Beaching patterns of plastic debris along the Indian Ocean rim

Abstract

Abstract. A large percentage of global ocean plastic waste enters the northern hemisphere Indian Ocean (NIO). Despite this, it is unclear what happens to buoyant plastics in the NIO. Because the subtropics in the NIO is blocked by landmass, there is no subtropical gyre and no associated subtropical garbage patch in this region. We therefore hypothesise that plastics "beach" and end up on coastlines along the Indian Ocean rim. In this paper, we determine the influence of beaching plastics by applying different beaching conditions to Lagrangian particle tracking simulation results. Our results show that a large amount of plastic likely ends up on coastlines in the NIO, while some crosses the equator into the southern hemisphere Indian Ocean (SIO). In the NIO, the transport of plastics is dominated by seasonally reversing monsoonal currents, which transport plastics back and forth between the Arabian Sea and the Bay of Bengal. All buoyant plastic material in this region beaches within a few years in our simulations. Countries bordering the Bay of Bengal are particularly heavily affected by plastics beaching on coastlines. This is a result of both the large sources of plastic waste in the region, as well as ocean dynamics which concentrate plastics in the Bay of Bengal. During the intermonsoon period following the southwest monsoon season (September, October, November), plastics can cross the equator on the eastern side of the NIO basin into the SIO. Plastics that escape from the NIO into the SIO beach on eastern African coastlines and islands in the SIO or enter the subtropical SIO garbage patch.

Modeling the Exposure of the Macaronesia Islands (NE Atlantic) to Marine Plastic Pollution

“…As expected, B surface particles intercepting Cabo Verde are more likely to pass through the highly productive (e.g., Mittelstaedt, 1991;Van Camp et al, 1991;Lathuilière et al, 2008) areas of intensive fishing activity along the northwest African coast, extending from the Canaries to the Mauritanian coast (Figure 7D). Indeed, Fernandes (2019) found that >60% (by mass) of the plastic debris collected in the northern side of Santa Luzia island in Cabo Verde were fishing gear.…”

“…Driven by the relaxation of the trade winds and consequent intensification of the North Equatorial Counter Current, the Mauritanian Current reaches its northernmost extension (20 • N) during summer and autumn, where it intersects the North Equatorial Current and turns southwestward in the direction of Cabo Verde. During winter and spring the opposite occurs, as the trade winds intensify and force the Canary Current upwelling system to reach its southernmost extension (15 • N, Mittelstaedt, 1991;Van Camp et al, 1991). Such dynamic variability at the Mauritania coast is expected to affect the Cabo Verde islands, located 450-600 km west.…”

“…On the other hand, the major pathway of incoming B mixed particles is depicted by the high proportions of particles that connect the archipelago to the Northwestern African coast in the east. As B mixed particles are less exposed to the wind, the transport of these particles is mainly influenced by: (i) the North Equatorial Current that detaches from the African Coast near Cape Blanc (Mittelstaedt, 1983); (ii) by the winddriven Ekman currents associated with the seasonal upwelling off the Mauritanian coast (Mittelstaedt, 1991;Van Camp et al, 1991);…”

Sources of marine debris for Seychelles and other remote islands in the western Indian Ocean

“…Martinez-Ribes et al, 2007;Turrell, 2020a). Elsewhere, however, particularly in the case of remote islands with minimal or no population, debris accumulating on the coast may have been transported over great distances by ocean currents, winds, and waves (van Sebille et al, 2020). These islands, many of which belong to small island developing states, are faced with the deeply inequitable situation of bearing the costs of removing waste they were not responsible for generating, contrary to the "polluter pays" principle (OECD, 1975).…”

“…assuming knowledge of some input distribution of marine debris and predicting where that debris is transported (e.g. Kaandorp et al, 2020;van der Mheen et al, 2020;Chassignet et al, 2021), or backward-in-time, i.e. simulating trajectories that lead to a site of interest and inferring debris sources based on where debris passed through in the past (e.g.…”