Abundance, distribution, and risk assessment of microplastics in freshwater fish, water and sediments in a subtropical river system.

Abstract

Plastic pollution has emerged as one of today’s most pressing environmental challenges, with a significant portion of it originating from microplastics (less than 5mm). Their buoyancy and endurance in aquatic habitats pose major risks to aquatic species. The fragmentation of weirs in our rivers is another significant issue faced by aquatic fauna, as weirs act as sinks for microplastic pollution, which can result in their consumption by freshwater organisms. The current study assessed sediment and water samples along the Luvuvhu River system to determine the abundance, distribution, and diversity of microplastics, using a combination of diversity indices and multivariate analysis by exploring the impacts of weir–induced alterations on microplastic accumulation, given that weirs can serve as plastic sinks and disrupt natural water flow. Fish species were collected from the Luvuvhu River. In the laboratory, microplastic abundances were quantified both upstream and downstream of weirs. The study’s finding generally showed high microplastic abundance during the cool–dry season in the surface water samples upstream (mean 409.4 ± 582.1 particles m–3) and downstream (mean 221.1 ± 363.6 particles m–3) of weirs and high during the hot–wet season upstream (mean 73.3 ± 61.9 particles dwt) and downstream (mean 30.8 ± 17.4 particles dwt) of weirs in the sediment samples. Microplastic types were dominated by fibres, with the colour scheme dominated by transparent in both the upstream and downstream sites. A significant difference was observed among sites for fragments, with film and beads being significant among locations. Low–density polyethylene (LDPE) and polypropylene (PP) were dominant polymers in surface water, while polyethylene terephthalate (PET) and polyethylene (PE) were dominant in sediment samples. Among the 215 fish (27 species) examined, microplastics were detected in 98.8 % of them. Microplastics in fish were dominant downstream of the weir during the cool–dry and hot–wet seasons. Particularly high microplastic abundances were found in Anguilla marmorata (range 0–29 particles), with fibres and transparent particles being the most prevalent. The results further showed high microplastic abundances in benthopelagic feeders, highlighting that habitat plays a great role in microplastic ingestion in fish. The most common polymer types found in fish were polypropylene (PP) and polyethylene (PE). Microplastic abundance was found to be higher during the hot–wet season (mean 5.2 particles) compared to the cool–dry season (mean 3.0 particles). This increase may be attributed to a larger number of plastic particles being washed into the river during the hot–wet season. Wastewater treatment systems and laundry washing are a primary source of microplastics pollution, particularly due to the observed dominance of fibres in the weirs. Microplastic abundances indicated that the Luvuvhu River is polluted, and weirs played a significant role in microplastic retention and accumulation. The present study's findings provide important information for stakeholders to integrate weir design and maintenance practices that minimize microplastic ingestion by fish and accumulation, while enhancing sediment flushing.