Water hyacinths as riverine plastic pollution carriers

Abstract. Plastic pollution is an emerging entity threatening freshwater and marine ecosystems. Rivers play an important role in the transport and retention of plastic from land to sea. Tropical rivers are among the most polluted globally, and are assumed to emit substantial amounts of plastic into the marine environment. Concurrently, tropical rivers are invaded by water hyacinths, a free-floating vegetation species native to the Amazon. Water hyacinths grow rapidly, forming dense mats of plants and other material including plastic pollution. With only limited anecdotal and scientific evidence of plastic-water hyacinth trapping, its full spatial extent along river systems remain unknown. Here, we demonstrate the consistent role of water hyacinths as carriers of plastic pollution along a river. Over 69 k plastic items and 57 k water hyacinth patches were identified along the 42 km most downstream section of the Saigon river, Vietnam. More than 73 % of all floating plastics were carried by water hyacinths, ranging between 58–82 % per specific location. The highest trapping ratio was found at the most upstream locations. Although water hyacinths only covered 1.3 % of the total river surface, the plastic concentration in water hyacinths was 197 times higher than in open water. Most downstream, the lowest water hyacinth coverage (0.2 %) corresponded to the largest difference between surface plastic concentration in water hyacinths and at the open water (factor 781). Previous work demonstrated the effective trapping of plastic pollution by water hyacinths at individual sites. Here, we show that plastic-water hyacinth aggregates consistently occur at the river scale. We quantified plastic and water hyacinths at five locations along the Saigon river, Vietnam, using drones and fixed cameras, in combination with a custom-trained YOLOv8 deep learning model. With our paper, we support the theory that water hyacinths effectively concentrate and carry plastic pollution along rivers. Further work on plastic-water hyacinth interactions is needed to better understand the transport, fate and impact of plastic in the world's most polluted rivers. Our results also support the idea of plastic monitoring from space using well-detectable floating vegetation as a proxy. Finally, our work suggests that current removal practices of water hyacinths may be optimized to also recover plastic pollution from rivers.