The purpose or this study was to evaluate the changes of dissolved silicon concentration and biomass of diatoms in southern part of Caspian Sea, 1270 plankton and water samples were collected seasonally from 77 stations for further quantitative and qualitative analysis.The results showed that the minimum and maximum density of dissolved silicon and biomass of diatoms were varied from 0.195 to 0308 ppm and 33 to 179 mg/m3 respectively. As well as the fluctuation of dissolved silicon concentration in different depths of photic zone. It was indicated that the maximum and minimum concentration of this element were found in the eastern area with 0.269 ppm and western area with 0.180 ppm. The most and the least biomass of diatoms were observed in the western and central areas with 180 and 89 mg/m3, respectively. The concentration or silicon in offshore waters was more than coastal waters and on the contrary, the biomass of diatoms from coastal to offshore waters indicated decreasing trend. In a conclusion, the concentration of dissolved silicon and biomass of diatoms could be affected by different physical parameters such as: water circulation, river currents, exchange of biogenic sediments in photic and aphotic zones. Also, there was a significant relationship between the biomass of diatoms and silicon concentration
