Trophic state” is often used to classify aquatic ecosystems according to biotic productivity. Carlson trophic state indices were used to assess the trophic situation in Gheshlagh reservoir (North-Western Iran), on which a dam was constructed three decades ago. Using data from a one year surface sampling, a comparison of trophic state index (TSI) between 4 sampling points in the reservoir and 1 sampling point under the dam, was performed. The results of TSI calculated based on chlorophyll a concentration and Secchi depth (SD) showed that the aquatic environment in the reservoir is eutrophied (50 < TSI < 80), while it is mesotrophic (40 < TSI < 50) under the dam. Moreover, the measured concentrations of chl. a and phosphate were consistent with the available data ranges on eutrophication. In addition, it was shown that algal biomass bloom occurred at the end of spring, all over the sampling points
