The Zooplankton of the Blue Nile

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El Moghraby, Asim Ibrahim
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University of Khartoum
The investigation of the problem involved studies on the whole of the Blue Nile in the Sudan, including the newly created Rosaries reservoir. The studies were carried out between November 1966 and June1970. The quantitative and qualitative cycles of the zooplankton were studied as well as the horizontal and vertical distribution, the factors controlling distribution, food of the zooplankton and diapauses. The reality of zooplankton was discussed and the role of the new dam at Rosaries was confirmed. The following conclusions were reached: 1) Creation of the Rosaries reservoir resulted in the build-up of a pure plankton community within the influence of the dam. 2. Crustacea were more numerous than Rottifers, and Cladocera were more numerous than Copepod. 3. The numbers of zooplankton increased 7.316 times more than in the river above the reservoir. The increase was particularly noticeable among Rotifers. 4 D. lumholtzi and Microcystis flos aquae become dominant immediately. 5. The seasonal cycle bf the plankton community had a maximum in November and anther in February, March and April. 6. Conditions in the reservoir were still not stablised four years after the first filling . 7. The zooplankton below the dam usually had a seasonal cycle similar to that in the reservoir, but with a temporal displacement. 8. Zooplankton usually decreased in density in the conditions of free flow below the Rosaries reservoir. 9. Density of zooplankton at the Sennar reservoir increased even above those at Rosaries. 10. Numbers usually decreased at Wad Medani and Hassaheisa but increased again at Khartoum. 11. The seasonal cycle of plankton had not changed years after the work of Telling and Rzoska. (1967) and was similar to that at Roseires. 12. Rotifers sometimes dominated population peaks C. dubia, C. cornuta and D. excisum appeared to have lost their dominance while T. galebi replaced T. neglectus. 13. The annual flood is the most important factor limiting numbers of plankton and it affects all other factors. During the period of low flow environmental factors are optimum for plankton in the Blue Nile. High pH values do not favour C. cornota but are important for the increase in population of B. calyciflorus. High concentrations of Na are in favour of the growth of populations of M. leuckkarti and T. galobi while high concentrations of Mg and SiO2 are inimical to populations of C. cornuta. Water storage, on the other hand, and low current flow favour the increase, in numbers of zooplankton. 14. The zooplankton was found to be over-dispersed horizontally. There was also unevenness in vertical distribution. 15. Different species of zooplankton were found to have differential feeding times and differential feeding stations. The food of the different species varied with season and locality. Evidence of food preferences were observed. 16. Different zooplankton species were found to go into diapause during the period of flood.
328 Pages
Zooplankton; Blue Nile