Metal concentrations in fish from the major African sub-regions (μg g-1 fresh weight)
|Western and Central Africa||0.034–0.053||0.03–0.19||0.43–0.48||0.18–0.70||3.0–5.6||0.63–1.1||3.8–5.4|
|Western and Central Africa||0.06–0.17||0.10–0.26||0.36–2.28||0.46–11.3||4.55–27.5|
|Western and Central Africa|
Comparison of metal concentrations in sediment from Africa and other areas of the world (μg g-1 dry weight)
|African inland waters||0.24||0.37||23.2||26.3||82.5||This study|
|African coastal waters||0.19||2.78||57.8||19.4||92||This study|
|North-east Ontario lakes||10.5–2900||130–448||Bradley and Morris, 1986|
|Narragansett Bay, USA||0.06–2.45||17–81||36–98||53–168||Eisler et al., 1977|
|River Tawe, Wales||39||862||326||5107||Vivian and Massie, 1977|
|Liverpool Dock, UK||109–613||90–1592||734–2087||Bellinger and Benham, 1987|
|Portsmouth Habour, UK||0.5–3.3||49–114||26–72||61–210||Soulsby et al., 1978|
|Evoikos Gulf, Greece||0.4–1.1||52–147||Angelidis et al., 1981|
|Straits of Malacca||ND*-125||6.5–35-3||1.0–26.3||Sen Gupta et al., 1990|
|Bahrain||13–106||0.02–0.05||1.70–15.1||5.60–10.0||Sen Gupta et al., 1990|
|Kuwait||50–170||0.09–0.23||3.3–68||20.1–21.9||Sen Gupta et al., 1990|
|Saudi Arabia||3–37||2.5–5.0||0.6–4.2||5.4–16.6||4.0–23||Linden et al., 1990|
|Hong Kong||22||96||Gomez et al., 1990|
|South China Sea||0.41–2.39||1.94–9.21||12.5–49.9||Gomez et al., 1990|
|Jakarta Bay||0.05–4000||5.0–400||10–780||60–7140||Gomez et al., 1990|
|Wellington Habour, New Zealand||22–6740||15–216||55–2270||Brodie et al., 1990|
|Fiji||<0.2||1.1–2.2||6.8–10||85–150||54–220||Brodie et al., 1990|
* ND - Not Detected
Comparison of metal concentrations in fish from Africa and other areas of the world (μg g-1 fresh weight)
|African inland waters||0.035||0.053||0.31||0.85||7.16||This study|
|African coastal waters||0.095||0.069||0.69||0.80||4.76||This study|
|British rivers||0.17||0.15||0.87||Mason, 1987|
|Northern Tyrrhenian Sea||1.21||<0.02||<0.20||0.37||3.92||Leonzi et al., 1981|
|Finnish lakes||0.77||Surma-Aho et al., 1986|
|Northern Indian Ocean||0.01||0.90||0.62||0.81||Sen Gupta et al., 1990|
|Bahrain||0.004–1.07||0.00003–0.071||0.10.0.47||Linden et al., 1990|
|Straits of Malacca||0.01–0.58||ND-0.10||ND-1.20||0.05–0.75||1.70–10.8||Gomez et al., 1990|
|Indonesia||0.02–0.20||0.02–0.03||0.09–0.68||0.33–0.68||0.30–9.96||Gomez et al., 1990|
|Gulf of Thailand||0.01–0.10||0.01–0.06||0.01–0.09||0.50–1.25||6.20–11.8||Gomez et al., 1990|
|Phillipines||0.01–1.10||ND-0.36||0.01–0.08||ND-4.43||0.20–58-4||Gomez et al., 1990|
|Hong Kong||ND-0.40||ND||ND-0.30||ND-1.10||0.80–25.4||Gomez et al., 1990|
|New Zealand||0.02–1.10||0.01–0.03||0.03–0.18||0.12–0.75||0.80–5.1||Brodie et al., 1990|
|Papua New Guinea||0.03–0.40||ND-0.10||ND-0.30||0.30–0.70||3.0–5.0||Brodie et al., 1990|
* ND - Not Detected
The above notwithstanding, the occurrence of trace metals in African aquatic systems is not excessive when compared to some other areas of the world. For example, mean mercury levels in fish were lower by an order of magnitude compared to values reported for mullets in the Tyrrhenian Sea, an area close to naturally occurring mercury deposits (Leonzi et al., 1981). They were, however, similar to levels in other tropical, less industrialized areas like Indonesia and Thailand (Gomez et al., 1990). The maximum cadmium concentrations were also low compared to fish from British rivers (Mason, 1987) and from the coast of the Philippines (Gomez et al., 1990), but they were within the same range as levels in other areas.
With the exception of lead, inland and coastal water sediments also had comparable or relatively low contents of Hg, Cd, Cu and Zn. Admittedly, these comparisons are based on data which exclude hot spots. However, the low occurrence of heavy metals in African aquatic environments indicate low inputs of contaminants containing trace metals, compared to the more industrialized regions.
In view of the expected increase in industrialization and urbanization in most African countries, it is still important to formulate pollution control policies that take into account the need to regulate discharges of contaminants into aquatic systems.
In spite of the actual relatively low inputs of contaminants, there is increasing awareness of the need to control waste discharges into the environment. In general, many African countries, aware of the possible detrimental effects, have formulated various laws to control aquatic pollution, although many of these are not enforced.
Since trace metals originate from both domestic and industrial discharges, the control of these sources would also serve to control trace metal contamination of the environment. In addition to legislation, control measures should include the formulation of standards and criteria, effluent treatment, monitoring, environmental training and education programmes. These aspects have been treated in detail for African inland waters by Biney et al. (1987) and Saad et al. (1990) and can equally well apply to coastal waters.
It is also important to add that, as part of the objectives of pollution control, emphasis should be placed on the need to minimize waste generation. Industries should be encouraged to adopt low- and non-waste technologies (LNWT) at all stages of a products life, i.e. raw material extraction, production, use and disposal. For planned and proposed industries the achievement of LNWT is possible through the assessment of their potential impacts on the environment and through the adoption of clean manufacturing processes in the design stage. Environmental auditing which involves self regulation should also be encouraged within the business community as part of an overall environmental management policy. The advantage here is that self-regulation is frequently more effective than reliance on official rules which may not cover every contingency.
This review of heavy metals in the African aquatic environment has shown that available data originate from only a few areas of the continent, are scattered and may be inconsistent in some cases. Besides, depending on the area, more information may exist on coastal than on inland areas or vice versa. It is also not possible to establish a trend in heavy metal accumulation since data cover only a narrow period of time. There is, therefore, a need to generate more data covering the different environmental compartments in all the African sub-regions.
Despite this inadequacy, some conclusions may be drawn from this review. Generally, lower concentrations of heavy metals occur in African aquatic systems compared to other areas of the world. Concentrations in inland and coastal environments exhibit no significant differences and on a continental level, the four geographical areas-Northern, Western, Eastern and Southern Africa - have similar low levels. There are however some hot-spots, such as lake Mariut, Lagos lagoon, Ebrié lagoon and Haartbeespoort dam.
With the expected increases in urbanization and socio-economic activities, there is the need to identify the sources and quantify the discharges of heavy metals into aquatic environments on a national basis. It is also important to formulate pollution control measures in each country which should cover legislation, standards and criteria, waste minimization, effluent treatment, monitoring, training, education and public awareness.
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