Background

Heavy metal levels have increased in the Niger Delta estuaries over the past decade, due to domestic, industrial, mining and agricultural activities (Kaoud, 2015). Wastes from these activities discharged and released into the marine environment may cause extensive ecological differences, due to their level of toxicity, persistence and accumulative behaviour in the organisms that dwell in the marine/estuarine ecosystem (Indrajit et al., 2011). Most heavy metals are the natural constituents of the earth’s crust from where they are taken up by organisms and transferred in to the food chain. A good number of heavy metals are present in trace amount, but high levels of accumulation may affect organisms through the food chain and pose risk to consumers of seafood when concentrations exceed permissible limits (El-Moselhy et al., 2014). Essential metals such as Cu, Zn, Co, Cr, Ni and Mn are needed in trace amounts (smaller than 0.01% of the mass of the organism) in the diet and their absence may cause serious problems (Ekweozor et al., 2017), while non-essential metals such as Cd, Pb, As, and Hg have no biological function and their presence even in very small quantities may be toxic. Nonetheless, it is clear that they are potentially hazardous to living organisms at high exposure levels and are extremely persistent in the marine environment and therefore should be routinely monitored (Igwemmar et al., 2013).

The metal concentrations vary with species which may be related to their feeding habits and bioaccumulation capacity (Akoto et al., 2014). Fish, as a predator, is able to concentrate high amount of heavy metals through the food chain and is an important and affordable source of protein to man (Al-Busaidi et al., 2011). It is important to monitor the levels of metals such as As, Cd, Cr, Ni, Pb, Hg, V, and Zn in water, sediment and fish in order to compare them with regulatory standards and guidelines for public health purposes. Fish species such as S. melanotheron are among top aquatic organisms exposed to high amount of heavy metal due to their feeding habits. They are able to filter out microscopic algae, invertebrates and detritus from the sediment (Ofori-Danson and Grace, 2016). Fish has various point of entry; skin, oral consumption of water, assimilation through the gills, food and non-food particles. Once absorbed, they are then carried in the blood stream to a storage point organ such as the liver for transformation and/or storage. Upper reaches of Bonny Estuary have been exposed to pollution pressures that derive both from human activities and natural sources. The communities that are situated along the shore of the river include the Okochiri, Ekerekana, Okari-ama, Oba-ama, Ogoloma and Kalio-ama communities. Health risk assessment for heavy metal level in fish is a very useful technique that provides information about any threat regarding heavy metal contamination to consumers. Therefore, their concentration in the aquatic environment needs to be assessed as this may impact and become threats to them. In this study, S. melanotheron was chosen based on its food and economic value in the Niger Delta. The concentration of the listed metals - As, Cd, Cr, Ni, Pb, Hg, V, and Zn- in the gill, muscle and liver of S. melanotheron was assessed to reveal its safety value in a creek of the Niger Delta.

1 Results and Discussion

1.1 Trace metal concentrations in water

The concentration of trace metal in water (Table 1) showed that Zn had the highest concentration which was found in stations 1 and 2 (0.008 ± 0.013 and 0.007 ± 0.001,2 mg/L) compared to Cr, Ni, V, Pb, Cd, Hg and As which recorded <0.001 mg/L in all stations. The concentrations were not different at p<0.05 and were below permissible limit for drinking water. This result agrees with the study of Otokunefor and Obiukwu (2005) for heavy metal concentration in surface water in the Okrika River.

1.2 Heavy metal concentration in sediment

The concentration of heavy metal in sediment was significantly higher than the concentration in water. However according to Sediment Quality Guideline (USEPA, 1999) the heavy metal concentration in sediment (Table 2) was also below permissible limit indicating absence of pollution levels in the sampled sediment. Ecological indices revealed that the metal contamination factor in the creek sediments across all sampling stations were of low contamination levels. The Pollution Load Index (PLI) in all the sampled stations were <1 which revealed that the study area was not polluted with the studied metals (Table 3; Table 4). This is in contradiction with a study of heavy metal concentration in the Choba section of the New Calabar River, Eastern Niger Delta (Nwankwoala and Angaya, 2017) that expressed the area as polluted however their result on CF is in agree with results obtained in this study. Analysis using Geo-accumulation Index (Igeo) revealed values <1 which also indicated that Okrika River was not polluted with the studied metals at the time of sampling.

1.3 Heavy metal concentration in fish tissues

There were varying patterns in metal concentration in the organs of the fish from the 5 sampled stations. Concentrations in the tissues of S. melanotheron generally followed the order of Muscle>Gill>Liver (Table 5), and were not different at p<0.05. The high concentration of heavy metal in the muscle of S. melanotheron in this study is in agree with Ekweozor et al. (2017) who also recorded high concentration of heavy metals in the muscle of fish but not in agreement with El-Moselhy et al. (2014) and Ugbomeh and Akani (2016) that observed lower concentration of all metals in the muscle. This implies that the affinity for the accumulation of heavy metal varies from fish to fish and from organ to organ. The muscle of fish is the edible part, and the concentrations of the heavy metals observed in this study is a health concern. The Cr concentration in fish muscle was above the FAO/WHO (2012) permissible limit of 0.5 mg/kg in three stations (S1, S3 and Control, S5). Pb was above FAO/WHO (2012) permissible limit of 0.5 mg/kg in the muscle of S. melanotheron from the POD (S2) and S3. Zn concentration was below permissible limit of 40 mg/kg in the muscle. Ni concentrations were above permissible limit FAO/WHO (2012) of 0.6 mg/kg in the muscle of fish caught in all stations. V concentration were all below permissible limit FAO/WHO (2012) of 0.5 mg/kg in all stations. Muscles may not to be active sites for bio-accumulation, but at chronic exposures in minute concentrations, fish muscles could concentrate heavy metals that exceed the permissible limits for human consumption and imply severe health implications.

In comparison with other tissues, Zn had the highest concentration (p<0.05) in all sampled stations, followed by Ni. Cr was not detected at the point of effluent discharge (S2) but detected in the gills of fish caught from S3 and S4. Gills are the main route of metal ion exchange from their surrounding medium as they have very large surface area that facilitate rapid diffusion of toxic metals (Dhaneesh et al., 2012). Highly increased concentration of metals in the liver may represent storage of sequestrated products in this organ (Hamilton and Mehrle, 1986). V and Pb were only detected in the muscle of fish from the POD (S2), S3 and S5. High concentration of V in the muscle of S. melanotheron is in agree with Tyokumbur and Umma (2017) who found higher concentration of V in the muscle of S. melanotheron. The concentration of Ni in the muscle of the fish disagrees with that of Tyokumbur and Umma (2017) that found higher concentration in the liver than other tissues of the fish. Cd, Hg, and As concentrations were below detectable limit in the tissues of the studied fish.

1.4 Bio-sediment accumulation factor

Bio-sediment accumulation factor was <1 for Cr in all the tissues in all the sampled station as shown in Table 6. Ni was ≥1 indicating bioaccumulation at the POD (S2) with a higher bio-accumulative potential in the muscle tissue (6.06). In S3, the bioaccumulative factor of Ni in the muscle was higher than in the liver of the fish. Fish from S4 showed bioaccumulation factor >1 in all the tissues. S. melanotheron from S5 also had BSAF for Zn higher than 1 in the muscle, liver and gills. Fish from S5 showed the highest bioaccumulation of Zn than in other stations. The bio-accumulative factor of Pb was <1. Other metals (Cd, Hg, As) were not detected.

1.5 Risk associated with consumption of S. melanotheron

The HQ for the heavy metals were <1 in all stations as shown in Table 7. The results from this study showed that consumption of this fish from the sampled area had no potential health implications for the individual heavy metals. Also the HI values were <1 which further indicates no threat to the people that consume S. melanotheron from the study area. However, there is need for constant monitoring of heavy metals in S. melanotheron as the bioaccumulation of these metals may over time become a health hazard for those who consume them.

2 Materials and Methods

2.1 Description of the study area

The sampling stations were established along the stretch of Upper Bonny Estuary and Okrika River, Rivers State, Niger Delta. The river is a brackish inter-tidal mangrove swamp. The vegetation consists of R. racemosa, L. racemosa, R. mangle, Nypa fructan, and Avicennia nitida that line the shores of the creek. Anthropogenic activities along the creek include point source of industrial effluent discharge, sand mining or dredging, fishing, navigation, washing, bathing, recreational and illegal bunkering activities. A major industrial outfit - the Port Harcourt Refinery Company (PHRC) - a point source of industrial effluent discharge generates several volumes of effluent channeled into the water body via a drainage system. The shoreline of the water body includes communities representing the names of the creeks which are the 5 main sampling stations as indicated in Figure 1 and described below.

Okochiri creek as station 1 (S1): Representing the upstream of the river N04^o44’36.4”, E007^o06’31.7”.

Ekerekana creek as station 2 (S2): The Point source of industrial effluent discharge into the river N04^o44’46.9”, E007^o06’06.0”.

Okari-ama creek as station 3 (S3): Representing 500 metres away from the point source N04^o44’23.2”, E007^o05’51.8”.

Ogoloma creek as station 4 (S4): Representing further downstream of the river N04^o43’59.0’ E007^o05’05.7”.

Bonny river (S5) (Control station) is the farthest from the point source N04^o43’16.0”, E007^o04’40.8”.

2.2 Sample collection and method

The surface water samples were collected at low tide at about 30 cm depth with a 2 litre plastic Hydrobios sampling bottle by lowering the corked bottle to the required depth before removing the cork to fill up to overflow. This was transferred to a clean 2 litre polyethylene container and fixed with 2 drops of concentrated HNO₃ to preserve the metals. The water samples were stored in an ice box at 4°C and taken to the laboratory within six (6) hours for analysis. All analyses were completed within two weeks of collection. The water sample was aspirated on to the Atomic Absorption spectrophotometer (AAS) model GBC Avanta Version 2.02 and the metal concentrations were recorded in ppm.

2.3 Sediment sample analysis

Sediment samples were collected from the intertidal at low tide with the use of a stainless Eckman’s grab. The samples were placed in pre-cleaned well labelled polythene bags and transferred in an ice chest to the laboratory. In the laboratory, samples were air dried for two to three weeks at room temperature until weight was constant. After drying the sediment sample, visible remains of organisms and debris were removed, sample was crushed in a mortar and sieved using a 200 µm sieve to normalise particle size. Samples were further homogenised in a porcelain mortar, sieved (using a 2-mm mesh size) and stored in a plastic container. Two grams of crushed sample was dissolved in 5 ml of ultra pure Nitric acid 10% in HCL solution. The solution was filtered into a cleaned and dried 20 ml standard volumetric flask. The solution was then aspirated on to the Atomic Absorption spectrophotometer (AAS) model GBC Avanta Version 2.02 and the metal concentrations were recorded in ppm.

2.4 Preparation of fish samples and determination of heavy metals

The fish was collected along the Okrika river in the Niger Delta from the five stations at monthly intervals for six months (6) from September 2016 to February 2017 using cast nets of mesh size 30 mm-50 mm and gill nets of mesh size 30-50 mm. Samples were taken randomly from marketable sizes. Fish samples were labelled, preserved in ice, transported to the laboratory and stored in the freezer prior to pre-treatment and analysis. 36 samples of S. melanotheron were used in this study. In the laboratory whole biota samples were properly cleaned with distilled water to remove debris and all external adherents, weighed to the nearest 0.01 g using a top pan electronic balance and measured in cm using a ruler to the nearest mm.

2.5 Sample preparation

Samples were prepared according to methods by APHA (1995) and Ademoroti (1996). The samples were thawed on a clean plastic sheet, and the gill, muscle and liver were excised using a dissecting kit. The tissues extracted were placed in cleaned silica dishes, dried and ashed (ignited at 500°C for 30 minutes). The samples were allowed to cool in a desiccator and crushed into powder. Portions of 2 g of sample were then weighed for acid digestion as was done for the sediment.

2.6 Quality control

Quality control was according to Ezeonyejiaku and Obiakor (2016).

2.7 Statistical analysis

The data collected for heavy metals was subjected to analysis of variance (ANOVA), Turkeys multiple comparison was used to compare the significant difference of the mean values amongst the sampled station. Probabilities of <0.05 were considered statistically significant. All statistical analyses were carried out with the SPSS 20 software program.

2.8 Biosediment accumulation factor

The bio-sediment accumulation factor (BSAF) (Deforest et al., 2007) was calculated as follows:

2.9 Contamination Factor (CF), Pollution Load index (PLI) and Geo-accumulation Index (Igeo)

The contamination level of sediment is expressed as a contamination factor (CF) calculated from:

CF = C_m sample/C_m background

Where: C_m sample = the concentration of a given metal in sediment; C_m background = the value of the metal equal to the Average Shale Value (ASV) (Turekian and Wedepohl, 1961) - Cr (90), Zn (95), Pb (20), Ni (50) and V (103).

2.10 Pollution Load Index (PLI)

Pollution load index (PLI) (Tomlinson et al., 1980), as a means of assessing the quality of the study area with respect to heavy metals contamination was evaluated as:

PLI = (CF₁ x CF₂ x CF₃ x ...... CF_n)^1/n

Where: n is the number of metals, CF is the contamination factor of individual metal.

When the value of PLI is 1, it indicates heavy metal load close to background levels, while values above 1 indicate pollution.

2.11 Index of Geo-accumulation (Igeo)

Geo-accumulation index (Igeo) (Muller, 1969) compares current concentration of heavy metals with preindustrial level. The geo-accumulation index (Igeo) values were calculated for the different metals using the equation:

Igeo = Log 2 (C_n/1.5 B_n) 

Where: C_n=Measured concentration of element in the sediments sample, B_n=Geochemical background for the element n.

The factor 1.5 is used to compensate for possible variations with respect to background lithological variations. Average shale values (ASV) by Turekian and Wedepohl (1961) in mg/kg of Cr (90), Zn (95), Pb (20), Ni (68) and V (103) were used as the background values to estimate Igeo.

2.12 Public health risk assessment of fish consumption

Potential risk associated with human consumption of fish muscle was assessed by comparing concentrations of the metal in fish tissue with the FAO/WHO regulatory standards (Ezeonyejiaku and Obiakor, 2016). The Hazard Index (HI) was evaluated following methods by USEPA (2000). Exposure level to the metals through consumption of fish muscle was obtained by calculating the average daily dose (ADD in mg/kg/d) using the equation:

ADD = (C×IR×EF×ED)/(BW × AT)

Where: C = the mean trace metal concentration in fish muscle (mg/kg); IR = the mean ingestion rate i.e, the fish consumption rate estimated to be 0.04 mg/kg for normal adults in the Niger Delta region of Nigeria (Anyakora et al., 2008; Maximilian et al., 2015); EF = Exposure Frequency (365 days per year); ED = Exposure duration over a life time (54 years) (World Bank, 2015); BW = Body weight (70 kg for Adults); AT = Average Life time (54 years × 365 days per year).

The risk assessment was then computed by calculating the Hazard Quotient (HQ), which is the non-cancer index of adverse health effects from the intake of trace metals in food expressed as:

HQ = ADD/RfD

Where RfD is the oral reference dose of trace metals (mg/kg/d) based on the upper level of metal intake for an adult person with an average body weight of 70 kg. The oral RfD values adapted from WHO (2008), FAO/WHO (2006) and the Integrated Risk Information System of USEPA (1985; 1998) are: Zn = 0.214, Pb = 0.003,57, Cd = 0.001, Cr = 0.003, and Ni = 0.02 mg/kg/d.

The hazard index (HI) (Guerra et al., 2012) is the sum of the Hazard Quotients for all heavy metals and it estimates the risk to human health through more than one heavy metal (USEPA, 1998). It was calculated by the equation:

HI = ΣHQ = HQ_Cr + HQ_Zn + HQ_Pb +HQ_Ni

An HI value <1.0 suggests that adverse health effects are not likely to occur, while an HI≥1 suggests that probable risk and adverse health effects are likely to occur.

Authors’ contributions

This work was carried out in collaboration between all authors. Authors IKE and APU designed the study and wrote the protocol, author AWG performed the statistical analyses and wrote the first draft of the manuscript. Authors AWG, MM, EN and IKE managed the literature searches and analyses of the study. Author APU did the final draft of the manuscript. All authors read and approved the final manuscript.

Acknowledgements

We want to thank Dr and Mrs Adolphus Anaero-Nweke for their financial assistance and the Rivers State University, Nkpolu-Orowurukwo Port Harcourt for enabling the project.

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