1. INTRODUCTION
In natural aquatic ecosystems, metals occur in low concentrations, normally at the nanogram to microgram per litre level. In recent times, however, the occurrence of metal contaminants' especially the heavy metals in excess of natural loads, has become a problem of increasing concern. This situation has arisen as a result of the rapid growth of population, increased urbanisation, expansion of industrial activities, exploration and exploitation of natural resources, extension of irrigation and other modern agricultural practices as well as the lack of environmental regulations.
At the global level the scientific community has investigated some of these problems and the results have been published in several reviews and books (Nriagu, 1989; Förstner and Wittmann, 1981; Salomons and Förstner, 1984). Most of the present introductory material is based on these sources.
Unlike other pollutants like petroleum hydrocarbons and litter which may visibly build up in the environment, trace metals may accumulate, unnoticed, to toxic levels. Thus problems associated with trace metal contamination were first highlighted in the industrially advanced countries because of their larger industrial discharges and especially by incidents of mercury and cadmium pollution in Sweden and Japan (Kurland et al., 1960; Nitta, 1972; Goldberg, 1976). In spite of the relatively low level of industrial activity in less developed regions such as Africa, there is nevertheless growing awareness of the need for rational management of aquatic resources including control of waste discharges into the environment. This becomes even more important in view of the expected increases in industrial and urban activities in all parts of the continent.
Existing information on various environmental problems has been reviewed by Dejoux (1988) in a monograph of pollution in African inland waters and by Phillips (1991) on a worldwide basis on tropical marine ecosystems. These publications showed that the existing information on Africa is scattered and scarce, and therefore demonstrated the need for a more precise and specific review of the occurrence of trace heavy metals in various aquatic environmental compartments in the continent.
For effective water pollution control and management there is a need for a clear understanding of the inputs (loads), distribution and fate of contaminants, including trace metals from land-based sources into aquatic ecosystems. In particular, the quantities and qualities need to be considered together with the distribution pathways and fate and the effects on biota.
The need to make an assessment of the level of heavy metal contamination in the African environment has led to the initiation of several pollution monitoring programmes and research work in various universities and scientific institutions in the region. The most relevant programmes are the Mediterranean pollution monitoring programme (MEDPOL) covering also North Africa, the West and Central Africa marine pollution and research programme (WACAF 2) and the Eastern Africa marine pollution and research programme (EAF/6).
During the last decade, there have also been considerable improvements in the sampling and analytical techniques for trace metals. These, coupled with international intercomparison exercises, have facilitated the generation of more reliable data. The present paper thus attempts to compile and analyze the available information on the occurrence of trace metals in both freshwater and marine ecosystems of Africa as a contribution towards the formulation of rational management policies for aquatic resources in the continent.
The decision to review freshwater and marine data jointly is a result of the need to have a holistic approach that could influence future control strategies.
In natural aquatic ecosystems, metals occur in low concentrations, normally at the nanogram to microgram per litre level. In recent times, however, the occurrence of metal contaminants' especially the heavy metals in excess of natural loads, has become a problem of increasing concern. This situation has arisen as a result of the rapid growth of population, increased urbanisation, expansion of industrial activities, exploration and exploitation of natural resources, extension of irrigation and other modern agricultural practices as well as the lack of environmental regulations.
At the global level the scientific community has investigated some of these problems and the results have been published in several reviews and books (Nriagu, 1989; Förstner and Wittmann, 1981; Salomons and Förstner, 1984). Most of the present introductory material is based on these sources.
Unlike other pollutants like petroleum hydrocarbons and litter which may visibly build up in the environment, trace metals may accumulate, unnoticed, to toxic levels. Thus problems associated with trace metal contamination were first highlighted in the industrially advanced countries because of their larger industrial discharges and especially by incidents of mercury and cadmium pollution in Sweden and Japan (Kurland et al., 1960; Nitta, 1972; Goldberg, 1976). In spite of the relatively low level of industrial activity in less developed regions such as Africa, there is nevertheless growing awareness of the need for rational management of aquatic resources including control of waste discharges into the environment. This becomes even more important in view of the expected increases in industrial and urban activities in all parts of the continent.
Existing information on various environmental problems has been reviewed by Dejoux (1988) in a monograph of pollution in African inland waters and by Phillips (1991) on a worldwide basis on tropical marine ecosystems. These publications showed that the existing information on Africa is scattered and scarce, and therefore demonstrated the need for a more precise and specific review of the occurrence of trace heavy metals in various aquatic environmental compartments in the continent.
For effective water pollution control and management there is a need for a clear understanding of the inputs (loads), distribution and fate of contaminants, including trace metals from land-based sources into aquatic ecosystems. In particular, the quantities and qualities need to be considered together with the distribution pathways and fate and the effects on biota.
The need to make an assessment of the level of heavy metal contamination in the African environment has led to the initiation of several pollution monitoring programmes and research work in various universities and scientific institutions in the region. The most relevant programmes are the Mediterranean pollution monitoring programme (MEDPOL) covering also North Africa, the West and Central Africa marine pollution and research programme (WACAF 2) and the Eastern Africa marine pollution and research programme (EAF/6).
During the last decade, there have also been considerable improvements in the sampling and analytical techniques for trace metals. These, coupled with international intercomparison exercises, have facilitated the generation of more reliable data. The present paper thus attempts to compile and analyze the available information on the occurrence of trace metals in both freshwater and marine ecosystems of Africa as a contribution towards the formulation of rational management policies for aquatic resources in the continent.
The decision to review freshwater and marine data jointly is a result of the need to have a holistic approach that could influence future control strategies.
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