Abstract
This study was carried out to determine the water quality of El-Temsah Lake by studying environmental variables, as well as phytoplankton abundance and community structure. Water samples were collected from three basins of the lake, namely Suez Canal pathway, the western lagoon (AL-Sayadin lagoon) and EL-Temsah Lake during August, October and December, 2012. A total of 118 taxa were identified belonging to seven classes Ochrophyta (66) (56%), Cyanophyceae (18) (15.2%), Dinophyceae (14) (12%), Chlorophyceae(11) (9.3%), Euglenophyceae (6) (5%), Cryptophyceae (2) (1.7%) and Silicoflagellates (1) (0.8%). The lake ecosystem is increasingly affected by various anthropogenic impacts, such as excess of nutrients, causing eutrophication, toxic contamination of industrial, agricultural and domestic origin which in turn induced two of phytoplankton blooms ,the first occurred at the western lagoon (3.1 x106 cells /l) during August due to the concurrent growth of Cryptomonas ovata and the second (2x 106 cells/l) occurred in October due to the predominance of potentially harmful diatom Pseudo nitzshia delicatissisma at EL-Temsah Lake. The most frequently dominant species found were Skeletonema costatum, Chaetoceros spp., Pseudanabaena limnetica, Oscillatoria spp., Pseudo nitzshia delicatissisma, Asterionella formosa. The qualitative and quantitative results of phytoplankton, as well as chemical data lead us to conclude that, in general, the water of EL-Temsah Lake seem to be highly affected by eutrophication and can be considered as eutrophic.

Key words: Phytoplankton, water pollution, El-Temsah Lake