Phytoplankton bloom (Cyanobacteria: Nodularia spumigena) in the southwestern Caspian Sea off Anzali, July 2021

This study focuses on density and species composition of phytoplankton during bloom in the southwestern Caspian Sea off Anzali. The eighteen sampls were took  between Apri and July 2021. The findings displayed, the  phytoplankton bloom speices was distanguited in the study as cyanobacteria Nodularia spumigena  Mertens as bloom in the southwestern Caspian Sea in 2005. A total of 8  phytoplankton species were distinguished during the bloom, the phytoplankton abundance were calculated  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 Nodularia spumigena abundance as 512,000,000 cells.l-1 provided the biggest contributions to the total phytoplankton during the bloom. Vertical distribution of  N. spumigena at the bloom region revealed that most N. spumigena 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  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  and 9.91 µM. l-1 during bloom. There is strong relationship between N. spumigena abundance and environments factors. The very low wind, strong heating of the upper layer almost 29 ºC and increasing of  nutrients levels were the most reasons of N. spumigena bloom in the southwestern Caspain Sea Anzali offshore.