Freshwater insects of different feeding guilds ingest microplastics in two Gulf of Guinea tributaries in Nigeria

Plastic pollution has enormous impacts on freshwater and marine ecosystem health, and it is one of the topmost environmental concerns of the current geological period (i.e. the Anthropocene). Thus, the goal of our study was to provide baseline information and bridge the information gap on the occurrence of microplastics (MPs) in African freshwater systems, using two tributaries of the Gulf of Guinea (Ogun and Osun Rivers) in Nigeria as a case study and three freshwater insect species of different feeding guilds as bioindicators. A total of 29 individuals of the insect species were chemically digested and subsequently analysed for MP presence under a digital microscope and a micro-Fourier-transform infrared (μFTIR) spectroscope. Collector-gatherers (Chironomus sp. and Siphlonurus sp.) recorded the highest MP load per gram wet weight, while the predatory Lestes viridis recorded the lowest. The highest diversity of polymers was recorded in Chironomus sp. of Ogun River, i.e. styrene ethylene butylene styrene, acrylonitrile butadiene styrene (ABS), chlorinated polyethylene, polypropylene (PP), and polyester, while two polymers each were recorded in Siphlonurus sp. (i.e. polyester and ABS) and L. viridis (i.e. polyester and PP) of Osun River. We conclude that collector-gatherers like Chironomus sp. and Siphlonurus sp. could be best employed as MP bioindicators in freshwater systems. However, their suitability as MP bioindicators should be further investigated in different freshwater ecosystems worldwide. Graphical abstract.