Macroinvertebrate diversity, bioavailability, and bioaccumulation of heavy metals reveal ecological stress in freshwater bodies of Southwest Nigeria

Anthropogenic pressures increasingly threaten freshwater ecosystems in the tropics, yet long-term data on biodiversity shifts and contaminant dynamics remain scarce. The study investigated macroinvertebrate assemblages and heavy metal accumulation in Opa Reservoir, Nigeria, and its tributaries over two annual cycles (2020–2021). Across nine sampling stations, 47 macroinvertebrate taxa were identified, with Insecta (27 species) and Gastropoda (16 species) dominating community structure. Gastropoda was higher in both seasons among the zones established on Opa Reservoir and in all sampling stations along Opa tributary, except in the Amafa River, where insecta was higher in the rainy season. Species richness and abundance peaked in the lacustrine zone, whereas diversity indices were highest in riverine zones. Bioaccumulation analyses revealed significant spatial variation in metal concentrations. Copper, chromium, and manganese increased from riverine to lacustrine zones in several Bulinus species, while lead and cadmium showed contrasting patterns across taxa and habitats. All the metals analysed were higher in gastropoda compared with other classes. Notably, Melanoides maculata and Bulinus spp. exhibited high tolerance and heavy metal loads, underscoring their potential as sentinel taxa. The prevalence of tolerant species alongside elevated trace metal burdens signals a decline in ecological integrity and raises concerns about contaminant transfer through aquatic food webs. These findings demonstrate that benthic macroinvertebrates serve as robust bioindicators of pollution in tropical reservoirs and highlight the urgent need for integrated watershed management and regional policies to mitigate freshwater biodiversity loss and human health risks.