Study On Spatiotemporal Characteristics Of Food Webs In Dongting Lake And Qiandao Lake

The freshwater ecosystem is suffering intense and multiple environment pressures,such as water pollution,ecological invasion and habitat destruction.Therefore,their structure and trophic level of food webs are undergoing dramatic changing.However,those issues,especial spatiotemporal heterogeneity of food web in Chinese large lakes or reservoirs are not researched or presented,the general characteristics is not reported either.In this study,in order to gian some direction on sustainable resourecs of aquatic ecosystem,the temporal and spatial characteristics of food webs in Dongting and Qiandao lakes are investigated and analysed.The main results are shown below.1.First,we hypothesized that spatial heterogeneity of the water environment in the same lake or reservoir would lead to spatial differences in the food web,and that primary food sources(pelagic source,benthic source and littoral source)might be the main factors affecting the trophic levels of consumers.Therefore,we constructed an evaluation method for spatial characteristics of food webs and verified the feasibility of this method in Dongting Lake which showed obviously spatial heterogeneity.The results showed that there were significant differences of the food web among each sub-lake,and the trophic level of consumers in Eastern Dongting Lake was significantly higher than that of in Southern Dongting Lake and Western Dongting Lake.The contribution proportion of primary food sources in different regions may be the main reason to lead to the diverse trophic level of consumers in different sub-lakes.Therefore,by exploring the role of primary food sources in each sub-lake,this study can fill the gap in the impact of primary food sources on the trophic level structure of consumers.At the same time,it also proves that the method of spatial characteristics evaluation of a food web is feasible.2.Based on the above research,this method is applied to explore the spatial characteristics of the food web in Qiandao Lake.Due to the obvious spatial heterogeneity between the upstream(named fish breeding area)and the downstream(named water protection area),it is necessary to explore the food web structure in different regions,so as to provide theoretical basis for solving water environmental problems on a regional scale.The results showed that the concentration of nutrients,and the densities of plankton and fish were higher in upstream than in downstream.Moreover,the trophic levels of consumers in the upstream(especially the demersal fish)were significantly higher than those of in the downstream.This chapter also showed that the diverse contribution proportion of primary food sources to the two sub-lakes was an important reason for the spatial differences of the food web.Meanwhile,it is also found that the trophic levels of consumers in the high nutrient sub-lake were significantly higher than those of in the low nutrient sub-lakes.By comparison,the trophict levels of dominant fish are higher in the high-nutrient sub-lake than the low-nutrient sub-lake in Dongting Lake and Qiandao Lake.The abundance and distribution of primary food sources are the main factors that affect the spatial differences in fish trophic levels and food web structure.This study analyzed the spatial heterogeneity of the water environment and food web characteristics of the Qiandao Lake ecosystem,which can provide a theoretical support for regional scale management of freshwater ecosystem.3.In freshwater ecosystem,seasonal variation of water environment also has certain influence on the food web structure.In this study,we focused on seasonal variations in trophic levels,a key attribution of the food web.To determine whether seasonal variations of fish trophic levels could be indicated by the change in food resources.Silver carp and bighead carp were used to explore the responses of the filter-feeding fish trophic levels to seasonal variations of food resources.Pearson' correlation analysis and structural equation model showed that seasonal variation of total phosphorus and water temperature could indirectly affect trophic levels of silver carp and bighead carp by mediating the abundance of phytoplankton and zooplankton directly and indirectly through the food chain.According to these findings,the seasonal variation of food resources could be an important indicator of the temporal dynamics of the food web trophic pattern in freshwater ecosystems.This is important for understanding the temporal dynamics of the food web in freshwater ecosystems.4.Finally,in order to have a more comprehensive understanding of the food web structure of Qiandao Lake ecosystem,this study constructed a food web based on ecosystem model,and made a quantitative analysis on the trophic level structure,as well as the energy flow and the ecosystem characteristics of the Qiandao Lake.Meanwhile,the current status and historical evolution of Qiandao Lake ecosystem were discussed.The Ecopath pedigree index(P index)was 0.544,with high data reliability.The fractional trophic levels ranged from 1 to 3.14,which is basically consistent with the result of stable isotope analysis.Energy flow was dominated by grazing food chain,which accounted for 56% of the total energy source.The parameters that could reflect the complexity of ecosystem,including the system connection index(CI),the omnivory index(SOI)and the Finn cycling index,presented to be decreased compared with those in previous years,indicating that the trophic interaction relationship in the Qiandao Lake ecosystem was weakened.It is possible that the structure of fishery resources has been changed with the increased numbers of silver carp and bighead carp in the ecosystem.Based on the results of the model,this study gives some suggestions on the fishery resources,hoping to play a positive role in the management of fishery resources.The results of this study reveal that the spatial-temporal distribution and abundance of primary food source are an important factor affecting the trophic level of consumers,which provide theoretical value for the research of the food web in the freshwater ecosystem.The results of this study also show that foraging strategies and community structure of fish also can change the structure of the food web.Besides,this study evaluated the food web structure,development status and stability of the Qiandao Lake ecosystem by building an ecosystem model,and proposed suggestions for the fishery resources,so as to provide reference for the management of fishery resources.