The Chari River supplies approximately 95% and the Yobe River supplies the remainder of the fluvial addition to Lake Chad. The water loss from the lake is mainly by evaporation resulting in an increase in conductivity and the sodium, potassium and magnesium content of the lake waters northward. However the calcium content of the waters decreases in the shallow-water areas and to the north. Factor analysis of element concentrations in the bottom interface sediments indicate that: (a) there is a strong association of iron-cobalt-chromium probably in the iron phase of sediments; (b) there is a moderate association of copper and chromium and to a lesser extent nickel, iron and cobalt with organic carbon and nitrogen probably resulting from these elements being utilized as micronutrients by plants and/or their later adsorption onto organic matter in the sedimentary process; (c) zinc, manganese and calcium appear to act independently in the sedimentary regime of Lake Chad.

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