Abstract
M.Sc. (Aquatic Health)
Previous studies showed that the upper reaches of the Taaibosspruit catchment in the
north eastern region of the Free State, contains elevated concentrations of mercury in the sediment. Additional laboratory-based bioaccumulation studies with sediment-dwelling Chironomidae larvae revealed that the mercury was in a bioavailable form, but it was uncertain to what extent this mercury was in the methylated organic form. Therefore, a study to assess the extent of inorganic- and methylmercury contamination in the water and sediment, as well as the uptake of this pollutant by aquatic and aquatic associated biota was undertaken. This study tested the hypothesis that bioaccumulation of mercury occurs in the different trophic levels of the upper reaches of the Taaibosspruit ecosystem. The objectives of this study were to determine the spatial and temporal physico-chemical characteristics of the water and sediment of the Taaibosspruit system; to determine the extent of mercury distribution within the water and sediment of the Taaibosspruit system; to determine the degree of bioaccumulation and possible biomagnification of mercury by the biota from the different trophic levels of the ecosystem in the Taaibosspruit system; to determine the distribution of cadmium, chromium, copper, lead, manganese and uranium within the water and sediment of this system, as well as the degree of bioaccumulation of these metals by invertebrates and fish; and to determine whether the aquatic macroinvertebrate community structures change along the mercury pollution gradient in the Taaibosspruit system. Sampling was done bi-annually at five sites during late high- and late low flow periods. The reference site is situated on a tributary of the Taaibosspruit, which is not directly influenced by discharges from the industrial complex. In situ water quality parameters were determined at each site and the following samples were collected: water, sediment,
aquatic rnacroinvertebrates, vertebrates (fish and amphibian species), as well as noninvasive tissue from birds (feathers). The physico-chemical characteristics of the water were determined by measuring the chlorophyll-a concentrations, nutrient levels and suspended solid content. The sediment was analysed for organic content, water content, as well as sediment particle size distribution. The methylmercury concentrations in the water, sediment and biotic components were determined through cold vapour atomic fluorescence spectrometry, whilst inorganic mercury, cadmium, chromium, copper, lead, manganese and uranium concentrations in these samples were determined through inductively coupled plasma mass spectrometry. All analyses were subjected to appropriate quality analysis and quality control procedures. The relationship between environmental mercury exposure and biological effects was determined using changes in the macroinvertebrate community assemblage structure as the biological endpoint. The invertebrate samples were retained for enumeration and lower taxonomic identification in the laboratory. Data were subjected to the appropriate univariate statistical analysis (oneway ANOVA and regression), as well as multivariate statistical analyses (redundancy and principal component) to study spatial and temporal trends in mercury distribution and associated biological responses. This study showed that elevated concentrations of some nutrients and other dissolved solids were present in the system and that the water quality from the upper catchment of the Taaibosspruit (near the industries) had an impact on the downstream sites. The highest mercury concentrations were found at the sites closer to the industrial complex. These concentrations were higher than the concentrations from the reference site and
when compared to international mercury guidelines, it was found to be too high.
Comparison of the mercury concentrations in the water and sediment with environmental
parameters also revealed that some of these parameters significantly influenced the
environmental mercury concentrations. It was found that the biota has bioaccumulated
mercury, which has resulted in the biomagnification of methylmercury by the biota from
the different trophic levels of the ecosystem from this system. The results of the other six metals showed that the highest concentrations were found at the sites closer to the
industries, with very high cadmium and copper concentrations in the water at the site
where wastewater from the industries enters the system. These high concentrations in the water, suspended particles and sediment resulted in the accumulation of the metals by the biota present in this system. The sediment-dwelling Chironomidae-larvae bioaccumulated these metals to higher levels than the other biota from this system. The response of aquatic macroinvertebrates to high mercury exposure was dearly visible, as the sites where the environmental mercury concentrations were the highest, had low species richness. The environmental variables also influenced the community structure of the macroinvertebrates by reducing the number of different families at the more impacted sites. The results have shown that some of the fauna in the Taaibosspruit system were reduced in favour of the more tolerant families, namely Chironomidae and Oligochaeta. Thus, it was found that Taaibosspruit system is impacted upon by the surrounding industries, namely by enhancing the bioaccumulation of mercury, and influencing the macroinvertebrate community structure.