The structure and dynamic of phytoplankton assemblages in Great Bitter Lakes were studied seasonally during
the period from May 1999 to February 2000. Population density varied between112198X103 cells. l-1 in spring
and 2382X106 cells. l-1 in winter. In terms of cell number, Bacillariophyceae was the most important group
(mean annual percentage contribution was 67%), which remained dominant throughout the study period.
Cyanophyceae ranked the second abundant group (14 %) dominated in autumn and winter. The contribution of
Dinophyceae and Chlorophyceae were 8.6 and 7.3% respectively. The other classes contributed with very low
share of the total phytoplankton. The relationship between phytoplankton composition and physico-chemical
variables was studied. CCA analysis indicated that water salinity, and temperature are the most important factors
affecting phytoplankton distribution. The trophic status index indicated that the lake ranked between
mesotrophic and moderately eutrophic. In winter, the lake was under blooming due to the luxuriance growth of
Nitzschia pseudo delicatissima Hasle. and Phormidium sp. The impact of pollution was significantly effective in
the western side of the lake compared to the eastern one.