The trend of changes in the Trohic State Index (TSI) in Chitgar Lake during 2013-2019

Lake of the Martyrs of the Persian Gulf (also known as the Chitgar Lake) is located in the north-western of Tehran. The construction of this artificial lake was done by Tehran Municipality was filled water of the Kan River in 2012. TN, TP, SD and Chl-a was used to determine trophic levels. Water sampling was performed by Rotner sampler and the water quality was investigated on 5 stations site during 2013-2019. Standard methods were used for Analysis of water parameters. To determine the difference between mean of the studied parameters, the One-way ANOVA variance analysis test and Duncan mean pair test was used, and the confidence level was 95%. Based on the study, the results indicated that during 2013-2019 the mean yearly EC was 432±65 µs/cm, pH was 7.79±0.45 and SD was 3.7±0.9 meter. The mean yearly of dissolve oxygen, total phosphorous, total nitrogen, respectively, were 8.3±1.4, 0.041±0.015, 2.828±1.342 as mg/l and chlorophyll-α was 5.2±2.7 µg/l. During the months under study, the water temperature varied between 5.8±0.4 to 28.9±0.2 as °C. The ratio of total nitrogen to total phosphorus was more than 30 which indicated phosphorus is limiting factor in the lake Eutrophication. Based on multivariate evaluation method, during 2013-2019 mean of TSI was 40.6±6.3 with range of 29-50. In the early years, TSI was oltraoligotrogh and during 2016-2017 the eutrophication state close to the mesotrophic state and then it has almost returned to its original level during the downward trend after the summer of 2018. Introducing fishes and refinery performance are two main factors that have contributed to the declining level of eutrophication in the lake.