en Ecology Ecology پژوهشي Orginal research papers Diatom species diversity and cell abundance are influenced by environmental factors. The effect of physico-chemical factors on interseasonal dynamics of planktonic diatom community structure was examined along Noor shore of Caspian Sea to determine which of the variables impact more on the diatom diversity, distribution and cell number. Overall, 144 samples were collected during four sampling campaigns along four transects of 2 km long each with three sampling sites in 2014 (February, May, July and November). In total, 62 diatom species were identified, of which 27 species were typical marine or brackish-water and the rest were marine-freshwater or freshwater. In the present study, diatom species diversity was highest in winter. As a result, Shannon-Wiener Index was 3.5 in winter, while in spring it was reduced to 0.8. Among the revealed species, the centric <em>Thalassiosira caspica</em> was the most abundant (up to 2.75&times;10⁶cells L^-1). Comparison of species and their abundances between coastal, middle and offshore stations, as well as all stations as a whole, showed that they differed (Test &chi; 2, <em>p</em><0.05) except for <em>Navicula</em> sp. (in summer) and <em>Ulnaria ulna</em> (in autumn and winter). Canonical correspondence analysis results showed that in particular temperature and salinity, in the second place transparency, phosphates and to a lesser extent dissolved oxygen and pH can affect diatom species diversity and abundance. Diatoms <em>Thalassiosira fasciculata,</em> <em>T. caspica</em> and <em>Melosira caspica</em> may be recommended as biological indicators along the shore of Caspian Sea. Abundance, Bacillariophyceae, Caspian Sea, CCA analysis, Diatom diversity, Phytoplankton 945 960 http://jifro.ir/browse.php?a_code=A-10-1371-2&slc_lang=en&sid=1 B. Zarei Darki zareidarki@modares.ac.ir 100319475328460041007 100319475328460041007 Yes Department of Marine Biology, Faculty of Marine Sciences, Tarbiat Modares University, Noor, Mazandaran Province, Iran S. Bigham 100319475328460041008 100319475328460041008 No Department of Marine Science and Technology, University of Hormozgan, Bandar Abas, Iran R. Patimar 100319475328460041009 100319475328460041009 No Department of Fisheries, Faculty of Natural Resources, Gonbad Kavous University, Gonbad Kavous, Iran