University of Khartoum

Observations on the Phytoplankton Biomass and Species Composition In Relation to Environmental Conditions in the White Nile River at Khartoum

Observations on the Phytoplankton Biomass and Species Composition In Relation to Environmental Conditions in the White Nile River at Khartoum

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Title: Observations on the Phytoplankton Biomass and Species Composition In Relation to Environmental Conditions in the White Nile River at Khartoum
Author: Manal Elseyed, Abdel Rahman
Abstract: Seasonal variations of some physical, chemical and biological variables were studied in the White Nile near Khartoum at fortnightly intervals for one year from May 2000 to May 2001. The major nutrients: phosphate, nitrate and silica varied between 0.017- 0.306 mg PO4 - P l-1, 0.05-0.39 mg NO3-N l-1 and 3.5 - 26.7 mg SiO2 l-1 respectively. The maximum concentrations of phosphate, nitrate and silica were recorded during the rainy season, while the minimum levels of phosphate were recorded in the winter during January and February. Low concentrations of nitrate were observed in summer (March). Low concentrations of silica were recorded during September and November at times of maximum development of diatoms. Fortunately lead, cadmium and nickel were found in very low negligible concentrations. This is an indication that the White Nile far from being polluted by heavy metals. Total phytoplankton biomass expressed as volume (µm3 ml-1) varied between 30.8 x 103 - 13.7x106 µm3 ml-1 with annual average of 1.3x106 µm3 ml-1. The maximum development of total biomass was recorded during late September and November with the peak in mid October. The minimum growth occurred during the rainy season when unfavourable conditions of reduced transparency due to high silt content and high current velocity prevailed. Chlorophyll 'a' content was estimated during the period June to December. It varied between 0.6 and 8.9 ىg Chl 'a' l-1. The maximum value recorded in October, coincided with high total biomass, while the minimum value recorded during the rainy season was associated with low algal densities. Diatoms were always dominant over other groups, contributing high percentage exceeding 89% to total phytoplankton biomass. Blue-green algae came second in dominance contributing 10.1%, while green algae constituted a very minor proportion, being less than 0.5% to total biomass. The contribution of euglenophycean and dinophycean algae was negligible. Chrysophycean and xanthophycean algae were not represented during the present study. Over 151 taxa belonging to 5 classes were identified to species level. In terms of species, green algae were better represented than the other groups by 78 taxa belonging to 37 genera, while diatoms and blue-green algae contributed 45 taxa belonging to 22 genera and 26 taxa belonging to 17 genera respectively. Euglenoids were represented by 2 genera and dinoflagellates by one genus only. Seven genera belonging to Bacillariophyceae and Cyanophyceae contributed significantly to total biomass (over 99%). Most of these major components show appreciable increase at the end of rainy season (late September). Aulacoseira granulata and its variety angustissima, constituting most of the total biomass, attained their peaks simultaneously with A. distans and Navicula spp. in mid October. Blue-green algae, Anabaena flos-aquae and Oscillatoria spp. dominated the plankton in late October. Fragilaria spp. and Synedra spp. attained their peaks in November and December respectively. Throughout winter, from late December to February, phytoplankton maintained minimum numbers. In summer from March to May only Cocconeis sp. attained its peak in early March with another smaller peak in May
Description: 153page
URI: http://khartoumspace.uofk.edu/handle/123456789/13175
Date: 2015-06-16


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