Environmental stressors influence the biological water quality of a lotic system in southwestern Nigeria

Deforestation in the Afrotropical realm is extensively encroaching on riparian corridors, causing increasingly negative impacts on freshwater biota. This study aimed to assess the biological water quality of an Afrotropical river in southwestern Nigeria whose riparian corridor is threatened by numerous environmental stressors. Water and macroinvertebrate samples from the Ojutu River, Osun State, were collected from March to November in 2019. The river’s extent of degradation was determined based on criteria such as riparian vegetation removal, grazing, farming, solid waste dumping, water abstraction and in-channel structures. Macroinvertebrate samples were scored using the International Organization for Standardization (ISO) Biological Monitoring Working Party (BMWP) system, which uses the presence of macroinvertebrate groups as a bioindicator of water quality in a river. The values obtained indicate that the water quality of the Ojutu River varied from an altered system (36 ≤ WQ ≤ 60) to a much-altered system (16 ≤ WQ ≤ 35) in the wet season, with evidence of mild pollution (61 ≤ WQ ≤ 100) in the dry season. The main drivers of biological water quality were in-channel structures, water abstraction, riparian vegetation removal and solid waste dumping. This study underscores the need to initiate resource management measures and enforce laws that could improve the ecological integrity of degraded riparian corridors in the Afrotropical realm.