Profiling microplastics in a forgotten river system in Southern Africa

Microplastics have been studied in rivers worldwide with far-reaching implications for aquatic ecosystems. What is less understood is how microplastics distribute through rivers, as microplastics do not distribute ubiquitously through a river system. This study described the microplastic profile of the Nyl, Mogalakwena and Limpopo Rivers in South Africa. The study aimed to determine the driving environmental factors of microplastic distribution in a river system over multiple seasons. The study discovered relationships between flow and microplastics in sediment. This relationship allowed microplastics to have significantly (p < 0.05) different distributions over seasons. Seasons with reduced flow had higher mean microplastic abundances in water (1436 ± 4492 particles.m⁻³) and sediment (1710 ± 4951 particles.kg⁻¹dw), which decreased in both water (59 ± 46 particles.m⁻³) and sediment (17 ± 11 particles.kg⁻¹dw) during the high flow season. Although microplastic abundances decreased with increased flow, a more homogenous distribution was detected through the entire system in the high flow period. The results suggest that microplastics could become trapped and increase significantly during reduced flow but become more evenly distributed during high flow seasons. The microplastics had also become bioavailable, being found in benthic macroinvertebrates in the river system at varying concentrations with a mean of 29 ± 33 particles.g⁻¹ww, which could not be related to environmental matrices. Significant differences (p < 0.05) were determined between microplastic polymers found in water compared to sediment in all seasons. The study is the first in this transboundary river system, which impacts multiple African nations and a RAMSAR accredited wetland of international importance.