Study On Nutritional Structure Of Fishes Food Web In The Mainstream Of The Yangtze River In The Three Gorges Reservoir Area Based On Stable Carbon And Nitrogen Isotopes

The construction of the Three Gorges Dam has blocked the original continuous river ecosystem and fragmented the aquatic environment of the Yangtze River,and the reservoir impoundmentand reservoir operation changed the original hydrological rhythm of the river.The change of environmental characteristics may have an impact on the structure of fish community and fish feeding,and eventually lead to the spatial and temporal differences of nutritional structure of fishes.In order to investigatethe temporal and spatial differences in the nutritional structure of fishes food webs in different river sections of the mainstream of the Three Gorges Reservoir and its response to environmental factors,based on analyses of carbon and nitrogen stable isotopes,the presentstudy investigatethe temporal and spatial differences of fishes food web structure,basic carbon source of food web,trophic level of fishes,food chain length,and isotope niche of fish community in Jiangjin,Mudong,Fuling,Yunyang,Wushan,Zigui and Jingzhou river sections,as well as the key environmental driving factors affecting them.The main research results are as follows:（1）A total of 42 species of fishes were collected which belonging to 4 orders,8families.Among them,the Cyprinidae accounted for themost,64.3%,followed by Bagridae,14.3%.All fish samples were analyzed for carbon and nitrogen stable isotopes,and the analysis results showed that theδ¹³C value of samples ranged from-20.85‰to-27.28‰,with an average of-23.82‰,theδ¹⁵N value of samples ranged from 6.31‰to14.90‰,with an average of 10.67‰;There were no significant differences inδ¹³C andδ¹⁵N values of fish communities among different river sections（P>0.05）;Theδ¹³C value of fish communities in the Wushan section during the flood season was significantly less than that in the dry season（P<0.05）,and theδ¹⁵N value of fish community in the Zigui section in flood season was significantly higher than that in dry season（P<0.05）,there were no significant seasonal differences in theδ¹³C andδ¹⁵N values of fish communities in the flood season and dry season in the other rivers（P>0.05）.（2）Four potential basal carbon sources（C3 plants,C4 plants,particulate organic matter（POM）and periphyton）were collected in different river sections in this study,Carbon and nitrogen stable isotope analysis showed that there were significant differences inδ¹³C values among the four basal carbon sources（P<0.05）,theδ¹⁵N value of POM was significantly higher than that of C3 plants,C4 plants and periphyton;There were spatial differences inδ¹³C andδ¹⁵N values of periphyton and POM in different river sections,while there was no significant spatial difference inδ¹³C andδ¹⁵N values of C3 and C4 plants（P>0.05）.The relative contribution ratios of four potential basal carbon sources to fish communities were calculated by Bayesian mixed model,the results showed that POM is the most important food carbon source of fishes food web,and its average contribution to fish reaches 61.9%,followed by C3 plants and periphyton,with an average contribution of 15.7%and 19.4%respectively,while C4plants have the lowest contribution to fish,with an average contribution of 3.1%.There are differences in the basic carbon sources of different feeding fish combinations.Among them,the most important basic food carbon source of piscivorous fish assemblages,benthivorous fish assemblages,omnivorous fish assemblages and herbivorous fish assemblages is POM,while the carbon source of planktivorous fish assemblages is mainly C3 plants.Affected by factors such as differences in the natural landscape of each river section,the relative contribution ratios of four basal carbon sources to fish communities in different river sections are also different,the basal carbon sources of fish communities in the main river section of the reservoir（Mudong,Fuling,Yunyang,Wushan,Zigui）is mainly POM,and the Jiangjin section are mainly POM and periphyton,while the Jingzhou section was mainly periphyton.（3）The fish trophic levels were estimated and the continuous nutrition spectrum of fishes was constructed with snails as the benchmark organism.The results showed that the trophic levels of fish ranged from 1.46±0.2 to 4.00±0.43,with an average of 2.67and the trophic level was 4.Among them,there is only one species（Coreius heterodon）with trophic grade greater than grade 4,and the species with trophic grade between grade 2 and 3 are the largest,accounting for 59.5%of the total species.The trophic levels of fish combinations with different feeding habits were different,and showed as piscivorous fishes>benthivorous fishes>planktivorous fishes and omnivorous fishes>herbivorous fishes,indicating that fish feeding preference was one of the key factors determining the nutritional position of fish.The trophic levels of the same species of fish or the fish combination with same feeding habits are different in different sections of the river,and the range of trophic levels of fish communities in Yunyang and Wushan sections in the middle reaches of the reservoir area was relatively wide,and the proportion of fish with high trophic levels were also relatively large,but overall the average trophic level of fish communities in different river sections was not significant（P>0.05）;Except Zigui river section,there was no significant difference in fish trophic level between flood season and dry season（P>0.05）.It showed that the differences in the environmental characteristics of different river sections and factors such as reservoir storage and reservoir operation mainly caused differences in the internal structure of fish food webs,but did not cause significant temporal and spatial differences in the average trophic level of fish communities.Compared with the initial stage of reservoir impoundment,the fish trophic level increased significantly,and the community structure and trophic characteristics of high trophic-grade fish have changed to some extent.In particular,Coilia brachygnathus with higher trophic level gradually expanded and became the dominant species in the main stream of the reservoir area,which might affect the trophic structure of fish in the Three Gorges Reservoir area.（4）The average food chain length（FCL）of fish was 4.06,ranging from 3.71（Jiangjin）to 4.31（Yunyang）across the seven river sections;The FCL tended to increase first and then decrease along the longitudinal gradient.The seasonal difference of FCL in Jiangjin,Mudong,Fuling and Jingzhou river sections is relatively small,and the species of fish with the highest trophic level are different in flood season and dry season;The seasonal difference of FCL in Yunyang,Wushan and Zigui river sections is relatively large,and the species of fish with the highest trophic level in flood season and dry season are the same.On the whole,the FCL of fish in flood season is significantly longer than that in dry season（P<0.05）,especially for the river section upstream of the dam,the closer the river section to the dam,the greater the difference.The linear relationship between FCL and environmental characteristics in different river sections showed that there was a significant positive correlation between FCL and resource availability defined by chlorophyll-a concentration（P<0.05）,while the relationship between other environmental factors and FCL was not significant（P>0.05）.Comprehensive analysis showed that the storage of the construction of the Three Gorges Dam has been speculated to increase the FCL in the impoundment river sections by increasing the resources availability.（5）Based on the carbon and nitrogen stable isotopic composition of fishes,the temporal and spatial differences of fish isotopic niche in different river sections were analyzed,the results showed that the core niche size（SEAc）of fish community in Yunyang section was significantly larger than that in other sections of the river,while that in Jiangjin and Mudong section was relatively small,the core niche size of fish community tended to increase first and then decrease along the river longitudinal gradient.There was a difference in the overlap rate of fish niche between two different river sections,among them,the overlap rate of fish core niche in Jingzhou river section and each river section in the upstream of the dam was low;The seasonal difference analysis results of isotope niche of fish community in the same river section showed that the core niche of fish community in flood season was significantly higher than that in dry season in Jiangjin,Zigui and Yunyang river sections,while the core niches of fish communities in Mudong,Yunyang and Wushan section in flood season were less than those in dry season,the core niche of fish community in Fuling river has no significant difference in flood season and dry season.In addition,the overlap rate of core niche in flood season and dry season in Jiangjin,Wushan and Zigui river sections is lower than that in other river sections.The results of relative importance analysis showed that the river cross-sectional area,chlorophyll-a concentration,water level variation coefficient and total phosphorus concentration had relatively strong explanatory power for most niche quantitative indicators,and further regression analysis showed that there was a significant positive correlation between river cross-sectional area and chlorophyll-a concentration and core niche size,nutrient length（NR）and average nutrient diversity（CD）of fish community（P<0.05）.Comprehensive analysis showed that the construction of the Three Gorges Dam may cause the niche of the fish communities upstream and downstream of the dam to diverge,and factors such as reservoir storage and reservoir operation not only change the normal change law of fish community niche on the longitudinal gradient of the river,but also affect the seasonal fluctuation law of fish niche by changing the size of ecosystem and resource availability.