Abstract
Rivers all over the world have supported the growth of human civilization since the first
towns appeared some 7000 years ago. As a result of this growth and the diversification
of activities, most of the world’s rivers have been negatively affected. Freshwater is
probably the most important resource of mankind. Not only is it vital for terrestrial life but
we consume water in such a range of activities that it can aptly be regarded as a ‘pillar of
our civilization’. The development of water conservation recycling and management
plans is therefore seen as vital. Cohen (1995) has recently highlighted the importance of
freshwater to the future of mankind and in this context it is becoming increasingly
important to view water as an economic resource in its own right.
Being a third world country, South Africa is a developing country with a constant
increase in population size. With the increase in the population, it is accompanied by
increasing and expanding mining and industrial sectors. Various metals resulting from
these sector’s activities are common pollutants in the rivers of South Africa. Although
some metals are essential elements that are necessary for normal growth and
metabolism in organisms, all metals can become toxic at elevated levels and therefore
the monitoring of the freshwater systems is necessary in order to protect the
environment and ultimately to protect humankind. Aquatic invertebrates appear to be
excellent biological monitors of heavy metal pollution. They are more tolerant of metals
than fish, they accumulate metals in relative proportion to the metal concentration in the
water, and they concentrate the metal by some predictable, reproducible factor.
However the life cycle of aquatic insects are short which limits the value in
bioaccumulation studies. In contrast, crabs are relatively long lived and form an integral
part of the food chain, as they are vital components in the diet of several animal species
including humans.
In the present study an integrated approach was employed in order to investigate the
metal pollution in the Klip River, Gauteng. Selected physico-chemical characteristics of
the water and sediment were analysed, as were the levels of metals bioaccumulated by
the freshwater crab, Potamonautes warreni. Different biological aspects were
investigated to identify the natural course of the life cycle, including moulting and
reproductive cycles, relative abundance and age structure, as well as several
behavioural patterns of this crab species.
Water and sediment samples were collected from the Klip River and Sedaven Dam
(reference locality) every month from August 1997 up to August 1998. Water quality
data revealed that although the quality of the water at all sampling localities were
relatively high, Lenasia (locality 1) and Dirk Fourie Stadium (locality 4) appeared to be
more impacted than the rest of the sampling localities including the reference locality.
Results of the metal analysis in the water samples indicated that levels of zinc, iron,
manganese and chromium were below the suggested South African guideline values
(laid down for the protection of aquatic life) and that elevated levels of cadmium, lead
and aluminium were recorded at localities 1,3 and 4. Results of the metal analysis in the
sediment samples indicated that there were elevated levels of zinc, lead, iron,
manganese, chromium and aluminium at localities 1 and 2.
Potamonautes warreni individuals were sampled every month to coincide with the water
and sediment samples and the metal concentrations were determined. The results of
this investigation showed that elevated levels of zinc, cadmium, lead, iron, manganese,
chromium and aluminium was recorded at locality 1 and this indicated that this locality is
subjected to above standard discharge. At locality 3 elevated levels of zinc, cadmium
and chromium was recorded and at localities 4 and 5 elevated levels of iron and
aluminium was recorded.
The results of the present study indicates that metal levels in P. warreni are indicative of
the environmental levels of metals to which these crabs are exposed. Higher metal
levels as compared to the habitat were found in the organisms at all localities. Locality 1
had a higher environmental level than the rest of the localities. Therefore, these
organisms are useful bioaccumulative indicators and should be incorporated into
biomonitoring programs in conjunction with fish. It must be stressed that further studies
be done on the mechanisms of regulation and elimination of certain metals from the
body of P. warreni and on sublethal effects of metals on the physiological processes
such as reproduction and growth, especially in juvenile crabs.
Prof. G.J. Steyn