South Africa is a semi arid country and the provision of water clean water to a steadily growing population is currently one of the major challenges facing governmental organisations. Water resources in South Africa are subject to many forms of pollution. resulting in eutrophication and salinisation. Hence, there is a need to monitor chemical and organic pollution in South African rivers. Chemical monitoring is expensive and not all the elements of water quality can be monitored and measured in a particular sample. The synergistic effects of water quality determinants cannot be demonstrated if only the chemical composition of a water resource is monitored. Biological monitoring can provide a rapid indication of water quality and at a lower cost than traditional monitoring. Organisms within a river are exposed to all water quality variables present in a system and can provide an integrated reflection of the health of their environment. Diatoms are found in all aquatic ecosystems and have demonstrable responses to many of the elements of water quality that have been identified as causing aquatic pollution. These elements include total dissolved solids, pH and plant nutrients such as nitrates and phosphates. The relationship between the structure of a given diatom community and the water quality to which the community is exposed, has lead to the development of several indices of water quality. Diatom indices of aquatic pollution have been developed in France, Belgium, Germany, Britain and Japan. Existing diatom indices have been tested for use in Finland, Poland, Britain, the Himalayas and South America. Several diatom indices were tested in this study for application in the Vaal and Wilge Rivers. The tested diatom indices correlated well with measured water quality variables such as pH and the chemical variables responsible for eutrophication and salinisation. The demonstrated correlations were comparable to those demonstrated by European authors. Several indices proved successful in indicating general water quality, namely the Biological Diatom lndex (BDI), the Specific Pollution sensitivity lndex (SPI) and the Generic Diatom lndex (GDI). The Eutrophication and Pollution lndex (EPI) successfully indicated levels of plant nutrients together with the ionic composition measured at various sites in the Vaal and Wilge Rivers. It is recommended that these indices be further tested in different regions within South Africa. / Thesis (M. Omgewingswetenskappe)--North-West University, Potchefstroom Campus, 2004.

South Africa is a semi arid country and the provision of water clean water to a steadily growing population is currently one of the major challenges facing governmental organisations. Water resources in South Africa are subject to many forms of pollution. resulting in eutrophication and salinisation. Hence, there is a need to monitor chemical and organic pollution in South African rivers. Chemical monitoring is expensive and not all the elements of water quality can be monitored and measured in a particular sample. The synergistic effects of water quality determinants cannot be demonstrated if only the chemical composition of a water resource is monitored. Biological monitoring can provide a rapid indication of water quality and at a lower cost than traditional monitoring. Organisms within a river are exposed to all water quality variables present in a system and can provide an integrated reflection of the health of their environment. Diatoms are found in all aquatic ecosystems and have demonstrable responses to many of the elements of water quality that have been identified as causing aquatic pollution. These elements include total dissolved solids, pH and plant nutrients such as nitrates and phosphates. The relationship between the structure of a given diatom community and the water quality to which the community is exposed, has lead to the development of several indices of water quality. Diatom indices of aquatic pollution have been developed in France, Belgium, Germany, Britain and Japan. Existing diatom indices have been tested for use in Finland, Poland, Britain, the Himalayas and South America. Several diatom indices were tested in this study for application in the Vaal and Wilge Rivers. The tested diatom indices correlated well with measured water quality variables such as pH and the chemical variables responsible for eutrophication and salinisation. The demonstrated correlations were comparable to those demonstrated by European authors. Several indices proved successful in indicating general water quality, namely the Biological Diatom lndex (BDI), the Specific Pollution sensitivity lndex (SPI) and the Generic Diatom lndex (GDI). The Eutrophication and Pollution lndex (EPI) successfully indicated levels of plant nutrients together with the ionic composition measured at various sites in the Vaal and Wilge Rivers. It is recommended that these indices be further tested in different regions within South Africa. / Thesis (M. Omgewingswetenskappe)--North-West University, Potchefstroom Campus, 2004.

Aquesta tesi tracta la jerarquia i l'heterogeneïtat dels sistemes fluvials que afecten l'estructura de les comunitats bentòniques de diatomees. A nivell regional, es van buscar diferents grups de punts i les seves espècies indicadores, es va estudiar la resposta de les comunitats de diatomees als gradients ambientals, es va avaluar la utilitat de diferents índexs de diatomees i es va buscar el millor sistema de classificació per a condicions de referència. A nivell de conca, es volien definir els factors que determinen la distribució longitudinal de la diversitat de les comunitats de diatomees. Finalment, a nivell d'hàbitat es van determinar quins factors afecten les algues i els cianobacteris a aquesta escala i es va examinar la contribució relativa de l'ambient i l'espai en la distribució de la biomassa i composició d'algues i cianobacteris. Per tant, els diferents capítols d'aquesta tesi han estat desenvolupats seguint aquest esquema. / This thesis deals with the hierarchy and heterogeneity of stream systems affecting the structure of benthic diatom communities. At a regional level, I search for different groups of sites and their indicator taxa, I studied the responses of the diatom communities to the gradients of environmental variables, I tested the usefulness of diatom indices and I searched for the best classification system for reference conditions. At a watershed level my interest was to define the factors that determined the longitudinal distribution of diversity of diatom communities. Finally, at a habitat level it was interesting to determine the factors affecting algae and cyanobacteria at this scale and examine the relative effects of environmental factors and space on the distribution of biomass and composition of benthic algae and cyanobacteria. Thus, the different chapters of the thesis had been approached following this scheme.