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)
Location | Hg | Cd | Pb | Cu | Zn | As | Fe(x103) | Reference | |
MEDITERRANEAN | |||||||||
Abu-Kir Bay, Egypt | 2.02 | 12 | 102 | 4.5 | Saad et al., 1981b | ||||
Port Said, Egypt | 3.2 | 14 | 50 | 2.5 | Saad et al., 1981b | ||||
Eastern Harbour, Alexandria, Egypt | 2.89 | 14 | 51 | 1.1 | Saad et al., 1981b | ||||
El Mex, Egypt | 2.18 | 24.1 | 35.4 | 1.47 | Saad et al., 1981 | ||||
South-western Mediterranean | 0.12 | Bernhard, 1988 | |||||||
GULF OF GUINEA | |||||||||
Ebrié Lagoon, Côte d'Ivoire | 0.35 | 57.6 | 37.0 | 187 | 52.40 | Kouadio and Trefry, 1987 | |||
Lagos Lagoon, Nigeria | 4.10 | 178.9 | 15.0 | 147 | 36.38 | Okoye et al., 1991 | |||
Atlantic coast, Nigeria* | 0.10 | 2.30 | 67.5 | 72.5 | 6.2 | Ndiokwere, 1984 | |||
INDIAN OCEAN | |||||||||
Knysna Lagoon, South Africa | 0.019 | 0.23 | 48.4 | 6.7 | 40.6 | Watling and Watling, 1982a | |||
UNPOLLUTED SEDIMENTS | 0.01–0.08 | 0.2–5.0 | 8–60 | 14 | GESAMP, 1985, 1988 |
Table VI
Mean metal concentrations in inland water fish (μg g-1 fresh weight)
Location | Hg | Cd | Pb | As | Cu | Zn | Mn | Fe | References | |
FINFISH | ||||||||||
Lake Mariut, Egypt | 0.15 | 3.7 | 7.6 | 0.9 | 11.2 | Saad et al., 1981a | ||||
Lakes ldku,Mariut, Egypt | 0.01 | 0.004 | 0.67 | 0.031 | 1.77 | 7.4 | El Nabawi et al., 1987 | |||
Nozha Hydrodome, Egypt | 0.05 | 3.14 | 8.0 | 12.6 | Saad, 1987 | |||||
Kpong Headpond, Ghana | 0.053 | <0.10 | 0.43 | 0.36 | 5.6 | 0.63 | 3.8 | Biney, 1991 | ||
River Wiwi, Ghana | 0.37 | 0.19 | 0.47 | 0.18 | 3.0 | Biney and Beeko, 1991 | ||||
Niger Delta, Nigeria | 0.034 | 0.03 | 0.48 | 0.70 | 4.8 | 1.1 | 5.4 | Kakulu et al., 1987a | ||
Lake Nakuru, Kenya | 0.044 | 0.05 | 0.17 | 0.36 | 2.0 | 22 | 1.8 | Greichus et al., 1978a | ||
Lake Victoria, Kenya* | 0.04–0.12 | 0.4–1.1 | 0.15–0.53 | 2.21–7.02 | 0.22–0.74 | 0.53–4.65 | Wandiga and Onyari, 1787 | |||
Lake Mcllwaine, Zimb. | 0.02 | 0.17 | 0.28 | 1.08 | 9.6 | 5.4 | Greichus et al., 1978 | |||
Hartbeesport Dam, S.A. | 0.02 | 0.05 | 0.26 | 0.66 | 11.8 | 1.6 | Greichus et al., 1977 | |||
Voelvlei Dam, S. Africa | 0.01 | <0.02 | 0.40 | 0.30 | 6.6 | 0.24 | Greichus et al., 1977 | |||
SHELLFISH | ||||||||||
Macrobrachium sp. | ||||||||||
Lower Volta R., Ghana | 0.04 | <0.10 | 4.36 | 11.0 | 16.1 | Biney, 1991 | ||||
Niger Delta, Nigeria | 0.02 | 0.04 | 2.47 | 8.5 | 14.1 | Kakulu et al., 1987a | ||||
Egeria radiata | ||||||||||
Lower Volta R., Ghana | 0.05 | <0.10 | 1.37 | 4.5 | 20.2 | 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)
Location | Hg | Cd | Pb | Cu | Zn | Reference | |
FINFISH | |||||||
Egypt | 0.077 | 0.004 | 0.07 | 1.65 | 4.23 | El Nabawi et al., 1987 | |
Senegal | 0.17 | <0.10 | 0.50 | 0.73 | 4.55 | Ba, 1988 | |
Côte d'Ivoire | 0.11 | <0.25 | <0.80 | 4.86 | Metongo, 1988 | ||
Ghana | 0.064 | <0.10 | 0.36 | 0.46 | 4.63 | Institute of Aquatic Biology, 1990 | |
Ghana | 0.24 | Ntow and Khwaja, 1989 | |||||
Nigeria | <0.10 | 2.28 | 11.3 | 27.5 | Okoye, 1991 | ||
Cameroon | 0.09 | 0.26 | Mbome et al., 1985 | ||||
Cameroon | 0.06 | <0.10 | 1.83 | 0.75 | 5.55 | Mbome, 1988 | |
Kenya* | 0.04–0.38 | 1.22–6.48 | 0.36–2.04 | 4.67–40.8 | Wandiga and Onyari, 1987 | ||
SHELLFISH | |||||||
Penaeus sp. | |||||||
Senegal | 0.17 | <0.10 | <0.50 | 4.68 | 13.9 | Ba, 1988 | |
Cote d'Ivoire | 0.042 | <0.25 | 6.02 | 17.9 | Metongo, 1988 | ||
Ghana | 0.033 | <0.10 | 0.82 | 6.16 | 14.9 | Institute of Aquatic Biology, 1990 | |
Nigeria | 0.18 | 5.10 | 23.6 | 240 | Okoye, 1991 | ||
Cameroon | 0.057 | <0.10 | Mbome et al., 1985 | ||||
Cameroon | 0.070 | 0.21 | 9.5 | 40.4 | Mbome, 1988 | ||
Crassostrea sp. | |||||||
Cote d'Ivoire | 0.125 | 0.65 | 24.5 | 1205 | Metongo, 1991 | ||
Nigeria | 0.17 | 2.09 | 5.80 | 628 | Okoye, 1991 | ||
Cameroon | 0.072 | 0.56 | Mbome et al., 1985 | ||||
Cameroon | 0.083 | 0.25 | 8.45 | 407 | Mbome, 1988 | ||
South Africa | 1.62 | 0.08 | 2.35 | 213 | Watling and Watling, 1982a | ||
WHO Limits | 0.5** | 2.0 | 2.0 | 30.0 | 1000 | 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)
Location | Hg | Cd | Pb | As | Cu | Zn | Mn | Fe | References | |
INLAND WATERS | ||||||||||
River Nile, Egypt | ||||||||||
Ceratophyllum (clean site) | <0.05 | 2.7 | 2.7 | 13.8 | Fayed and Abd El-Shafy, 1985 | |||||
Ceratophyllum (industrial site) | 0.30 | 22.2 | 36.4 | 117.0 | Fayed and Abd El-Shafy, 1985 | |||||
Lower Volta River, Ghana | ||||||||||
Ceratophyllum | 0.37 | 0.99 | 17.4 | 12.2 | 45.4 | 3332 | 2579 | Biney, 1991 | ||
Pistia stratiotes | 0.31 | 0.93 | 22.6 | 12.6 | 39.8 | 2259 | 3852 | Biney, 1991 | ||
Potamogeton octandrus | 0.25 | <0.20 | 9.4 | 5.3 | 12.5 | 2370 | 1113 | Biney, 1991 | ||
Vallisneria aethiopica | 0.13 | 1.33 | 23.2 | 12.6 | 42.9 | 1809 | 3560 | Biney, 1991 | ||
Lake Mcllwaine, Zimbabwe | ||||||||||
Blue-green algae | 0.26 | 1.5 | 78 | 2.9 | 190 | 220 | Greichus et al., 1978 | |||
Harbeespoort Dam, S. Africa | ||||||||||
Algae | 1.6 | 0.06 | <0.10 | 1.5 | 2.7 | 39.0 | 96 | Greichus et al., 1977 | ||
Eichhornia | 0.71 | 0.23 | 2.6 | 4.1 | 12.0 | 42.0 | 840 | Greichus et al., 1977 | ||
COASTAL WATERS | ||||||||||
Accra, Ghana | ||||||||||
Ulva fasciatus (Green algae) | <0.10 | <0.2 | 8.3 | 6.9 | 24.8 | 163 | Environ. Management Associates, 1989 | |||
Sargassum vulgare (Brown algae) | <0.10 | <0.2 | 8.5 | 7.2 | 37.8 | 342 | Environ. Management Associates, 1989 | |||
Polycavernosa dentata (Red Algae) | <0.10 | 1.4 | 8.6 | 4.5 | 33.0 | 452 | Environ. 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)
Matrix | Hg | Cd | Pb | Cu | Zn | Mn | Fe | As | References | |
INLAND WATERS | ||||||||||
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 | 21.7 | 29.5 | 39.1 | 318 | 56821 | Biney, 1991 | |||
Macrophytes | 0.29 | 0.89 | 18.8 | 11.2 | 37.6 | 2560 | 2922 | Biney, 1991 | ||
Shellfish | 0.19 | <0.2 | 6.0 | 38.2 | 69.1 | 33.2 | 80.1 | Biney, 1991 | ||
Finfish | 0.29 | <0.2 | 2.3 | 2.0 | 30.7 | 3.4 | 19.0 | Biney, 1991 | ||
Hartbeesport Dam, S.Afr. | ||||||||||
Sediment | 0.60 | 0.87 | 63 | 41 | 260 | 680 | 75 | Greichus et al., 1977 | ||
Algae | 1.60 | 0.06 | <0.1 | 2.7 | 39 | 96 | 1.5 | Greichus et al., 1977 | ||
Macrophytes | 0.71 | 0.23 | 2.6 | 12 | 42 | 840 | 4.1 | Greichus et al., 1977 | ||
Finfish | 0.52 | 0.05 | 1.0 | 2.9 | 120 | 12 | 2.3 | Greichus et al., 1977 | ||
Lake Victoria, Kenya* | ||||||||||
Sediment | 0.55–1.02 | 6.02–69.4 | 0.19–78.6 | 2.54–265.2 | 53.7–616 | 1180–52880 | Wandiga and Onyari,87 | |||
Finfish | 0.04–0.12 | 0.39–1.08 | 0.15–0.53 | 2.2–7.02 | 0.12–0.74 | 0.53–4.65 | Wandiga and Onyari,87 | |||
COASTAL WATERS | ||||||||||
Mediterranean, Egypt | Saad et al., 1981 | |||||||||
Sediment | 2.18 | 24.1 | 35.4 | 151 | 1470 | El Nabawi et al., 1987 | ||||
Finfish | 0.02 | 8.25 | 21.2 |
Table X
Metal concentrations in sediment from the major African sub-regions (μg g-1 dry weight)
Sub-region | Hg | Cd | Pb | Cu | Zn | Mn | Fe(× 103) | |
INLAND WATERS | ||||||||
Northern Africa | 0.15–0.20 | 7.3–10.6 | 38.0–85.6 | 94–139 | 387–958 | 0.46–58 | ||
Western and Central Africa | 0.21–0.33 | 0.16–0.20 | 13.4–16.7 | 24.7–30.3 | 16–62 | 295–352 | 55–60 | |
Eastern Africa | <0.05 | 0.27–1.02 | 6.02–18.1 | 0.96–6.2 | 2.54–140 | 53–550 | 1.18–69 | |
Southern Africa | 0.02–0.28 | 0.19–1.0 | 9.0–17.8 | 10.5–41.0 | 36–289 | 150–350 | 12–16 | |
COASTAL WATERS | ||||||||
Northern Africa | 0.12 | 2.02–3.20 | 12–14 | 35–51 | 1.1–4.5 | |||
Western and Central Africa | 0.10–0.35 | 2.30–4.10 | 57.6–67.5 | 13–37 | 73–187 | 36–52 | ||
Southern Africa | 0.019 | 0.23 | 48.4 | 6.7 | 41 |