PRINCIPLES FOR THE SUSTAINABLE UTILIZATION OF THE OKAVANGO DELTA ECOSYSTEM, BOTSWANA

A broad understanding of the structure and functioning of the Okavango Delta ecosystem has provided a basis for evaluating the kinds of perturbations that are likely to affect the ecosystem as a whole. Sediment introduced into the system results in constant changes in the distribution of water on the fan surface. This promotes the occurrence of a variety of habitats in different stages of wetting and drying, and accounts for the overall habitat diversity in the system. It also promotes regeneration of saline soils that are locally toxic to vegetation on islands in the Okavango Delta. Water abstraction from the lower reaches of the fan by activities such as dredging may be rendered useless by changes in the distribution of water on the fan over relatively short time spans. However, water abstraction in itself is not necessarily a problem. Ideally it should be none fr om the apex of the fan, provided it is small relative to the total inflow (1-2%), and does not disrupt sediment supply to the system from source areas, such as by the construction of a weir ol dam. An additional impact of the construction of an impoundment in the catchment would be an. increase in the total dissolved solid concentration of inflowing water. The system is adapted to low total dissolved solid concentrations and, by affecting the nature of plant communities at the apex of the fan, the system as a whole could be affected. The dominance of transpiration over evaporation in this wetland ecosystem results in the accumulation of dissolved substances, notably silica and calcium and magnesium carbonate below surface, where they are biologically not deleterious. Sustained removal of vegetation may result in salinization of surface water, and would have a large impact on the ecosystem. Additional impacts that could alter the structure and functioning of the ecosystem include eutrophication that may result from agricultural development in the catchment, which may profoundly affect the nature of vegetation communities in the upper reaches of the fan, and thus the patterns of sediment and water dispel sal. We no not regard the present utilization of al eas around the periphery of the Delta as a direct threat to the ecosystem itself.