The Development of Plankton in Relation to Hydrological Regime in the Blue Nile

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Abstract
A considerable development of phyto- and zooplankton occurs seasonally in the Blue Nile at Khartoum. Details of seasonal succession are described from quantitative and qualitative records made chiefly during 1951-53 (zooplankton) and 1955-56 (phytoplankton). They are interpreted in relation to hydrological, physical and chemical characteristics, with some evidence from longitudinal upstream surveys of plankton development. Concentrations of plankton were very low during the annual phase of high flood water, when adverse conditions of rapid flow and high turbidity existed. Subsequent population increase was rapid, particularly by the diatom Melosira granulata and the crustaceans Moina dubia and some Cyclopid Copepods; it was probably favoured by the filling of an upstream reservoir and relatively high concentrations of some plant nutrients. The reduction of nitrate-nitrogen to below 10-20 μ g/l was possibly responsible for the primary check to the increase of Melosira, as in the neighbouring White Nile. Later increase of the blue-green alga Anabaena flos-aquae f. spiroides produced higher population densities, some supersaturation of dissolved oxygen, increase of pH to values of 8· 9 or more, and probably contributed to a depletion of phosphate. A decline of both phyto- and zooplankton occurred about March-April, and a second maximum appeared in May-June before the flood; the environmental control of these events is poorly understood. Both plankton maxima at Khartoum were partly the result of multiplication in flowing river conditions below the reservoir; the first maximum involved a sharp increase of plankton in the seasonally filled reservoir. Quantitative aspects of planktonic growth and production under these two hydrological situations are discussed and contrasted. The importance of the reservoir in enhancing plankton development is supported by evidence from older records. Brief comparison is made with other examples of dense plankton development in the Nile river system, both in reservoirs and in flowing river conditions.