is required, in the middle and eastern parts of the Indian Ocean, to observe the gradient of floating plastic concentration identified in this study, across different seasons and understand the long-term impact on marine fauna.\u003c/p\u003e \u003cp\u003eMacroplastics beached on the eastern side of Madagascar (Ampanihy, Ampahiry, Tromelin), mostly came from Southeast Asia. The South equatorial current, monsoons, and inadequate waste management in this region contribute to the influx of plastic debris towards islands of the Southwest Indian Ocean like the Seychelles \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, Saint Brandon\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e, and the Maldives\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e,\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e. In the Mozambique Channel, our study recorded more plastic debris beached on Juan de Nova than on Europa. However, among the 1,000 top most plastic-emitting rivers, two are located in Mozambique and one in Kenya\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e. In the Mozambique Channel, mesoscale eddies\u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e can carry floating plastic debris to remote islands such as Juan de Nova and Europa\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. In our study, mesoplastics were more abundant than macroplastics on the beaches, however our concentrations may be underestimated, because plastic debris can be buried in the sand\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e,\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e or remain trapped in the coral reefs\u003csup\u003e\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e\u003c/sup\u003e. As these uninhabited islands are protected areas with biodiversity hotspots, species can directly interact with beached plastic, impacting their life\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e.\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eIdentifying the use of plastic pollution on these remote uninhabited islands can be crucial to reducing future marine debris deposition. The \u0026ldquo;brand audit\u0026rdquo; in our study, indicated that macroplastics collected on the remote beaches located both in the east and west of Madagascar, mainly originated from Southeast Asia (more than 50%) and were primarily associated with \u0026ldquo;food packaging\u0026rdquo;, specifically branded mineral water bottles identifiable by caps or bottles, with Aqua\u003csup\u003e\u0026reg;\u003c/sup\u003e from Danone\u003csup\u003e\u0026reg;\u003c/sup\u003e being the most prevalent. This brand represents the first mineral water sold by Danone\u003csup\u003e\u0026reg;\u003c/sup\u003e in Indonesia\u003csup\u003e\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e,\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e. Similar observations were done in various studies on remote islands such as the Seychelles\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e, Saint Brandon\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e, and countries including South Africa\u003csup\u003e\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e\u003c/sup\u003e and Madagascar\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eWhile this study sheds further lights on the origin and extent of floating plastic pollution in the Southwest Indian Ocean, its long tem fate remains unclear. Models of plastic debris dispersion predict a residence time of around 50 years, with plastic debris gradually moving towards the South Atlantic and a total disappearance of the predicted accumulation after 100 years\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e. Systematic collection of plastic debris in both the South Indian and South Atlantic Oceans, along with the identification of their origins and age, could help verify these model predictions and provide a better understanding of the long-term fate and persistence of the South Indian garbage patch.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe would like to express our gratitude to the donors of The Ocean Cleanup for funding the doctoral project and the Western Indian Ocean Marine Science Association for the Marine Litter Monitoring Project in Madagascar (MALIMO). We also thank the French government (DEAL) for the DEMARRE project, for their support under the 2019-2022 Convergence, Transformation Contract (CCT) and by Région Réunion. We thank the European Regional Development Fund and Region Reunion for OSIRIS campaign precisely for COMBAVA and PRIMO projects. Our sincere appreciation goes to the Sea Sustainable Trust for providing training in South Africa to monitor plastic debris. We extend our thanks to all the volunteers and organizations who have contributed to the expeditions cruises and beach monitoring project: Madagascar, MALIMO project (Cetamada association, Alida Tihelle, Givio Mareva, Jonesse Lydivano, Antonio Fotaka, Angelico Monrose, Aquatic Service ONG); Terres australes et antarctiques françaises (all volunteers between August 2020 and July 2023: Michaël Arlandis, Marie-France Bernard, Sophie Bertrand, Audrey Cartraud, Clément Clasquin, Marine Delmas, Quentin d’Orchymont, Florian Falaise, Antoine Goguelat, Raphaël Gouyet, Nicolas Guillerault, Rémi Joly, Bérenger Laurent, Camille Legrand, Cédric Roy and Robin Thibault); Reunion Island, IOTA, PLAST, DEMARRE, projects (BESTRUN association, Christopher Graziano, Valentin Lauféron, Daniel Rasbash, Loïc Sabadadichetty, Captain Michel Guillemard of the \u003cem\u003eLys\u003c/em\u003e vessels from \"Travaux sous Marins de l’Océan Indien\", Master students from BESTALI 2022-2023 at the University of Reunion Island, Lisa Rolland and Amanda Lejeune, Ifremer); offshore expeditions: MADCAPS project (\u003cem\u003eS.A Aghullas II\u003c/em\u003e vessel, Exploration de Monaco, April Burt from Seychelles Island Foundation, Chloé Thibault), SIOM1 (Antsiva shooner, Captain Nicolas Tisné), Ecole Bleue Outre Mer 2022 (\u003cem\u003eMarion Dufresne II,\u0026nbsp;\u003c/em\u003eIfremer); We are also grateful to GLOBICE association and Directive Marine South Indian Ocean (DMSOI) for their commitment to monitoring plastic observation from \u003cem\u003eOSIRIS II\u003c/em\u003e vessel patrols and volunteers (Paul Lallement, Alain Dubois (MMCO), Emmanuelle Leroy, Michael Mwang’ombe (Watamu Marine Association), Tristan Simille (Goodbye Plastic), Ina Madler, Vincent Quinquempois, Rémi Trimouille, Lana Barteneva (MMCO), Bernard Rota, Erwan Bailby (Cetamada), and Marine Malen). Thank you very much to Lisa Weiss for her review in improving the paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eM.T. and L.L. designed the study; M.T., A.F., A.R., A.S., G.F., M.C., P.M., M.A., S.J., P.J., S.J.R., M.E., A.T.H., M.L.C., L.L managed expeditions cruises or beach monitoring or directly facilitated the expeditions; M.T., A.F., A.R., G.F., V.M., M.C., T.M., S.J collected the samples; M.T., A.R., M.C., V.M., J.G., analysed the samples at laboratory; M.T. conducted the data analyses and the calculations, and prepared figures and tables; M.T. wrote the manuscript. All authors reviewed and edited the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePart of this study is funded under Western Indian Ocean Marine Scientific Association (WIOMSA) Marine Litter Monitoring Project, as well as by the donors of The Ocean Cleanup. The DEMARRE project is supported by the French government (DEAL) under the 2019-2022 Convergence and Transformation Contract (CCT) and by Region Reunion. OSIRIS campaigns from 1-5 was funded by European Regional Development Fund and Region Reunion for COMBAVA project. OSIRIS campaigns from 6-9 was funded by European Regional Development Fund and Region Reunion for PRIMO project\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eM.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., M.A., S.J., T.M., P.J., A.T.H., M.L.C declare no competing interests.\u0026nbsp;All the remaining authors declare no competing interest. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupplementary information: Tables S1, S2, S3, S4, S5, S6, S7 and Figures S1, S2, S3, S4, S5\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets generated during manta trawling are available in the [Raw data of plastic debris collected in the Southwest Indian Ocean by manta trawling] repository, [10.6084/m9.figshare.26828164].\u003c/p\u003e\n\u003cp\u003eThe data generated for brand audit is included in this published article (and its Supplementary S7).\u003c/p\u003e\n\u003cp\u003eThe datasets concerning sea surface survey and beach monitoring are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eJambeck, J. 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Lett.\u003c/em\u003e\u003cstrong\u003e7\u003c/strong\u003e, (2012).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Indian Ocean, plastic debris, concentration, origin, manta trawling, beaches","lastPublishedDoi":"10.21203/rs.3.rs-4982071/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4982071/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMarine plastic pollution is increasing. The Indian Ocean is understudied compared to the Pacific and Atlantic Oceans. This study investigates plastic pollution in the Southwest Indian Ocean using a multi-faceted approach that includes both floating (visual survey and manta trawls) and beach-collected plastics, assessing their concentration, composition, and origin. Through 19 oceanographic campaigns and 153 uninhabited beach surveys, a total of 101,055 pieces of marine litter were identified, with 95% being plastics. Floating macroplastics were predominantly found near remote island waters, particularly at Glorieuses (10\u003csup\u003e3\u003c/sup\u003e items.km\u003csup\u003e-2\u003c/sup\u003e). Meanwhile, an increasing gradient of floating microplastic concentrations was observed from 40°E (10\u003csup\u003e3\u003c/sup\u003e items.km\u003csup\u003e-2\u003c/sup\u003e) to 65°E (10\u003csup\u003e5\u003c/sup\u003e items.km\u003csup\u003e-2\u003c/sup\u003e) along 30°/33°S. High concentration of beached macroplastics where observed on the east coast of Madagascar and Tromelin. Mesoplastics were more abundant than macroplastics, on remote islands. Floating and beached plastic debris were mainly hard fragments, mostly made of polyethylene (floating, beached: 72%, 57%) or polypropylene (26%, 34%). The majority of macroplastics identified in the brand audit, was mainly mineral water food packaging (81%) from Southeast Asian manufacturers. Our results will inform national management and provide evidence to support international plastic treaty negotiations on legacy plastics.\u003c/p\u003e","manuscriptTitle":"Plastic debris (\u0026gt; 500µm) concentration gradient detected across the Southwest Indian Ocean","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-10-07 07:29:16","doi":"10.21203/rs.3.rs-4982071/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-11-15T06:16:29+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-21T12:44:59+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-23T17:43:31+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-20T13:46:21+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-19T09:47:10+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"223284250710548436988140179338857702148","date":"2024-09-15T04:18:12+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"235077432683525943822682456363387662950","date":"2024-09-14T10:41:18+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"22633271926564551662488605153987589398","date":"2024-09-12T17:16:34+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"307352819427509885207521413968212660419","date":"2024-09-12T09:19:11+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"298836378306939099843341219156741774107","date":"2024-09-12T08:00:55+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-09-12T07:04:37+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-09-07T05:08:14+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2024-09-05T17:45:27+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-09-04T08:32:06+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2024-08-27T06:46:08+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"8495b20a-7783-4a4d-9140-e07700d3ba64","owner":[],"postedDate":"October 7th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":38098214,"name":"Physical sciences/Chemistry/Polymer chemistry/Polymer characterization"},{"id":38098215,"name":"Earth and environmental sciences/Environmental sciences/Environmental impact"},{"id":38098216,"name":"Earth and environmental sciences/Ocean sciences/Marine chemistry"}],"tags":[],"updatedAt":"2025-07-07T16:00:25+00:00","versionOfRecord":{"articleIdentity":"rs-4982071","link":"https://doi.org/10.1038/s41598-025-02893-0","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2025-07-01 15:57:11","publishedOnDateReadable":"July 1st, 2025"},"versionCreatedAt":"2024-10-07 07:29:16","video":"","vorDoi":"10.1038/s41598-025-02893-0","vorDoiUrl":"https://doi.org/10.1038/s41598-025-02893-0","workflowStages":[]},"version":"v1","identity":"rs-4982071","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4982071","identity":"rs-4982071","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}