Abstract
M.Sc.
Estuaries are regarded as South Africa's most productive ecosystems due to the important functions that they perform, such as providing nursery areas and feeding sites for juvenile macro-invertebrate and fish species. Furthermore, ecologically healthy estuaries are not only of critical importance since they facilitate the provision and recirculation of nutrients, they also provide conduits for catadromous or anadromous fish and act as buffers during floods. In South Africa these functions are continuously threatened by residential and/or industrial developments. It is thus essential to determine the ecological integrity (structure and function) of these systems. An ecological integrity study was carried out on the Mvoti Estuary, KwaZulu-Natal during 2004/2005. An ecological integrity study was also carried out on the Amatigulu/Nyoni Estuary, KwaZulu–Natal during the same period and was used as a reference study. The ecological integrity of the Mvoti Estuary was assessed in terms of the effects of selected abiotic drivers on specific biological responses. The study was carried out according to the guidelines of the ecological reserve determination methodologies and the resource directed measures for aquatic resources as set out by the Department of Water Affairs and Forestry. Surveys were carried out during low flow (September 2004) and high flow (March 2005) periods. Abiotic and biotic monitoring was carried out at four sites on the Mvoti Estuary and five sites on the Amatigulu/Nyoni Estuary. The abiotic components, such as the substrate composition and physico-chemical properties of the water and sediment, was analysed using standard methods. Biological monitoring included the study of the benthic macro-invertebrate-, ichthyofaunal- and avifaunal communities. The biotic and abiotic components were analysed using various indices, where applicable. The ecological integrity of a system can be assessed on the basis of its ability to carry out its natural functions. Results show that various anthropogenic activities in the upper reaches of the Mvoti River contributed to the highly modified state of the Mvoti Estuary. The abiotic drivers of the aquatic system that contributed to the modified state of the Mvoti Estuary is unacceptable water quality, a loss and/or modification of habitat and an altered hydrological pattern. As a result of the abiotic drivers, results show that the
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ichthyofaunal-, macro-invertebrate- and, to a lesser degree, the avifaunal communities also reside in a modified state. Physico-chemical, geomorphological and hydrological changes in this system resulted in the Mvoti Estuary suffering a loss in both its biological and ecological function as well as aesthetic value. It is apparent from the above that there is not only a need for rehabilitation, but also a need for an effective and continuous management strategy. This management strategy can only be successful if the biomonitoring of the system includes the effects at both economical and social levels.