The increase in population, the development of urban communities and the human activities (industrial and agricultural, etc.) have caused an increase in the discharge of urban, agricultural and industrial wastewater into aquatic ecosystems. The algal proliferation in water basin increases under the influence of organic and inorganic substances in wastewater and lands, leads to the decrease in water quality. Zhaveh Dam was built on the Sirvan River, after the crossing of the two main branches of the Gheshlagh and Gavehroud Riveres, with the aim of being used in the agricultural and industrial sectors. There are gardens and agricultural lands around of Gavehroud. The Gheshlagh branch enters to the reservoir of Zhaveh Dam, after passing through the suburbs of Sanandaj city and industrial areas and receiving all kinds of sewage. In this research, for the first time, the structural pattern of phytoplankton and algal periphyton in the branches of Gheshlagh and Gavehroud (Sanandaj-Kurdistan) was studied during in the water year (from fall of 2020 to summer of -2021). The aims of this study are to determine the temporal and spatial changes of the species composition, abundance and biomass of phytoplankton and algal periphyton, to investigate water quality and trophic state based on algal indicators, and to determine the kind of using of the dam. In this research, 4 stations were selected from the upper of the Gheshlagh branch to near the crest of the dam (reservoir of the dam) and one station in the branch of Gavehroud. According to the results, 157 species of microalgae were identified in 7 phyla: Bacillariophyta, Pyrrophyta, Cyanophyta, Chlorophyta, Euglenophyta, Xantophyta and Cryptophyta in water and periphyton samples. The highest number of species was found in the Bacillariophyta (42% of the total number of species), followed by Cyanophyta and Chlorophyta (about 22% each). Phytoplankton abundance were changed from 13 (autumn, station 4) to 2635 (summer, station 3) million cells/m3. Abundance of algal periphyton was recorded from 17000 million cells/cm2 (summer, station 4) to 454 thousand cells/cm2 (autumn, station 5). According to Pearson's correlation test, Bacillariophyta and Cyanophyta respectively had the highest importance in changes of phytoplankton and algal periphyton abundance Phytoplankton biomass changes were from 39.3 (autumn, station 4) to 4727 (spring, station 5) mg/m3. In algal periphyton, the minimum (0.4) and maximum (4002) biomass (mg/cm2) was recorded in the spring season at (station 1) and (station 3), respectively. Also, Bacillariophyta, Cyanophyta, and Chlorophyta formed 36, 23, and 25% of the dominant species in the biomass, respectively. The Nitzschia, Diatoma, Navicula, Aphanothece, Lyngbya., Oscillatoria, Spirulina, Schroederia were the most imortant taxa of each phyla of phytoplankton. In algal periphyton, the highest abundance were belonged to Gloeotrichia (Station 4), Oscillatoria (station 5) and Cladophora taxa. They showed the highest abundance in the summer season (station 2). The Shannon's index (species diversity) of phytoplankton showed its maximum (3.12) and minimum (0.34) in autumn and winter seasons respectively in station 2. In algal periphyton, the maximum amount of Shannon's index was obtained in the winter (2.76) at station (5). The minimum Shannon index (0.01 to 0.05) was reported in summer and spring seasons atall stations except station (3). The water quality was generally in the "relatively polluted" group based on the Shannon index of phytoplankton. However in the autumn, with the water quality was classified in the "severely polluted" group. the water quality based on the Shannon index of algal periphyton was also generally classified (except for the summer season) in the "relatively polluted" group. In the summer season, the quality decreased to "severely polluted" in all stations. The saprobic index (organic pollution index) of phytoplankton showed changes from 1.5 to 2.5 units during the study period. This range of saprobic index indicates water quality with "moderate pollution". Navicula cryptocephala, Cyclotella meneghiniana, Lyngbya sp., Nitzschia palea and Stigeoclonium amoenum were same species in periphyton and water in the “severe pollution” class. The seasonal rainfall and the results of statistical tests of algae characteristics (total abundance, phyla and dominant species, saprobic, Shannon, evenness and number of species) indicate the division of seasons into two periods of dry (spring and summer) and rainy seasons (autumn and winter). Also, this test in the stations showed that the stations are divided to 3 groups: (1), (2 and 3), (4and 5) based on phytoplankton and 4 groups: (1), (2 and 3), (5) , (4) based on algal periphyton. The Diatom Trophic Index (TDI) in water and periphyton samples were often more than 3.5, which is indicates to hypertrophy condition,with ”very high  utrients” concentration. In general, in this study, the water quality in the lowest level of pollution and the best condition indicated that the water was suitable for transport boats (non-recreational), but it required advanced purification for use in general purposes and drinking water. In relation to fisheries and aquaculture, it was only suitable for breeding resistant species and even for use in industry and agriculture, it needed preliminary treatment.
Keywords: Phytoplankton, Algal periphyton, Pollution, Trophic level, Water quality, River tributries, Zhaveh Dam, Kordestan
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