In this study, species composition, density and biomass of zooplanktons including Rotatoria, Protozoa and Meroplanktons were sampled using Juday net with mesh size of 100 µ at depths of 5, 10 and 20 meters in southern Caspian Sea (Babolsar) in two seasons of cold (Feb. 2015) and warm (June 2015) . This study identified Asplanchna sp. and Synchaeta sp. from Rotatoria, and Balanus sp., Pelecypoda larvae, Balanus cypris and Nereis sp. larvae from Meroplankton. Globogeria sp. was the only protozoa observed in this study. Two way ANOVA analysis showed that sampling site and season had significant effect on density and biomass of zooplankton (P<0.05). Duncan's multiple range test showed that the density and biomass of zooplankton groups were significantly higher in the station A1 than two other stations (P<0.05). Moreover, the density and biomass of zooplankton were significantly higher in cold season compared to those of warm season (P<0.05). The highest density and biomass belonged to Asplanchna sp. from Rotatoria in the cold season with the density of 23381.47±2907.94 org.m^-3and the biomass of 476.6±58.2 mg.m^-3. The highest density and biomass of zooplankton was observed at the depth of 5 meter, and density and biomass were decreased by increasing the depth. According to the results of this study, species composition, density and biomass of zooplanktons including Rotatoria, Protozoa and Meroplanktons were very low in the south of the Caspian Sea (Babolsar).

Keywords: Protozoa, Southern Caspian Sea, Zooplankton, Rotatoria, Meroplankton

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