Study On Ecosystem Evolution Of Lake Xihu,Dali Over The Past 50 Years Based On Sedimentary Records

In recent decades,intense human activities have caused a dedine in many lake eosystems in Yunan Province,rendering the transformation of the lake from a clear macrophyte-dominated state to a turbid phytoplankton-donminated state.Improved understanding of the ecological changes in lake ecosystem has significant implications for management.In this study,a small lake in Dali Prefecture of Yunnan Province,i.e.,Lake Xihu,was selected.Combined with diatom records and physicodemical proxies from the lake sediments,this paper focuses on the long-term ecological changs in Lake Xihu,Dali since the mid-1960 s.The results show that since 1966,the change of grain size index indicates not only the decrease of water level,the enhancement of hydrodynamics and the tendency of drought,but also the enhancement of erosion and the increase of turbidity in the lake.The TOC,TN and TP of sediments indicate that the nutrient input of the lake has been increasing continuously since the mid-1960 s,and the nutrient level of the lake has been improving continuously.The C/N in the sediments of the Lake Xihu,Dali showed that the organic matter in the Lake Xihu,Dali was influenced by both internal and external sources and exogenous organic matter.After 2004,the C/N showed a continuous decrease,which indicated that the proportion of endogenous organic matter in the lake increased gradually,and the internal productivity of the lake increased significantly.CONISS cluster analysis results show that the Lake Xihu,Dali has undergone a significant shift in stable states over the past 50 years.Since 1966,the diatom assemblages in the Lake Xihu,Dali has undergone three stages of changes,and its structure has undergone two significant changes.From 1966 to 1998,the benthic-epiphytic species doninated in diatom assemblages.From 1999 to 2014,benthic-epiphytic types still dominated the diatom cassemblages,and the structure of diatom assemblages showed the internal replacement of benthic-epiphytic species.From 2015 to 2018,the components of planktons diatoms indicating higher nutrient levels increased significantly,while the components of benthic-epiphytes decreased relatively.In the process of the transformation of diatom community species types,the species diversity also changed.From mid-1960 s to around 2014,species richness and Shannon-Wiener diversity index were relatively stable.After 2014,both diversity indices increased significantly.Principal component analysis based on the diatom assemblages in temporal scale showed the response of diatoms succession to nutients.Redundancy analysis and orrelation analysis also confirmed that nutrient enrichment was the main driving force for the sucession of diatom assemblages and species diversity change in Lake Xihu,Dali.In addition,the enhancement of lake hydrodynamics,the enhancement of river basin erosion and climate change are the important reasons for the diatom community succession.Correlation analysis between dominant diatoms and driving factors showed that different species of diatoms had different response characteristics to different driving factors.The correlation analysis between diatom diversity index and driving forces showed that the input of nutrient,mainly phosphorus,was the main reason for the increase of diatom community species diversity.The response pattern of diatom species richness and Shannon diversity index to the driving factor(total phosphorus)was linear positive correlation,and with the increase of total phosphorus,diatom species richness and Shannon diversity index also showed an upward trend.The STARS analysis showed that the ecosystem of the Lake Xihu,Dali had undergone a significant steady-state transformation in the past 50 years.Prior to 2000,the benthic-epiphytic species(i.e.Cocconais plaentula,Staurosira construens,Gomplonema angustum,and Acinanthidium minutisimum)doninated in diatom assemblages,indicating oligotrophic conditions;since 2000,benthic-epiphytic species(i.e,Encyonopsis micraephala and Navicula crptocephadla);Before 2015,the aquatic vegetation coverage rate of the Lake Xihu,Dali was relatively high,which was a grass-type clear water.Eutrophic planktonic species(i.e.,Cyclotella atomus,Cyclotela meneghiniana,Stephanodiscus hantschi,and Aulacoseira granulata)indicating mesotrophic to eutrophic conditions dominated successively after 2015,the lake ecosystem gradually changed to algal turbidity state.In the past 50 years,climate change and human activities(i.e.,livestock farming,fertilizer,sewage discharge,fisheries aquaculture and agricultural reclamation)have enhaned the accumulation of nutrients in the lake.The continuous loading of nutrients promoted the propagation of planktonic algae,and also the productivity of the lake,rendering the transformation to a turbid phytoplankton-dominated state.This study can provide a reference for the restoration of the ecosystem in Lake Xihu,Dali and the comprehensive management of lake basin,and provide a typical case for the study of the stable transition of the shallow lake ecosystem on the plateau.