Spatio-temporal Patterns Of Hydrological Regulation And Ecological Responses Of Lakes In Northwest Yunnan

As an important part of inland waters,lakes serve as essential habitats for aquatic organisms,and provide resource support and ecosystem service for catchment development.The province of Yunnan,one of the five major lake regions in China,hosts a large number of freshwater lakes characterized with various types,complex hydromorphology and diverse environmental gradients.With changing climate,extreme droughts have occurred more frequently and lasted longer,leading to a shrinking of smaller waters and a significant loss of regional water resources.In the context of expanding industrialization and urbanization,Yunnan lakes have experienced rapid deterioration of water quality over the last few decades due to hydrological regulation,eutrophication and extreme droughts,which lead to the loss of biodiversity and the degradation of ecosystem functioning.More recently,anthropogenic regulation dramatically altered lake hydrology through damming,diking,lake separation and water diversion.To date,ecological assessment of hydrological regulation is predominantly derived from hydrological and limnological monitoring in regualated lakes,while their deviation from baseline status and their dependence on impoundment intensity are rarely examined due to insufficient monitoring data.As a natural collector on the signals of environmental changes that occur within lake waters and their surrounding catchments,lake sediments can provide a continuous and long-term trajectory of limnological changes and ecological responses through multi-proxy analyses.Therefore,it is viable to combine multi-disciplinary research methods to identify the spatio-temporal patterns of hydrological fluctuation and long-term ecological consequences for lakes of Yunnan along a gradient of impoundment intensity.In order to uncover the regional pattern of lake hydrological fluctuation over time,remote sensing images,monitoring data and documentary records were used to quantify the annual changes in surface area of 13 lakes in the northwest Yunnan over the past 30 years.Furthermore,we conducted multi-proxy sediment analyses in three lakes with varying degrees and types of anthropogenic disturbance for quantifying the long-term limnological changes in combination with data derived from remote-sensing images,monitoring data and historical documents.On the basis of chronological construction,we analyzed sediment physical（e.g.grain size,magnetic susceptibility,loss on ignition）and geochemical proxies（e.g.nutrient elements,stable isotopes）to provide the time series of limnological changes and ecological responses,together with biological proxies（e.g.diatom assemblages,algal pigments）.Furthermore,we compared the variation of limnological changes and ecological responses with varying impoundment intensity both among lakes and within lake basins.The main conclusions are listed as follows:1.The surface area variation of Northwest Yunnan lakes was characterized by spatio-temporal heterogeneity with dramatical fluctuations during the last three decades.Lake area data extracted from remote sensing images in the dry seasons served as an reliable indicator for hydrological fluctuation,which can reflect the inter-annual trend of water level change.Regulated lakes often showed a greater variation in water level than the unregulated lakes.Small and shallow lakes are often characterized by more pronounced water level fluctuation（WLF）.Regional climate change is a key factor driving the change of lake surface area,while intensified catchment development and water demand can also play a synergistic role in driving lake hydrological change over time.2.During the past two centuries,regional climate change has led to an increase in hydrological fluctuation and a decrease in water level of Jiehu Lake,an alpine shallow lake without human disturbances.Hydrological changes increased the abundance of benthic diatoms but played a less important role in driving diatom assemblages than air temperature and nutrients.The sediment records of magnetic susceptibilities well reflected the variation of regional precipitation due to their significant link through catchment processes.Sediment grain size components served as an integrated signal for lake hydrological properties linked with regional precipitation,basin erosion and surface runoff.The sediment content of both pigments and total nitrogen（TN）displayed a significant increase over time,with a synchronous decrease in the bulk sediment C/N ratio andδ¹³C signature,indicating a long-term increase in lake production and autochthonous organic matters.It was further revealed that nutrients,air temperature and hydrodynamics were among the key factors in driving the long-term variation in diatom succession and algal production.They independently accounted for 22.5%,9.3%and 6.0%of the total variance in diatoms,respectively,while there also existed a strong interaction between lake hydrology and nutrients（22.0%）.3.Haixi Lake,a shallow lake from northwest Yunnan,was converted to a deep reservoir with pronounced hydrological regulations since 1957 AD.A water depth inference model was constructed from the clay components of 27 surface sediments across the lake basin.The historical change in water level was then reconstructed based on this model,showing a significant correlation with the observed water depth and monitored lake area for the last few decades in Haixi Lake.There exsited a long-term trend of water level increase and a gradual decrease of littoral habitats,causing the loss of benthic diatoms and an increasing proportion of autochthonous organic matters.The autochthonous TOC,derived largely from phytoplankton,increased remarkably due to the increasing interaction of water level fluctuation,climate warming and nutrient enrichment.Meanwhile,the increased intensity in lake impoundment starting from 1990 AD has significantly reduced sediment burial and storage of organic carbon,the later of which are dominated by autochthonous organic matters.In the context of regional warming,the increased rate of decomposition can enhance the formation of inorganic carbon and therefore the emission of greenhouse gases.Water depth and associated hydrological variables further showed a stronger impact on diatom communities during the damming stage,when compared to the pre-disturbance and reinforcement stages（independently accounted for>35%of the total variation）.There also existed a threshold level in both water depth（e.g.¹0 m）and the magnitude of WLF（e.g.² m）,beyond which there can exist a significant loss of diatom species richness and a decrease in community dissimilarity,especially in the reinforcement stage（i.e.since 1990 AD）.4.Erhai Lake,a large lake from northwest Yunnan,was dammed for hydroelectric generation in 1973 AD,which led to a significant decrease of water level and the amplification of WLFs.Over sediment surveys revealed the spatial heterogeneity in the temporal changes of nutrient cycling,algal production and diatom community succession across three lake basins.There existed a strong asynchrony in the long-term changes of algal production and cyanobacteria blooming across the lake basins.Furthermore,our study reveals a significant role of hydrological regulation in driving ecosystem changes,particularly in the shallow basin,in which water depth was identified as the key driver on diaton succession and cyanobacterial blooms,independently accounting for>10%of the total variations.In the context of regional warming and cultural eutrophication,our results highlight that reduction of nutrient input,moderate control of water levels and restoration of submerged macrophytes should be combined for the mitigation of algal blooms and sustainable ecological restoration of this highly stressed lake.5.Compared with the undisturbed lake of Jiehu,hydrological regulation significantly changed water levels and the amplitude of water level variation,which largely accounted for the shrinkage of littoral habitats in lakes of Haixi and Erhai.Lake biogeochemical cycling（e.g.carbon and nitrogen）varied largely depending on the types and intensity of hydrological regulation over time.Increased water levels and weakened hydrological fluctuation enhanced nutrient retention,promoting the growth of plankton and a shift in the organic carbon transfer from a benthic pathway to a planktonic one.Climate warming,water level fluctuation and eutrophication were identified as key environmental factors in causing spatial heterogeneity in the temporal patterns of the productivity-diversity relationship（PDR）across lakes.With intensitified hydroloical regulations,the independent explanatory power of hydrological fluctuation in driving diatom changes increased from⁶%to>25%.In summary,hydrological fluctuation,climate change and eutrophication play important roles in impacting ecological status of two regulated lakes,with a larger role of water regulation than in an unregulated lake from northwest Yunnan.There also exist an increase in primary production and a significant change in diatom composition in the three study lakes since¹950s,which are strongly associated with pronounced changes in water level,thermal regime and littoral habitats.With increased frequency and magnitude in water regulation,the link of lake hydrology with both nutrients and air temperature is strengthened in driving diatom changes,suggesting the role of hydrological fluctuation in modulating the impact of other stressors.Furthermore,there may exist strong spatial heterogeneity in the temporal pattern of limnological changes within lakes with complex morphology and land use types.Our results highlight the necessity of implementing a strategy of moderate hydrological regulation for sustainable management and water resource use of impounded lakes in the context of ongoing eutrophication and warming climate.