Seasonal variations of phytoplankton abundance and community organization were observed over a one-year period (June 2008 to July 2009) along Tajan River is among significant rivers of Caspian Sea water basin. Phytoplankton biovolume showed seasonality, with the highest levels during summer periods and lowest levels during the winter period. Phytoplankton abundance was correlated to water and air temperature patterns, which may be altered by long-term changes in climate. The main purpose of this study was to document relationship among nutrient concentrations with phytoplankton distribution and composition. The correlation coefficient between physico-chemical parameters and phytoplankton of river are significantly associated with most of the physico-chemical characteristic. From the results, it is evident that the Tajan River water is eutrophicated with high values of BOD, COD, phosphate, nitrate, sulphate, TDS, monovalent, divalent cations and dominated by members of Oscillatoria, Navicula, Nitzschia, Euglena, Cymbella, Synedra.
