Temporal and special variations of physicochemical factors and special richness of phytoplanktons in Manjil reservoir

One of the most valuable water reservoirs in the north of Iran is Manjil Dam Lake which can have many other uses apart from agricultural water supply and power generation. For this reason, in order to qualitatively evaluate the water during one year and monthly, physical-chemical and comprehensive factors of phytoplankton have been studied as trophy indices in 5 stations in the lake. Results showed that average annual water temperature, light penetration depth, nitrite, nitrate, total phosphate, and dissolved oxygen were 15.33±6.21°C, 1.8±.35 m, 0.017±0.001 mg/l, 1.816±0.629 mg/l ,0.013±0.113 mg/l and 8.38±0.383 mg/l, respectively. Moreover, 22 different phytoplankton genera were identified that Chlorophyta, Cyanophyta, Pyrrophyta, Bacillariophyta, and Chrysophyta had frequencies of 42%, 44.3%, 6.3%, 2.1%, and 0.05%, respectively. Also, ecological indicators in terms of diversity and richness of phytoplanktons species in Manjil Dam Lake demonstrated that species composition changed with time and physical and chemical factors were determined to be the most important causes of these changes. However, certain genera were mainly predominant during the year and the change in Shannon-Wiener index showed a small amount. From the perspective of the trophy status based on the models used, it was determined that this reservoir dam was completely oligotrophic which was due to high levels of sediments entered from water resources of this dam, reducing the light penetration depth by a large degree.