Amounts, Sources, Fates and Ecological Impacts of Marine Litter and Microplastics in the Western Indian Ocean Region: A Review and Recommendations for Actions

The Western Indian Ocean (WIO) is a region where biodiversity is increasingly being impacted by anthropogenic marine debris, but information is scattered widely in the literature. Herein, we reviewed 147 marine litter studies from the WIO region covering the time period from 1973 (first study published in the region) to the end of 2021, in order to (1) synthesize available information on the quantities and composition of marine litter and microplastics in the main ecosystem compartments, (2) identify the main sources and pathways of litter and (3) summarize which organisms have suffered from interactions with marine litter. This information was evaluated to (4) determine important knowledge gaps and (5) develop recommendations to address the issue in the region. The oceanography of the WIO is dominated by the western boundary currents (WBC), which are characterized by high current velocities and frequent onshore flows, potentially influencing the dynamics of marine litter in the region. The literature review confirms that higher macrolitter densities (mostly plastic packaging) on the seashore were generally found on urban, recreational beaches close to point sources (urban run-off, beachgoers). Identification of litter types and local concentrations indicate that most litter does not disperse far from the source, suggesting that shorelines in the WIO region are important sinks for litter. Some buoyant litter also reaches WIO countries after extended oceanic journeys and onshore transport in the WBCs, mainly from Southeast Asia, but most foreign PET bottles are dumped illegally from ships. Many studies (37%) reported interactions between organisms and marine litter; ingestion (103 species) and entanglement (58 species) were documented most frequently, and all four species of sea turtles studied had ingested plastic debris. Major knowledge gaps identified were related to the studied compartments and countries (most studies have been conducted on sandy beaches and in South Africa), to the quantification of litter from land-based versus offshore sources and to the determination of litter impacts on organisms, at both the individual and population level. Reducing current levels of plastic pollution requires reusable materials and better waste management, as well as international efforts to reduce the input of litter from shipping and fisheries. Despite local particularities (e.g. socioeconomic systems, hydrology, geomorphology), the general patterns and processes in the WIO region (high local retention, some input from offshore sources) likely apply to marine litter dynamics in other WBCs, requiring similar solutions.