Abstract

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This study evaluated the temporal dynamics of phytoplankton species diversity and abundance in relation to some physico-chemical parameters in Lake Manyara for a period of fourteen consecutive months (between July 2007 and August 2008). The aim was to relate the temporal variability in the physico-chemical parameters of the lake to the abundance and diversity of phytoplankton. Three sampling sites were established in the lake, and data were collected once every month. Electrical conductivity, salinity, water temperature, dissolved oxygen; turbidity, transparency and pH were measured in-situ using a water quality checker Water samples for nutrients and phytoplankton abundance and species composition were analysed at the University of Dar es Salaam. In the laboratory, the concentration for inorganic nutrients were determined using a Shimadzu UV-spectrophotometer. Salinity and pH values in the lake ranged from 0.1 ‰ to 40 ‰ and from 6.23 to 10.67 respectively. However, there were shift in phytoplankton community dominance between Anabaena sp to Arthrospira sp likely due to differences in salinity and pH thresholds requirements. Nitrate, phosphate and ammonium concentrations ranged from 0.5 to 3.2 μg/l, 0.5 to 570 μg/l and 0.5 to 10.5 μg/l respectively while conductivity varied between 8.7 to 84.5 mS/cm. Other cyanobacterial genera, Phormidium, Oscillatoria, Spirulina, Aphanocapsa and Synechoccocus were also common. The results indicated that phytoplankton assemblage was positively correlated with ammonium, phosphate, dissolved oxygen, conductivity and nitrate.

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