The distribution and occurrence of heavy metals in the sediment, water column and fish (Heterotis niloticus, Labeo senegalensis, Synodontis clarias, Hyperopisus bebe, Alestes baremose, Chrysichthys nigrodigitatus, Tilapia mariae, Parachanna obscura, Pharactolaemus ansorgii, Clarias gariepinus, Protopterus annectens, Auchenoglanis occidentalis, Gymnarchus niloticus, Sarotherodon galilaeus, Mugil cephalus, Schilbe mystus, Cynoglossus senegalensis and Clarias walkeri) of the Anambra River were investigated over a year period in five locations adopting standard ecological and chemical methods. Following the establishment of the presence and concentration of the heavy metals (As, Cd, Cr, Cu, Fe, Pb and Zn) and physicochemical properties in the principal media and fish, the biological effects of the most hazardous heavy metals in the media were investigated in laboratory bioassays on single and multiple mixtures on the basis of 96-h LC50 index. Bioaccumulation studies of sub lethal concentration of the heavy metals were evaluated over 28-day duration in laboratory bioassay. The ranges of concentration of heavy metal types measured in the Anambra River environment in both rainy season and dry season were as follows: Sediment : (AS: not detected, rainy season, and not detected, dry season; Cd: 0.0025 0.001mg/kg – 0.0060 0.001mg/kg, rainy season and 0.0020 0.000 mg/kg – 0.0060 0.000 mg/kg, dry season; Cr: 0.0010 0.000 mg/kg – 0.0065 0.006 mg/kg, rainy season and 0.0010 0.000 mg/kg-0.0030 0.000mg/kg, dry season; Cu: 0.0505 0.013 mg/kg – 0.1255 0.021mg/kg, rainy season and 0.0415 0.001mg/kg-0.1250 0.000mg/kg, dry season; Fe: 0.0310 0.006mg/kg-0.0880 0.018 mg/kg, rainy season and 0.2060 0.001mg/kg-0.1750 0.000mg/kg, dry season; Pb: 0.0015 0.0010mg/kg-0.0060 0.007mg/kg rainy season, and 0.0010 0.000mg/kg-0.0050 0.000mg/kg, dry season; Zn: 0.1955 0.127mg/kg-0.4220 0.154mg/kg, rainy season and 0.5200 0.000mg/kg-0.9300 0.000mg/kg, dry season). Water: (AS: not detected, rainy season and not detected, dry season; Cd: not detected, rainy season and not detected, dry season; Cr: 0.0010 0.001mg/L – 0.0040 0.001mg/L, rainy season and not detected, dry season; Cu: 0.0015 0.001-0.0270 0.024mg/L, rainy season and 0.0040 0.000mg/L-0.0200 0.000mg/L, dry season; Fe: 0.0035 0.001mg/L-0.0410 0.001mg/L, rainy season and 0.0150 0.000mg/L – 0.0600 0.000mg/L, dry season; Pb: 0.0010 0.000mg/L-0.0030 0.000mg/L, rainy season and 0.0020 0.000mg/L-0.0040 0.000mg/L, dry season; Zn: 0.0360 0.004mg/L -0.2585 0.036mg/L, rainy season and 0.1480 0.000mg/L-0.4100 0.000mg/L, dry season). Fish: (AS: not detected, rainy season and not detected, dry season; Cd: 0.0016 0.001mg/kg-0.0053 0.001mg/kg rainy season, and 0.0010 0.000mg/kg-0.0045 0.001mg/kg, dry season; Cr: 0.0010 0.000mg/kg-0.0043 0.0042mg/kg, rainy season and 0.0010 0.000mg/kg-0.0050 0.000mg/kg, dry season; Cu: 0.0162 0.006mg/kg-0.0408 0.017mg/kg, rainy season and 0.0040 0.001mg/kg -0.0811 0.064mg/kg, dry season; Fe: 0.0032 0.002mg/kg-0.0368 0.007mg/kg, rainy season and 0.0050 0.001mg/kg -0.1660 0.119mg/kg, dry season; Pb: 0.0010 0.000mg/kg-0.0070 0.007mg/kg, rainy season and 0.0010 0.000mg/kg-0.0055 0.001mg/kg, dry season; Zn: 0.0362 0.035mg/kg-0.8700 0.099mg/kg, rainy season and 0.0060 0.001mg/kg – 0.4460 0.009mg/kg, dry season). There was no season and location variations (P>0.05) in the heavy metal concentrations sampled for water and sediment, but in fish, concentrations exhibited
variation (P<0.05) in Clarias gariepinus, Mugil cephalus, Heterotis niloticus and
Protopterus annectens over the sampling regimes. In the water column, only pH and temperature varied significantly (P<0.05) over the season. A toxicity ranking within limits of variations, showed Cd to be most toxic to Clarias gariepinus followed by Cr and Pb in a decreasing order of toxicity in single action toxicity test. The 96-h LC50 values for the single action were obtained as 8.280mg/L, 70.183mg/L and 63.546mg/L for Cd, Pb and Cr, respectively. The binary mixture in predetermined ratio (1:3, 1:5 and 5:3) of the test heavy metals (Cd-Pb, Cd-Cr and Cr-Pb) showed the 96-h LC50 values to be 17.699mg/L, 61.444mg/L and 26.263mg/L, respectively. The resultant toxicity levels of the 96-h LC50 values based on the computed synergistic ratio (S.R) were in conformity with the model of synergism (S.R= 1.034-3.965) except in few cases where the interaction was in agreement with model of antagonism (SR = 0.468 and 0.135 for Cd-Pb and Cd-Cr, respectively). The triple action studies (Cd-Pb-Cr) of the ratio (1:3:5) with 96-h LC50 value of 17.878mg/L on equiwild interactions of the single element conformed mostly with the model of synergism (SR = 3.534 – 3.926) except the antagonistic interaction of Cd (SR = 0.463). Laboratory bioaccumulation studies in Clarias gariepinus exposed to sub lethal concentration (equivalent to 1/10th and 1/100th of 96-h LC50 of Cd, Cr and Pb in single and joint mixtures) revealed that the test animal bioaccumulated in their tissues varying amount of the metals depending on the type of metal, length of exposure time and the concentration of the metals in test media. During the single and joint action tests, the test animal increasingly accumulated the metals steadily with time exposure over the 28-day experimental duration. In the joint metallic mixtures (Cd-Pb, Cd-Cr, Cr-Pb and Cd-Pb-Cr), it was found that the level of Cd, Cr and Pb bioaccumulated over a 28-day period was significantly reduced compared to the concentration of respective metals accumulated during the single action studies. The ecotoxicological implications of the measured levels of heavy metals in the principal media of the Anambra River (water, sediment and fish) and the results of laboratory bioassays with respect to environmental protection strategies were discussed. Recommendations were outlined to minimize and properly manage heavy metal polluted water bodies.