African Journal of Aquatic Science 2000, 25: 105–122 Printed in South Africa — All rights reserved Copyright © NISC Pty Ltd AFRICAN JOURNAL OF AQUATIC SCIENCE ISSN 1608–5914 The macroinvertebrates of the Cunene River from the Ruacana Falls to the river mouth and assessment of the conservation status of the river FC de Moor 1 *, HM Barber-James 1 , AD Harrison 2 , CR Lugo-Ortiz 1 1 Department of Freshwater Invertebrates, Albany Museum, Grahamstown 6139, South Africa 2 111a Berg Road, Fish Hoek, Cape Town 7975, South Africa *Corresponding author, e-mail: f.demoor@ru.ac.za Received 15 August 2000, accepted 17 December 2000 The proposed construction of a second hydroelectric power-generating dam on the Cunene (Kunene) River on the Namibia-Angola border, more than 100km downstream of the Ruacana hydroelectric power plant, will have a major influence on the aquatic biota of this river. In order to assess the potential impact of this impoundment a more detailed inventory of the biota in the river prior to this development was needed. Unfortunately the river presently does not represent an entirely undisturbed system as the operation of the Ruacana hydroelectric power station since 1970 has had a long-term effect on its biota. In the low-flow mid-summer season in particular, large areas of the river become irregularly inundated and exposed, sometimes on a daily basis, making them unsuitable for macroinvertebrate colonisation. There are six impoundments along the Cunene River upstream of the Ruacana Falls in Angola. These, however, do not have any major disruptive effects on daily flow variation in the Cunene River in Namibia although whole system biological consequences could be considerable but are unstudied. The Cunene River has a diverse freshwater fish fauna but, prior to the surveys reported here, the aquatic macroinvertebrates had been poorly studied. Surveys conducted by staff of the Department of Water Affairs (DWA) of Namibia and the Albany Museum, Grahamstown, in 1997 and 1998 have resulted in 216 aquat- ic macroinvertebrate species being recorded from Ruacana to the river mouth. The lower Cunene River, flowing through a very arid region, is biogeographically isolated and therefore highly vulnerable to change. The biota recorded reflects a fauna of widespread species and several elements of tropical origin. Several undescribed species may reflect some endemics but because of limited knowledge of the tropical rivers, this cannot be ascertained with certainty. From the aspect of conservation, the river contains a diversity of species with an abundance of filter-feeding species. Further surveys conducted during different seasons will undoubt- edly record more taxa. The Namibian Government is planning to construct a sec- ond hydroelectric power plant on the Cunene (Kunene) River. There are several possible sites for the plant, all of which involve the building of dams to allow for the storage of from one to three years of MAR (mean annual runoff) of water. As hydroelectric power generation demand can fluc- tuate seasonally, daily and on an hourly basis the storage of such a large volume of water is necessary to facilitate the supply of electricity on demand (Public Meeting 1997). It is important to consider the socio-political and biological fac- tors that may be detrimentally affected by the construction of such a hydroelectric power plant. From a biological view- point, the possibility that modified flow regimes may lead to an increase in pest species in either the newly created standing water lake or in the regulated river downstream of the impoundment have to be considered. If not managed carefully, water releases downstream of the hydroelectric plant can also detrimentally influence the biota. Large areas of riverine substrate are already lost to potential colonisation by macroinvertebrates as a result of irregular fluctuations in water levels of the river created by the Ruacana hydroelec- tric power scheme. The inundation and desiccation of river- ine substrates caused by water releases (sometimes sever- al times per day) to meet demand for hydroelectric power can lead to large areas of riverbed being devoid of aquatic macroinvertebrates and plants (Fisher and Lavoy 1972). Irregular increases or decreases in flow can therefore have deleterious influences on the riverine biota and severely reduce the natural ecological functioning of the river. Functionally, aquatic macroinvertebrates are important processors of organic matter. They play a vital role in purify- ing water and also provide a valuable food resource for larg- er animals within and outside the system. In order to contin- ue functioning optimally, the component species in a river system require regular inputs of nutrients, sediments and water flow. Specific river systems evolve particular assem- blages of species forming functional communities within reaches. A reduction or increase in flow, or changes in sed- iment transport or nutrient loads, would lead to changes in community structures through loss of certain species and Introduction Keywords: Cunene (Kunene) River, Namibia, freshwater, macroinvertebrates, water quality, hydroelectric power, minimum flow require- ments, species diversity