Temporal And Spatial Variations Of Food Web In Lake Taihu Under Human Disturbance

Due to the important ecological service functions,shallow lake has been developed at high intensity by human,and has formed a closed social-ecological system(SES).More and more attention has been paid on how the shallow lake ecosystem response to human disturbances.Under long-time external stressors,the resilience of system will decline gradually,and the regime shift of system will happen eventually.In some large shallow lakes,there are two different types of stable states coexisted:the algae-and macrophyte-dominated stable state,and then may drive different ecosystems.Food web,which is composed by the trophic interactions among species within habitats,can provide an intelligible perspective for understanding the response of ecosystem to human disturbances.Lake Taihu is the third largest freshwater lake in China.For a long time,Lake Taihu has provide valuable ecosystem service functions to human,and has endured strong human disturbances,such as the increased nutrient inputs,levee construction and aquaculture.All these disturbances have resulted in the degradation of ecosystem structure and function.In view of the importance of Lake Taihu,ecologists have carried out a lot of intensive studies at Lake Taihu,and these researches have lay a foundation of the further research.Here,we hope to do some explorations on the temporal variations and spatial variations on the response of ecosystem to human disturbances.Based on the understanding of research status and progress,we used document surveys,field investigation,ecological models and experimental data to study the response of ecosystem to human disturbances.The results indicated that:(1)Lake Taihu SES experienced a regime shift in the early 1990s,and its development during 1960s-2000s can be divided into four stages:normal development stage(1960-late of 1960s),preliminary degradation stage(late of 1960s-late of 1980s),disordered stage(late of 1980s-late of 1990s)and reorganization stage(late of 1990s-2009).The highest system resilience coefficient was 9.136 in 1960,while the lowest system resilience coefficient was-1.130 in 1990,then the system resilience coefficient shook up until 2009.The loss of resilience results in the regime shift of SES eventually.(2)During the long-term eutrophication process,the structure of Lake Taihu food web simplified,the proportion of species and linkages lost in the upper part of food web was highest,the connections within the lower part of food web enhanced,and these resulted in the energy exchange barycenter within food web declined.Due to the decrease of total energy transfer efficiency,energy cannot transferred to the top predators smoothly,the length of food chain declined.The long-term eutrophication downed the food web of Lake Taihu both in structure and process.(3)The species richness of phytoplankton and zooplankton were higher in the macrophyte-dominated zone,while the species richness of zoobenthos and fish were higher in the algae-dominated zone.Compared to the macrophyte-dominated zone,the extreme dominated species in the algae-dominated zone were mainly the species which can tolerance the turbid water and small size fish.(4)Lower ?13C and higher ?15N of basal sources and consumers were found in the algae-dominated zone compared to the macrophyte-dominated zone.Except for the filter-feeding fishes,the ?13C and ?15N of basal sources and consumers in these two lake zones showed significant differences.In the algae-dominated zone,one important energy pathway was found:phytoplankton-zooplankton-small zooplankton-preferred carnivorous fish.Meanwhile,three important energy pathway were found in macrophyte-dominated zone:?SPOM-filter-feeding fish;? phytoplankton-zooplankton-big zooplankton-preferred fish;? submerged macrophyte-herbivorous fish.These spatial variations might be the result of concerted actions of human disturbances and biological characteristics.(5)Both in algae-dominated zone and macrophyte-dominated zone,the C,N and P contents and ratios of environmental samples and species in the lower trophic levels had higher spatial heterogeneity,while species in the higher trophic levels had the ability to remain the ecological stoichiometry stable.The C:N,C:P and N:P of environmental samples in the algae-dominated zone were higher than that in the macrophyte-dominated zone,these indicated the higher nutrient loading in the algae-dominated zone.However,the C:N of phytoplantkon in the algae-dominated zone was significant higher than that in the macrophyte-dominated zone,this have relationship with its low food quality,and this also influenced the elment ratios of the consumers in food web.Stoichiometric imbalance were found both in these two lake zones.In general,long-term human disturbances reduced the resilience of Lake Taihu SES,simplified the food web structure and weaken the energy transfer to the upper trophic levels;the different human activities and limnological characteristics in the algae-and macrophyte-dominated zone resulted in the spatial variations of species characteristics,energy pathway and material flow.In the restoration and management of Lake Taihu,we should put more attention the cumulative impacts of long-term external stressors,take adaptive management based on the development rules of ecosystem.In addition,spatial heterogeneity should be considered and targeted measures should be took in different lake zone.