oa Botswana Notes & Records - Hydrobiological studies of the Okavango Delta and Kwando/Linyanti/Chobe River, Botswana: I surface water quality analysis

Volume 27, Issue 1
  • ISSN : 0525-5090



The surface water quality of the Okavango Delta has been investigated since 1989 in a programme under the auspices of the Botswana Department of Water Affairs. These studies have mainly focused on the central section of the Delta, the Jao/Boro River with associated swamp and floodplain areas. The chemical composition of water flowing out of the Delta is different in many respects from its initial composition in the Okavango River. There is a gradient of increasing concentration towards the outlet in all chemical constituents of the main channels while spatial and temporal variations in the swamp areas are more complex, reflecting the great heterogeneity of habitats within the swamps. While evaporation contributes to concentration of all solutes, part of the changes can be attributed to biological processes. These transformations which are more clearly defined for N, P, and C components are related to specific physical attributes of the sampled sites, such as vegetation cover and extent of flooding. Whilst a seasonal pattern in variation of water chemistry is apparent, the data suggest that modifying effects of the swamps on water chemistry are limited. River channels are mainly oligotrophic habitats. Swamps and floodplains vary between oligotrophic and mesotrophic, and isolated water bodies and outlet areas are clearly mesotrophic to eutrophic. The data further suggest differential removal of solutes and a net ion accumulation in the system during the study period. In general, channel waters, which flow considerably faster than the swamp waters, are poor in most solutes and trace elements, and moderately richer in silica, dissolved organic carbon, and organic nitrogen. Waters of the Delta's outflow at Maun are of poor quality, turbid, and affected by anthropogenic inputs. Overall, phytoplankton concentrations were low in river channels and only a few algal groups were recorded in this habitat. By contrast, phytoplankton communities were well developed in swamp areas and isolated water bodies. Preliminary identification of the dominant zooplankton taxa showed that river channels in the upper Delta were almost devoid of zooplankton while high densities were found in terminal areas of the Delta. A preliminary investigation of water chemistry and microbiota of the Kwando-Lioyanti Chobe River suggest that these waters are slighdy more acidic, and contain higher sulphate concentrations than the Okavango River.

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