Ecological Risk Assessment , Uptake Pattern Analysis and Depuration Studies of Heavy Metals Using Selected Fish Species and Cultured Gill Epithelial Cells
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos
Aquatic pollution by heavy metals remains a recurrent problem globally due to their persistence, toxicity and ability to accumulate in biological systems. This study investigated the pollution trend, ecological and public health risks associated with heavy metal pollution in the Lagos Lagoon as well as pattern of interaction among heavy metals that can be exploited for remediation of exposed aquatic organisms. Heavy metal content in surface water, sediment and biota were analyzed using Atomic Absorption Spectrophotometry (AAS) and concentrations were compared to documented data from previous studies while risk indices and empirical Sediment Quality Guidelines (SQGs) were used to evaluate ecological and public health risks. Pattern of interaction among heavy metals in fish species (Sarotherodon melanotheron and Clarias gariepinus) were evaluated in laboratory bioassays and radioactive isotope was used as tracer to monitor pattern of uptake, effect of multiple exposures and water chemistry on uptake rates of a representative heavy metal in an edible shrimp species (Gammarus pulex). Effects of heavy metal exposure on target gene expression in cultured fish (Oncorhynchus mykiss) gill epithelia were also evaluated using Reverse Transcription quantitative Polymerase Chain Reaction (RT qPCR). Significantly (P < 0.05) lower concentrations of most of the heavy metals analyzed in surface water and sediment samples were found in zone 1 comprising Tincan Island, Iddo and Banana Island stations compared to the other zones (2 - 5) in both dry and rainy seasons. However, Lead (Pb) had a significantly (P < 0.05) higher concentration (0.03 ppm) in surface water collected from zone 1 compared to those collected from zones 2 - 4 (0.02 ppm in each zone respectively) during both seasons. Metal pollution trend analysis showed that concentration of selected heavy metals have decreased over the last two decades especially Pb with a concentration of 11.90 ppm in 1991 and 237 ppm in 1995 compared to 0.03 ppm in 2013 in surface water samples collected during the dry season. However, concentrations of Cadmium (Cd) increased significantly (P < 0.05) from values of 1 ppm in 1991 and 0.77 ppm in 1995 to 5.34 ppm in 2013 in surface water samples collected during the dry season. Cadmium, Arsenic (As) and Mercury (Hg) were found to be the main contributors to ecological risks associated with heavy metals in sediment of the Lagos lagoon and edible species analyzed were found not to currently pose public health risk with all heavy metals analyzed having Health Risk Index (HRI) < 1. Zinc (Zn) and Chromium (Cr) were found to significantly (P < 0.05) enhance depuration of the non essential heavy metals accumulated by exposed test organism (Clarias gariepinus). Zinc enhanced depuration of Pb, Cd and Hg by 13.99%, 16.67% and 10% respectively in flesh of exposed test organisms. The shrimp species (Gammarus pulex) was found to efficiently regulate internal Zn concentration as shown by decreasing residual Zn concentrations in the species (20, 916 pmol g -1; 10,321 pmol g -1 and 9,587 pmol g -1) which corresponded to decreasing period of acclimatization in synthetic fresh water devoid of Zn. The heavy metals Cd, Cobalt (Co), Copper (Cu), Silver (Ag) as well as Calcium (Ca) were also found to inhibit Zn uptake in the shrimp species. Zinc and Cd up regulated expression of target genes Metallothionein A & B (MTa and MTb) in cultured gill epithelia while Pb inhibited the expression of the genes. The study has shown that the health and integrity of the Lagos Lagoon ecosystem is threatened by heavy metal pollution aggravated by increasing concentrations of Cd, As and Hg, hence there is the need to continuously monitor pollution trends and also to re-evaluate and enforce safety limits for the deposition of heavy metals in the lagoon and adjoining aquatic ecosystems. The study has also revealed that essential heavy metals have the potential of reducing body burdens of non-essential heavy metals accumulated by organisms. It is therefore suggested that essential heavy metals should be deployed in developing eco-friendly in situ methods for remediating exposed organisms in heavy metal polluted ecosystems.