Using Stable Carbon And Nitrogen Isotopes To Study The Food Web Structure Of Lake Xingkai In China

The study of food web provides information about the composition,connectivity and integrity of biological communities,and is important to our understanding of the energy flow and material circulation in aquatic ecosystems.Stable isotope analysis is an effective method for exploring food web structure,trophic cascade,and species invasion in freshwater ecosystems.We used carbon(13C,12C)and nitrogen(15N,14N)stable isotopes to study the ecosystems of Lake Xingkai and Lake Xiaoxingkai,in terms of food web structure,trophic cascade,invasive species,and spatial and temporal dynamics of primary productivity and its contribution to consumers.We also explored the effects of environment changes and hydrological connectivity on ecosystem function in the Lake Xingkai basin.Our findings provide a scientific basis for ecosystem assessment,management and sustainable utilization in the Lake Xingkai basin.The spatial and temporal variations of phytoplankton community structure were significantly different between Lake Xingkai and Lake Xiaoxingkai.Phytoplankton species were dominated by Chlorophyta,Bacillariophyta and Cyanophyta.Phytoplankton abundance,biodiversity,biomass and species richness of Lake Xiaoxingkai were higher than those of Lake Xingkai.The dominant species changed from A.azotica(2011)to C.meneghiniana(2014),most likely due to increased nutrient concentrations caused by agricultural activities.Throughout our study,the water quality of Lake Xingkai was better than that of Lake Xiaoxingkai,as evidenced by lower nutrient concentrations,clearer water,and fewer algal blooms.Phytoplankton community structures of the two lakes were affected by different environmental variables.The main environmental variables influencing phytoplankton abundance were pH,Fe2+,Fe3+ and water temperature in Lake Xingkai,while phytoplankton abundance in Lake Xiaoxingkai was mainly affected by Fe2+,water temperature,TOC and BOD5.Microalgae and C3 plants were the energy base of the food web in Lake Xiaoxingkai.Particulate organic matter(POM)was the main source for primary consumers.The range of the trophic level of consumers was from 2.00 to 4.38.The trophic levels of some large carnivorous fishes were at the top of the food chain,including Culter ilishaeformis,Channa argus and Silurus asotus.Sediment was the main source for benthic invertebrates,while microalgae was the main source for small herbivorous and omnivorous fishes.C3 plants contributed to benthic fishes,whereas zooplankton was the main source for omnivorous and carnivorous fishes.POM and terrestrial organic matter(TOM)were the energy base of food web in Lake Xingkai.TOM was the main component of POM.The trophic level of main consumers in Lake Xingkai were relatively concentrated,ranging from 2.00 to 3.23.The large carnivorous fish,Esox reicherti and Culter ilishaeformis,had high trophic levels and were at the top of the food chain.POM contributed to small herbivorous and omnivorous fishes.TOM was the main source for zooplankton.Whereas zooplankton was the main source for big omnivorous and carnivorous fishes.There is a significant positive correlation among body length,body weight,and trophic level in Culter dabryi shinkainensis in Lake Xingkai,indicating that the trophic level increased with the growth of Culter dabryi shinkainensis.There is an obvious diet shift in the growth of Culter dabryi and Culter alburnus.There was no significant temporal difference of carbon and nitrogen stable isotopes of POM in Lake Xiaoxingkai.However,carbon and nitrogen stable isotopes of POM and zooplankton showed significant spatial variation in Lake Xingkai and Lake Xiaoxingkai.TOC,TC and IC were the main environmental factors affecting nitrogen stable isotope of POM.Hydrological connectivity increased the similarity of phytoplankton community structures between the two lakes.However,it also caused nutrients to be transported from Lake Xiaoxingkai into Lake Xingkai,resulting in deterioration of water quality in Lake Xingkai.Moreover,hydrological connectivity also caused invasion of Protosalanx hyalocraniu that were introduced in Lake Xiaoxingkai into Lake Xingkai.This in turn affected the native fishes in Lake Xingkai by competition and predation.Compared with Lake Xiaoxingkai,the influence of the invasion of Protosalanx hyalocraniu on the food web structure and ecosystem function was higher in Lake Xingkai than in Lake Xiaoxingkai.