Patterns Of Lake Cladoceran Community Change In Response To Multiple Stressors During The Last Century ——A Case Study From Yunlong Tianchi And Dali Xihu

The Yunnan-Guizhou Plateau Lake District is one of the five great lake regions in China.There exist a large number of lakes,a variety of lake types.In recent decades,Yunnan lakes have encountered a variety of challenges such as hydrological regulation,fish introduction,and eutrophication.These multiple environmental stressors have seriously affected the environment health and ecological safety of lakes and reservoirs.The sediments of lakes and reservoirs can provide reliable and continuous information on environmental and biological changes in water bodies and watersheds.Furthermore,zooplankton such as cladocerans are located in the intermediate position of lake trophic levels.Meanwhile,cladocerans can leave identifiable remains in sediments that can be reliably used to infer the changes in lake biota.To date,few studies have been conducted in uncovering the long-term patterns of Cladocera community response to multiple stressors for alpine lakes in northwestern Yunnan.To explore the long-term impact of climate change and human activities on the lake environment and zooplankton communities（i.e.cladocerans）in the past three hundred years,this study focused on two lakes（Yunlong Tianchi and Dali West Lake）which show a different level of anthropogenic disturbance for sediment analyses.In combination with modern lake surveys and historical documents,I carried out multiindex analyses of surface sediments and sediment cores,aiming to reconstruct the history of environmental change and cladoceran variation at both lakes.Based on the spatial surveys and time series of cladoceran community variation（i.e.species compsition,body size,biomass）,the patterns of spatial and temporal variation of cladocera communities were examined along the gradient of human disturbances such as hydrological fluctuation and eutrophication through the comparative analysis of both lakes.Furthermore,statistical methods were used to analyze the similarity of ecological changes between both lakes,and to quantify the role of main environmental factors in driving the variation of cladoceran communities.The main findings of this paper are listed as follows:（1）Over the temporal and spatial scales,the cladoceran community in the lake of Yunlong Tianchi was dominated by planktonic taxa such as Bosmina.On the spatial scale,there also exists a threshold water depth（about 7.7 m）that affects the structuring of cladoceran communities in Yunlong Tianchi.On spatial scales,the body size of bosminids（antennae length,shell length,and spine length）did not vary significantly along the water depth gradient.However,over time,after the introduction of zooplanktonvorous fish in 1969,the body size of bosminids gradually decreased reflecting an increase in fish predation pressure which may also increase with lake depth.（2）Over the past 300 years,the water level of Yunlong Lake has shown a pattern of "low water level-rising water level-falling water level-rising water level",and the cladoceran community has shown an evolutionary pattern of "benthic-planktonic-benthic-planktonic".After the dam was built in the 1950 s,the water level rose significantly and the habitat in the coastal zone was lost,while the planktonic cladoceran increased significantly and dominated,and the benthic cladoceran gradually decreased.（3）The lake of Dali Xihu has been significantly affected by human activities,and its sedimentary cladoceran community is mainly composed of Bosmina,Alona and Chydorus.Since the 1950 s,the sediment contents of Chl-a,TC,and TN have increased significantly,and showed an accelerating trend after 1997,indicating a continuous increase in lake primary productivity and autochthonous organic matters over time.Since the 1950 s,the sediment C/N ratio has dropped significantly,indicating that the catchment input of nutrients accelerated the process of lake eutrophication due to agricultural expansion.（4）Both lakes have experienced the process of hydrological regulation and nutrient level rise in the past 100 years.Over time,aquatic environment and cladoceran communities have changed significantly since the 1950 s at both lakes.Since the 1950 s,the coarse-grained matter has first increased and then decreased,indicating the enhancement of hydrodynamic force caused by dam construction and the weakening of hydrodynamic force after dam construction.In the 1950 s,the nutrient levels（TN,TC）of Yunlong Tianchi and Dali Xihu showed an overall upward trend.At the same time,the C/N of the two lakes increased rapidly in 1950 s and then gradually decreased.All the above phenomena showed the increase of external input and that of lake nutrient level,both of which were caused by the influence of human activities during the dam construction.A comparison of both study lakes in northwestern Yunnan shows that the difference in the intensity of human activities have led to obvious differences in lake environmental changes and ecological response patterns over time.In general,the environment and biological community in the nutrient-poor lake of Yunlong Tianchi are mainly affected by the fluctuation of lake water level（16.8 %）and by the introduction of fish,while the nutrient-rich lake of Dali Xihu is mainly affected by eutrophication（3.5 %）.In conclusion,changes in hydrological fluctuation and nutrient levels are important environmental factors in affecting the environmental changes of both lakes in northwest Yunnan.During the last centuries,climate change has played a greater role in affecting water level,lake environment and cladoceran assembalges in Yunlong Tianchi,an alpine lake that is less affected by human activities.In Dali West Lake,which is strongly affected by catchment development,the long-term impact of human activities has led to an acceleration of eutrophication and biotic changes.Therefore,in the context of continuing climate change,it is vital to control nutrient input,fish introduction and hydrological regulation for effective management of alpine lakes in northwestern Yunnan.