Aras River zooplankton population changes Ali Mohsenpour Azari,Saeed Chodar Rezai*,Yusofali Asadpoor, Alireza Asem,Reza  Ahmadi, Saber Shiri Iranian Artemia Reference Center, Urmia, Iran  Summary: This study is a section of "the effect of environmental factors of the Aras River and reservoir on Astacus leptodactylus growth and development" project. Sampling was made from 4 sites along river (Ghochali, pompage, poldasht and Ghargholog) by 50 µm mesh size net. All samples were fixed by 4% formalin and studied by an inverted microscope in Iranian Artemia Reference Center. The results showed that zooplankton fauna of Aras River included 4 phylum (Arthropoda, Protozoa, Ciliophora, Rotifera) 12 families (Bosminidae, Brachionidae, Vorticellidae, Strombididae, Stentoridae, Lecanidae, Arcellidae, Cyclopoidae, Amoebidae, Colepidae, Prorodontidae, Spirostomidae), 14 genera. Amoeba had the highest density (32.58%) and Spirostemum, coleps and strombidium from ciliophora and lecans from rotifera had the lowest densities (1.6%). Ciliophora included 6families and 6 genera and the highest diversity. Pompage site had the highest zooplankton density (38.6%). Brachionus and Cyclops (eutroph water indicator general) were abundant in this site so this site was eutroph. Ghargholog had the lowest density (11.8%) and the least diversity (3 genera). This was related to its location which near residential area with higher pollution loads, so that zooplankton density was reduced and macro zooplankton genera such as Bosmina, Brachionus, Cyclops and copepode were not able to thrive in this region of the Aras River. Home