Temporal And Spatial Pattern Recognition Of Cladocera Community Change In Yunnan Typical Lake Under Multiple Environmental Pressure

With the intensification of human activities and catchment development, lakes in Yunnan have been impacted by multiple environmental stressors(i.e. eutrophication, fish introduction and macrophyte degradation) over the past few decades, which led to the deterioration of lake environment and degradation of ecosystem function. Zooplankton, an important component of lake food webs, is among the most sensitive indicators for lake ecosystem health. Despite of mounting studies on lake ecology and water quality for Yunnan, there existed a strong lack of long-term lake monitoring data, particularly those on zooplankton surveys. Therefore, it is urgently needed to identify the long-term patterns of zooplankton changes in Yunnan lakes, which can serve as the basis for ecological evaluation and restoration of impacted lakes. In the past few decades, lake sediment records have been widely used to identify the spatio-temporal patterns of lake environment and ecosystem changes. Cladocera is an important component of zooplankton and their sediment microfossil records can reliably provide long-term history of zooplankton changes. Therefore, we focused on the sediment cladoceran records to uncover regional patterns of ecological and environment changes in six lakes of Yunnan for the past century.The main findings of this study include:1) From 1980 s, the sediment Chl-a level at Xingyun Lake rised rapidly with a spike in the abundance of Chydorus sphaericus which is known to tolerate nutrient enrichment, while oligotrophic Bosmina longispina gradually declined. PCA results show that the lake eutrophication accounted for >80% of the total variance in cladoceran community changes. Furthermore, lake productivity was also the most significant driver of total cladoceran production(r2=0.95, p<0.001, n=33). In addition, the gradual loss of macrophytes due to lake eutrophication and water level fluctuation may have contributed to the loss of benthic cladocerans.2) Our sediment records confirmed with the modern survey data and experimental results in that some cladoceran species(such as Bosmina and Daphnia) are most sensitive indicators for heavy metal pollution. In the lakes of Yangzong and Datun, severe arsenic pollution levels(i.e. lake-water arsenic concentration > 100 ?g/l) can lead to the collapse of planktonic cladocerans such as Bosmina and Daphnia( i.e. a loss by >10-100 times), while eutrophic and metal-tolerant species(such as C. sphaericus) generally showed increased production. Therefore, plankton communities and lake ecosystems can experience catastrophic changes as a result of arsenic pollution if a threshold level is reached.3) The regional survey of zooplankton changes in four large lakes which have all experienced the introduction of icefish suggested that the change of predation regime can vary depending on lake trophic level and productivity. For example, in Fuxian lake where the invasive icefish grew most successfully(i.e. the proportion of icefish production among the total fish catch > 80%), the size of Bosmina decreased obviously shortly after the icefish introduction. In contrast, the average size of bosminids overall showed an increasing trend after the icefish introduction in Dianchi and Xingyun which had low to moderate proportions(i.e. < 40%). Both lake eutrophication and fish predation accounted for the majority of the total variance in bosminid flux data across lakes(53.5-81.2%), except in Xingyun(i.e. 32.0%). Specifically, the independent role of predation pressure showed a unidirectional decrease over a nutrient gradient, decreasing from 26.4% in Fuxian to 1.5% in Xingyun. The strength of nutrient effect was similar(20.6-22.1%) across most of the lakes but was more than doubled in Erhai(46.4%). Furthermore, there existed a clear interaction between both forcing in affecting zooplankton production, showing a unimodal shape in the driving strength of fish predation across lakes.The preliminary results of this study show that, in the absence of long-term lake monitoring data, sedimentary cladoceran records can provide long-term history of zooplankton and food web responses to multiple environmental stressors, such as eutrophication, industrial pollution and fish introduction. Therefore, sediment-based lake studies can serve as the basis for ecological evaluation and restoration for impacted lakes in Yunnan.