Stable Isotope Analyses Of Food Web Structure Under Different Nutrient Level Lakes

Fuxian Lake and Xingyun Lake are a pair of"sister lakes"in the central Yunnan Plateau.Due to different degrees of human activities,the water quality,biological species composition and resource status of Fuxian Lake and Xingyun Lake are obviously different.At the same time,the introduction of invasive species Neosalanx taihuensis has had an irreversible impact on the food webs of both lake.However,the research on the nutritional niche and interspecific relationship in Yunnan Plateau lakes is relatively weak,and the comparative study on the structure of food webs between different types of lakes is still insufficient.It is urgent to study the nutritional structure of food web and the characteristics of nutritional relationship among organisms in Yunnan lake ecosystem,so as to explore the stability mechanism of food web structure.Based on the investigation of water quality and aquatic organisms carried out in Fuxian Lake and Xingyun Lake in 2019,this paper measured the?¹³C and?¹⁵N value of phytoplankton,aquatic plants,zooplankton,zoobenthos and fish by stable isotope technology.Using the statistical analysis method of R language,we discussed the carbon and nitrogen isotope distribution pattern and fractionation mechanism of different tissues of fish,constructed models to calculate the trophic level of consumers,and quantitatively analyzed the contribution of carbon source,trophic level and the core ecological niche relationship between populations and species in the food webs.The isotopic niche was used to analyze the composition of food resources and interspecific and intraspecific nutritional interactions of important fish communities,evaluate the impact of invasive species on indigenous fish,and identify the similarities and differences of food web structure among lakes.The purpose is to provide scientific reference for the study of material circulation,energy flow and ecological restoration of lake ecosystem in the future.The main results are as follows:(1)In this study,carbon and nitrogen contents and stable isotopes of 31 tissues of Ctenopharyngodon idellus and 17 tissues of Anabarilius grahami collected from Fuxian Lake were determined to quantitatively evaluate the ecological processes of elemental assimilation and stable isotope fractionation among different tissues.It is shown that there exists significant carbon isotopic fractionation among tissues,with a range of-20.66‰?-11.62‰for Ctenopharyngodon idellus and-27.55‰?-19.71‰for Anabarilius grahami.Meanwhile,the nitrogen isotope fractionation is relatively moderate,with a maximum range of 1.88‰and 4.60‰,respectively.The?¹³C values show a significantly negative correlation with carbon contents of tissues.With a 10%increase in carbon content,there exists a depletion of?¹³C value by?1.83‰(Ctenopharyngodon idellus)and?1.35‰(Anabarilius grahami),indicating that the absorption and synthesis of carbon in tissues are accompanied by significant isotopic fractionation.However,the?¹⁵N values of different tissues may be affected by essential amino acids but not associated with nitrogen contents,due to that high content of essential amino acids in tissues often leads to a depletion of ¹⁵N value.Combined with the analysis of intestinal contents,isotopic fractionation coefficients between the dorsal muscle of Ctenopharyngodon idellus and intestinal content(Vallisneria natans)are 0.40‰for carbon isotope and 2.66‰for nitrogen isotope,suggesting carbon and nitrogen isotopic compositions of dorsal muscle are reliable indicator of food source and trophic level.For fish without obvious dietary change over life time,the?¹³C value of mucus and the?¹⁵N value of scales(after correction)can be used as alternative indices for dorsal muscles.Our results show that carbon and nitrogen contents and isotope fractionation of fish tissues provide an important way to identify the trophic structure and the pathway of trophic flow in lakes of southwest China.Furthermore,the alternative sampling of non-lethal tissues such as mucus and scales can be of great potential for effective conservation of endangered fish.(2)The structure of lake food web is complex,and accurate calculation of consumer trophic level is important to the construction of lake food web.In this paper,three models were used to calculate the trophic level of consumers,and the best model was selected according to the feeding characteristics of consumers.Results showed that the calculation of trophic level of consumers in Fuxian Lake was more suitable for model A with separate calculation of planktonic food web and benthic food web,while Xingyun Lake was more suitable for model B based on the contribution proportion of mixed carbon sources.This was mainly due to the differences in the composition of food resources between Fuxian Lake and Xingyun Lake and the different nitrogen isotope signals of different food resources.The planktonic food web and benthic food web in Fuxian Lake are relatively developed.Model A could avoid the inaccuracy caused by adopting a benchmark for large deep-water eutrophic lakes.For eutrophic lakes,the algal biomass is large,and the contribution of planktonic carbon source in the whole lake food web is absolutely dominant.Therefore,the calculation method based on planktonic carbon source in model B was more in line with the reality of Xingyun Lake.(3)The results of carbon source contribution showed that the initial carbon sources used by consumers in Fuxian Lake food web were 57.8%of planktonic carbon source,31.2%of benthic carbon source and 11.0%of terrestrial carbon source.The Xingyun Lake is dominated by planktonic carbon sources,and the contributions of three carbon sources were 77.3%,12.2%and 10.5%respectively.The food chain lengths of Fuxian Lake and Xingyun Lake were close,which were 3.66 and 3.73respectively.The consumers at the top of the food chain were Pelteobagrus fulvidraco in Fuxian Lake and Silurus grahami in Xingyun Lake respectively.In terms of niche,the trophic niche of different feeding fish in Fuxian Lake overlaped to some extent,and separated from each other in Xingyun Lake,indicating that the structure of food web is closely related to the primary productivity in lake.The omnivorous fish in both lakes have wider basic food resources and larger core ecological niche area than carnivorous fish,suggesting that omnivorous fish had stronger adaptability.(4)The isotopic analysis of Anabarilius grahami in Fuxian Lake showed that?¹³C is-22.84±0.65‰and?¹⁵N is 10.63±1.70‰,and its ecological niche highly overlaped with the invasive species Neosalanx taihuensis.It indicated that the competition for food resources between Neosalanx taihuensis and Anabarilius graham had squeezed the original living space of Anabarilius graham.Another invasive species Ctenogobius,with?¹³C of-22.33±0.57‰,which was on the same food chain of Anabarilius graham,while?¹⁵N of Ctenogobius is 13.14±1.28‰,which was significantly higher than that of Anabarilius graham,indicating that Ctenogobius could feed on eggs and juvenile fish of Anabarilius graham.In addition,in recent years,the spawning ground and habitat of juvenile Anabarilius graham had been damaged by attached algae.It is due to the influence of many factors that the original stability of the food web structure of Fuxian Lake had been destroyed,which is the main reason for the decline and slow recovery of the population of Anabarilius graham.