The trophic status of the Zayandeh River dam lake in the spring and summer, 2017

The Zayandeh River dam lake, supplies freshwater for municipal, agricultural and industrial activities of three central Iranian provinces. Monthly water sampling was conducted in the spring and summer 2017 at four stations in the lake to determine trophic state. Electro-conductivity, temperature, pH, turbidity, total suspended solids, total dissolved solids, nitrate, phosphate, dissolved oxygen, biological oxygen demand also chlorophyll-a levels were measured in the samples. The maximum value of Trophy State Index (TSI) was recorded in May and the minimum value in September. Based on TSI, the Lake was oligotrophic in the spring and summer. However, in the May, the lake was in mesotrophic state, probably due to floods, runoff and drainage of farmlands. Due to high temperatures and biological activity in the summer, nitrate and phosphate decomposition increased. On the other hand, agricultural activities decreased compared to the spring, resulted in decreases in the concentration of nutrients, especially nitrate. Therefore, the lake is in oligotrophic state from June to September.