The effect of development on seasonal wetlands on the Cape Flats, Western Cape, South Africa

Abstract

Includes bibliographical references (leaves 101-113).Seven decades ago, the pre-eminent limnologist, Miss Edith Stephens described the Cape Flats as "a paradise for the aquatic biologist". At that time the area was characterised by numerous temporary or seasonal wetlands that filled and dried in concert with the seasons. Since Miss Stephen's observations, the number of seasonal wetlands on the Cape Flats has dwindled alarmingly and very few remain. This dissertation attempts to account for this and provide guidance for the management of the remaining seasonal wetlands. The dissertation shows how development (characterised by the urban and agricultural land-use) has radically altered the nature of seasonal wetlands and that Typha capensis can be used as an indicator of the ecological value or integrity of these wetlands. Two case studies have been selected to test this premise. The various interrelationships between vegetation, hydrology, nutrients, land-use and ecological value are explored and the proliferation of wetland communities of Typha capensis is shown to be an indicator of negative impacts on seasonal wetlands. The literature review indicates that the proliferation of Typha capensis signifies a decline in habitat diversity and biodiversity (species richness). Typha capensis has been shown to be influenced by streamflow and nutrient input (the plant thrives in shallow areas, permanently inundated with nutrient-rich waters). Therefore, changes to the total area occupied by Typha capensis can be used to illustrate how development has affected the hydrology, habitat diversity, biodiversity and ecological value of seasonal wetlands. The primary source of information was aerial photography, of varying scales, dated from the early 1940s to 2000 and acquired from the Department of Land Affairs: Land Surveys and Mapping. Identifying, mapping and interpreting land-use changes and changes to Typha capensis formed the basis of the research. Water chemistry information, obtained from the Scientific Services Department of the City of Cape Town, and an extensive literature review supplemented the photographic information. Zeekoevlei / Rondevlei wetland and the Khayelitsha wetlands are used as case studies. Together they have been affected by a significant range of impacts generated by different manifestations of development including Wastewater Treatment Works, high-income residential areas, catchment hardening, manipulation of drainage patterns, informal settlement, informal grazing, agricultural runoff and horticultural market gardens. The pattern of land-use change within the selected wetlands' catchments (Lotus River catchment and Kuils River catchment), the impacts of the observed land-use changes, and the impacts to Zeekoevlei / Rondevlei and the Khayelitsha wetlands with respect to seasonality, Typha capensis and ecological value are presented and discussed in detail. In general, urban areas have increased over time to dominate the two catchments. Agricultural areas were consolidated into a few areas while the areas of open space diminished rapidly. Surface and stormwater runoff from "hardened" catchments, irrigation of farmland, and treated effluent from Wastewater Treatment Works all drastically increased, In addition, the concomitant influx of nutrients (nitrates, nitrites and phosphorus) polluted the rivers and wetlands, making them eutrophic and promoting the proliferation of large stands of Typha capensis. The impact of these changes was the loss of seasonality, habitat diversity and biodiversity. Specific recommendations are made for the long term management of Zeekoevlei, Rondevlei and the Khayelitsha wetlands. The proposed management strategy is based on selected management objectives i.e. what is the wetland being managed for? It might not be possible to fully rehabilitate them to a pristine state but management as recreational areas, conservation areas and even educational areas is possible. Key points of the recommended management plans include: * maintaining the winter drawdowns at Zeekoevlei / Rondevlei; * implementation of a dredging and Typha clearing programme; * strategically placed reed beds to purify inflow; * a fire programme for the Khayelitsha wetlands; and * instating an environmental awareness course at the Khayelitsha wetlands. The importance of an Integrated Catchment Management Plan is emphasised. Catchments should be managed as a whole, recognising the relationships between planning, land-use and water resources