Assessing the water quality through constant monitoring is the basis of planning to control and reduce pollution. Since biological communities are sensitive to water quality and are affected by polluting factors, they can reflect the water quality in aquatic ecosystems. The purpose of this study is to identify the zooplankton, macrobenthoz communities and determine the water quality using biological indicators in the lower reaches of the Tajan river for the purpose of releasing juvenile fish. In this research, 7 sampling sites were selected along the Tajan river from the downstream of Tajan bridge to the mouth of the river and sampling was done monthly from May to October 2022. In zooplankton sampling, 100 liters of water were filtered by a plankton cone net. Sampling of Macrobenthoz was done by grab (225 cm2) device and Surber sampler (1600 cm2) with three repetitions in each sampling site. A total of 24 zooplankton species were identified during one-year sampling in different sampling sites from the holoplankton and zoobenthoz groups. The results showed that the main zooplankton population was made up of the Holoplankton group with a frequency of 90.9% and zoobenthoz was 9.1% of the population, the holoplankton population was influenced by Rotifera. Rotifera 62.2%, Coppepoda 24.3%, Protozoa 3.6% and Cladocera 0.8% constituted the zooplankton population of the region. The min and max average zooplankton diversity was measured in sampling sites 4 and 3, 7 as 1.4±0.4 and 1.9±0.2, 1.9±0.3, respectively, but there was a significant difference between the sampling sites were not seen (P>0.05). The results showed that all the sites are on average in the quality class of β-mesosaprobity with relatively polluted water. During this survey, 8 families including Chironomidae, Simuliidae, Baetidae, Hydropsychidae, Gammaridae, Lumbriculidae, Spionidae and Erpobdellidae were identified. Analysis of the distribution of different macrobenthic families in different sites showed that Lumbriculidae was the most present in different sites, as it was seen in all the sampling sites. The results showed that although the Chironomidae family ranked second in terms of distribution, it had the highest percentage of relative abundance (38.9%). Macrobenthic species diversity with the highest amount in sampling site 2 showed a significant difference with all the sites (P<0.05) and sampling site 7 with the lowest amount had a significant difference with the sites 1, 2 and 3 (P<0.05).  Based on the Hilsenhoff biological index, the degree of organic pollution and water quality in different sites ranged from high quality class (no pollution) in the sites 6 and 7 to medium class with relatively significant pollution in the sites 1 and 2. According to the results, it can be said that these indicators are suitable for evaluating the water quality of Tajan river and more suitable indicators should be used for the sites 6 and 7. The site 4, at a distance of 7 km from the sea coast (Soteh) with little pollution, it had the best water quality conditions, so it seems that the best place for releasing fish fry in terms of water quality conditions is in Soteh. Due to the proximity of the water quality distance in the study area, habitat parameters should be taken into account to determine the exact location of releasing the fry.
Keywords: Tajan River, Zooplankton, Macrobenthoz, Biological indicators, Fish fry
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