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Annex IV


7.3 Concentration of metals in aquatic fauna

Heavy metal concentrations in freshwater fish muscle are presented in Table VI. Some differences between water bodies were observed with respect to the levels of certain elements in finfish. For example, zinc showed relatively higher values in samples from Lake Nakuru, Kenya, followed in decreasing order by those from Zimbabwe and South Africa, Egypt, Nigeria and Ghana. Likewise, copper concentrations were higher in samples from Egypt and lakes Nakuru and Mcllwaine. Although data for iron were scare, the concentration of this element also seemed higher in samples from Egypt.

However, on the whole, the levels of metals in inland water fish muscle were below WHO limits, except for lead in Macrobrachium sp. from the Niger Delta, Nigeria and lower Volta River, Ghana.

Data on heavy metal concentrations in marine finfish and shellfish are summarized in Table VII. As for inland waters, the levels in marine organisms were generally below WHO limits except for some hot-spots. Cadmium and mercury for example showed the same low trends in both fin- and shellfish. In contrast, marine organisms from Nigeria had lead levels which exceeded the WHO limit. This may be attributed to a higher state of contamination from the use of leaded gasoline.

In support of what has been reported in several studies (Hellawell, 1986; Kakulu and Osibanjo, 1986; Kakulu et al., 1987a; Institute of Aquatic Biology, 1990), shellfish had higher concentrations of most metals. The highest concentrations of cadmium, copper and zinc occurred in Crassostrea sp. which has a great capacity to accumulate contaminants and is a biological indicator of pollution.

7.4 Concentration of Metals in Aquatic Flora

Aquatic plants have been shown to accumulate heavy metals in their tissues and therefore have been used as biological indicators for metal pollution monitoring in the aquatic ecosystem. Table VIII shows the distribution of heavy metals in aquatic plants. Generally, the levels in aquatic plants from inland waters were higher than in those from coastal waters. The variability in the levels of heavy metals in different regions could be ascribed to biological variation between the species rather than environmental factors. Nonetheless, higher concentrations of cadmium were found in Ceratophyllum from industrial areas in Egypt compared to relatively unpolluted areas (Fayed and Abd El-Shafy, 1985). Furthermore, it is significant that an excessively high value of lead (78.0 μg g-1) was found in blue-green algae from Lake Mcllwaine, Zimbabwe (Greichus et al., 1978) compared to the rest of the region.

Table V
Mean metal concentrations in marine sediments (μg g-1 dry weight)

 Abu-Kir Bay, Egypt 2.02 12    102 4.5Saad et al., 1981b
 Port Said, Egypt 3.2   14      50 2.5Saad et al., 1981b
 Eastern Harbour, Alexandria, Egypt 2.89 14      51 1.1Saad et al., 1981b
 El Mex, Egypt 2.18 24.1     35.4   1.47Saad et al., 1981
 South-western Mediterranean0.12          Bernhard, 1988
 Ebrié Lagoon, Côte d'Ivoire0.35     57.637.0187 52.40Kouadio and Trefry, 1987
 Lagos Lagoon, Nigeria 4.10178.9   15.0147 36.38Okoye et al., 1991
 Atlantic coast, Nigeria*0.10    2.3067.5      72.5   6.2 Ndiokwere, 1984
 Knysna Lagoon, South Africa0.019  0.2348.4  6.7     40.6  Watling and Watling, 1982a
UNPOLLUTED SEDIMENTS0.01–0.080.2–5.08–60  14   GESAMP, 1985, 1988

* Median values

Table VI
Mean metal concentrations in inland water fish (μg g-1 fresh weight)

 Lake Mariut, Egypt 0.15  3.7    7.60.911.2  Saad et al., 1981a
 Lakes ldku,Mariut, Egypt0.01    0.004 0.67   0.0311.77  7.4  El Nabawi et al., 1987
 Nozha Hydrodome, Egypt 0.05  3.14  8.0 12.6  Saad, 1987
 Kpong Headpond, Ghana0.053<0.10   0.43 0.36  5.6  0.633.8Biney, 1991
 River Wiwi, Ghana0.37  0.190.47 0.18  3.0  Biney and Beeko, 1991
 Niger Delta, Nigeria0.0340.030.48 0.70 et al., 1987a
 Lake Nakuru, Kenya0.0440.050.170.362.0  22  1.8 Greichus et al., 1978a
 Lake Victoria, Kenya* 0.04–0.120.4–1.1    0.15–0.532.21–7.020.22–0.740.53–4.65Wandiga and Onyari, 1787
 Lake Mcllwaine, Zimb.  9.65.4 Greichus et al., 1978
 Hartbeesport Dam, S.A. Greichus et al., 1977
 Voelvlei Dam, S. Africa 0.01<0.02  0.400.30  6.6  0.24 Greichus et al., 1977
Macrobrachium sp.         
 Lower Volta R., Ghana0.04  <0.10  4.36 11.0   16.1  Biney, 1991
 Niger Delta, Nigeria0.02  0.042.47 8.514.1  Kakulu et al., 1987a
Egeria radiata         
 Lower Volta R., Ghana0.05 <0.10  1.37 4.520.2  Biney, 1991
WHO Limits   0.05**2.0  2.0   30    1000          Kakulu et al., 1987a

* Range values
** Action level adopted in many countries

Table VII
Mean metal concentrations in marine fish (μg g-1 fresh weight)

 Egypt0.077   0.0040.071.65        4.23El Nabawi et al., 1987
 Senegal0.17   <0.10   0.500.73        4.55Ba, 1988
 Côte d'Ivoire0.11   <0.25    <0.80          4.86Metongo, 1988
 Ghana0.064  <0.10   0.360.46        4.63Institute of Aquatic Biology, 1990
 Ghana0.24       Ntow and Khwaja, 1989
 Nigeria <0.10  2.2811.3        27.5Okoye, 1991
 Cameroon0.09   0.26   Mbome et al., 1985
 Cameroon0.06   <0.10  1.830.75       5.55Mbome, 1988
 Kenya* 0.04–0.38  1.22–6.48  0.36–2.04      4.67–40.8Wandiga and Onyari, 1987
Penaeus sp.      
 Senegal0.17  <0.10  <0.50  4.68    13.9Ba, 1988
 Cote d'Ivoire0.042<0.25   6.02    17.9Metongo, 1988
 Ghana0.033<0.10  0.826.16    14.9Institute of Aquatic Biology, 1990
 Nigeria 0.185.1023.6240Okoye, 1991
 Cameroon0.057<0.10     Mbome et al., 1985
 Cameroon0.0700.21    9.5    40.4Mbome, 1988
Crassostrea sp.      
 Cote d'Ivoire0.1250.65 24.51205  Metongo, 1991
 Nigeria 0.172.09    5.80628Okoye, 1991
 Cameroon0.0720.56   Mbome et al., 1985
 Cameroon0.0830.25     8.45407Mbome, 1988
 South Africa 1.620.08    2.35213Watling and Watling, 1982a
 WHO Limits0.5**2.0  2.0   30.01000  Kakulu et al., 1987a

* Range values
** Action level adopted in many countries

Table VIII
Mean metal concentrations in aquatic plants (μg g-1 dry weight)

River Nile, Egypt         
 Ceratophyllum (clean site) <0.05    2.7   2.713.8  Fayed and Abd El-Shafy, 1985
 Ceratophyllum (industrial site) 0.3022.2 36.4117.0     Fayed and Abd El-Shafy, 1985
Lower Volta River, Ghana         
 Ceratophyllum0.370.9917.4 12.245.433322579Biney, 1991
 Pistia stratiotes0.310.9322.6 12.639.822593852Biney, 1991
 Potamogeton octandrus0.25<0.20    9.4   5.312.523701113Biney, 1991
 Vallisneria aethiopica0.131.3323.2 12.642.918093560Biney, 1991
Lake Mcllwaine, Zimbabwe         
 Blue-green algae0.261.5   78   2.9 190       220 Greichus et al., 1978
Harbeespoort Dam, S. Africa         
 Algae1.6   0.06<0.10 1.5  2.739.0    96 Greichus et al., 1977
 Eichhornia0.710.  840 Greichus et al., 1977
Accra, Ghana         
 Ulva fasciatus (Green algae)<0.10    <0.2    8.3   6.924.8   163Environ. Management Associates, 1989
 Sargassum vulgare (Brown algae)<0.10    <0.2    8.5   7.237.8   342Environ. Management Associates, 1989
 Polycavernosa dentata (Red Algae)<0.10    1.4  8.6   4.533.0   452Environ. Management Associates, 1989

7.5 Comparison between metal contents in sediments and biota

Comparisons between heavy metal concentrations in sediments and biota of selected African waters are shown in Table IX. In Egypt all metals except cadmium showed higher values in the sediments than in fish (Saad, 1985a, 1987). In Ghana only iron and lead followed this pattern, whereas the other metals gave higher values in certain flora and fauna (Biney, 1991a). In Kenya the metals accumulated in higher concentrations in sediments than in fish (Wandiga and Onyari, 1987). In Southern Africa the same pattern occurred with few exception (Greichus et al., 1977). The levels of accumulation of metals in the different flora and fauna did not follow the same pattern.

7.6 Comparison between different sub-regions in Africa

In Tables X and XI are presented the trace metal concentrations in sediments and in fish muscle and shellfish from the major African sub-regions, Northern, Western, Eastern and Southern Africa. The data presented are ranges of means based on Tables IV and V for sediments and Tables VI and VII for fin- and shellfish. Hot-spots, i.e., abnormally high concentrations were excluded form the calculations since the objective is to compare actual background levels from the different sub-regions in Africa.

An inspection of Tables X and XI shows inadequacy of data especially for the coastal and marine areas. In spite of this, the four regions exhibit comparable concentrations of trace metals in both inland and coastal fish and sediment. Mercury for example, occurred in finfish within the narrow ranges of 0.01 to 0.053μg g-1 fresh weight for inland fishes and 0.06 to 0.17 for marine fishes. Corresponding values for cadmium were 0.004 to 0.19 and 0.004 to 0.36 μg g-1 fresh weight.

Where comparable data were available, coastal fishes showed slightly higher maximum values of trace metals than inland fishes. This was also true for sediments and may be due to the data originating mainly from coastal lagoons which are normally heavily influenced by anthropogenic activities.

7.7 Comparison of African data with some other areas of the world

The levels of heavy metals in sediments and finfish from African inland and coastal waters are presented alongside data from some other areas of the world in Tables XII and XIII. The means and ranges for African waters were calculated from Tables IV and V for sediments and Tables VII and VIII for fish, excluding the hot spots.

Comparison of such data may be difficult since data calculated for the whole African region are being judged in relation to selected individual areas and sites of the world which may not be representative for their regions. Moreover, different species of fish and fractions of sediments were analyzed. Also, information on sex and weight is often lacking, and comparison is further complicated by the differences in data presentation. For example, analytical results may be presented as means or ranges on a dry or wet weight basis.

Table IX
Comparison of trace metal concentrations in sediments, fauna and flora (μg g-1 dry weight)

Lake Mariut, Egypt         
 Sediment 0.07 91     162 4747 Saad, 1985a
 Finfish 0.25 23        59   257 Saad, 1985a
Nozha Hydrodome, Egypt         
 Sediment 0.16 133         156 8628 Saad, 1987
 Finfish 0.47 34         41   109 Saad, 1987
Lower Volta River, Ghana         
 Sediment <0.2     21.729.5         39.131856821 Biney, 1991
 Macrophytes0.290.89 18.811.2         37.62560   2922 Biney, 1991
 Shellfish0.19<0.2       6.038.2         69.1      33.2          80.1 Biney, 1991
 Finfish0.29<0.2       2.32.0       30.7        3.4          19.0 Biney, 1991
Hartbeesport Dam, S.Afr.         
 Sediment0.600.8763   41    260680 75     Greichus et al., 1977
 Algae1.600.06<0.1 2.7  39  96 1.5Greichus et al., 1977
 Macrophytes0.710.23  2.612      42840 4.1Greichus et al., 1977
 Finfish0.520.05  1.02.9120 12 2.3Greichus et al., 1977
Lake Victoria, Kenya*         
 Sediment 0.55–1.026.02–69.40.19–78.62.54–265.253.7–6161180–52880 Wandiga and Onyari,87
 Finfish 0.04–0.120.39–1.080.15–0.532.2–7.020.12–0.740.53–4.65 Wandiga and Onyari,87
Mediterranean, Egypt        Saad et al., 1981
 Sediment 2.18 24.1    35.41511470 El Nabawi et al., 1987
 Finfish 0.02 8.2521.2    

* Range values

Table X
Metal concentrations in sediment from the major African sub-regions (μg g-1 dry weight)

Sub-regionHgCdPbCuZnMnFe(× 103)
 Northern Africa 0.15–0.207.3–10.638.0–85.694–139387–9580.46–58
 Western and Central Africa0.21–0.330.16–0.2013.4–16.724.7–30.316–62295–35255–60
 Eastern Africa<0.050.27–1.026.02–18.10.96–6.22.54–14053–5501.18–69
 Southern Africa0.02–0.280.19–1.09.0–17.810.5–41.036–289150–35012–16
 Northern Africa0.122.02–3.20 12–1435–51 1.1–4.5
 Western and Central Africa0.10–0.352.30–4.1057.6–67.513–3773–187 36–52
 Southern Africa0.0190.2348.46.7  41  

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