The Nile Origin, Environments, Limnology and Human Use
The book discusses the Nile basin from a holistic point of view, giving due attention to its geological history, hydrology, climate, and the full diversity of subsystems like lakes and swamps. It discusses at length the biota of the basin, and derives gen
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Abstract Phytoplankton is widely but unevenly developed in regions of the Nile system. These are successively examined here with regard to community composition and abundance, interpreted in relation to upstream–downstream changes, timesequences, and probable limiting factors. Features in the headwater lakes are largely distinct from those in subsequent flowing sections of the river and in its reservoirs with seasonal or long-term retention. Regions with such retention are generally those with greatest, usually seasonal, plankton development, although some exceptions exist. Community composition is mostly dominated by diatoms (e.g. Aulacoseira granulata) or Cyanobacteria (e.g. Anabaena flos-aquae f. spiroides), but there are many species of green algae and some flagellates (e.g. Pediastrum, Volvox). A remarkable desmid association has been described from the small Lake Ambadi of the Sudd region. Diatoms are generally the pioneers in seasonal sequences or successions, which are often ended by the influx of relatively turbid though nutrient-rich floodwater. Sequences of species of estimated abundance have been studied intensively in relation to environmental factors in some reservoir-influenced regions. In the Blue Nile there is now a cascade system of two reservoirs linked by conditions of free-flow but subject to strong annual floodwater from Ethiopia. Events in the upper and more recent reservoir have brought about changes in those observed downstream. At Lake
J.F. Talling () Formerly: Hydrobiological Research Unit, University of Khartoum, Sudan. Now: Hawthorn View, The Pines, Bongate, Appleby, Cumbria CA16 6 HR, UK e-mail: [email protected] F. Sinada Hydrobiological Research Unit, Department of Botany, Faculty of Science, University of Khartoum, Sudan e-mail: [email protected] O.E. Taha and E.M.H. Sobhy National Institute of Oceanography and Fisheries, Fish Research Station, El-Khanater El-Kharia, Cairo, Egypt e-mail: [email protected]; [email protected]
H.J. Dumont (ed.), The Nile: Origin, Environments, Limnology and Human Use, © Springer Science + Business Media B.V. 2009
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Nasser there is a community-differentiation between the main lake and its lateral arms or khors (e.g. Khor el Ramla). Here, and further downstream in Egypt, there have been maintained assessments of total abundance by chlorophyll-a estimation. In these, and in species biodiversity, there is evidence for some long-term changes. Nutrient-phytoplankton relationships include possible limitation by low concentrations of inorganic nitrogen and – for a common diatom – by those of carbon dioxide associated with pH levels above 9.0. Light-limitation is also expressed in determinations of photosynthetic productivity, available from several upstream and downstream regions. This productivity can be high, conditioned by community abundance, vertical light penetration, and a frequently high specific activity per unit measure of biomass.
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Introduction
Several reasons may lead to vigorous and varied develop
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