en Ecology Ecology پژوهشي Orginal research papers Seasonal variations of zooplankton communities were studied in the southern Caspian Sea for 4 successive seasons and 8 transects with 5 stations from 5 to 100 m depths which sampled during 2009. The zooplankton population constituted of 73.33% copepods, 24.21% rotifers, 2.23% cladocerans and 0.23% protozoans. Copepods were dominant at all stations with a density between 1456±531 ind. m-3 (Transect 8) to 4524±1215 ind. m-3 (transect 2). The predominant species of copepods was Acartia tonsa constituting 99.50% of all copepod populations. They were most abundant during warm months of summer in the upper layers (surface to 20 m depth) while rotifers replaced them during cold season. The maximum density of zooplankton was observed in the west decreasing towards the eastern parts of the sea. Zooplankton, Caspian Sea, Copepods, Rotifers, Acartia tonsa 437 448 http://jifro.ir/browse.php?a_code=A-10-1272-92&slc_lang=en&sid=1 M Rowshan Tabari rowshantabari@yahoo.com 100319475328460010156 100319475328460010156 Yes S. M. R Fatemi 100319475328460010157 100319475328460010157 No R Pourgholam 100319475328460010158 100319475328460010158 No R Mousavi Nadoushan 100319475328460010159 100319475328460010159 No