Introduction: This study is in line with the plan to farm trout in cages in the southern areas of the Caspian Sea (Mazandaran waters-Kelarabad) which was carried out in 2012. Methods: Sampling was conducted on a monthly basis and at 3 stations. The goal of this study was to examine the condition of planktonic groups, density, biomass and diversity in the location of fish cages and comparison with the control station. Results:The results showed that the phytoplankton group of 5 phylum and zooplankton of 6 phylum were identified which the population of phytoplankton were only in the spring season the highest and their biomass in station 1 while the population density of zooplankton in most seasons at station 1 had the highest sampling. The density and biomass of phytoplankton and zooplankton in spring and winter were more than other seasons that can be due to the activity of farming rainbow trout in the cages. Over the years, under the dominance of phytoplankton diatoms (84%) and dominant zooplankton population was also Copepods (67%) based on ISI index. Shannon diversity index of phytoplankton varied in the range of 1.25 to 2.73 and for zooplankton in the range of 0.24 to 1.41. There was a significant difference between the intensity and biomass of different seasons (P < 0.05). Conclusions: The conclusion is that the phytoplankton and zooplankton communities at the location of fish cages culture was by far more than the control station which shows the effect of fish cage culture on planktonic organisms and possibly algae blooming in larger size (using several cages and several places for the development of aquaculture) and it irreversible effect on the closed ecosystem of the Caspian Sea in the future.