Eco-partitioning and indices of heavy metal accumulation in sediment and Tilapia zillii fish in water catchment of River Niger at Ajaokuta, North Central Nigeria

In this study the distribution and accumulation indices of some heavy metals in sediments and Tilapia zillii fish in freshwater catchment of River Niger by Ajaokuta Steel Company (ASC), North Central Nigeria were investigated. Water, bottom sediments and Tilapia zilliifish samples were collected upstream and downstream of the drainage column by ASC, Ajaokuta. The sample were digested according to standard methods and analysed for Cd, Mn, Cr, Ni, Cu, Zn and Pb using flame atomic absorption spectrometer. Accumulation indices or factor (AI or AF) of the investigated heavy metals were defined using the ratio of mean concentration Co in component/organism and that in the surrounding water Cw at steady state (AI/AF = Co/Cw). Sediment accumulation indices (AI) of the metals were: Cd, 5.4; Mn, 3.4; Cr, 1.6; Ni, 12.5; Cu, 1.6; Zn, 25.9 and Pb, 411.6, while the AI of the metals in  fillets of T. zillii were Cd, 3.0; Mn, 2.1; Cr, 1.6; Ni, 5.1; Cu, 4.6; Zn, 3.2 and Pb, 14.0. Seasonal climate changes induces little marginal or no changes in the AIs of the metals except for Pb (841.5, 411.6) and Cd (11.6, 5.4) in sediment, and Pb (32, 14) and Cd (5.3, 3.0) (p<0.05) in fish fillets. Thus significant changes in metal AIs may be the consequence of their concentration levels in the aquatic ecosystem. Therefore, accumulation indices or factors may be an estimate of ecosystem status, and may be a useful tool for monitoring and predictive risk assessment (MPRA) purposes.   Key words: Partitioning, accumulation indices, heavy metals, fish fillet, bottom sediment.