The Three Gorges Dam is the largest hydroelectric power station project in the world.After the impounding of the Three Gorges Reservoir(TGR),its hydrological characteristics(water depth,water flow velocity and water temperature)markedly changed.Meanwhile,the external nutrients,e.g.terrestrial plant,debris,soil organic matter,come from the Yangtze River upstream and stagnate in the TGR after the impounding of the TGR.As no study was carried out investigating the comprehensive effects of the above factors on the fish community structure and their food web in the TGR,the stable carbon and nitrogen isotope(δ¹³C and δ¹⁵N)technology was applied in this study to evaluate the food web of fish and nutrient relationship caused by the seasonal variation in the main stream of the TGR from Wushan to Wanzhou(summer,July 2010;winter,December 2010),and the data of 2005 in summer were also provided to compare their changes.The results of δ¹³C indicated that the values of both particulate organic matter(POM)and periphytic algae samples were (-25.62‰)-(-23.72‰) and (-19.81‰)-(-19.47‰),respectively,and no significant variation of δ¹³C values was observed for primary producers(POM and periphytic algae)between summer and winter in 2010(P>0.05).However,as the POM and periphytic algae(endogenous nutrients)were the main basal carbon origin for the webs of the fish food,and the input of the exogenous nutrient substance was an important supplementary way for the consumers in TGR.In summary,an increasing trend of the δ¹³C values for consumers and the chain length for the webs of the fish food were obtained from 2005 to 2010.Our findings will be helpful to the management of the fish resources and the protection of the ecological restoration in the TGR.