Wetlands are known as “kidney of the earth”, functioning ecologically, environmentally and productively in the aspects of flood storage, climate regulation, pollution control, soil erosion reduction, landscaping, and maintenance of biological diversity and productivity. Eutorphication is increase of primarily productivity in water ecosystems as result of increase of nutrients, especially phosphorus and nitrogen in ecosystems. Eutorphication lead to decrease of water quality and occurrence of algal blooms. In addition, eutorphication affect biodiversity. With increase of human activity such as development of agriculture and increase of wastewater release to aquatic ecosystems, assessment of trophic status seemed to be very necessary. Choghakhor wetland is important ecosystems because is located in Iran’s central plateau and has a high biodiversity. The aim of this research was the assessment of trophic status and change in throughout the year. Sampling is carried out from May to March 2010 in eight stages with time interval of 45 days. In order to assessment of trophic status in Choghakhor’s wetland 10 station selected and physico-chemical parameters (temperature, dissolved oxygen, pH, alkalinity, depth, Secchi disk depth and nutrients) as well as biological parameters (chlorophyll a and macrophytes coverage) were measured in throughout the year. In addition, phytoplankton species diversity and density were determined in order to assessment of trophic status change effect on structure, diversity and density of phytoplankton assemblage. Results showed there was not significance difference between stations in all stages ( P 0/05). With respect to Redfield index and high positive correlation between chlorophyll a and total phosphorus, probably phosphorus is limiting factor in this ecosystem. This ecosystem ), total nitrogen (r=0.448), temperature (r=0.338), macrophytes coverage (r=0.324), Secchi disk depth (r=0.201) as well as thermal stratification. To determine of trophic state, three common indices of trophic Key words Choghakhor Wetland, Eutrophication, Carlson Index, TRIX Index, Redfield Index, Trophic state.
