| In the context of global change,lake ecosystems have been degraded to varying degrees on a global scale,and the ecological problems faced by lakes in arid regions have become more severe.Lakes in arid regions are the main links in the regional water resources cycle,and their changes are sensitive to climate change and human activities.In the past 2000 years,when humans were making light use of natural conditions,the state of aquatic ecosystems in lakes in arid regions was stable,and in the past 200 years,they were strongly affected by climate warming and human activities,and their stable aquatic ecosystems were broken.In the past 200 years,due to the strong influence of climate warming and human activities,the fluctuation and frequency of various elements of the ecosystem have begun to increase.Eco-environmental problems such as salinization,eutrophication,and declining biodiversity have emerged.For the alpine lakes in arid regions,there have been numerous paleoecological studies that have revealed that the aquatic ecology of lakes in remote areas including the Arctic and high mountains has undergone tremendous changes in the past 200 years.Because the alpine lakes are not directly disturbed by human activities,there are two reasons to drive the trans formation of their ecosystems: one is the climate warming;the other is the atmospheric pollution caused by human activities,such as nitrogen deposition,acid deposition and other processes.However,there are few systematic studies on the evolution of lake aquatic ecosystems and their response to climate change and human activities on the long-term scale based on ancient limnology.The study on the difference of the state transition mechanism of lake aquatic ecosystem with different impacts on human activities is lacking.This study selected two lakes in the arid region of northwestern China,the alpine lakes Shuanghu of the Altai Mountains,the oasis lakes——Bosten Lake in the south of the Tianshan Mountains as research subjects to explore the long-term ecological evolution history of the two lakes and their response to climate and human activities.After dating the sediments from Shuanghu borehole,biological indicators such as diatoms,and environmental surrogates such as magnetic susceptibility,grain size,TOC,and TN were used to reconstruct the ecological changes and environmental changes during the past two thousand years.Then we discussed the ecological change and its possible driving chanism.As for Bosten Lake,by studying literature and collecting data,relevant data including indicator data(Paleo Lake and tree rings),instrumental survey data,and socio-economic data were extracted,and on this basis,the construction was also constructed to construct the evolutionary sequence of the Bosten Lake aquatic ecosystem.We use Climate change and human activities sequences to conduct comprehensive comparisons and explore the possible driving mechanisms of paleoecological changes in Lake Bosten.The following are the main conclusions:1.A total of 21 genera and 94 species were identified in 138 samples of SH12 B.The main diatom species of Shuanghu include Fragilar ia,Achnanthes,and Navicula,and they are individual smaller diatoms.According to their life forms dividided the main diatom species into phytoplankton species,facultative phytoplankton species and benthic species.The dominant species are facultative phytoplankton and benthic species,accounting for 80-99% of the diatom assemblages,and mainly include Fragilar ia construns,F pinnata,Achananthes minutissima,and Navicula atomus.2.Aquatic ecological Changes in the Shuanghu Lake in the Past Two Thousand Years:1)The abundance of benthic diatoms is low,but its species are numerous,and most of them are small.This shows that the transparency of lake water is poor,and it is in a state of oligotrophic for a long time;The dominant species are Fragilar ia from beginning to end.The genus is very advantageous in alkaline,oligotrophic alpine lakes,and can grow normally under conditions of poor ice transparency.In addition,there's no new species have emerged.In summary,the diversity of diatoms in the Shuanghu Lake in the past was stable.The dominant species is the cold-resistant Fragilaria,and the relative abundance of planktonic diatoms is extremely low.So we suppose the lake's ecological environment was stable and its structure and function remained unchanged.Based on sediment susceptibility,grain size and TOC,the paleoenvironment of the Shuanghu Lake over the past two thousand years has been found to be stable,with little fluctuation in dryness and humidity.Moreover,Shuanghu is located in the western part of Altai and is far away from the influence of human activities.2)In the past 200 years,the magnetic susceptibility of the sediments in the Shuanghu Lake has suddenly increased.At the same time,the relative abundance of the eutrophic species Navicula atomus in the diatom assemblage has reached the highest in the whole drill.After a comprehensive analys is of the lake basin process and atmospheric deposition,it is believed that the main reason is that the impact of atmospheric deposition has increased,making the magnetic particles and nutrients entering the lake,especially N.3.Based on the records of various micro-paleontological indicators of Bosten Lake core(chirontodendron,diatom,zoonotic,etc.),combined with other climate data and modern observational data,we have constructed the Bosten Lake ecosy-stem state and regional climate change sequence for the past 1000 years.We consider that climate are the main factors controlling the ecological changes of lakes on the millennium and centennial scales.However,after 1950,the frequency and amplitude in various elements of the ecosystem were in a state that had never existed since natural climate change dominated in the past 200 or even 1,000 years,and climate change was not enough to explain the reasons.Therefore,it is believed that the main reason for the abnormal fluctuations in the Bosten Lake ecosystem over the past 50 years is the human activities that are superimposed on climate change.For oasis lakes,human activities are directly involved in their material and energy cycles,thereby changing their ecosystems;For alpine lakes,although humans cannot directly affect them,they can indirectly affect the lake ecology through changing natural processes such as atmospheric deposition.4.For oasis lakes,the dominant controlling factor for the transformation of ecosystems on the millennium scale is the climate.In the past 200 years,human activities have replaced the climate as the dominant factor in changing the lake ecosystem,and it changes lake ecosystems by directly participating in their physical and energy cycles;For alpine lakes,climate change is a dominant factor in changing its aquatic ecosystem,but humans can indirectly affect their aquatic ecology through changes in atmospheric deposition and other natural processes. |
PDF Full Text Request