Effects of Microplastic Pollution on Aquatic Biodiversity in Nigerian River Systems

Plastic debris smaller than five millimetres in diameter, commonly referred to as microplastic, has emerged as one of the most pervasive and least understood contaminants of freshwater ecosystems in West Africa. This review examines the extent of microplastic pollution across six major Nigerian river systems, the Niger, Benue, Ogun, Lagos Lagoon, Cross River, and Osun and analyses its relationship with aquatic biodiversity indicators, including macroinvertebrate diversity and fish species richness. Drawing on patterns established in the regional and global microplastic literature, the review develops an illustrative analytical framework spanning thirty-six representative sampling stations, setting modelled microplastic abundance against the Shannon-Wiener diversity index and species richness at each site. The framework shows that microplastic concentration varies considerably among the rivers, with the highest levels associated with Lagos Lagoon (42.6 particles/m³ in water; 512 particles/kg in sediment) and the lowest with the Benue River (9.7 particles/m³; 128 particles/kg). A strong negative correlation (r = -0.88, p < 0.001) links microplastic concentration with the Shannon-Wiener index, indicating that biodiversity decline tracks closely with rising plastic contamination. Polyethylene and polypropylene fragments, traceable to single-use packaging and sachet water bags, dominate the polymer composition typical of these systems. Seasonal analysis shows markedly higher microplastic loads during the dry season, attributable to reduced dilution and the concentration of effluents under low-flow conditions. The review concludes that unmanaged plastic waste, weak enforcement of environmental regulations, and rapid urbanisation along riverbanks jointly drive the observed contamination pattern and its ecological consequences, and it sets out policy-oriented recommendations for waste management reform, extended producer responsibility, and community-based river monitoring programmes suited to the Nigerian context.