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Lakes play a vital role in water conservation, biodiversity, tourism and job creation. However, seasonal changes, inflow of sewage and erosion increase the amount of nutrients discharged into the lakes and affect the structure of the ecosystem and the diversity of its planktons through the reduction of water quality. The purpose of this study was to identify the species composition and abundance of phytoplankton and zooplankton communities in Karon-4 lake to investigate the trophic indicators and water quality of this lake. For this purpose, after identifying 4 stations, sampling was done in 4 stages during 2017. Based on the results, 30 species of phytoplankton (belonging to 7 groups) and 58 species of zooplankton (belonging to 7 groups) were identified. The highest density of phytoplankton was observed in late summer and autumn, indicated the high values of soluble oxygen of the lake in these seasons beacause of increased photosynthetic activities. Most of the observed zooplankton species were also in the spring season and related to Copepoda, Rotifer and Cladocer. The average Shannon-Wiener and Margalf indices for phyto and zooplanktons indicated the improvement of living conditions and the possibility of the presence of more diverse species of zooplanktons due to the development of phytoplanktons in this lake. Therefore, in order to prevent algal bloom, in addition to controlling external factors (entry of various pollutants), attention should also be paid to internal conditions governing this dam. Finally, based on the abundance and diversity of species observed, this lake was classified as oligo-mesotrophic. [ABSTRACT FROM AUTHOR]