Physical and Chemical Water Characteristics

Abstract

An outline is given of the physical and chemical features of waters in the Nile system, including headwater lakes. Temperature is governed by the large range of latitude and altitude, and is mostly within 15-30°C. Differences with depth can be appreciable under conditions of low to minimal flow and in deeper water columns. They introduce a density stratification with both diurnal and seasonal components, which influences the vertical distribution of plankton and dissolved gases. A long-lasting seasonal stratification is seen only in the deeper headwater lakes, and in Lake Nasser outside the winter season. Light penetration with depth is generally low, especially in silt-laden water associated with turbulent flow and floodwater. During water storage in reservoirs it is subject to the two opposing influences of silt deposition and phytoplankton development. Another influence, that of dispersed colour of partly organic origin accompanied by Fe enrichment, occurs during passage of water through the large Sudd swamp region. Concentrations of two gases, oxygen and carbon dioxide, are influenced by atmospheric exchange, photosynthesis of phytoplankton and submerged macro-phytes, and the decomposition of organic matter, also to the limitation of vertical exchange by density stratification and mats of surface vegetation. Severe oxygen depletion at depth occurs in Lake Victoria and L. Nasser, and can affect the whole river in the Sudd swamp region. Conversely, a near-surface super-saturation of oxygen occurs by day in waters with abundant phytoplankton, conditions in which pH is often above 9.0 by the photosynthetic consumption of CO 2. Otherwise pH is generally in the range 7-9. Salinity, as the total concentration of major ions, is extensively documented by electrical conductivity. It is strongly reduced in floodwater, but otherwise tends to increase downstream. In Egypt there are instances of relatively high salinity, induced mainly by evaporation in some laterally derived water-bodies (e.g. L. Qarun) or by the influx of seawater in some Delta lakes. The Blue and White Niles differ in cationic composition, the divalent ions Ca 2+ and Mg 2+ being predominant in the 368 J.F. Talling former and the monovalent ions Na + and K + in the latter-for which river there is a large and variable contribution from Lake Albert. The HCO 3 − /CO 3 2− complex (summed as titration alkalinity) is dominant among anions, is closely correlated with conductivity, and falls to low levels only in parts of the Sudd swamps. Longitudinal and seasonal surveys have shown much variation in the major plant nutrients PO 4-P, NO 3-N, NH 4-N and Si. A major input of PO 4-P comes from the contribution of Lake Albert. Chemical exchanges between silt and water are likely to be important, with relevance for bacterial and algal communities. Floodwater is generally with considerable concentrations of these nutrients, especially NO 3-N. Subsequent falls reflect the changing character of drainage and consumption in the seasonal and spatial abundance of phytoplankton. These changes have been closely documented for the White and Blue Niles near Khartoum.