Biological indicators of water quality in an urban waterway : can diatoms reflect short term spatial and temporal changes in water quality?
Kriel, Gideon Petrus
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With water being such a limited resource in South Africa, numerous settlements have developed around the main sources of fresh water - rivers. Rapid urbanization and economic development have resulted in unfavourable changes in the hydrology and ecology of river systems. Rivers remain the only sources of fresh surface water and although numerous, these are often small or ephemeral, or both. Thus, there exists a need for careful management of water with respect to the availability and quality of this resource. Rivers and streams in urban environments are often characterised by long canalised sections. One of the main reasons for this is flood prevention, especially when riverside property is at risk because existing floodplains and high flood levels were not considered. Canalisation alters the shape and dimensions of a stream's channel which in turn could have major effects on stream ecosystems, such as interruption of lateral and vertical exchange of water and nutrients. The monitoring of water quality in South Africa has, traditionally been confined to interpretations based on the physical and chemical properties of the water. However, the collection of urban water quality data is costly, while the cumulative impacts of urbanisation on stream impacts may be reflected best by the resident biota. It is often the case that biota commonly used to indicate stream conditions are absent in canalised urban rivers. However, the group of algae known as diatoms are present in the majority of these rivers and were tested in this study for their ability to act as bio-indicators in these particular aquatic environments. The European and other diatom indices tested in this study were able to reflect the unpredictable and extreme conditions present in the urban channels, with correlation coefficients of correlations between diatom indices and measured environmental variables often exceeding 0.60. It was also shown that diatoms could be used to indicate changes in trace metal concentrations, and that diatoms better indicated the influence of a highly contaminated tributary on the Mooi River than measurement of physio-chemical variables. It was also shown that epilithic and epiphytic substrata could be used interchangeably in urban canals. Thus this study has shown that the diatoms can be successfully applied as bio-indicators of water quality in urban environments.