This study determined the concentrations of Copper (Cu), Iron (Fe), Lead (Pb) and Zinc (Zn) in four fish species (i.e Brycinus nurse, Chrysichthys nigrodigitatus, Hemichromis fasiatus, and Tillapia zilli). This was evaluated in whole fish and water collected from two locations (Okhoro and Bridge) of Ikpoba River, Benin City, Nigeria, in order to ascertain the magnitude of impact by these heavy metals on the resources of the investigated ecosystem. Heavy metal concentrations in water and fish were analysed using an atomic absorption spectrophotometer. The mean concentrations of the heavy metals in water were Cu (0.047-0.063mg/l), Fe (0.067-0.167mg/l), Pb (0.0137-0.0185mg/l) and Zn (0.47-0.53). The mean concentrations of the heavy metals in fish were Cu (B. nurse 1.100mg/kg, C. nigrodigitatus 2.106mg/kg, H. fasiatus 2.060mg/kg, T. zilli 1.480mg/kg,), Fe (B. nurse 78.20mg/kg, C. nigrodigitatus 85.80mg/kg, H. fasiatus 83.30mg/kg, T. zilli 84.64mg/kg), Pb (B. nurse 0.040mg/kg, C. nigodigitatus 0.1408mg/kg, H. fasiatus 0.296mg/kg, T. zilli 0.0452mg/kg), Zn (B. nurse 36.40mg/kg, C. nigrodigitatus 54.54mg/kg, H. fasiatus 88.80mg/kg, T. zilli 37.92mg/kg). The mean concentrations of metals in fish species were not significantly different (P > 0.05), the mean concentrations of metals in water were also not significantly different (P > 0.05) between stations. Metals were bioaccumulated by the fish species at various stations with higher values of Cu and Fe at station 2 and Pb and Zn at station 1. Heavy metal concentrations in fish and water were discussed with reference to the World Health Organisation (WHO) limits for food fish and water, the mean levels of Fe, Pb and Zn in fish exceeded the WHO limits for fish and fishery products. Therefore it was advocated that regular monitoring of heavy metals in fish and water in the River be carried out in order to curtail further negative impacts.
Africa is blessed with a lot of inland water bodies. These aquatic ecosystems could be lagoons, creeks, rivers, streams e.t.c. which play important role in the socio-economic lives of the riverine populace. The inhabitant of these areas depends on these water bodies as a source of livelihood, recreation among other things (Ndimele et al., 2011a). Apart from this, some of these aquatic ecosystems are major nursery grounds for fish species, so they are also important for their continuous existence (Kumolu-johnson, 2004). Fish constitutes an important and cheap source of animal protein to human beings and a large number of people depend on fish and fishing activities for their livelihood. All these benefits are threatened by industrialization in a bid to meet the growing demands of the world population. Nations are investing massively in industrialization, industrial, agricultural and domestic activities have led to the pollution of the Nigerian environment and subsequently increased the problem of waste disposals, however they are not committing enough funds to develop processes that will treat the waste generated by these industries as well as mitigate their effects on the environment and man (Ndimele et al., 2011b). A lot of industrial effluents are emptied into the aquatic environment untreated, in few cases where they are treated, the products of the treatment plants are still potentially harmful to aquatic flora, fauna and even man. One of the common components of industrial effluents is heavy metals (Kumolu-johnson et al., 2005), among environmental pollutants, metals are of particular concern, due to their potential toxic effect and ability to bioaccumulate in aquatic ecosystems (Censi et al., 2006). Therefore, knowledge of the changing concentration and distribution of heavy metals and their compounds in various compartments of the environment is a priority for good environmental management programmes all over the world (Don-Pedro et al., 2004).
Over the last few decades there has been growing interest in determining heavy metals levels in aquatic environment and attention was drawn to the measurement of contamination levels in public food supplies particularly fish. Increasing human influences through heavy metal pollution have however led to the depletion of our fish resources and substantial reduction in the nutritive values (Srivastava and Srivastava, 2008). As a result of these heavy metals pollution several endemic fish species have become threatened. Heavy metal pollution is a serious and wide spread problem due to the toxic, persistent, biodegradable and bio-accumulation properties of these contaminants (Yuan et al., 2009), they are considered as major environmental pollutants causing cytotoxic, mutagenic, and carcinogenic effects in animals (Rauf et al., 2009).
Heavy metals are general collective term which applies to the group of metals and metalloids with a specific gravity or density greater than 4g/cm3 (Duffus, 2002), they are chemical elements with a specific gravity that is at least five times the specific gravity of water. The specific gravity of water is 1 at 4°C (39°F). Simply stated, specific gravity is a measure of density of a given amount of a solid substance when it is compared to an equal amount of water. Heavy metals including both essential and non-essential elements have a particular significance in ecotoxicology, since they are highly persistent and all have the potential to be toxic to living organisms (Storelli et al., 2005). Heavy metals such as copper, iron, chromium and nickel are essential metals since their play an important role in biological systems, whereas cadmium and lead are non-essential metals, as they are toxic, even in trace amounts (Fernandes et al., 2008).
1.1 SOURCES OF HEAVY METAL POLLUTION IN THE AQUATIC ECOSYSTEM
Heavy metal pollution is a serious and widespread environmental problem due to their toxicity; they enter the environment through various natural methods and human activities, and can accumulate in fish and other organisms (Kalay and Canli, 2000). In Nigeria, the major sources of heavy metals pollution are industrial effluents discharged from various processing industries. This increases the influx of metals, which can be transported by wind and water and thus become available to plants and animals. In Benin metropolis, large scale wastes are generated by battery recharging workshops, poultry houses, gasoline servicing stations, rubber and plastic processing factories, markets and sawmills which use wood preservatives rich in resin acids, dioxins and pentachlorophenol (Oguzie and Okhagbuzo, 2009). Mismanagement of these wastes which are dumped in soils and unsanitary landfills are subjected to weathering and leaching processes by rain and other atmospheric influences resulting in the release of hazardous substances such as cyanides, minerals, heavy metals and organic acids which get to underground water systems and inland water bodies untreated. Their effects render underground and surface waters unsafe for human, recreational and agricultural use. Heavy metals are discharged into inland water bodies mainly through anthropogenic sources including mining activities, industrial and domestic effluents, urban storm-water run-offs, leaching of metals from garbage and solid wastes dump e.t.c. they accumulate in soils, plants and sediments from where they are released into inland water bodies. The receiving water bodies’ loads of pollutants are increased during the rainy season due to flood run-off water and atmospheric precipitation, during the dry season, the action of wind becomes more pronounced in the transport of hazardous substances which bioaccumulate in inland water bodies, the cumulative effects of the pollutants which eventually enter the food chain could cause severe physiological disorders and a host of other problems to aquatic organisms including fish. According to Goodwin et al. (2003), aquatic organism have the ability to accumulate heavy metals from various sources including sediments, soils erosion and run-off, air deposition of dust and aerosol and discharges of waste water. Botkin and keller (1998), categorised the sources of heavy metals as non-point or point sources, non-point sources are diffused and intermittent and are influenced by natural factors such as land use, climate, hydrology, topography, native vegetation and geology. The primary anthropogenic sources of heavy metals are point sources such as mines, foundries, smelters, and coal-burning power plants, as well as diffuse sources such as combustion by-products and vehicle emission.
1.2 OCCURRENCE AND FORMS OF SOME SELECTED HEAVY METALS
Lead is one of the oldest metals known to humans. It is a metal that occurs naturally in the earth although lead is found uncombined in nature. It is not very abundant, its relative rates being smaller than those of other metals as the aluminum, iron, e.t.c. however it is more abundant than cadmium. It has many industrial uses and is found in trace amount everywhere in the human environment (Environmental protection agency, 2012).
The major sources of lead emission have historically been from fuels in on-road motor vehicles, the point sources of lead and its compounds may be released to the environment from mining activities because they occur in the earth’s crust, in nature they occur in small amount in the earth crust usually present as lead sulfide, lead oxide, or lead carbonate. Lead is also emitted from volcanoes and forest fire.
Copper is widely distributed in many parts of the world mainly as mineral combinations with iron, sulfur, carbon and oxygen. It is a chemical element with the symbol Cu and atomic number 29 (Wikipedia, 2013). Copper occurs both in combined state and free state, it also contains many ores, the important ores of copper are copper pyrites (CuFeS2), cuprite and copper glance.
Copper is essential to all living organisms as a trace dietary minerals because it is key constituent of the respiratory enzyme complex cytochrome c oxidase. In molluscs and crustacean, copper is a constituent of the blood pigment hemocyanin, which is replaced by the iron- complexed hemoglobin in fish and other vertebrate.
Cadmium is a relatively rare element and natural, its abundance in the lithosphere is estimated at about 0.5 parts per million in the earth’s crust the only important ore of cadium is greenockite or cadmium sulfide (CdS). No cadmium has been found in commercial quantities and no more is mined solely for recovering cadmium.
Cadmium is found throughout the environment from natural sources and processes such as the erosion and abrasion of rocks and soils, and from singular events such as forest fires and volcanic eruptions. Surface waters containing in excess of a few micrograms of cadmium per liter have probably been contaminated by industrial wastes from metallurgical plants, plating works, plants manufacturing cadmium pigments, textile operations or by-effluents from sewage treatment plants. Cadmium is also present as a pollutant in phosphate fertilizer (Fleischer, 2011).
Zinc is a natural component of the earth’s crust and an inherent part of our environment. Zinc is present not only in rock and soil, but also in air, water and the biosphere. They are normally associated with lead and other metals including copper, gold and silver (International Zinc Association, 2011). Deposits containing zinc form from hot or hydrothermal fluids generated within the earth, some fluids may reach the ocean floor in areas of underwater volcanic activity to form volcanogenic deposits. Other fluids may escape to the surface through cracks or faults into small shallow lakes or seas and under suitable conditions, lead-zinc-silver deposits may form.
Rivers generally contain between 5-10 ppb zinc, winds and water carry minute amounts of zinc to lakes, rivers and seas where it collects as sediment or is transported further. It is estimated that natural emissions of zinc amount to 5.9 million metric tonnes each year, in comparison anthropogenic emissions of zinc to the atmosphere as a result from man’s activity are estimated at 57,000 metric tonnes per year worldwide.
Iron is an abundant element in the universe, it is found dissolved in ground waters and the ocean to a limited extent. It is made up of 5 percent of the earth’s crust and second in abundance to aluminum among the metals and fourth in abundance behind oxygen, silicon and aluminum, iron which is the chief constituent of the earth’s core is the most abundant element in the earth as a whole, contributing to the creation of the Earth’s magnetic field (Buzzle, 2011). In the earth crust the free metal is rare, occurring as terrestrial iron (alloyed with 2-3% nickel) in basaltic rocks.
Weathering processes releases the elements into waters, both mineral water and drinking water contains iron carbonate, in deep sea areas the water often contains iron fragments the size of a fist. Iron compounds are applied as pigments in glass production, chemicals, iron fertilizers or pesticides, aluminum products containing iron are discharged on surface water.
Manganese is a mineral element that is both nutritionally essential and potentially toxic. It comprises approximately 0.085% to 0.095% of the earth’s crust and component of many rock types, particularly those of metamorphic and sedimentary origin (Wikipedia, 2012). It is associated with iron ores of sub marginal concentration, predominate ores of manganese include pyrosulite (MnO2), manganite e.t.c. Total manganese concentrations in surface water showed a typical seasonal trend, with the highest annual manganese concentrations observed during high run-off periods. It is used in industrial processes and in various consumer products. The major man-made sources of environmental manganese include municipal wastewater discharge, sewage sludge, and to a lesser extent by emissions from the combustion of fuel additives.
1.3 JUSTIFICATION OF THE STUDY
Ikpoba River is an important fresh water system in Edo State with rich fisheries production. At present, there are a number of factories established in the area as well as farmers using pesticides and fertilizers in their farm. The factory effluents, pesticides and fertilizers as well as wastes from the nearby markets and workshops may finally get discharged into the water body. Ikpoba River (bridge) and reservoir are hotspots where washing, bathing and fishing takes place. Diverse amount of wastes from these activities and effluents from a quarry company engaged in mining and crushing stones are also constantly discharged to these freshwater ecosystems and there is the possibility of heavy metals building up in water and aquatic organisms especially in fishes.
The present study sets out to assess the levels of heavy metals in water and whole fish species in Ikpoba river with a view to determining whether the concentration level of these metals constitute health hazards to consumers.
1.4 OBJECTIVE OF THE STUDY
The specific objectives of the study were to;
- determine the heavy metals (Pb, Mn, Cu, Fe) concentration in water and fish from the Ikpoba river since fish is an important component of the human diet in this zone.
- to ascertain if fish species bioaccumulated heavy metals from their environment.
- compare the levels of heavy metals in fish and water with standards set for fish and fishery products by the Food and Agricultural Organisation (FAO) of the United Nations and World Health Organisation (WHO).
- make recommendation on the safety of heavy metals concentration in the river.