Determining the role of catchment geochemistry on the chemistry of water, sediment and fish from impoundments within selected large catchments in South Africa

Ph.D. (Zoology)The study concept was derived following an information requirement related to the need for the development of a scientifically sound technique to minimize illegal entries at major South African freshwater fishing tournaments. The hypothesis was that fish that are in equilibrium with the environment that they live in should reflect the chemistry of that environment. Therefore it would be possible to chemically link the fish to a specific impoundment and identify whether the fish are indeed from that impoundment or whether they were brought there to be illegally entered at a fishing tournament. The project area consisted of selected lakes within the Vaal, Mgeni, Crocodile (West) and Olifants River catchments in South Africa. The catchments were chosen to contain major sport fishing lakes and to have different sizes, different sources of pollution, different underlying geological compositions and different climates. Soil analyses data from the regional geochemical mapping program of the Council for Geoscience as well as lake sediment and lake water analyses were used to trace chemical elements during weathering, erosion and deposition. This was done to identify unique characteristics that are able to distinguish between water and sediment from individual lakes. It was found that Rb, Sr, Ba, Cu, Ni,Th, Pb, Sc, K, Mg and Ca could be effectively used as single elements or element ratios to distinguish between lakes. Large variation in sediment compositions within individual lakes limited this distinction. Anomalous catchment soil concentrations related to the underlying geology or anthropogenic contamination can however be traced to lake sediments. Rare earth elements are not uniformly distributed in lake sediments or lake waters and sampling localities should be carefully considered when comparing lakes. Leachable soil sulphate distribution patterns from coal mining areas above Lake Witbank and Lake Middelburg extend downstream through Lake Loskop to Lake Arabie (Flag Boshielo). Leachable fluoride, abundant in the Pilanesberg Alkaline Complex extends northwards along rivers draining the complex. The mineralogy of lake sediments was also compared with the major element composition of these sediments as well as with the anion composition of lake water. Genetic rock classification diagrams could be used effectively to show correlations between catchment soil and lake sediment samples both in terms of mineralogy and chemistry. Lake water and fish tissue samples were collected and analysed to investigate the link between element concentrations in lake water and otolith, fin spine, muscle, liver and gill tissues. Using the Sr/Ca elemental ratio, a species-specific correlation was identified between lake water, otolith, spine and gill tissue samples. The best discrimination between fish species was achieved using a Na/Ca versus Mg/Ca elemental ratio diagram of gill tissues. The best discrimination between fish from different lakes was achieved using a Ba/Mg versus Sr/Mg elemental ratio diagram for spine tissue. Sediments from most lakes in the project area showed elevated Hg values during the dry season as well as elevated Hg levels in sediments from the deeper/central part of these lakes. Lake Sterkfontein has a low sediment Hg concentration but contains fish with the highest Hg tissue concentration in the project area. When comparing the Hg distribution in fish muscle and spine tissue from the project area there is a better correlation with species than with location. The Sr isotope ratio of lake water shows a remarkable correlation with the Sr isotope ratio of fish from the same lake. Analytical data showed that fish within a specific lake all have the same Sr isotope ratio in their spines regardless of species, age, sex and condition. The Sr isotope ratio of water generally increases with distance from the source within tertiary catchments and is determined by the prevalent geology of the formations being weathered. In large rivers like the Vaal River where pollution also plays a role the pattern is much more complicated. The Pb isotopic composition of soil samples from Pretoria and the surrounding areas correspond well to the major underlying geological units. Superimposed on this pattern is an anomalous anthropogenic Pb component possibly related to leaded fuel, which extends from the Pretoria city center northwards along the road and railway networks. Water and sediments from Lake Bon Accord and Lake Marais also contain this anthropogenic signature. The hypothesis that fish, which are in equilibrium with the environment that they live in, should reflect the chemistry of that environment, is therefore not rejected since the results of analysing lake water and fish tissues from 23 South African lakes, within 4 major catchments showed definite correlations. Comparing single elements is the least effective method of establishing such correlations. Comparing elemental ratios is a much better method, while comparing isotope ratios is the most effective method