Microplastic assessment approaches for African freshwater biota: a review

Microplastic pollution is a growing global concern with direct and indirect environmental health impacts. Africa hosts some of the most heavily polluted water bodies, exacerbated by limited management resources and research capacities. To evaluate the state-of-the-art in African freshwater microplastics approaches, we review studies that assessed pollution in freshwater organisms and appraise the field sampling and laboratory techniques used. Thirty-seven studies were included that analysed the status of microplastic concentration, ingestion, and abundance in African freshwater organisms. Of these, 11 studies conducted experimental work in laboratory settings, whereas the remainder were field-based. Studies were biased taxonomically and geographically, with 24 on fish, 10 on macroinvertebrates, and one each on birds and amphibians, and with studies predominantly in a few countries, mainly South Africa. Most of the studies were thus conducted in southern Africa, followed by east Africa, finding fibres to be the most dominant microplastic type, followed by fragments. Laboratory studies predominantly used pellets, polystyrene microbeads, polyethylene, polypropylene, polyvinyl chloride, nylon 66, and polyethylene terephthalate to determine their impact on organisms such as Clarias gariepinus, Oreochromis niloticus, Tilapia sparrmanii, Daphnia magna, Raphidocelis subcapitata and Tetrahymena thermophila. Microplastic extraction and separation from fish and aquatic macroinvertebrates are mostly done using potassium hydroxide (KOH), hydrogen peroxide (H2O2), nitric acid (HNO3) and sodium hydroxide (NaOH). Furthermore, instrumental analytical techniques for microplastics included the use of microscopes and Fourier-transform infrared (FT-IR) or attenuated total reflectance-Fourier transform infrared spectroscopy for polymer verification. Although Africa ranks highly in unmanaged plastic waste, studies on the prevalence of freshwater microplastics and their interactions with freshwater organisms in natural ecosystems remain scarce. Therefore, it is recommended that more studies are conducted to address the substantial gap, given the importance of freshwater biota in biomonitoring, especially in countries with a complete absence of studies on freshwater microplastic pollution.