| The Three Gorges Dam (TGD) is located in Yangtze River in China, which was built since1993and fully finished in2009. Yangtze River is by far the world's largest hydroelectric project and it possesses the most abundant area in freshwater fish species. As the TDG had significantly changed the eco-environment of the Three Gorges Reservoir in aquatic conditions, so what extent it has posed to the aquatic animals lived in this region is urgently to be solved. Xiaojiang River, located in the middle of TGR, is a tributary of the Yangtze River. To release the eco-environment pressure of Xiaojiang River, the Hanfeng Lake Dam was build in the Kaixian city, and an environment pattern of "reservoir of the river reservoir" was formed when the Hanfeng Dam was completed in the TGR, which provides a ideal ecological model to study the mainstream of TGR. In the present study, the Xiaojiang River of the TGR was chose to study the diversity of fish community, the food shift of fish, the eco-environment, the food web model and the energy origins of dominant economic fishes in in2010(the low level water and impoundment period, respectively), based on the methods of resource investigatio and combined with stable isotope analysis.The main results of present study as following(1) From March to December in2010,7sampling sites along Xiaojiang River were selected and the results indicated that the1701fish samples we obtained belongs to6orders,9families,47genus and56species, of which29and42species were collected from Hanfeng Lake and the down reaches of Xiaojiang River, respectively. Accordingly, the Cypriniformes and Siluriformes were accounted for69.5%and12.5%of the total species. Moreover, our data also indicated that Zacco platypus, Hemiculer tchangi, Silurus asotus, Carassius auratus, Cyprinus carpio, Culter mongolicus mongolicus, Culter ilishaeformis, Pelteobagrus vachelli and Pelteobagrus nitidus were the dominant economic fishes in the Xiaojiang River. In summary, the species number of catches possessed the highest index in spring, and the fish compositions in Hanfeng Lake received a greater seasonal change than that of down reaches, while a contrary result in aspect of fish richness was obtained. However, the richness of catches in Xiaojiang River was lower than that in other tributaries of TGR.(2) Mongolian culter (Culter mongolicus), which is one of the main economic fishes in Xiaojiang River after inpoundment in the TGR, plays an important role in aquatic ecosystem. The diet shift of Mongolian culter in various ontogenetic stages has a deep impact on the aquatic ecosystem. Therefore, based on the stable isotope analysis, combined with the identity of contents in stomach and intestines, we herein discussed the Mongolian culter food groups from backwater of Xiaojiang River region. Our results indicate that the δ13C and δ15N values of small Mongolian culter whose length is less than200mm are-24.50‰±1.15‰and12.17‰±1.54‰, respectively, they are carnivorously omnivorous with2.9of trophic level; those big ones more than200mm in length are-23.87‰±1.12‰and13.54‰±1.12‰, respectively, their trophic level is3.3and diet type is carnivorous. The fact that the δ13C and δ15N values of large Mongolian culter are significantly higher than that of small ones (P<0.01), prove a diet transfer took place during their growth, but there is a overlap of isotope values between large and small Mongolian culter for their sharing the foods from the same source. This study helps to predict the "top-down effect" caused by the enhancement and releasing of fishes and a variety of fisheries resources management activities such as fishery capture in Xiaojiang River region.(3) The stable isotope approach was used and we compared the difference of isotopic signatures of the same species between the impacted areas (A and B sites) and the control areas (C and D sites). So the aims of the present study were trying to investigate the effects of human sewage and agricultural non-point source input in organisms from the different trophic levels (primary producers and primary consumers) in the Hanfeng Lake, an upper reaches of the Yangtze River. The results showed that the carbon and nitrogen stable isotope values of POM and sanil samples were various in different groups. In impact groups, the values of POM samples were-25.93‰～24.63‰forδ13C and4.12‰～9.86‰for δ15N, the values of snail samples ranged from-14.28‰～-21.60‰(δ13C) and7.97‰～19.99‰(δ15N); In control groups, the values of bothe POM and snails samples were-25.62～-22.51‰(δ13C),0.01～6.56‰(δ15N),-22.96～-19.21‰(δ13C),6.75～8.89‰(δ15N), respectively; The δ13C values for either POM and snails (primary consumers) were not significantly different between different sampling groups (P>0.05), while the change of δ15N in different sampling groups varied greatly (P<0.05). The relative trophic position of primary producer, such as periphytic algae, POM (mainly composed of phytoplankton) and primary consumers (snails), are higher in impact groups than in those of control groups, whereas there is no significant difference in the relative trophic levels of the other higher trophic level consumers, such as carp (Cyprinus carpio), crucian(Carassius auratus) and Amur catfish(Silurus asotus). Therefore, the nitrogen stable isotope signature was proved to better describe the information on the absorption and enrichment of anthropogenically derived material in food webs of the Hanfeng Lake. Compared with the other organisms(e.g. periphytic algae and fish), POM and snails were more sensitive to pollution than other species, and could be more appropriate to serve as an indicator of assessing the quality of environment. Moreover, the organisms from both impacted groups (site A and site B) in the Hanfeng Lake have been profoundly influenced by anthropogenic inputs of nutrient matter, and site B received a more stronger interference than site A. In conclusion, it has important significance to strengthen the protection of the environment of the Hanfeng Lake, and control the effluent discharge amount for protecting the water quality of the Xiaojiang River and even the TGR.(4) Under the anthropogenic disturbance, the food webs in the Xiaojiang River, a tributary of the TGR, were investigated via stable carbon and nitrogen isotopes. Meanwhile, the importance of external and internal carbon sources in different impoundment periods (in summer and autumn) was elaborated by means of stable isotope analysis. Overlap in the carbon of primary producers made it difficult to determine unambiguously the food resources of many fishes. However, isotope analysis suggested that the carbon sources of many fishes in the Xiaojiang River mainly came from internal carbon in the low water level period; while in the impoundment period, total carbon source of Hanfeng Lake was primarily from internal carbon, while that of down reaches of the Xiaojiag River was mainly from internal and external nutrient substances. Nitrogen isotope signatures of many fishes indicated that815N was inclined to decrease from the low water level to the impoundment period. Probably, a large variety of external food sources were available for fishes during the impoundment period. Food webs in the Xiaojiang River involved two or three trophic levels. There existed three trophic levels throughout the whole year, except Hanfeng Lake where there were two trophic levels in summer. Fishes ate all sorts of food, often feeding on food of more than one trophic level. In the current study, it was found that①Hanfeng Lake Dam had produced a deterring effect on carbon sources of Xiaojaing River in the impoundment period,②the feeding habits of some fish species were changed after the finally successful impoundment in the TGR. Therefore, further investigations are still required.(5) To analysis the major energy origins of these7species economic fishes, the IsoSource Model were applied. The results showed that the the microalgae were the main energy sources of the7species economic fishes in different seasons, while a decreasing trend for economic fishes in impoundment period were also observed. The contribution ratios of terrestrial C4plants in impoundment period was higher for economic fishes'food source than that of the low water level period, especially for crucian (Carassius auratus) and Amur catfish (Silurus asotus), and they ranged from38%-54%and32%-50%, respectively. During the impoundment stage of TGR, terrestrial C4plant contributed more than30%of the total organic carbon of these two fishes, suggesting that the contribution percent of terrestrial C4plant increased for economic fishes'food source with the processing of impoundment in TGR. Therefore, we assumed that the inputs of terrestrial nutrient matters are indispensable for the food supply for integral food web in TGR with increasing of the Three Gorges Project runtime.
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