A Comparative Study On Environmental Changes And Cladocera Community Responses In Two Alpine Lakes Of Yunnan In The Last 200 Years

Alpine regions are sensitive to climate change.Alpine lakes are an important component of freshwater ecosystems,characterized by low environmental carrying capacity and simplified structure of biological community.Compared to lakes below the tree line,lakes above the tree line are characterized by being recharged by melt waters,low water temperature,underdevelopment of soil and vegetation,strong watershed erosion,and low concentrations of lake-water dissolved organic carbon（DOC）,which is liable for stronger ultraviolet radiation for aquatic organisms.Therefore,changes in the biological communities of alpine lakes located above the tree line may be significant in the context of global change.Regional warming and atmospheric nitrogen deposition have become important environmental stressors in alpine regions.Therefore,a comparative analysis of catchment processes,limnological changes,and biological communities in alpine lakes along the gradient of tree lines should be applied to reveal the long-term patterns and mechanisms of alpine ecosystems under multiple stressors.The existing research on long-term patterns of community response in alpine lakes mainly focuses on autotrophic indicators such as algae,strongly lacking the long-term patterns in the response of higher trophic level organisms（such as zooplankton）.In this study,two alpine lakes（Dinggong Co and Wodi Co）,located above and below the tree line in northwest Yunnan,respectively,were selected for sediment investigation and comparative analyses.Firstly,the environmental change history of both lakes was reconstruction through multi-proxy sediment analyses（total nitrogen,total phosphorus,pigments,stable nitrogen isotopes,etc.）in combination with previous records on climate reconstruction.Meanwhile,the succession sequence of cladoceran community（species structure,biomass,etc.）was examined and established.Through comparative analysis of both lakes,the similarities and differences and key processes of cladoceran communities in response to climate warming and atmospheric nitrogen deposition in the past 200 years were identified,and the main patterns and driving mechanisms of ecological changes in two alpine lakes above and below the forest line were revealed.The main results and research conclusions of this thesis are as follows:（1）Sediment records show that Wodi Co（3842 meters above sea level and 20.7meters of water depth）exhibits significant ecological and environmental changes.In the past two hundred years,the nutrient level（such as TN content）and primary productivity（such as chlorophyll pigment concentration）of water bodies have generally shown an upward trend.Correlation analysis shows that atmospheric nitrogen deposition and exogenous input processes in the watershed are the main factors affecting the increase of TN,while regional warming and nutrient enrichment promote the continuous increase of primary productivity in the lake.Since the 1960s,the warming trend in the Wodi Co Basin has been significant,with the lake nutrient level and chlorophyll concentration showing an accelerated upward trend.According to the borehole records,the dominant species of cladocera community are the planktonic genus species（Daphnia longispina,etc.）,and before 1900 AD.The relative abundance of D.longispina was relatively stable（40.83±8.02%）,followed by a downward trend and a significant decline between 1948 and 1965,before rising significantly again and becoming the main dominant species.RDA analysis further showed that temperature,chlorophyll a,and TN were the main factors affecting the succession of cladoceran communities.The results of variance decomposition showed that the synergistic effects of temperature,nutrient level,and chlorophyll concentration on the community structure of cladocerans were significant,and the independent interpretation of TN had significantly increased since the 1960s.It can be seen that temperature and nutrient levels had a significant independent impact on the community structure of cladocerans before the 1960s in the eutrophic Lake Wodi Co,while the increase in water nutrient levels and primary productivity has become the main factors affecting the changes in the community structure of cladocerans since the 1960s.（2）Analysis of sediment proxy indicators indicates that Dinggong Co（4132meters above sea level and 8.4 meters of water depth）has experienced fluctuations in nutrient levels over the past 200 years.Before 1860s,the contents of TN and TC were lower,the primary productivity（chlorophyll a concentration）decreased,and the nutrient level of lakes was lower,during 1860-1980s,the concentrations of TN,TC,and chlorophyll all increased,and the nutrient level of the lake increased;Since the1980s,the contents of TN have decreased again.The temperature reconstruction data indicate that the region has been continuously warming over the past 200 years,with no significant increase in chlorophyll a,total phosphorus,and total nitrogen in the sediments of Dinggong Co,and an increasing trend in C/N values,indicating that the input of lake organic matter is mainly terrestrial,and the driving force of climate warming on the increase of lake nutrient level is not obvious.Dinggong Co depositδ¹⁵N has been significantly negative since 1980,indicating a process of atmospheric nitrogen deposition in this area.The dominant species in the Dinggong Co sediments are the facultative planktonic species Chydorus sphaericus（81.8±15.1%）,and the benthic species Alona guttatal/rectangula（15.7±12.4%）also account for a relatively high proportion in the borehole.Planktonic Daphnia longispina,D.pulex,Bosmina longispina,and Bosmina longirostris are fewer.The results of RDA analysis showed that chlorophyll concentration and TIC content were the main environmental factors driving the changes of cladoceran community in Dinggong Co.（3）The regional climate change backgrounds of Wodi Co and Dinggong Co are similar,but the different limnological characteristics of the two alpine lakes may lead to differences in environmental change and community response patterns.Dinggong Co is located upstream of Wodi Co,and Dinggong Co is located above the forest line,while Wodi Co is located below the forest line.The altitude difference between the two lakes is nearly 300 m,and the water depth and area of Wodi Co are greater than Dinggong Co.In the past two hundred years,the primary productivity（Chl-a）of both lakes has shown an upward trend,and the increase in Wodi Co is greater than that in Dinggong Co.Of two lakesδ¹⁵N There was a different degree of decline inδ¹⁵N,and Wodi Co was wrongδ¹⁵N significantly decreased during 1800-1960s,followed by an upward trend,whileδ¹⁵N has only decreased significantly since the 1980s in Dinggong Co.In the deep-water lake Wodi Co,the cladoceran community is dominated by planktonic species D.longispina and D.Pulex.In the relatively shallow lake Dinggong Co above the forest line,the cladoceran community is mainly composed of epiphytic species C.sphaericus is the dominant species,among which the benthic species A.gutatta/rectangula、The relative proportion of A.affinis has increased and there are fewer planktonic species.In summary,climate change,atmospheric nitrogen deposition,and watershed surface processes are the main environmental factors driving environmental changes in alpine lakes in northwest Yunnan.However,there are significant differences in the ecological environment change process and biological community response characteristics of alpine lakes with different limnological characteristics and forest line locations.The differences in the biological communities affecting the two lakes are mainly driven by different water depth conditions.Wodi Co deep water lakes are conducive to the growth of planktonic species（such as D.longispina）,while Dinggong Co shallow water lakes are conducive to the growth of benthic species and facultative floating species that prefer to attach to submerged plants.For the deep water Wodi Co below the forest line,nitrogen deposition and climate warming have led to an increase in lake nutrient levels and affected the construction of Wodi Co Cladocera communities,promoting an increase in cladoceran biomass.The main influencing factors for the structural composition and long-term change pattern of Cladocera community in Dinggong Co are water level fluctuations and changes in nutrient levels.Climate warming has driven the increase in nutrient levels in lakes,but due to the impact of precipitation and watershed inputs,the characteristics of changes in nutrient levels in lakes are not particularly obvious.In summary,this paper reveals that the long-term impact of regional warming,atmospheric sedimentation,and watershed surface processes on the phytoplankton community in alpine lakes,as well as the modern model of enhanced coupling with algae,It is helpful to identify the key processes of the ecological environment evolution of alpine lakes in the context of global change and scientifically predict the main direction of their future changes.