Maryam Ghashghaie - PhD student, Agriculture Faculty, Bu Ali Sina University, Hamedan, Iran.
Abdollah Taheri Tizro - Associate Professor, Agriculture Faculty, Bu Ali Sina University, Hamedan, Iran.
Ramtin Pakzad - Technician of Plant Physiology Laboratory, Science Faculty, Bu Ali Sina, Hamedan, Iran.

This study aimed to investigate trophic state of Ekbatan Dam Lake. In this research two month of July and January were chosen to study the quality of water behind the dam andcompare the trophic state of the reservoir in two different seasons. 15 stations were selected in different points of Ekbatan Dam Lake for each time and sampling was accomplished from the surface layer. In this study sechi disk, nitrate, phosphate andchlorophyll-a were measured. Then using Calson index the rate of eutrophication was estimated for all points. Also quality zonation was done using GIS for water qualityparameters and TSI (Trophic State Index). Results of the study showed that the trophic state of the lake is oligotrophic to eutrophic in different stations in July while it is eutrophic for all points in January. Based on the estimated values of this index, stations located at the entrance of the lake show more eutrophic situation in July. Also phosphorous is the limiting nutrient in this system. This research showed that eutrophication is a serious problem in Ekbatan reservoir and needs more attention.

Carlson Index, Ekbatan Dam, Eutrophication, TSI, Water Quality