Respoase Of Chironumid Subfossil Ussamblages To The Palcoenvironmental Of Xijiu Lake

With the rapid development of the social economy,the environmental deterioration of Xijiu Lake,such as eutrophication,heavy metal pollution,has become more and more serious during the past decades.The lake locating in the first station of the West Taihu area is a typical shallow lake of the Taihu Basin.This work studied the change of assemblage structure and deformities of the chironomid subfossil in the Xijiu Lake sediments and made further analysis on the transitional processes of ecosystem and heavy metal pollution processes in the past 40 years.The main results are as follows.1.In this study,a total of 1423.5 chironomid head capsules were analyzed.According to the ecological properties of the chironomid,they were further divided into three groups: the euryphagic species,the primary macrophyte-related species,and the secondary macrophyte-related species.The proportion of each group in the sediments accounted for 51.2%,8.3% and 21% respectively.The dominant species,represented by Tanypus chinensis,were euryphagic species.The total chironomid Deformities Incidence(DI)was 2.6%.The respresentative deformed species were Microchironomus tabarui,Microchironomus tener,Procladius choreus and Chironomus plumosus.The main types of mentum deformities include missing teeth,K?hn gaps and additional teeth.2.Five heavy metal elements were analyzed in this study(Cr,Cu,Zn,Pb and Hg).The level of heavy metals peaked between 1990 and 2000.The result of cluster heatmap showed that Hg pollution was severer than the other four heavy metal elements and always maintained in high-level state after 1990,while Cr,Cu,Zn and Pb showed significant zoning characteristics.3.The result showed that Xijiu ecosystem had undergone a regime shift during the past 40 years based on the biological indicators(chironomid subfossils)and geological parameters(TN and TP)in sediment core XJ2018.RSI index and F-test were calculated and conducted based on the change of chironomid assemblages and ecosystem transform methods.The highest RSI index(RSI=-2.45)and the peak of F-test were both found in 1990,which was distinct by high-level aquatic vegetable coverage and dominant macrophyte-related taxa(Cricotopus trifasciatus,Dicrotendipes spp.).After this period,Xijiu Lake transformed into phytoplankton dominatedturbid-water stage which was dominated by some euryphagic species(Tanypus chinensis,Microchironomus tabarui).From 2000 to 2010,the ecosystem was further deteriorated,during which macrophyte-related taxa almost disappeared,while euryphagic species accounted for the vast majority of chironomid communities.After 2010,the restoration of macrophyte-related taxa showed that the overall environment of Xijiu Lake had been improved as some measures for ecological protection were implemented by the local government,though the phytoplankton dominated turbid-water state remained unchanged.4.Combined with the analysis of heavy metal elements and chironomid deformities,the heavy metal pollution processes in Xijiu Lake can be roughly divided into four stages.The results of RDA analysis showed that Cr,Cu and Pb were highly correlated with the deformity species.Before 1990,heavy metal pollution was low but showed a gradual rising trend,which accorded with DI index.From 1990 to 2000,Cu turned into a high-level state while Pb and Cr turned into medium state as a result of rapid development of heavy metal industry in Yixing City.Also,the DI index was significantly high due to heavy metal pollution in this period.With Yixing City starting its industrial transformation in 2000,the environment has improved gradually,which can be proved by the decreased heavy metal elements and DI index.However,after 2010,DI index increase again,and Cu,Pb and Zn returned to the medium pollution state.