Study On The Food Web Structure And Sources Of Organic Carbon In Lake Fuxian Based On Stable Isotopes And Fatty Acid Composition

The research of food web structure and source of organic carbon and the pathway of carbon flow is the effective method to understand and explain the process of ecosystem structure and function.Many factors could change the lake food web structure,such as the change of nutrient level,biological invasion,or others.The change process could occur from bottom to top,or from top to down of the food web.Allochthonous organic carbon plays an important role in lake food web,especially in some oligotrophic lakes,allochthonous organic carbon is the main organic carbon source supporting lake food web.Here we used stable isotope and fatty acid biomarkers to study the seasonal changes of food web structure,the sources of organic carbon and the pathway in Lake Fuxian.The zooplankton community structure in Lake Fuxian for spatial and temporal variation analysis shows that the zooplankton density was variable obviously throughout the year.There was a strong seasonal variation,but with no significant correlation with the variable of nutrient.Zooplankton community were mainly distributed in the epilimnion and the thermocline,the hypolimnion was very low,but the Phyllodiaptomus tunguidus Shenet Tai distributed from water surface to bottom in Lake Fuxian.The study of spatial and temporal variation in values of?¹³C and?¹⁵N of particulate organic matter?POM?,Bosmina sp.and Phyllodiaptomus tunguidus Shenet Tai show that the seasonal changes of isotope values were significant,but not significant correlation with depth.The result concluded that plankton food in Lake Fuxian affected by seasonal variable of physical and chemical factors.In Lake Fuxian,the?¹³C annual average values were Phyllodiaptomus tunguidus>POM>Bosmina sp.,the?¹³C annual average values of Bosmina sp.was lower 1.01‰than Phyllodiaptomus tunguidus.It reflected the different sources of organic carbon between Phyllodiaptomus tunguidus and Bosmina sp..And the?¹⁵N annual average values were Phyllodiaptomus tunguidus>Bosmina sp.>POM,the?¹⁵N annual average values of Bosmina sp.was lower 2.61‰than Phyllodiaptomus tunguidus.The trophic level position of Bosmina sp.was lower than Phyllodiaptomus tunguidus,but less than 1 level.It reflected the different feeding habits between Phyllodiaptomus tunguidus and Bosmina sp..During lake stratification,the?¹⁵N values between zooplankton and POM showed a strong correlation.This result esplained the zooplankton had strong feeding selected.Species composition and feeding habits in zooplankton community is the precondition of establishing lake food web structure.In this study,we determined the primary consumer Bosmina sp.as the effective baseline to establish the planktonic food web in Lake Fuxian through the analysis of the stable isotope values of POM and zooplankton.Bosmina sp.as plankton food web baseline,we calculated the trophic levers of Phyllodiaptomus tunguidus and POM were 2.77and 1.73 respectively.The trophic lever of Phyllodiaptomus tunguidus was higher than Bosmina sp.,it indicated that Phyllodiaptomus tunguidus was more carnivorous.Food web structure and organic carbon source were variable in Lake Fuxian seasonally.Carbon and nitrogen stable isotope and fatty acid composition analysis showed that the sources of organic carbon in Lake Fuxian ecosystem were main phytoplankton and allochthonous organic carbon.The?¹³C values of phytoplankton and allchthonous organic carbon seasonal change obviously.POM was a mixture of phytoplankton,submerged plant and terrestristal detritus,etc.The?¹³C values of POM reflected the various components?¹³C Characteristics that phytoplankton and allchthonous organic carbon were the maior components,and the submerged plant was least in POM.The result of the analysis of the fatty acids in POM showed allchthonous organic fatty acid content was high in the summer,but less in the winter.This phenomenon was corresponding to the content variable pattern of allchthonous organic matter in Fuxian Lake.The?¹³C values of Bosmina sp.was lower than POM,it meaned Bosmina sp.utilizatived less allchthonous organic matter,which was mainly of phytoplankton.The?¹³C values of Phyllodiaptomus tunguidus was higher than POM,explained Phyllodiaptomus tunguidus utilizatived more allchthonous organic matter.Bosmina sp.had more bacteria and the allchthonous organic matter special fatty acid content in the winter,it indicated that Bosmina sp.reserved more allchthonous organic carbon by feeding more bacteria.And Phyllodiaptomus tunguidus had lower phytoplankton special fatty acid content in the summer,but to winter it had increased.The result corresponded the phytoplankton biomass variable in Lake Fuxian.Fish were mainly planktonic and benthic diets in Lake Fuxian.There were variable of diets within species and difference season significantly.In the summer,the initial carbon source of Neosalanx taihuensis,Hyporhamphus intermedius and Hemiculeer leucisculus were mainly from phytoplankton.And Triplophysa fuxianensis,Pseudorasbora parva,Ctenogobius giurinus,Anabarilius sgrahami,Carassiusaur atusauratus,Pelteobagrus fulvidrac were mainly from submerged plant and allchthonous organic detritus.In the winter,the initial carbon source of Neosalanx taihuensis,Hyporhamphus intermedius,Anabarilius sgrahami and Ctenogobius giurinus were mainly from phytoplankton,and Triplophysa fuxianensis,Abbottina rivularis,Pseudorasbora parva,Ctenopharyngodon idellus,Cyprinus carpio and Hemiculeer leucisculus were mainly from submerged plant and allchthonous organic detritus.The diet of Triplophysa fuxianensis,Hemiculeer leucisculus,and Ctenogobius giurinus changed significantly.With the changed of diet,the trophic level of the fish changed too.In Lake Fuxian planktotrophic fish had the highest the trophic position,roughly near the fourth trophic level,and the trophic level of benthic fishes was lower,generally near the third trophic level.In the winter,the trophic position of benthic omnivorous fish fuxian Triplophysa fuxianensis and Cyprinus carpio were between Phyllodiaptomus tunguidus and Bosmina sp.,and trophic position of vegetarian Ctenopharyngodon idellus was lower than Bosmina sp.but higher than POM.Stable isotope analysis combined with fatty acid composition could complement information each other,it made the analysis more accurate to illustrate the seasonal variable of food web structure and source of carbon and the characteristic of transmission effectively in Lake Fuxian.The specific fatty acid carbon stable isotope values of consumers changed seasonally in the food web of Lake Fuxian.They were mainly influenced by seasonal difference food source and their own biochemical synthesis process.During the transference in the food web,the?¹³C values of specific fatty acids would become depleted or enriched,and in different specific fatty acid the?¹³C value changed difference.The?¹³C values of specific fatty acids displayed some characteristics when they transfer within the food web in Lake Fuxian.The characteristics accorded to the different?¹³C value of food source.16:1?7 was as a phytoplankton fatty acid biaomarker,the?¹³C value showed a trend of decline in the food chain transfer process.And 24:0 was as the terrigenous organic fatty acid biaomarker,the?¹³C value depleted to herbivore zooplankton,while enriched to carnivorous zooplankton and fish.It was different that the?¹³C value of specific fatty acid in different fishes.And the seasonal change rule of the?¹³C value of specific fatty acid in different fishes,while in different kinds of fatty acids the seasonal change rule of?¹³C values were not similar.The result suggested the specific fatty acid carbon stable isotope analysis was useful when the source of food for the consumer was diversity and the variable characteristic undefined in the food web.