Synthesis and Implications

Synthesis and implications This ecological perspective of the fresh and brackish (inland) waters of the subcontinent has provided an introduction to the limnological literature of southern Africa. It has drawn together some of the research results which, while appearing disparate, form the warp and weft of a limnological pattern for the subcontinent, without which the response of the subcontinent's water resources to the diversity of changes of anthropogenic origin cannot be adequately assessed, and proper management decisions taken. In so doing it has emphasized those studies and investigations which, because of the extent of limnological knowledge and understanding of warm water systems, either rivers or lakes, they have generated, should be incorporated rapidly into management policies. The hydrological effects of the ever increasing demand for water by the growing population and industrialization of southern Africa have to be viewed against the climatic variability of the subcontinent. Preston-Whyte & Tyson (1988) report that between 9000 and 4000 BP wetter conditions prevailed over southern Africa than exist at present. Thereafter wet cool and dry warmer conditions reflected, in general, the major climatological epochs throughout the world, for example the neoglacial advance between 2000 and 3000 BP, and the medieval warm epoch of 1000 AD. Attention has already been drawn to the quasi-periodic rainfall oscillations for the summer rainfall regions from 1910/11 until the present day. Preston Whyte & Tyson (1988) have established that the dry spells are "more persistently dry than the wet spells wet". It is during such dry periods that the land is at its most fragile, and because of our collective ignorance the hazard of desertification in the subcontinent has increased materially. The advance of Karroid conditions has been estimated by Acocks (1953) as some 1.6 km per year. This advance is checked during periods of good rains if, and only if, the damaged terrestrial ecosystems are allowed to recover. This does not often happen in southern Africa and the desertification potential of the subcontinent is inexorably realized. Thus without a real understanding of the periodicity of long-term climatological cycles against which to judge the short-term hydro logical management solutions imposed by Man, southern Africa faces an uncertain hydrological future! The limnological character of the subcontinent has been defined. With the exception of the winter rainfall Region 4, principally of the southwestern 388 Cape, South Africa, the rainfall of the continent occurs predominantly in the summer months. While the application of time series analysis has demon strated a dominant twenty year cycle of above average and below average rainfall, within this cycle the variation in intensity and frequency of summer rainfall is large. The wildness of the rivers of the subcontinent is well recorded, stemming not only from the strong seasonality of rainfall, but also from its variability. These facts, coupled with the low average rainfall, and the lowest mean annual runoff (MAR), 8.6%, of all the countries studied (Alexander 1985; this volume Fig. 5.2), have forced the construction of an array of major and minor dams on the continental rivers and the planning, design and construction of a variety of inter-basin transfer systems. Five limnological regions have been proposed, defined by their geomorpho logical, geochemical and climatic features. The rivers and their associated wetlands are the primary limnological feature of the subcontinent. During the past 150 years of urban and industrial development the hydrological proper ties of the rivers have changed materially: the majority are now regulated to a lesser or greater degree so that the limnologicallandscape is now constructed out of reservoirs and their connecting streams. The impact of this perturbation upon the original riverine environment has been largely ignored until the work of Davies (1979) and O'Keeffe (l986a) drew attention to the ecological issues at stake, and in particular the downstream effect of flow regulation by dams upon the physical and chemical changes in the river flow and ecology of the complex riverine communities it supports. Contrary to what was expected there are instances of reservoirs materially improving the quality of the river flow, for example within the Buffalo River system near East London. The reservoirs act as settling and biological oxidation systems and by these metabolic processes effectively reset the energy phase by the addition of allochthonous carbon simulating the primary energy resources of the headwa ters. Current research programmes in South Africa, in particular, may well lead to a modification of the serial discontinuity concept of Ward & Stanford (1983). Because the river environment is so sensitive to man-induced changes, stress has been laid upon the development of a composite classification so as to provide guidelines with which to assess the impact of development on river sections of similar physiographic structure. A first approach was one in which a hierarchial system based on ecoregions, regulation types, river zones and water chemistry have been used to assess the expert status of river sectors for the major river catchments of South Africa. This has been further developed and refined by the development of a specifically designed conservation system, the River Conservation System (O'Keeffe et al 1987), which provides a consistent yet flexible way of assessing rivers, simplifying the diffuse intuitions and value judgements of conservationists and making them comprehensible to engineers and catchment managers. The consequences of river regulation to the main stem river have been 389 critically examined by Ward et at (1984), and stressed for the subcontinent in Chapter 9. Of particular note is that the impoundment policy of the 1960's, which included the Orange River Project, did not have the effect of overcom ing regional water shortages as was expected. This is largely as a result of reservoirs being constructed in areas of low human density. As a corollary this has been of benefit to the maintenance of water quality, removed as they are from major urban centres. Their value lies more directly with irrigation, the generation of hydroelectric power, and in some instances flood regulation. The supply of potable water has, or will in future, come to rely heavily upon sophisticated inter-basin transfers of comparatively large volumes of water to dilute the increase in total dissolved solids and guarantee supply during periods of drought. Sustained turbulent flows down river courses previously subject to long periods of low or zero flow create a material change in the community structure of the invertebrate animals (notably insect larvae) which inhabit river margins, stony runs and sediments. The work of F.M. Chutter and his colleagues in the Vaal River, and of one of us (J.H.O.) in the Fish River, shows clearly the variation in interspecific tolerance, for example, within the Simulium complex as channel flow varies. Where flows are seasonably variable, S. nigritarsus and S. medusaeforme are dominant. As soon as river flow increases and is sustained as a consequence of regulation, the mammophilic S.chutteri is selected for and causes such irritation to livestock, grazing on lands adjacent to the river, that the lands can no longer be used. The effect of changes in the hydrodynamic character of the river flow at the most sensitive life cycle stadia in the control of this insect pest demonstrates the value of fundamental research in the solution of practical management problems. Similarly, studies of the feeding efficiency of S.nigritarsus and S.chutteri by H.Barber, as yet unpublished, point to both velocity and the degree of turbulence, as measured by Reynolds number, as important factors bringing about change in community structure. S. chutteri feeds more effectively at higher Reynolds number (Re = 2000) than does S. nigritarsus which feed optimally at the transition between laminar and turbulent flow (Re = 500 - 800). These and many of the other areas of hydrological or hydrodynamic change described in the text demonstrate that bio-engineering solutions to water orientated problems clearly depend, or should depend, upon the timeous intervention of specialists from other essential disciplines. There is a need to recognize that there will always be a response to any alteration in the hydrological cycle. The associated biological systems are not immune from Newton's third law! Our responsibility is to predict the magnitude of the response(s), and assess the problems, if any, which may arise either immedi ately or over long periods of time. These issues on a wider continental or intercontinental basis have been given detailed consideration by Davies (1979) and Davies & Walker (1986) and specifically in the subcontinent by Petitjean & Davies (1988). 390 The reservoirs, in common with those in other southern lands such as Australia, are largely monomictic, with the onset of stratification dependent upon surface heating and wind mixing of the warm buoyant surface layers downward. We have established, however, that there is considerable variation in the appearance and duration of both the seasonal and the diurnal thermo clines. Much of this variation is due to (a) river input and (b) high insolation during periods of low windspeeds. These appear to act antagonistically to bring about a redistribution of warm surface waters by entrainment or an increase in stability. Quite high temperature and therefore density differentials develop during summer. A 4 or 5°C drop across the metalimnion, representing a density differential of '" I kg m-3, maintains stratification in monomictic reservoirs. In these respects the reservoirs of the subcontinent do not differ greatly from the reservoirs of Australia or in the latitudinally equivalent regions of Brazil and Argentina (Tundisi 1981; Matsumara-Tundisi et a11981; Bonetto & Di Persia 1984). These reservoirs also exhibit a feature which has been found here, namely that monomixis is by no means as widespread as was previously thought.

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