Spatio-temporal Patterns Of Algal And Carbon Cycling Changes In Response To Catchment Development And Climate Change In Deep Lakes Of Northwest Yunnan

With the expansion of global population and social economy,there exists an increasing trend in the intensity and type of catchment development.Deterioration of environmental quality and ecological function has been increasingly found in surface waters.In the context of regional climate warming,more and more lakes in Yunnan have experienced multiple stressors,such as eutrophication,vegetation degradation,hydrological control and extreme climate events.In recent decades,deterioration of water quality,algae blooms and species loss have led to the decrease of ecological health and water resources,undermining the sustainability of regional development.To date,much of the limnological research have focused on water resources and environmental assessment while sediment studies that reveal long-term changes of lakes are weekly linked with modern processes.In particular,there is a general lack in the coupling of catchment disturbance and lake response,as well as between lake-water quality and biotic response.When compared to shallow lakes,deep-water lakes?i.e.maximum water depth>10 m?are characterized by abundant water resources,longer water retention time,stable thermal stratification and less developed littoral zone,showing a high degree of environmental sensitivity and ecological vulnerability.Therefore,this study combines limnological monitoring and paloelimnological methods to explore the couping processes in the lake-catchment-climate system in affecting ecological changes of deep lakes,and provides scientific data for ecological assessment and catchment development of deep lakes in Yunnan Province.In this study,a total of five deep-water lakes in northwest Yunnan were selected,with a focus on Lugu and Chenghai lakes of different catchment development history.Firstly,the weathering rate of catchment soil carbonate rocks under typical vegetation types and the seasonal variation of lake water environment were identified and compared between both catchments.Meanwhile,the bimonthly and spatial distribution of algal biomass and diatom communities were investigated through their relationship with water quality and carbon cycling at both lakes.Secondly,the history of catchment development and lake environment was reconstructed by applying multiple sediment proxies to reflect the changes in catchment erosion?i.e.magnetic susceptibility?,lake hydrology?i.e.grain size?,carbon cycling?i.e.carbonate content,lake-water TOC?,and nutrient dynamics?i.e.nitrogen elemental and stable isotope signals?.The long-term patterns of algal changes were revealed by analyzing sediment pigments and diatom assemblages in both deep lakes.Multivariate statistical analyses were used to tease out the driving factors for carbon cycling and algal changes.Finally,the spatio-temporal patterns of catchment disturbance in driving algal changes and carbon cycling were assessed through a comparative study of five deep lakes in the same climate context.Through seasonal investigation,sediment analyses and lakes comparison,this study therefore uncovered the regional patterns in lake carbon cycling and algal responses to catchment development,and provided important scientific basis for ecological assessment and catchment management of deep lakes in Yunnan.The main conclusions of this study include:?1?Modern surveys showed that the weathering rate of carbonate rocks was significantly related to the vegetation type in the catchment,as well as a seasonal variation in the lake-water inorganic carbon concentrations.The weathering rate of carbonate rocks in the catchment of Lugu Lake varied significantly under different vegetation types,and the dissolution rate of carbonate rocks in forested soils was 1.6times higher than that of shrubs.While shrubs predominated in the catchment of Chenghai Lake,the carbonate dissolution rate varied little among the different types of shrubs.However,the weathering rate(16.9 mg·m^-2·d^-1)in Eucalyptus-dominated forests was significantly lower than that in other land-use types(90.6 mg·m^-2·d^-1).Compared to Lugu Lake,the average weathering rate??was much lower than that in Lugu Lake??.The catchment of Chenghai Lake was domanented by shrubs?²6%?,while the forest coverage of Lugu Lake was⁴7%.Comparing these two different catchment,the positive shifts of vegetation?i.e.from shrub to forest?accelerated weathering rate strongly.In Lugu Lake,lake-water CO₃^2-concentrations were mainly affected by seasonal runoff in the catchment,while the water column change of HCO₃^-,a main carbon source for aquatic plants,was mainly driven by biological assimilation which is also evidenced in Chenghai Lake.?2?Lake-water cycling of organic carbon and algae distribution displayed a strong seasonal pattern at lakes of Lugu and Chenghai.Due to that the catchment forest coverage was significantly higher at Lugu lake?⁴7%?than that of Chenghai?²0%?,the seasonal variation in precipitation led to an opposite pattern in lake-water DOC concentration between both lakes.Furthermore,lake-water chlorophyll a?Chla?showed a significantly positive correlation with DOC?P<0.001,r=0.57?only in Chenghai Lake,indicating that primary production was the main driver of lake-water organic carbon level.Nutrients such as TP and TN showed the highest value in May?dry season?,and declined during the rain season in Lugu Lake.Due to lower catchment vegetation coverage,TP values in Chenghai increased during the rain season when there was an increased nutrient input of non-point sources.In Lugu Lake,the nitrogenophilic diatoms dominated throughout the year indicating a phosphorus-limiting nutrient scenario,while the high pH values in Chenghai Lake may promote the dominance of the alkaphilic species such as Cyclotella meneghiniana,with the high nutrient content favoring the increase of Nitzschia spp.Meanwhile,the increase of water temperature was associated with a predominance of small-sized diatoms at both lakes,reflecting that a strenthened thermal stratification may drive the succession of diatom assemblages.While water temperature,silicate and nutrient content explained7.8%,5.7%and 10.3%of the bimonthly variation of diatoms in Lugu Lake,water temperature,conductivity and nutrients explained 30.7%,13.7%,6.1%for diatom changes in Chenghai Lake,respectively.?3?Sediment records showed that intensified catchment development altered the long-term pattern of carbon cycling and algae changes at both lakes which also varied depending on the lake-specified history of catchment disturbance.Since the 1950s,large-scale deforestation in the Lugu Lake catchment has led to a rapid increase in soil erosion intensity?e.g.magnetic susceptibility?,resulting in a simultaneous and continuous decrease of organic carbon?lake-water TOC?and inorganic carbon?sediment carbonate?input.Specifically,lake-water TOC decreased from 6.6±0.2 mg/L to 3.9 mg/L,sediment carbonate decreased from 12.1±1.2%to 7.2±3.2%).In contrast,catchment erosion in Chenghai Lake was relatively stable and sediment carbonate level showed little change during the last two centuries.Meanwhile,sediment nutrient and lake-water TOC levels increased rapidly since the 1980s,reflecting that eutrophication promoted the algal biomass and the organic carbon cycling.With a continuous decrease of water TOC level in Lugu Lake,the increase of transparency may favor the photosynthetic process and algal grwoth.Therefore,the degradation of catchment vegetation significantly reduced the carbon sink of Lugu Lake,while the rapid accumulation of endogenous organic matter in Chenghai indicated that its organic carbon cycling was mainly driven by lake eutrophication.Meanwhile,regional warming has strenthened the intensity of thermal stratification at both deep lakes,leading to a gradual dominance of small-sized planktonic diatoms during the last few decades.?4?The comparative analyses of five deep lakes showed that eutrophication and regional warming significantly promoted algae production and enhanced the organic carbon sink,while hydrological regulation,such as dam and sluice construction,often favored the precipitation of inorganic carbon.In regard to the regional heterogeneity of catchment disturbance,there,however,existed difference the forcing of climate warming,hydrological regulation and eutrophication in causing limnological changes.For example,nutrients were found to be the most important driver in accounting for44.8%of the diatom changes across deep lakes.The temporal change of lake productivity varied over different time scales in response to these external forcing.At seasonal and annual scales,lake-water Chla was often enhanced with increased nutrient levels,while climate warming can serve as a major factor in promoting algal production on a decadal scale.Overall,catchment disturbance has intensified with forest degradation,eutrophication and hydrological regulation in deep lakes over the last century in northwest Yunnan,leading to a significant change in carbon cycling and algal response that varies with the external forcing.Catchment deforestation can significantly reduce the sink of both organic and inorganic carbon in deep lakes.Lake regulation and eutrophication often promote the lake burial of inorganic and organic carbon,respectively.Regional warming,through enhancing the thermal stratification in deep water lakes,has led to a gradual dominance of smaller-sized diatoms,which can be of important ecological signficance through trophic interaction.Therefore,restoration of catchment forests,control of nutrient input,as well as measures of hydrological management,such as enhancing hydrodynamics,for mitigating the ecological impact of regional warming,should be combined for sustainably protection and ecological restoration of deep lakes in northwest Yunnan.