| Climate change and human activities have become important factors in driving environmental changes of lakes.In the high-latitude and high-altitude areas that have not been directly affected by human activities,there has been increasing evidence on obvious ecosystem changes in alpine lakes.Alpine lakes are usually characterized by small area,limited nutrients,simplied food web strcture,and high sensitivity to climate change and nutrient input.Monitoring data have shown that there exists high flux of nitrogen deposition and a significant increase of regional temperature in the southeastern margin of Qinghai-Tibet Plateau.Previous studies have shown that there may exist strong heterogeneity in lake response patterns in this region depending on lake types and catchment characteristics.In this study,three small-sized lakes(Gaigong Cuona,Wodi Co,and Bigu Tianchi)below the tree line but with different water depths were selected for sediment survey and comparative analyses.Through the dating of sediment profiles and multi-proxy analyses,multiple algal indicators(i.e.,diatom assemblage and algal pigments)were analyzed in combination with data collection of nitrogen deposition and climate reconstruction.The history of lake environment and ecological changes was reconstructed for the last three centuries in three alpine lakes,and the long-term patterns and drivers of diatom community and primary productivity changes were identified and compared across three lakes.Key results of this study are summarized as follow:(1)There are significant differences in the environmental evolution history of the three alpine lakes in the past 300 years.The water environment of Gaigong Cuona experienced three distinct stages.From about 1726 to 1953,the catchment erosion and lake hydrodynamic changes were relatively stable with limited variation in nutrient level,and the carbon burial was mainly derived from authochotonous input.From 1953 to 1988,basin erosion was intensified,lake nutrient level increased and terrigenous organic matter input increased.During 1988-2019,the water dynamics of the lake weakened,the nutrient level first decreased and then increased,and the input of organic matter from both inside and outside the lake was relatively strong.In the past 300 years,the external basin erosion and internal hydrodynamic changes of Wodi Co were stable,and the nutrient level of the lake showed an overall upward trend,and the nutrient level increased rapidly during 1956-1983.The carbon source of the lake was mainly affected by the internal input.In the past 300 years,the erosion of Bigu Tianchi has been weakened,and the nutrient level of the lake has been on the rise.The organic matter in the lake mainly comes from plankton and soil organic matter.(2)The diatom community analysis showed that the species association,succession pattern and diversity of the three alpine lakes were different.The plankton species(82%)dominated the diatom community in Gaigong Cuona,and Pantocsekiella accounted for the highest proportion in the plankton diatom community.The diatom community assemblages in Gaigong Cuona showed no obvious stage characteristics,but the diatom biodiversity changes showed significant stage characteristics.The diatom community of planktonic and benthic diatom about each half in Wodi Co,give priority to with small Pantocsekiella planktonic diatom,benthic diatoms in diatoms Staurosira construens and Staurosirella pinnata accounted for the highest,and the combination of diatom community change without obvious stage characteristics;Diatom biodiversity also did not have obvious stage characteristics,but there was a brief decline in diatom biodiversity from 1950 to 1970.The diatom community in Bigu Tianchi was dominated by benthic species(accounting for about 99%),and the diatom community association showed an obvious change.That is,after 1968,the relative abundance of diatom species Achnanthidium minutissimum increased while the relative abundance of diatom species Gomphonema parvulum and Encyonema minutum decreased.Since 1968,the biological diversity of diatoms in Bigu Tianchi has decreased significantly,and the relative abundance of Gomphonema bigutianchnensis,its endemic diatom species,has decreased significantly.(3)The sediment pigment index showed that the primary productivity(such as pigment index)of the three lakes showed a different change track.Among them,the primary productivity of Gaigong Cuona showed an overall increase,and showed a rapid increase after 1991,and the biomass of cyanobacteria increased significantly in the same period.Similarly,the primary productivity of Wodi Co showed an overall increase,with a rapid increase during 1938-1962.The biomass of cyanobacteria changed little in the early stage,increased obviously from 1930,then decreased,and then increased rapidly after 1998.However,the primary productivity and biomass of cyanobacteria decreased in the shallow lake Bigu Tianchi.(4)Multi-indicator analysis showed that the atmospheric sedimentation process continued to input nitrogen and other nutrients into alpine lakes,resulting in the increase of lake nutrient levels and changes in algae.Under the long-term background of nutrient increase,the primary productivity of the lake and the biomass of blue-green algae increased as a whole,and the structure of diatom community also changed.The increase of lake nutrient level led to the increase of relative abundance of eutrophic diatoms(such as S.construens)and the decrease of relative abundance of eutrophic diatoms(such as Pantocsekiella comensis)in Wodi Co.Similarly,the increase of nutrient level also promoted the abundance of the eutrophic diatom(such as A.minutissimum)in Bigu Tianchi,and became the dominant species with the largest proportion.(5)Regional warming promoted the increase of primary productivity and biomass of cyanobacteria in the deep water lakes(Gaigong Cuona and Wodi Co)by increasing water temperature,strengthening thermal stratification and lengthening growing season.In addition,regional warming also accelerated the degradation of pigment in the sediments of the shallow lake Bigu Tianchi.Regional warming will strengthen the thermal stratification of the deep water lake,and facilitate the dominance of small Pantocsekiella species diatoms in the lake.For shallow lake area,increasing temperature will increase lake water temperature,resulting in the decrease of diatom species which prefer low temperature,and the dominance of diatom species which are more tolerant to water temperature change.In addition,regional warming may shorten the glacial period of alpine lakes,resulting in a decrease in the proportion of small Fragilaria diatoms in the lake.The results suggest that lake morphology may result in different responses of algae to regional warming in alpine lakes.The results of this paper show that in the southeastern margin of the Tibetan Plateau,where regional warming and atmospheric sedimentation are significant,the algal response patterns of alpine lakes show heterogeneous characteristics related to lake types and basin characteristics.The diatom community in the deep water lake had no obvious stage characteristics,while the diatom community structure in the shallow water lake changed obviously after 1968.At the same time,there were significant differences in the environmental factors driving the diatom community changes in the three lakes.With the decrease of lake water depth,the driving effect of TN on the diatom community gradually increased.The diatom community in the shallow lake Bigu Tianchi was the most sensitive to environmental changes.The three alpine lakes have similar areas,and shallower water depths mean smaller lake capacity,less buffering of air temperature and diluting of nutrient inputs,and potentially more sensitive algae responses to environmental changes.The primary productivity of the three lakes also showed similar differences with that of the diatom community.The primary productivity of the deep-water lakes Gaigong Cuona and Wodi Co showed an overall upward trend,while the chlorophyll concentration of the Bigu Tianchi continued to decline due to the shallow water depth and stronger degradation of sediment pigment.At the same time,the environmental factors driving the primary productivity changes of the two deep water lakes(Gaigong Cuona and Wodi Co)are also different.For example,the changes of primary productivity in both lakes are affected by temperature and hydrology changes,but TN has a stronger driving effect on primary productivity in the shallower Wodi Co.Under the background of continuous regional warming and atmospheric subsidence,the environmental characteristics and algae distribution of alpine lakes show a long-term succession track on the centennial scale,and the response patterns of alpine lakes show great differences.Therefore,the environmental protection and ecological assessment of alpine lakes need to consider the process differences of response patterns of different types of lakes and their future changes. |
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