Environment Changes Over The Past 2000 Years Documented By Cladoceran Fossils In Bosten Lake And Jili Lake,Xinjiang,China

Lakes in the Xinjiang region play an important role in maintaining regional biodiversity and ecological balance due to the shortage of water resources and fragile ecosystem.Bosten Lake and Jili Lake are two large freshwater lakes in the Xinjiang region and meltwater is an important source of supply water for the two lakes.Under the dual disturbance of human activities and climate change,a series of ecological problems,such as eutrophication,water shrinkage and salinization,and biodiversity decline,have occurred in Bosten Lake and Jili Lake.Based on the analysis of cladoceras fossils in lake sediments,grain size,carbonate content and other geochemical indicators,as well as other published climate records in the region,we reconstructed the evolutionary sequence of cladoceras communities in Bosten Lake over the past 2000 years and Jilihu Lake over the past 1800 years and discussed the history and driving factors of ecological environment change in Bosten Lake and Jili Lake.The main research results of this paper are as follows:1.The evolution history of cladoceran community in Bosten Lake over the past2000 years can be divided into 5 phases: Phase 1(0-280AD),the cladoceran community was dominated by littoral species and A.affinis was dominate species,while the absolute abandunce of planktonic species was low,indicating the freshwater environment of the lake;Phase 2(280-450AD),the absolute abundance of C.sphaericus increased significantly and became the dominant species,indicating the degradation of the lake environment;Phase 3(450-1570AD),the diversity and species richness of cladoceran community showed an increasing trend and A.affinis became the dominant species again,indicating the improvement of lake environment;Phase 4(1570-1850AD),C.sphaericus became the dominant species again and the diversity and species richness of cladoceran community decrased to the level of phase 2,indicating the re-degradation of the lake environment;Phase 5(1850-2000AD),the absolute abundance of planktonic species increased rapidly,especially B.longirostris,while the absolute abundance of littoral species decreased,indicating the rapid rise of the trophic state of Jili Lake.2.The evolution history of cladoceran community in Jili Lake over the past 1800 years can be divided into 4 phases: Phase 1(250-290AD),B.longirostris was the dominant species in the early stage,but its abusolute abundance decreased dramtically later,indicating the water level of the lake experienced a rapid rising and falling process;Phase 2(290-460AD),C.sphaericus and L.acanthocercoides became the dominant species,indicating the lake environment gradually degraded and the water level decreased;Phase 3(460-1520AD),the abusolute abundance of C.sphaericus and L.acanthocercoides decreased and O.tenuicaidis became the dominant species,indicating the improvement of lake environment and the water level increased;Phaes4(1520-1750AD),C.sphaericus basically disappeared,and L.acanthocercoides,L.leydigi and A.quadrangularis became the dominant species,indicating the water level decrased and the macrophytes were not developed;Phase 5(1750-2018AD),the absolute abundance of B.longirostris increased rapidly and became the dominant species,while the absolute abundance of other littoral species decreased or even disappeared,indicating the rapid rise of the trophic state of Jili Lake.3.The results of cladocerans in Bosten Lake and Jili Lake suggest a lower lake water level and a higher salinity occurred during the cold,high-precipitation periods of 250-500 AD and 1520-1820 AD,and that a higher lake water level occurred during the warm,low-precipitation period of 500-1520 AD.Therefore,the water level fluctuations of Bosten Lake and Jili Lake were modulated mainly by temperature-driven meltwater input on the centennial timescale.During cold period,low temperature limited the melting of mountain snow and glaciers,which led to the decrease of runoff into the lake and the decrease of lake water level;in contrast,during warm periods,high temperatures promoted the melting of mountain snow and glaciers,leading to increased runoff into lakes and rising lake levels.4.Human activities was the main factors caused the deterioration of lake environment in Bosten Lake and Jili Lake over the past century.Damming in the river basin has caused the water level dropped and the salinization of water in both lakes;Aquaculture and sewage discharge were the main causes of lake eutrophication.In addition,the project that channeled the Irtysh River into Ulungur Lake made the saline water Ulungur Lake to flow back into Jili Lake,which accelerated the process of salinization and eutrophication of Jili Lake.