Résumé
Seasonal variations in phytoplankton abundance and their composition were studied at five stations in the middle region (between Al-Hindiya barrage to Kifil City) of the Euphrates River in Iraq between March, 2004, and February, 2005. A total 151 taxa of phytoplankton were identified, belonging to Bacillariophyceae (98), Chlorophyceae (33), Cyanophyceae (14), Euglenophyceae (2), Xanthophyceae (2), and Dinophyceae (2). The total abundance of phytoplankton cells varied from 136 to 5312 cells l-1 with maxima in spring and fall. Bacillariophyceae were the most abundant group at all stations. Some species of phytoplankton occurred continuously during the study period (Cyclotella ocellata, C. meneghiana, Cocconeis placentula, Nitzchia spp, Meringosphaera spinosa). The study recorded four species as new records for Iraqi. The phytoplankton was indicative of oligotrophic conditions although it showed some signs of organic pollution near cities.