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Today, due to population increase and anthropogenic activities together with sewage and agricultural waste water entrance, aquatic ecosystems have been exposed to high pollutions. Phytoplankton is a group of water floating algae that have crucial roles in providing nutrients and oxygen for other organisms, nitrogen and CO_2 fixation. These organisms are considered as primary producers in aquatic ecosystems. They are found in various water habitats all over the world, affected by environmental variables such as pH, light and temperature and used for determination of water pollution degree and quality. Phytoplankton composition and density may be used as a complementary indicator of water trophy state. Phytoplankton communities indicate short and long term variations of aquatic systems. One of the most obvious problems in freshwater ecosystems is algal bloom or over growth of some blue- green algae which can decrease oxygen, and in some cases bring about toxin excretion and fish and human kills. In addition, many of bloom forming cyanobacteria produce secondary metabolites which can create sever poisoning in mammals including human. The genus Microcystis is a key bloom forming cyanobacteria in aquatic ecosystems. Populations of this genus form intense blooms in water bodies that has attracted more attentions in recent years. Various species of this alga have been distributed in stagnant and eutroph freshwater around the world. Microcystis has been defined by genetic criteria such as 16S rRNA molecular sequencing, but its classification in levels lower than genus is unclear and the presence of its classical morphospecies is doubtful. However, this genus creates sever blooms in eutrophic waters all over the world and many species produce toxins. Therefore, identification of its natural diversity in the levels lower than genus has high importance. However, several characteristics of Microcystis morphotypes which are classified a traditional species, actually are present and observed in different regions of the world. At present, they can be considered as morphospecies that belong to one genotype and have similar ecology. These traditional species with definite phenotypic and ecophysiological characteristics cannot be eliminated completely. Their identification is essential for ecological and ecotoxicological studies. Interistic transcribed separator (ITS) gene is a section of genome which is located between 16s rRNA and 23s genes. This gene has more heterogeneity than 16s rRNA; so it is used to identify many genera of cyanobacteria. The Aras reservoir located in the north-west of Iran plays important roles as fisheries, drinking and agricultural water supply and recreational activities in the region. The present thesis was undertaken to: analyze the Microcystis sp. populations by molecular methods such as ITS in Aras Reservoir. Samples for molecular analysis were collected from 10 sampling sites on 18 August 2013. Samples for molecular study of Microcystis were collected from two different depths (surface and 1 m depth) and transferred to laboratory without any treatments. Microscopic images of Microcystis were sent to professor Komárek and was confirmed.

Today, due to population increase and anthropogenic activities together with sewage and agricultural waste water entrance, aquatic ecosystems have been exposed to high pollutions. Phytoplankton is a group of waterfloating algae that have crucial roles in providing nutrients and oxygen for other organisms, nitrogen and CO2 fixation. These organisms are considered as primary producers in aquatic ecosystems. They are found in various water habitats all over the world, affected by environmental variables such as pH, light and temperature and used for determination of water pollution degree and quality. Phytoplankton composition and density may be used as a complementary indicator of water trophy state. Phytoplankton communities indicate short and long term variations of aquatic systems. One of the most obvious problems in freshwater ecosystems is algal bloom or over growth of some blue- green algae which can decrease oxygen, and in some cases bring about toxin excretion and fish and human kills. In addition, many of bloom forming cyanobacteria produce secondary metabolites which can create sever poisoning in mammals including human. The genus Microcystis is a key bloom forming cyanobacteria in aquatic ecosystems. Populations of this genus form intense blooms in water bodies that has attracted more attentions in recent years. Various species of this alga have been distributed in stagnant and eutroph freshwater around the world. Microcystis has been defined by genetic criteria such as 16S rRNA molecular sequencing, but its classification in levels lower than genus is unclear and the presence of its classical morphospecies is doubtful. However, this genus creates sever blooms in eutrophic waters all over the world and many species produce toxins. Therefore, identification of its natural diversity in the levels lower than genus has high importance. However, several characteristics of Microcystis morphotypes which are classified a traditional species, actually are present and observed in different regions of the world. At present, they can be considered as morphospecies that belong to one genotype and have similar ecology. These traditional species with definite phenotypic and ecophysiological characteristics cannot be eliminated completely. Their identification is essential for ecological and ecotoxicological studies. Interistic transcribed separator (ITS) gene is a section of genome which is located between 16s rRNA and 23s genes. This gene has more heterogeneity than 16s rRNA; so it is used to identify many genera of cyanobacteria. The Aras reservoir located in the north-west of Iran plays important roles as fisheries, drinking and agricultural water supply and recreational activities in the region. The present thesis was undertaken to: analyze the Microcystis sp. populations by molecular methods such as ITS in Aras Reservoir. Samples for molecular analysis were collected from 10 sampling sites on 18 August 2013. Samples for molecular study of Microcystis were collected from two different depths (surface and 1 m depth) and transferred to laboratory without any treatments. Microscopic images of Microcystis were sent to professor Komárek and was confirmed. Iranian Fisheries Science Research Institute Published