This study concentrated on temporal distribution and species composition of phytoplankton connected with physicochemical variations between January 2003 and November 2004 in the South Western Caspian Sea. During the study, 75 phytoplankton were distinguished in total and average phytoplankton densities in 2003 and 2004 were 1.47×10 5 ±6.05×10 4 and 1.97×10 5 ±7.18×10 4 cells L -1 , respectively. Contributions of diatoms (66.0% in 2003; 57.0% in 2004) and cyanophytes to total phytoplankton (17.0

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... in 2003; 36.0% in 2004) were higher than any other group. Diatoms Dactyliosolen fragilissimus and Thalassionema nitzschioides, and cyanophytes Oscillatoria sp. and Anabeanopsis raciborskii provided the biggest contributions to the total phytoplankton. Average DIN concentration in 2003 was higher (3.86±1.65 µM) than the average in 2004 (3.37±1.13 µM). However, average DIP concentration in 2003 was lower (1.45±1.37 µM) than the average in 2004 (1.99±0.67 µM). Average DIS concentration was two-fold higher than average DIN concentration. Average DIN:DIP without NH 4 (2.66±1.66 in 2003; 1.69 ± 1.17 in 2004) and DIS:DIP ratios (5.61±3.44 in 2003; 3.93±2.49 in 2004) were clearly lower than Redfield ratios. Nutrient ratios and phytoplankton densities revealed that system was eutrophic