| In recent decades, many lakes in Yunnan have experienced the impacts of multiple environmental stresses(such as eutrophication, water diversion, extreme climate). Despite of intensive measures taken for pollution remediation, a lack of the systematical knowledge on lake environment and ecosystem processes has prevented the effective restoration and sustainable management of impacted lakes. As a large and shallow system, Xingyun Lake, currently hyper-eutrophic, was reported to have experienced deforestation, lake reclamation, exotic fish introduction and rapid industrial development during the last century. While Chenghai Lake from Northwest Yunnan is known to be a large, deep and closed system and has experienced an increase of primary productivity more recently.In the context of relatively short time series of lake monitoring data, this study applied paleolimnological techniques to reconstruct the history of lake environment changes for the lakes of Xingyun and Chenghai during the last 200 years. Furthermore, we aim to evaluate and compare the differences in the temporal patterns of ecological responses through combining the multi-proxy sedimentary records(i.e. diatoms, pigments, grain size and isotope) with archived data(i.e. meteorological data and documents). Sedimentary records show that the nutrient level and primary productivity of both large lakes have increased significantly in the past few decades, but there existed differences on the timing and the magnitude of nutrient enrichment. The background nutrient level was likely much higher in Xingyun Lake, with a rapid process of eutrophication starting from late 1950 s. The eutrophication process started later(i.e. the late 20 th Century) for Chenghai Lake, but with an accelerating trajectory similar to that found in Xingyun. Sediment grain size data recorded hydrological fluctuation and extreme climate events for both lakes, indicating the significant impacts of both climate change and water diversion projects.The trajectory of sediment diatom communities showed significant changes over the last two centuries at both lakes, with a clear shift of dominant diatoms from oligotrophic species to eutrophic ones. Furthermore, hydrodynamic changes also played an important role in structuring the community reorganization. When the nutrient level of Xingyun Lake was low, the diatom community structure was relatively stable with the coexistence of several dominant species(e.g. Aulacoseiragranulata, A. subarctica. Cyclostephanos dubius, Fragilaria crotonensis). In the beginning of twentieth Century, Cyclotella rhomboideo-elliptica, the oligotrophic species dominant in the downstream lake of Fuxian, was found to spike in Xingyun, corresponding well with the documented water flow from Fuxian to Xingyun due to the blocking of the outlet river. With the continuous accumulation of nutrients in Xingyun, C. rhomboideo-elliptica gradually decreased before it disappeared shortly and was replaced by several eutrophic taxa(such as Aulacoseira granulata, A.subarctica, Cyclostephanos dubius and Fragilaria crotonensis). Multivariate statistical analysis further showed that, in addition to nutrient enrichment, hydrodynamic changes and climate warming also significantly affected the diatom community succession. In comparison, diatom assemblages showed more frequent and significant changes in Chenghai. Similarly, C. rhomboideo-elliptica also dominated in Chenghai Lake when the lake trophic level was low. With the accumulation of nutrients, the relative abundance of C. rhomboideo-elliptica decreased gradually and disappeared decades ago, a pattern similar to that found in Xingyun Lake. More recently eutrophic species(such as C. meneghiniana and C. dubius) gradually became dominant in Chenghai. Similarly, hydrological changes also affected the diatom community succession in Chenghai Lake, where the increase of Aulacoseira spp. and Nitzschia spp. indicated more turbulent water conditions, in good timing with the installation of water diversion from Xianren River to Chenghai.Principal component analysis(PCA) was applied to identify the major directions of diatom community structure changes. The results showed that the changes of diatom communities in Xingyun Lake reflected three environmental gradients, accounting for 44.81%, 24.39% and 11.1% of the changes in diatom assemblages, respectively, which may represent the trophic level, hydrological changes and climate warming. The selection of minimum adequate models showed that nutrient level was the most important environmental factor in Xingyun, followed by the proxies of C/N and cladoceran biomass. Statistical results showed that the diatom PCA axis 1 has explained > 80% of the changes for diatom community structure in Chenghai. The minimum adequate models showed that nutrition level independently explained 33.7% of the total variation of diatom communities in Chenghai. While lake depth and hydrological changes as indicated by the C/N ratio and grain size, respectively, also played an important role in driving diatom changes. In brief, the temporal pattern of diatom community changes in the shallower lake of Xingyun was morecomplicated than that in Chenghai Lake, suggesting that shallow lakes might be more sensitive to environmental change and human disturbance due to limnological differences.In conclusion, our results showed that multiple stressors such as eutrophication, artificial water diversion and climate warming were commonly found at both Xingyun and Chenghai. Despite that a similar trend of diatom community shift was found for both lakes, there exist a more complex model of diatom changes in response to climate warming and hydrological changes in the shallower and more eutrophic system of Xingyun. Our results highlight the control of nutrient input for the prevention of lake eutrophication, and also it is necessary to take into account the hydrological conditions, macrophyte restoration and climate-mitigation policies for sustainable restoration of impaired lakes. |
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