Plastics in the Indian Ocean – sources, fate, distribution and impacts

. Plastic debris are the most common and exponentially increasing human pollutant in the world’s oceans. The distribution and impact of plastic in the Pacific and Atlantic Oceans have been the subject of many publications but not so the Indian Ocean (IO). Some of the IO rim countries have the highest population densities in the world and mis-management of plastic waste is 20 of concern in many of these rim states. Some of the highest plastic-polluted rivers end up in the IO with all this suggesting that the IO receives a tremendous amount of plastic debris each year. However, the concentration, distribution and impacts of plastics in the IO are poorly understood as the region is under-sampled compared to other oceans. In this review, we discuss sources and sinks, which are specific for the IO as well as unique atmospheric, oceanographic and topographic features of the IO such as reversing wind directions due to the monsoon, fronts and upwelling regions that control plastic distribution. We 25 identified hotspots of possible plastic accumulation in the IO, which were different in the two hemispheres. In the northern Indian Ocean, the majority of the plastic material will most likely end up being beached due to the absence of a sub-tropical gyre, whereas in the southern Indian Ocean, the garbage patch is not well defined and there may be leakage of plastics into the southern Atlantic Ocean. Hotspots of predicted plastic accumulations are identified here as well as the vast knowledge gaps about the plastic issue of the IO and point to the most striking future investigation topics. “super convergence pathway” connecting the southern IO to the South Pacific Ocean. Their results showed particles being transported eastwards, passing close by the southern Australian coastline from the southern IO into the Pacific Ocean. However, trajectories of ocean surface drifters do not confirm the existence of this super convergence pathway. van der Mheen et al. (2020c) suggested that this may 375 be due either to the lack of observed drifter locations along the southern Australian coast, or due to an under-representation of the westwards flowing Flinders Current (Middleton and Cirano, 2002; Wijeratne et al., 2018, section 4.2) in the ocean circulation model used to force particle tracking simulations by Maes et al. (2018). The existence of a super convergence exists. The understanding of the exact processes of biofouling, fragmentation, and sinking as well as the timescales on which these occur is limited. As far as we are aware, no studies have focussed on this specific to the IO. However, 400 productivity derived from satellite images revealed strong variability in biological production in the IO, and the IO is one of the most productive regions in the global oceans mainly due to intense upwelling during the southwest monsoon (Qasim, 1977). This high surface productivity results in a high export flux of organic particles from the euphotic zone to the deep sea (Ittekkot et al., 1996; Guptha et al., 1997). Organic particles attached to microplastics in combination may increase their density and thus can accelerate the transport of both to the seafloor. As a result, sinking of plastics may be very relevant in the IO. 405 Woodall et al. (2014) estimated that 4 billion fibres per km 2 were present in the IO region (seamount sediments; 500-1000m depth), and as a result suggested that the deep sea is a major sink for microplastic debris, which should also be investigated in the future. plastics are transported by ocean surface currents, Stokes drift, and wind. If plastics are transported only by ocean currents, van der Mheen et al. (2019) suggested that the garbage patch would span the entire width of the IO basin as a result of the 435 unique South Indian Counter Current (section 4.2) that flows eastwards across the entire width of the basin. It is however unknown which forcing mechanisms predominantly transport plastics. As a result, it is unclear which of these simulated garbage patches represents best the reality in the subtropical southern IO. Sampling studies in this region are too scarce to provide any conclusions (section 3) about the dimension and the dynamics of plastic amount and the time plastic is 440 contained in the subtropical IO garbage.


Introduction
Historically, the motivation for the development of synthetic materials like plastics was for the conservation of elephants that inhabit countries along the Indian Ocean (IO) rim in southern Asia and Africa. In the late 19th century, the game of billiards was extremely popular where the billiard balls were made from ivory coming from elephant tusks. This was the start of 35 elephants becoming threatened with extinction (Freinkel, 2011). A New York billiards manufacturer devised a contest in 1850's, rewarding anyone who came up with a suitable alternative material for the billiard balls (Seymour and Kauffman, 1993). John Wesley Hyatt produced the first semi-synthetic material, which he called celluloid; however, it was highly flammable and volatile and produced a minor explosion when the balls hit each other. Hyatt never received his reward, but he found several uses where celluloid could replace natural materials. Celluloid was advertised as a saviour of the environment, 40 because it would no longer be necessary to ransack the environment for scarce natural resources (Meikle, 1997). In the early 1900s, Leo Baekeland produced the first fully synthetic material, which he dubbed Bakelite (Crespy et al., 2007). Unlike celluloid, Bakelite could be moulded into any shape and thus was extremely diverse in its applications. Its invention triggered the development of many more synthetic polymers around the world. During World War II, when import and export of natural resources were limited, the military used synthetic materials for anything from tyres to parachutes (Beckman, 2018). As a 45 result, plastic production became a mass-produced product and nearly quadrupled during the war (Freinkel, 2011).
After World War II, plastic producers turned their attention to the consumer market and the production of plastic materials has increased exponentially since the 1950s (PlasticsEurope, 2019). Since a large percentage of all plastics are single use, "throwaway" packaging items, plastic waste has increased at a similar rate. Since the 1950s, the global human population has 50 generated an estimated 8300 million tonnes of plastic, of which over 75% (6300 million tonnes) has been discarded as waste (Geyer et al., 2017). Almost 80% of this plastic waste either ends up in landfill or in the environment. As a result, several million tonnes of plastic waste are estimated to enter the global oceans every year from coastlines (Jambeck et al., 2015), from inland sources transported by rivers (Lebreton et al., 2017;Schmidt et al., 2017;; or directly from ocean-based sources such as the global fishing industry (Richardson et al., 2019), offshore platforms and commercial and recreational shipping. 55 Plastic materials are durable and very slow to degrade and can persist in the marine environment for decades to centuries (Barnes et al., 2009). Around 35% of all plastic materials produced globally have densities higher than that of seawater, and thus can sink to the seafloor. The remaining 65% of plastics float in the ocean and can travel enormous distances in the ocean.
Photodegradation and other weathering processes at the sea surface lead to fragmentation, increasing micro-and nanoplastic 60 abundance (Andrady, 2011). Plastics also accumulate biofouling while in the ocean, which can change the overall density and lead to plastics moving vertically in the water column. As a result, plastics are ubiquitous throughout the marine environment and have been found on remote islands (Duhec et al., 2015;Lavers and Bond, 2017), in polar ice  https://doi.org/10.5194/os-2020-127 Preprint. Discussion started: 11 February 2021 c Author(s) 2021. CC BY 4.0 License.

Sources
Plastic waste enters the IO from coastal sources transported by wind and tides, from sources far into the hinterland transported by rivers, and directly from ocean-based sources. The Indian Ocean is surrounded by 34 countries with population densities of around 100 people per km 2 on average, with Australia the least populated country (3 people per km 2 ) and India the most populated country (400 people per km 2 ). Hoornweg and Bhada-Tata (2012) estimated that the total amount of plastic waste 100 produced by all Indian Ocean rim countries in 2010 was around 41 million tonnes. In comparison, the total amount of plastic waste produced in 2010 by the USA and China alone, is estimated to be 38 and 59 million tonnes, respectively. Despite the relatively low plastic waste production around the IO rim, a large percentage ends up in the environment because of poor waste management in most Asian and African countries. Hoornweg and Bhada-Tata (2012) estimated that around 73% of plastic waste is inadequately managed along the IO rim and is released into the environment. 105 Jambeck et al. (2015) estimated the global amount of plastic waste that entered the ocean in 2010 from coastal populations living within 50 km of the coastline. Based on a fixed percentage of mismanaged plastic waste entering the oceans (15% and 40% for the low-and high-range estimates respectively), they estimated that between 4.8 and 12.7 million tonnes of plastic entered the global oceans. Around 15% of global ocean plastic entered the IO (Figure 1a). However, there was an error in the 110 Jambeck et al. (2015) estimate of the amount of mismanaged plastic waste that enters the ocean in Sri Lanka with this error originating from the World Bank Data (Hoornweg and Bhada-Tata, 2012) which overestimated the plastic waste entering from Sri Lanka by a factor 10. We have corrected this, as shown in Figure 1a, as well as in the estimate that approximately 15% of global ocean plastic enters the IO.

Land-based sources
115 Lebreton et al. (2017) and Schmidt et al. (2017) both estimated the amount of plastic waste entering the oceans through rivers. Similar to Jambeck et al. (2015), their estimates are based on a percentage of mismanaged plastic waste but also include the influence of river catchment geography and river discharge. In addition, these estimates were calibrated based on available measurements of river plastic debris around the world, ranging between sizes of 0.3 mm to 0.5 m. Lebreton et al. (2017) estimated that between 1.15 and 2.41 million tonnes of plastic waste enter the global ocean per year, up to 20% of which enters 120 the IO (Figure 1b). Schmidt et al. (2017) estimated that between 0.47 and 2.75 million tonnes of plastic waste enter the global ocean per year, of which around 15% enters the IO. The estimates of the amount of plastic waste entering the oceans through rivers by Lebreton et al. (2017) and Schmidt et al. (2017) agree relatively well with each other. In contrast, the estimates by Jambeck et al. (2015) of the amount of plastic waste entering the oceans through coasts are an order of magnitude higher.

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The largest coastal and riverine plastic sources in the IO are in the northern hemisphere around the Bay of Bengal and on the eastern side of the Arabian Sea ( Figure 1). The input of plastic waste from rivers mainly depends on the river discharge, which https://doi.org/10.5194/os-2020-127 Preprint. Discussion started: 11 February 2021 c Author(s) 2021. CC BY 4.0 License. varies seasonally. The highest river discharges occur during the wet season, which occurs during the boreal summer in the northern IO. Lebreton et al. (2017) estimated that plastic waste input from rivers in the IO peaks in August (Figure 1c). In the southern hemisphere, the largest coastal and riverine sources of IO plastic waste are from Indonesia and eastern Africa ( Figure  130 1b).

Ocean-based sources
Plastic waste can also enter the ocean directly from ocean-based sources such as the fishing industry, commercial and recreational shipping, and offshore platforms. Although the International Convention prohibited the dumping of waste from vessels in 1988 for the Prevention of Pollution from Ships (MARPOL), accidental losses and illegal dumping still contribute 135 to plastic debris. Abandoned, Lost, or Discarded Fishing Gear (ALDFG) by the fishing industry can produce large quantities of plastic debris, i.e. monofilament lines and nets primarily made from synthetic materials (Sheavly and Register, 2007;Bond et al., 2012). Since 2000, around 13% of the global marine capture of fishes originated from the IO region (Pauly and Zeller, 2016). It is therefore likely that ALDFG is a significant source of plastic debris in the IO. Estimates of plastic waste entering the ocean from fishing vessels (e.g. ghost nets) or offshore platforms do not currently exist. 140 Finally, plastic waste originating from south-east Asia can also be transported to the IO by the Indonesian Throughflow (section 4.1). However, these sources are currently undocumented and need to be further investigated.

Observations
Plastics have been sampled in the IO in coastal waters, the open ocean, sediment, and organisms since the 1980s (Table 1)  145 with most sampling studies being conducted on beaches. However, measurements are relatively scarce, and plastics have been regularly under-sampled in the IO (Fig. 2a) compared to the Atlantic and Pacific oceans. Samples of plastic debris consists of different plastic polymers and are generally classified into different type, and size categories. Commonly used type categories are plastic fibres, fragments, films, and pellets. Size categories of marine plastic debris can range from 1 μm to 1 m or larger and are often divided into nanoplastics, microplastics, mesoplastics, macroplastics, and sometimes megaplastics (van Sebille 150 et al., 2020). Size categories as defined by GESAMP (2018;Frias and Nash, 2019) are: <0.1 mm (nanoplastics); 0.33-1.00mm (small microplastics); 1.01-4.75mm (large microplastics); 4.76-200 mm (mesoplastic); and, > 0.200 mm (macroplastics).
However, these categories can vary widely between authors (Frias and Nash, 2019) and it is beyond the scope of this review to define them all.

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The most abundant polymers identified on beaches in the IO are high-and low-density polyethylene (HDPE and LDPE, respectively); high-and low density polypropylene (HDPP and LDPP, respectively); polystyrene (PS); polypropylene (PP); https://doi.org/10.5194/os-2020-127 Preprint. Discussion started: 11 February 2021 c Author(s) 2021. CC BY 4.0 License. and polyethylene terephthalate (PET). In water and in sediment, the most abundant polymers are HDPE, LDPE, PE, PP, and PS (Sruthy and Ramasamy, 2016;Koongolla et al., 2018;Robin et al., 2020). However, all types of plastics were found in water and sediment samples (fibres, fragments, films, and pellets). The few studies undertaken on benthic invertebrates (Taylor 160 et al., 2016;Courtene-Jones et al., 2017;Thushari et al., 2017;Naidu et al., 2018;Jamieson et al., 2019) and commercial fishes (Baalkhuyur et al., 2019;Karthik et al. 2018)  Global open ocean plastic samples were standardised by van Sebille et al. (2020) and the plastic concentrations from these 165 samples in the IO can be quantitatively compared (Figure 2a). In contrast, the methods and units used in the sampling of plastics on beaches vary widely and offer only a qualitative confirmation that plastics have been found on beaches throughout the IO (Figure 2b). In general, the collection and preservation, optimised protocols for microplastic extraction (water, sediments, fauna), and identification methods (e.g. visual or analytical) vary between studies (as illustrated in Table 1) and are still under development. This therefore highlights the need for a standardised global protocol for the study of plastic debris 170 and should be a major priority in ocean plastic research going forward.

Transport
Approximately 35% of all produced plastic materials have densities higher than that of seawater (PlasticsEurope, 2019) and, therefore will sink to the seafloor after entering the ocean. To the best of our knowledge, there have been no studies in the IO regarding sinking plastics and the transport of plastics through the water column and along the seafloor. Most plastic materials 175 float in the ocean and are transported by ocean surface dynamics (currents, wind, and waves). However, buoyant plastic debris can be vertically distributed in the upper few metres of the water column because of wind-and wave-induced mixing (Kukulka et al., 2012;Kukulka and Brunner, 2015;Brunner et al., 2015). Depending on the sea state and the characteristics of the plastic debris, sampling studies in the North Atlantic Ocean found plastic debris mixed up to 5 m depth in the water column (Reisser et al., 2015;Kooi et al., 2016). In addition, the density of plastic debris can change under the influence of biofouling and 180 degradation. As a result, initially buoyant plastics may then sink. These plastics can also become de-fouled in the water column because of foraging and decreasing light intensities, which can lead to plastics oscillating in the water column (Andrady, 2011;Kooi et al., 2017). Fallouts of plastic into the water column and the deep sea have recently been detected below the subtropical garbage patch in the North Pacific Ocean (Egger et al., 2020). In the IO, mixing of buoyant plastics in the water column and sinking of initially buoyant plastics have not yet been studied. In this section, we therefore focus only on the transport of 185 buoyant plastics by ocean surface dynamics in the IO. https://doi.org/10.5194/os-2020-127 Preprint. Discussion started: 11 February 2021 c Author(s) 2021. CC BY 4.0 License.
Buoyant plastics drifting at the ocean surface are transported by different forcing mechanisms. Recent reviews by Zhang et al. (2017) and van Sebille et al. (2020) discuss these processes and their influence on the transport of plastics in detail. Ocean surface currents play a dominant role in the transport of buoyant objects but wind (Breivik and Allen, 2008) and surface gravity 190 waves (Röhrs, et al., 2012) can have a significant influence on an object's behaviour. Ocean surface currents are forced by many different mechanisms such as wind, waves, tides, and density gradients (Talley et al., 2011;van Sebille et al., 2020). In combination with the Coriolis force, these forcing mechanisms result in Ekman currents, geostrophic currents, and Stokes drift that transport plastics. In addition to the indirect influence of the wind (i.e. forcing ocean surface currents and generating waves), the wind can also have a direct influence on the transport of buoyant objects through "windage", where the wind acts 195 on an object's surface area exposed to air (Richardson, 1997). Currents, wind, and waves have large temporal and spatial variations, both horizontally and vertically. Ocean surface currents typically have a pronounced vertical profile, with current speeds decaying rapidly with depth (Laxague et al., 2017) and which can have a significant influence on the transport of buoyant objects (van der Mheen et al, 2020b). The different forcing mechanisms that transport buoyant plastics are rarely independent and interact with each other in complex ways. In addition, depending on their specific characteristics (e.g. size, 200 shape, density), buoyant objects can react differently to the same forcing conditions (Maximenko et al., 2012).

Plastic transport and accumulation along ocean fronts
Plastics entering the ocean from land-based sources are subject to many physical processes on the continental shelf before they are transported into the deeper ocean or beach on coastlines. Several studies investigating plastic debris in the IO have mainly focussed on the biology and biogeochemistry (Roy et al., 2015;Sarma et al., 2015;Sarkar et al., 2018) and only a few studies 205 have addressed the transport of plastics between inshore and offshore regions. Physical processes that lead to convergent flows are one of the most important features for the transport of buoyant plastics. Convergent flows promote downwelling causing an accumulation along the convergent flow boundary of buoyant plastic debris.
Convergent flows occur along ocean fronts, defined as the boundary between two distinct water masses or a region where the 210 rate of change of selected physical properties is much greater than the surrounding areas (Bowman and Esaias, 1977;Belkin and Cornillon, 2007;D'Asaro et al., 2018). Fronts occur from hundreds of metres to many thousand kilometres and some are short-lived, but most are quasi-stationary and emerge at the same location on seasonal time scales (Belkin and Cornillon, 2007). Ocean fronts are considered hotspots of marine life (Belkin et al., 2009) because flow convergence at fronts channelling nutrients towards the fronts and stimulating increased production at different trophic levels (Owen, 1981;Baltar et al., 2016;215 Sarma et al., 2015;Woodson and Litvinb, 2015;Sarkar et al., 2018). Aggregations of plankton, larvae, and eggs are often found on the surface. Here, as the water sinks at the front due to convergent flow buoyant material will remain at the surface.
Predators such as fish and higher order biota are found above and beneath the front. Due to the accumulation of buoyant debris, including plastics, along fronts, the risk of marine organisms interacting with plastics is high.

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There are many types of fronts that are formed through different physical processes. They include: river plumes (Luketina and Imberger 1988;O'Donnell et al., 1998;Karati et al., 2018;Cole et al., 2020); shelf-sea tidal fronts (Simpson and Hunter, 1974;Nahas et al., 2005;Sharples and Simpson, 2020); shelf break fronts (Sharples and Simpson, 2020); upwelling fronts (Brink, 1987); and, fronts formed through interaction between flow and topography (Haury and Hamner, 1986;Pattiaratchi et al., 1987). Fronts appear as a visible band along the sea surface with differences in temperature and salinity (and as a result, 225 density) on either side of the front. All these different types of fronts occur in the IO and are extremely important for the accumulation of plastics.
Buoyant plastics entering the IO from land-based sources such as rivers may accumulate at many different frontal systems before they are transported offshore. During the Southwest monsoon season (section 4.2), coastal fronts are generally formed 230 by the interaction of two different water masses moving in opposite directions, where the low saline runoff waters from rivers are trapped as a salinity front by the high saline upwelling waters. Examples of such fronts are along the southern coast of Sri Lanka (de Vos et al., 2013).As a result, buoyant plastic debris is trapped in the frontal zone along these water masses (Naidu et al., 2021 in subm.) referring to locations of future investigations. Naidu et al. (2021 in subm.) also found that the abundance of plastics, mainly fibres and fragments, were an order of magnitude higher in the frontal zone compared to outside of it off 235 the west coast of India. Similarly, Hajbane et al. (2021) found that close to an offshore reef located in the eastern IO, concentrations of plastics along a coastal front were up to two orders of magnitude higher than in surrounding waters. Frontal systems can also be eroded under storm conditions (e.g. Chen et al., 2020) and as a result, plastics accumulated along coastal fronts may beach on coastlines.

Indian Ocean surface dynamics and plastic transport pathways 240
Compared to other ocean basins, the IO has several unique topographic, atmospheric, and oceanic features that influence the transport of buoyant plastics. The IO is a unique ocean because three continents and spans temperate to sub-polar regions surround it. To the north the IO is bounded by the Indian subcontinent, to the west by Africa and the Middle East, to the east by Indonesia and Australia, and in the south by the Southern Ocean. There are two distinctive tropical basins in the northern IO: the Arabian Sea in the west and the Bay of Bengal in the east. The IO is connected to the Pacific Ocean through the 245 Indonesian Archipelago that allows tropical water inflow from the Pacific Ocean. In the southern IO, the eastern boundary formed by the African continent does not extend beyond 35 o S, which allows for a connection between the southern IO and the South Atlantic Ocean (Gordon, 2003;Lutjeharms, 2006) and facilitates plastic transport between these oceans leading to an debris exchange between the garbage patches (van der Mheen et al., 2019; section 4.3). The presence of the land mass in the northern IO results in there being no subtropical gyre. Instead, temperature differences between the northern land mass and 250 https://doi.org/10.5194/os-2020-127 Preprint. Discussion started: 11 February 2021 c Author(s) 2021. CC BY 4.0 License. the ocean drive the monsoon system, which dominates the atmospheric and oceanic dynamics in the region. These dynamics determine the transport of buoyant plastics in the IO (van der Mheen et al., 2020a) and are described in more detail here.

Northern Indian Ocean surface dynamics and plastic transport pathways
The atmospheric dynamics in the IO are characterised by bi-annually reversing monsoon winds due to seasonal differential heating and cooling of the continental land mass and the ocean (Schott et al., 2009). The Southwest (SW) monsoon generally 255 operates between June and October and the Northeast (NE) monsoon operates between December through April (Tomczak and Godfrey, 2003). The transition periods between the monsoon seasons are the First Inter-Monsoon (May) and Second Inter-Monsoon (November). The influence of the monsoon system is not limited to the northern IO. Unlike in the other oceans, there are no steady equatorial easterly trade winds in the northern IO. Instead, they only have an easterly component during the NE monsoon season and are westerly during the rest of the year. In addition, the SW monsoon season starts with strengthened 260 south-easterly trade winds in the southern IO (Findlater, 1969;Joseph and Sijikumar, 2004).
These strong, seasonally reversing winds drive the ocean surface currents and circulation patterns in the northern IO (Stramma et al., 1996;Shankar and Shetye, 1997;Schott and McCreary, 2001;Shenoi et al., 2004). During the SW monsoon season, the flow in the northern IO is predominantly towards the east (Figure 4a (EICC). At the eastern boundary, the South Java Current (SJC) is variable but flows predominantly to the north west along the Java coast (Sprintall et al., 2009), in the same direction as the Indonesian Throughflow (ITF), which is strongest during the SW monsoon (Sprintall et al., 2009). At the western boundary of the northern IO, the Somali Current (SC) flows northeastwards during this season. 275 During the NE monsoon season, the ocean surface currents reverse direction and the flow in the northern IO is predominantly towards the west (Figure 4b), from the Bay of Bengal towards the Arabian Sea (Schott et al., 2009). Both the WICC and the EICC along the Indian coasts reverse direction and the SMC reverses and becomes the Northeast Monsoon Current (NMC), which is weaker than the SMC (de Vos et al., 2014). Together with the East African Coastal Current (EACC), the SC supplies 280 the eastwards South Equatorial Counter Current (SECC). The SECC feeds into the SJC, which flows south-eastwards along the coasts of Sumatra and Java during this season. Another unique feature in the equatorial IO is the strong eastward flowing https://doi.org/10.5194/os-2020-127 Preprint. Discussion started: 11 February 2021 c Author(s) 2021. CC BY 4.0 License.
Wyrtki jets (Wyrtki, 1973) that develop along the equator during the First and Second Inter-Monsoon. These jets are at their strongest during the Second Inter-Monsoon (Qui and Yu, 2009).

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The main features to note from the description of the ocean surface currents in the northern IO is the reversal of major currents with the monsoon season. Particle tracking simulations illustrate that these reversing currents transport buoyant objects between the eastern and western section of the northern IO with Sri Lanka and the Maldives in the central section ( Figure 5; van der Mheen et al, 2020a). In addition, the net surface transport in the northern IO is eastwards (Schott et al., 2009), because of the strong eastwards SMC, as well as the eastward flowing Wyrtki jets between monsoon seasons. This net eastward 290 transport, combined with eddies developing year-round in the Bay of Bengal and potentially trapping plastics, result in plastics being present in the Bay of Bengal throughout the year. In contrast, the Arabian Sea, is mostly depleted of plastics during the SW monsoon season ( Figure 5). However, if sources of plastics increase it is likely that concentrations will also increase. As plastics are transported back and forth between the Arabian Sea and the Bay of Bengal, they frequently come close to coastlines. Because of this, it is likely that a large amount of plastic beaches and accumulates on northern IO coastlines (van 295 der Mheen et al., 2020a; section 5.2).
To illustrate the influence of these unique features in the northern IO on the transport of buoyant objects, a particle tracking simulation was undertaken using surface advection fields from the global ocean circulation model HYCOM and the particle tracking model ICHYTOPOP. Passive particles (100,000) were released at a location to the south of Sri Lanka (Figure 4)

Southern Indian Ocean surface dynamics and plastic transport pathways
In the southern IO, similar to other ocean basins, there is a wind-driven subtropical gyre (Figure 4) which in this instance has several unique features. The gyre is bounded in the north by the South Equatorial Current (SEC), which flows westwards and is relatively steady all year round. At the western boundary of the subtropical gyre, the Agulhas Current (AC) flows poleward along the southern African coastline (Beal et al., 2011). The Agulhas Retroflection (AR) and Agulhas Leakage (AL) provide 320 a connection between the southern IO and South Atlantic Ocean (Gordon, 2003;Lutjeharms, 2006). At the eastern boundary, the Leeuwin Current (LC) flows poleward along the western Australian coastline. This is opposite to the equatorward flow expected for a wind-driven subtropical gyre (Pattiaratchi and Woo, 2009). Along the southern Australian coastline, the Flinders Current (FC; Middleton and Cirano, 2002) flows westward from the Pacific Ocean into the southern IO. In the south, the gyre is bounded by the Antarctic Circumpolar Current (ACC). The South Indian Counter Current (SICC) flows eastward through 325 the centre of the subtropical gyre (Lambert et al., 2016), which is opposite to the direction expected from Sverdrup theory (Palastanga et al., 2007;Wijeratne et al., 2018). Similar subtropical counter-currents exist in the other oceans but the SICC is unique because it flows across the full width of the basin and splits into three separate branches as it flows between the southern tip of Madagascar and the western coast of Australia (Menezes, 2014).

Transport of plastics between the northern and southern Indian Ocean
Strong ocean surface currents can act as transport barriers for buoyant objects, preventing buoyant objects from crossing these 340 currents (Bower, 1991;Brambilla and Talley, 2006). Due to strong equatorial currents, ocean surface drifters do not tend to cross the equator (Maximenko et al., 2012). It has therefore been suggested that buoyant plastics generally remain in the hemisphere where they originally entered the ocean (Lebreton et al., 2012). However, as previously mentioned, the easterly trade winds are not steady in the IO and as a result, the NEC and SECC are not steady either. The surface of the IO also appears more connected between hemispheres than the other oceans (Froyland et al., 2014). van  Based on these results, it seems possible for buoyant plastics to cross the equator in the IO. It is however unclear how frequently this occurs.

Plastic transport between the Indian, Atlantic, and Pacific oceans
To the best of our knowledge, no studies have currently focussed on the transport of plastics from the Pacific Ocean into the 360 IO through the ITF.
Plastics may also be transported from the IO into the Atlantic and Pacific oceans. Based on ocean surface drifter datasets, van der Mheen et al. (2019) showed that buoyant objects that are transported not only by ocean surface currents, but also by Stokes drift due to waves, and by wind, can move from the southern IO subtropical garbage patch (section 5.1) into the South Atlantic 365 Ocean. Using Lagrangian particle tracking simulations, Dobler et al. (2019) confirmed that plastics that are transported by Stokes drift as well as ocean surface currents move from the southern IO past the South African coastline into to the South Atlantic Ocean. Trajectories of undrogued ocean surface drifters (i.e. transported by ocean surface dynamics, including ocean surface currents, wind, and wave effects) also indicate that transport between these two areas of the ocean is possible (van der Mheen et al. 2020a). 370 Based on Lagrangian particle tracking simulations, Maes et al. (2018) suggested that there is a "super convergence pathway" connecting the southern IO to the South Pacific Ocean. Their results showed particles being transported eastwards, passing close by the southern Australian coastline from the southern IO into the Pacific Ocean. However, trajectories of ocean surface drifters do not confirm the existence of this super convergence pathway. van der Mheen et al. (2020c) suggested that this may 375 be due either to the lack of observed drifter locations along the southern Australian coast, or due to an under-representation of the westwards flowing Flinders Current (Middleton and Cirano, 2002;Wijeratne et al., 2018, section 4.2) in the ocean circulation model used to force particle tracking simulations by Maes et al. (2018). The existence of a super convergence https://doi.org/10.5194/os-2020-127 Preprint. Discussion started: 11 February 2021 c Author(s) 2021. CC BY 4.0 License. pathway between the southern IO and the South Pacific Ocean along the southern Australian coast therefore still needs further investigation. 380

Fate
Buoyant plastic debris tends to accumulate in subtropical garbage patches in the world's ocean (e.g. Moore et al., 2001;Lebreton et al., 2012;Maximenko et al., 2012;van Sebille et al., 2012;Eriksen et al., 2013;van Sebille et al., 2015). However, the large discrepancy between the estimated amount of plastic waste entering the ocean and the amount of plastic floating in the garbage patches highlights a fundamental gap in the understanding of the fate of plastic debris and it is likely that there are 385 large and unknown sinks (e.g. van Sebille et al., 2015). Several theories propose what these additional sinks may be including: (1) biological sinks (i.e. ingestion of plastics by marine organisms; e.g. Davison and Asch, 2011); (2) polar ice sheets acting as temporary sinks Peeken et al., 2018), which is not relevant to the IO; (3) coastlines acting as sinks (Lebreton et al., 2019;van der Mheen et al., 2020a); and (4) sinking of plastics due to density changes by fragmentation and biofouling . 390 Ingestion of plastics can occur at the ocean surface, in the water column, and on the seafloor. Estimates of plastic ingestion by vertebrates (van Franeker, 2011), (Davison and Ash, 2011), indicate that the global ingestion of plastics could be on the same order of magnitude as the amount of plastics accumulating in subtropical garbage patches (van Sebille et al., 2015). However, to the best of our knowledge, no estimates of the amount of ingested plastic by organisms in the IO currently exists. 395 Sinking and settling of plastics on the seafloor due to fragmentation and biofouling may be a major sink of plastic debris in the ocean . However, these estimates were based on mass balance models and no evidence of the size of this sink currently exists. The understanding of the exact processes of biofouling, fragmentation, and sinking as well as the timescales on which these occur is limited. As far as we are aware, no studies have focussed on this specific to the IO. However, 400 productivity derived from satellite images revealed strong variability in biological production in the IO, and the IO is one of the most productive regions in the global oceans mainly due to intense upwelling during the southwest monsoon (Qasim, 1977). This high surface productivity results in a high export flux of organic particles from the euphotic zone to the deep sea (Ittekkot et al., 1996;Guptha et al., 1997). Organic particles attached to microplastics in combination may increase their density and thus can accelerate the transport of both to the seafloor. As a result, sinking of plastics may be very relevant in the IO. 405 Woodall et al. (2014) estimated that 4 billion fibres per km 2 were present in the IO region (seamount sediments; 500-1000m depth), and as a result suggested that the deep sea is a major sink for microplastic debris, which should also be investigated in the future.
Due to a lack of information on ingestion and sinking of plastics specific to the IO, we only consider the southern IO subtropical garbage patch and the accumulation of plastics on coastlines as in more detail.

Subtropical garbage patch
In the subtropical oceans, between 20 to 40 °N and °S, wind-driven Ekman transport leads to convergence and associated downwelling. Because buoyant plastics remain at the ocean surface, they accumulate in so-called garbage patches in these 415 regions. In the IO, subtropical convergence, downwelling, and associated accumulation of plastics only occurs in the southern hemisphere, because the subtropics in the northern IO is blocked by landmass (section 4.2). van der Mheen et al. (2019) showed that the southern IO subtropical garbage patch is unique and behaves very differently to the garbage patches in the Atlantic and Pacific oceans.

Accumulation on coastlines
Plastics can also beach and accumulate on coastlines. This is most likely to be particularly relevant in the northern IO, as it is surrounded by land on three sides. Reversing ocean surface currents and winds driven by the monsoon system, transport plastics back and forth from the western to the eastern side of the basin (section 4.2), frequently transporting plastics close to 445 coastlines. van der Mheen et al. (2020a) determined beaching patterns of plastics released from river sources in the IO using Lagrangian particle tracking simulations.
The simulation indicated that for plastics entering the northern IO from rivers, coastlines of countries bordering the Bay of Bengal were most affected by beaching plastics (Figure 6). This is most likely due to the large source locations of particles 450 ( Figure 1b) and ocean dynamics of the Bay of Bengal. Although there are no downwelling regions in the Bay of Bengal, anticyclonic and cyclonic gyres develop throughout the year (Paul et al., 2009) that may trap plastics. In addition, the annual mean flow along the equator is to the east, directed from the Arabian Sea towards the Bay of Bengal due to the stronger SW monsoon (Schott et al., 2009;de Vos et al., 2014). van der Mheen et al. (2020a) hypothesised that plastics are present in the Bay of Bengal throughout the year (as shown in Figure 5). Countries that were among the top 15 that received the most beached 455 plastics in their simulations were Bangladesh, Myanmar, India, Malaysia, Indonesia, Sri Lanka, Thailand, Pakistan, the Maldives, and Somalia. Of these, only Somalia does not border the Bay of Bengal and does not have significant nearby inputs of plastic waste from rivers. Instead, van der Mheen et al. (2020a) hypothesised that plastics reach Somalia as the North Equatorial Current and Somali Current transport them westwards during the NE monsoon season. Incidentally, all the seven major countries surrounding the Bay of Bengal are believed to have least-managed plastic waste that is prone to enter the sea 460 (Jambeck et al., 2015). The Maldives seemed to have received a relatively large percentage of plastics in the simulations even though it has no riverine plastic sources of its own. They attributed this to the Northeast Monsoon Current and the Southwest Monsoon Current flowing past the Maldives in reversing directions twice a year. Beached plastics have also been found in The Maldives in sampling studies by Imhof et al. (2017).

465
In the southern IO, countries along the eastern African coast, Madagascar, Mauritius, Réunion, the Cocos (Keeling) Islands, and Christmas Island were heavily affected by beaching plastics from river sources (van der Mheen et al., 2020a). In the simulations, most of these plastics originated from rivers in Indonesia.
In many locations mentioned here, studies have sampled beached plastics (section 3). However, a quantitative comparison 470 between studies and model validation is not possible because of the different methods used by these studies. In addition, the simulation results by van der Mheen et al. (2020a) presented here, are simplified, and based on global ocean models. However, the dynamics of beaching plastics is complex and strongly influenced by small-scale coastal ocean dynamics (Isobe et al., 2014), as well as local morphology of the coastline (Zhang, 2017). It is unknown what the influence is of these small-scale https://doi.org/10.5194/os-2020-127 Preprint. Discussion started: 11 February 2021 c Author(s) 2021. CC BY 4.0 License. effects on basin-scale beaching patterns in the IO. Finally, plastics do not necessarily remain beached indefinitely, but can also 475 re-float and re-enter the ocean (Zhang, 2017;Lebreton et al., 2019). As a result, it is unknown how much plastic is stored on coastlines in the IO, as well as how permanent this sink is. Thus, long-term (multi-decadal) monitoring and field studies are necessary along IO coastlines.

Impacts
Plastic debris can harm the marine environment, communities, and economies in many different ways. The review by Law 480 (2017) discusses the different harmful effects of plastic debris, as well as existing evidence for their impact in detail. In this section, we briefly discuss different harmful effects of plastic debris and provide more detail about the potential impacts specific to the IO and IO rim countries. In section 6.4, we also provide an overview of the different policies and initiatives that are emerging in IO rim countries to prevent, mitigate, or clean-up plastic debris in the IO.

Impacts Ghost gear 485
Approximately 100 million tonnes of marine capture fishes have been landed globally each year since 2000, approximately 13% of which originated from the IO (Pauly and Zeller 2016). The IO supports a range of marine ecosystem services including numerous and expansive fishing ventures with 7%, or one million tonnes, of IO landings considered large commercial pelagic fishes, e.g. tuna and billfishes. The commercial pelagic fisheries of the IO alone are worth over US$ 1300 million annually.
Abandoned, lost, and discarded fishing gear (ALDFG; section 4.2) becomes "ghost gear", which continues to entangle and 490 injure or catch wildlife, for decades or centuries as it slowly degrades which additionally increases the pressure on marine wildlife. Entangled and caught fauna of ALDFG starts initially with sea turtles, marine mammals, sharks, and large predators for the first few months, changing to crustaceans when gear fragment into smaller parts (Macfadyen et al., 2009;Wilcox et al., 2015;Stelfox et al., 2016). A single ghost net may decrease fishing revenues by US$ 20 thousand annually (Lachmann et al., 2017) and can consume as much as 30% of landings within a single fishery (Laist and Liffman, 2000). Although ALDFG is 495 thought to diminish in catching efficiency over time, catching potential of gillnets were estimated to be 20-30% of original levels for long periods and at least 5% after more than two years of abandonment (Macfadyen et al., 2009;Stelfox et al., 2016).
Some catch rates were estimated to be 81 kg/day for tangle nets over 1.5 years, an average of 64 kg/day for deep water gillnets over a 45-day period and 0.6 kg/day for traps left for 3-6 months (Macfadyen et al., 2009).

500
ALDFG affects the tourism industry through a multitude of factors, including the removal of iconic marine species and wildlife in general, as well as contributing up to 90% of shoreline debris and degrading of the perceived beauty of an area (Gunn et al., 2010;Lachmann et al., 2017). These devaluations decrease benefits from coral reef tourism (US$ 36 billion) (Spalding et al., 2017) and global exports of aquarium fish (US$ 15 billion) (Raja et al., 2019). Ghost gear is also considered to be destructive https://doi.org/10.5194/os-2020-127 Preprint. Discussion started: 11 February 2021 c Author(s) 2021. CC BY 4.0 License. to both natural habitats, i.e. coral reefs, and manufactured objects, i.e. boats (Laist and Liffman, 2000;Gunn et al., 2010). 505 Many of the countries bordering the IO are small island developing states (SIDS) where more than 70% of the population lives along the coast and is highly dependent on marine ecosystem services (Canales et al., 2017). In addition, 60% of aquarium fish are exported from developing nations (Raja et al., 2019). Thus, the adverse cultural, societal, and sustenance impacts of ghost fishing may disproportionately affect IO rim developing nations (Wong, 2011;Guillotreau et al., 2012;Lachmann et al., 2017). 510

Emerging policies and initiatives on plastic in Indian Ocean rim countries
Of the top 20 countries ranked by mass in estimated mismanaged plastic waste (MMPW), nine of them are located along the IO rim (Indonesia, India, Thailand, Malaysia, Bangladesh, South Africa, India, Pakistan, and Myanmar). Plastic waste generated by these countries amounts to ~15% of the worlds' total MMPW (Jambeck et al., 2015), putting pressure on these countries to address the issue of MMPW. Until now, local and national actions have been the primary approach for mitigating 515 plastic pollution (Vince and Hardesty, 2016). The choice of measures at national or local level is left to the IO rim country administrations -in line with the principle of subsidiary. For example, some countries have refundable deposit schemes for bottles (Schuyler et al., 2018). Targeted deposit schemes can help reduce littering and boost recycling and have already helped several countries achieve high collection rates for beverage containers (Lavee, 2010;Dace et al., 2013;Schuyler et al., 2018,). In India, plastic pollution is being fought at various levels, by state governments, NGOs, and individuals across the country. 530 Initiatives range from banning of plastic to beach clean-ups. Several Indian states have banned or regulated the use of plastic, but India still struggles to manage its huge plastic waste. For microplastics, and their presence in consumer goods, regulations are still under development. The National Green Tribunal in January 2017 asked the Union Government to test leading cosmetics brands for microplastics. The Bureau of Indian Standards (BIS) classified microbeads as "unsafe" for use in cosmetic products and banned the use of microbeads in cosmetics in October 2017, but it will only be implemented in 2020. 535 https://doi.org/10.5194/os-2020-127 Preprint. Discussion started: 11 February 2021 c Author(s) 2021. CC BY 4.0 License.

Summary
The Indian Ocean covers~20% of the Earth's surface and compared to other oceans has unique physical characteristics (northern landmass, connection to the Pacific Ocean through Indonesia, and connection to the Atlantic Ocean past South Africa, Monsoon) that strongly influence the circulation patterns and therefore the fate of plastics. This paper examined the sources, transport, sinks and impacts of plastics in the IO. The main sources of plastics are from rivers (terrestrial sources) 540 with the majority input in the northern Indian Ocean, particularly the Bay of Bengal (Figure 7). There are also land-based sources from Indonesia into the southern Indian Ocean. Inputs from Australia are negligible due to absence of rivers and low population density in Western Australia. In the northern IO, reversing monsoon driven currents transport plastic material between the Bay of Bengal and the Arabian Sea with many plastics making landfall in Somalia, The Maldives, Sri Lanka, east and west coasts of India, Myanmar and western Sumatera (Figure 7). There is no garbage patch in the northern IO however, 545 in the southern IO there is evidence for a garbage patch. However, it is not well defined with very few measurements indicate that it may extend across the whole length of the ocean basin. The main beaching region in the southern IO is the coast of northern Madagascar. There also could be leakage of material into the southern Atlantic Ocean garbage patch past South Africa.

Knowledge gaps 550
The main gap in knowledge for the IO is the scarcity of data in both the surface and deeper ocean. Only a few data points are available for the whole IO including along coastlines and the deeper ocean ( Figure 2). This is a very large constraint, which needs addressing. As demonstrated in this paper, we can simulate the transport of plastics using numerical models but confirmation of these results is of paramount importance. However, the simulations pinpoint to the regions that require attention in future investigations. For example, it is unclear which type of these simulated garbage patches (leaky with low 555 concentrations and on the western side of the IO basin, or stable with high concentrations spanning the entire width of the IO basin) best represents the behaviour of the garbage patch in the subtropical southern IO. For the same reasons, it is also unknown how much plastic is contained in the subtropical IO garbage patch as well as how long it is likely to remain in this garbage patch (both as a result of escaping the IO into the South Atlantic Ocean and as a result of other factors such as sinking and ingestion of plastics). 560 The main sources of plastics considered in this paper were derived from rivers mainly in the northern Indian Ocean. There is most likely transport of plastics from the from south east Asia through the Indonesian Throughflow (section 4.1). However, these sources are currently undocumented and need to be further investigated.

995
Locations of plastic samples taken on beaches, in sediment, and in organisms around the Indian Ocean (references to individual studies listed in Table 1). Because sampling methods vary widely between studies, a quantitative comparison of measured plastic concentrations between these studies is not possible.