Seasonal Variation in Macroinvertebrate Diversity as Bioindicators of Water Quality in the Osun River, Osun State

Rivers in South West Nigeria face increasing pressure from agricultural runoff, domestic sewage, and small-scale industrial activities. Biological indicators offer a sensitive and cost-effective means of detecting long-term changes in water quality. This study evaluated seasonal variation in macroinvertebrate diversity as bioindicators of water quality in three reaches of the Osun River, Osun State. Sampling was conducted quarterly during wet and dry seasons using standardised kick-net sampling. Macroinvertebrates were identified to the family level and used to calculate diversity indices and apply the Biological Monitoring Working Party score adapted for West African conditions. Simultaneous physicochemical measurements included dissolved oxygen, pH, conductivity, biochemical oxygen demand, and turbidity. Results showed higher macroinvertebrate diversity during the dry season at upstream sites, while wet season flows brought increased sediment and nutrient loading downstream, suppressing sensitive taxa. Pollution-sensitive Ephemeroptera and Trichoptera families were absent from downstream sites receiving agricultural drainage throughout both seasons. Biotic scores corroborated physicochemical evidence of moderate to severe organic pollution in downstream reaches. The study recommends integration of macroinvertebrate bioassessment into the Osun State environmental monitoring framework and protection of riparian buffer strips to intercept agricultural runoff before it reaches the river channel. Keywords: macroinvertebrates, bioindicators, Osun River, water quality, seasonal variation.