| Under the context of realizaing national goal for carbon peaking and neutrality,accurate assessment of regional carbon sink is key for formulating emission reduction policies and action plans.Lakes,an important part of the hydrosphere,have great potential in carbon storage.As a key component of the terrestrial carbon sink,lake carbon sink contributes greatly to the global terrestrial missing carbon.Karst regions account for about 15%of the world’s land,and its carbon sink is the largest one on earth.In recent decades,along the gradient of regional warming,vegetation degradation and eutrophication,the productivity and carbon burial patterns have changed significantly in some lakes of Yunnan.However,in areas where karst landscapes is well developed,lake burial of both organic and inorganic carbon can be affected by catchment development.Vegetation degradation would reduce soil organic carbon storage through decreasing catchment output,while rock weathering and carbonate dissolution rate will reduce basin output and lake carbon storage.Therefore,under the background of continuous catchment development and enhanced nutrient export,algal production will be increased,while the source,content and burial flux of both organic and inorganic carbon may be altered in a synchronous way in lakes.In this thesis,research methods of limnology and sediment investigation were combined for conducting a comparative analysis of two lakes（Changhu and Yuehu lakes）with a contrasting degree of watershed land use in karst region of Shilin County in central Yunnan.In combination with seasonal monitoring and historical records,we conducted multi-proxy analsyses of sediment cores,including the catchment erosion（i.e.magnetic susceptibility（MS））,catchment allochthonous input（C:N ratio and C stable isotope）,lake hydrology（particle size）,nutrient levels（total phosphorus and nitrogen）,primary production（i.e.algal pigments and diatoms）,for reconstructing the history of environmental changes and the long-term variations of algae and carbon burial in both lakes over the past century.Meanwhile,long-term patterns,synchronous variations and driving processes of algae and carbon burial in response to watershed development were quantitatively identified and compared.The main conclusions of this thesis are summarized as follows:（1）The seasonal surveys showed that there exist strong discrepance in seasonal limnological changes between Changhu Lake（mesotrophic level,deep water）and Yuehu Lake（eutrophic level,shallow water）.The basin of Changhu Lake,currently characterized by higher cover of forests（33.43%）,may experience relatively low nutrient export in both dry and rain seasons,resulting in in-significant seasonal differences in lake-water nutrient and Chlorophyll a（i.e.Chl-a）contents.Yuehu Lake basin was mainly covered by cropland（60.98%）currently,and the lake-water concentrations of both nutrients and Chl-a were higher in rain season than those in dry season.With the seasonal increase in air temperature and lake stratification,there was no significant correlation of water temperature with either diatom variations or nutrient level（p>0.05）in Changhu Lake.However,in the mixed and shallow-water lake of Yuehu,the changes of diatom community showed significant correlations with both nutrients and water temperature（p<0.01）.（2）The lake-water dissolved organic carbon（DOC）and inorganic carbon（DIC）showed obvious seasonal changes at both lakes.The contents of DOC and DIC in rain season were higher than those in dry season in Changhu Lake,reflecting a seasonal change in the strength of chemical weathering and soil leaching in the basin.The DOC concentration in Yuehu Lake also showed significant seasonal variation,which may reflect the changes of crop growth and lake organic matter input.However,there was no significant seasonal difference in DIC,which may reflect a stable influence of well cultivated catchment,resulting in a relatively stable export of DIC in the basin over time.Furthermore,the lake-water levels of DIC in both lakes is higher than those of DOC,indicating that inorganic carbon is an important component in carbon cycling of Karst lakes.（3）The results of sediment algal pigments showed that the primary productivity of the two lakes continued to increase over the past century,especially since the 1980s.With the increase of nutrient input,there was a significant positive correlation between nutrient input and multiple pigment concentrations（such as Chl-a and Myxoxanthophyll）（p<0.001）.The results of the sediment diatoms showed that the sediment diatoms were mainly benthic species（relative abundance>96%）in Changhu Lake,and the changes of nutrient and hydrodynamic conditions were identified as the main environmental factors in driving the changes of diatom community.The pattern and trend of diatom variations in Yuehu Lake were similar to those of Changhu Lake,where benthic species were similarly dominant（relative abundance>95%）and the main drivers for diatom community change were nutrient and hydrodynamic proxies.Therefore,our results of sediment algal proxies showed that nutrient level was an important driver for algal production and community structure over time.（4）Sediment multi-proxy analyses also recorded the long-term fluctuation in soil erosion and allochthonous input,and their strong link with temporal variations of carbon burial in two lakes.The sedimental results showed rapid accumulations of carbon content and burial rate in both lake over recent decades.In Changhu Lake with a higher land cover of forest,the bulk and organic C/N ratio were significantly positively related to the MS signal（r=0.95 and 0.89,p<0.001）,and showed significant negative correlation with the contents of inorganic and organic carbon（r=-0.94,p<0.001 and r=-0.52,p<0.01）.Meanwhile,MS signal showed postive correlation with bulk and organicδ13C（r=0.95 and 0.93,p<0.001）,reflecting a significant impact of catchment input on sediment carbon burial.In contrast,the bulk and organic C/N ratio were negatively related to MS signal（r=-0.54,p<0.01 and r=-0.67,p<0.001）and the bulk C/N ratio showed no relationship with the sediment inorganic content（p>0.05）.Meanwhile,MS signal showed negative correlation with bulk and organicδ13C（r=-0.88 and-0.76,p<0.001）in Yuehu Lake,reflecting that the cropland expansion may have enhanced lake eutrophication and autochthonous input.In both lakes,nutrient enrichment and algal production enhanced the rapid accumulation of organic carbon.（5）Our results suggest that there may exist synchronic variations in sediment carbon burial over time,however,the long-term patterns are different between lakes that may be due to their contrasting degree of catchament development.Specifically,there existed a significantly synchronous change between organic and inorganic carbon flux（r=0.72 and 0.85,p<0.001）in both lakes,with sediment flux being positively related to bulk C/N ratio（r=0.48 and 0.45,p<0.001）.However,the correlation analysis showed a significantly synchronous change between organic and inorganic carbon content in Changhu（r=0.59,p<0.001）but not in Yuehu（p>0.05）.Furthermore,the burial rate of inorganic carbon was significantly higher than that of organic carbon in Changhu Lake,while the burial rate of organic carbon was consistently higher than that of inorganic carbon in the nutrient-rich Yuehu Lake,reflecting differential forcing of vegetation degradation and eutrophication in karst lakes,respectively.Over the gradient of enhanced watershed development,forest degradation can reduce the loading of catchment carbon export,while agricultural expansion and eutrophication would promote the growth of algae and the rate of organic carbon accumulation over time.In conclusion,catchment land use and lake nutrient levels have played important roles in driving the long-term trajectories and synchronic patterns of carbon accumulation and algae change in lakes.Therefore,the cycling of inorganic carbon should be considered in the assessment of carbon pools in karst areas.Meanwhile,the main results of this study can provide important data and scientific basis for lake protection,ecological restoration and carbon estimation in karst regions. |
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