en Ecology Ecology كاربردي Orginal research papers This study focuses on density and species composition of phytoplankton during bloom in the southwestern Caspian Sea off Anzali. The eighteen sampls were took&nbsp; between Apri and July 2021. The findings displayed, the&nbsp; phytoplankton bloom speices was distanguited in the study as cyanobacteria<em> Nodularia spumigena</em> &nbsp;Mertens as bloom in the southwestern Caspian Sea in 2005. A total of 8&nbsp; phytoplankton species were distinguished during the bloom, the phytoplankton abundance were calculated&nbsp; as 8,640,000 cells.l^-1. Contributions of cyanophyta (5,720,000 cells. l^-1) to total phytoplankton were higher than any other group. The cyanobacteria<em> Nodularia spumigena</em> abundance as 512,000,000 cells.l^-1 provided the biggest contributions to the total phytoplankton during the bloom. Vertical distribution of&nbsp; <em>N. spumigena</em> at the bloom region revealed that most <em>N. spumigena</em> abundance occurred in the upper layer (0-5 m) almost 5,000,000 cells.l^-1. After bloom period, the number of phytoplankton species increased sharply and identified 18 species. Bacillariophyta were dominant (60,000 cells.l^-1). Cyanobacteria (12,600 cells.l^-1) were the second most important group, contributing to the total phytoplankton abundance. The phtytoplankton abundance was declined &nbsp;sharply almost 83,400 cells.l^-1 after bloom as compared with bloom period. The total phosphorus and nitrogen consenterations rised sharply as 0.59 &nbsp;and 9.91 &micro;M. l^-1 during bloom. There is strong relationship between <em>N. spumigena</em> abundance and environments factors. The very low wind, strong heating of the upper layer almost 29 &ordm;C and increasing of &nbsp;nutrients levels were the most reasons of <em>N. spumigena</em> bloom in the southwestern Caspain Sea Anzali offshore. Nodularia spumigena, Abundance, Taxa, Temperature, Wind, Off Anzali 849 858 http://jifro.ir/browse.php?a_code=A-10-185-5&slc_lang=en&sid=1 S. Bagheri siamakbp@gmail.com 100319475328460046408 100319475328460046408 Yes Inland Waters Aquaculture Research Center, Iranian Fisheries Science Research Institute, Agricultural Research Education and Extension Organization (AREEO), Anzali, Iran S. Khatib sepidehkhatib@yahoo.com 100319475328460046409 100319475328460046409 No Inland Waters Aquaculture Research Center, Iranian Fisheries Science Research Institute, Agricultural Research Education and Extension Organization (AREEO), Anzali, Iran J. Sabkara jsabkara@yahoo.com 100319475328460046410 100319475328460046410 No Inland Waters Aquaculture Research Center, Iranian Fisheries Science Research Institute, Agricultural Research Education and Extension Organization (AREEO), Anzali, Iran Y. Zahmatkesh younes_zahmatkesh@yahoo.com 100319475328460046411 100319475328460046411 No Inland Waters Aquaculture Research Center, Iranian Fisheries Science Research Institute, Agricultural Research Education and Extension Organization (AREEO), Anzali, Iran