Indentification Of The Patterns Of Recent Environmental Changes And Algal Responses In Two Alpine Lakes Of Northwest Yunnan

Ecosystem health of shallow lakes has been subjected to the pressure derived from multiple stressors such as climate warming,anthropogenic disturbances and atmospheric nitrogen deposition over regional to global scales.However,lakes with contrasting type and intensity of catchment development can display distinct patterns of ecological responses in the context of global warming.Here.we selected two subtropical and alpine shallow lakes,northwest Yunnan,with contrasting impact derived from natutal forcing and anthropogenic disturbances,with Taiji Lake(3,912 m a.s.l.)in prestine conditions and Xihu Lake(1,968 m a.s.l.)impacted by intensive catchment development.Through combining multi-proxy sediment analyses with documentary data,we aim to track the trajectories of environment changes and algal responses,for identifying key environmental stressors and comparing their strength in driving the changes in primary productivity and diatom assemblages over the last two centuries.Finally,we evaluate the degree of heterogeneity in the long-term patterns of environmental drivers and biological responses between both lakes.The main conclusions were listed as follows.The sediment records and modern monitoring data together showed that Xihu Lake had experienced intensive anthropogenic impacts since the 1950 s and the major stressors included eutrophication(i.e.TN,?15N and Chl-a)and lake regulation(i.e.grain size proxies).Particularly,the terrestrial input of nutrients,as evidenced by our geochemical signals(i.e.C/N ratio and stable isotopes),corresponded well with the documented regulation of this lake.The archived data on catchment development further suggested that population and agriculture expansion,followed by increased cow density,has caused the long-term process of eutrophication.In contrast,Taiji Lake was an oligotrophic system predominantly impacted by climate change with little evidence of direct anthropogenic disturbances.Furthermore,hydrologic and physicochemical properties of Taiji Lake could be modulated by regional warming,through decreased snow cover,increased meltwater,and enhanced development of catchment soil and vegetation.While a moderate change in the ?15N signal of sediments suggested that atmospheric nitrogen deposition might not be a major factor in driving the nutrient change of Taiji Lake.The ecological status of both lakes has showed strong differences in the type of environmental drivers,timing,and magnitude of change.Primary production,as inferred by sediment pigments,has increased significantly from the 1960-1970 s in both lakes,but their driver was identified as nutrient enrichment in Xihu Lake and climate warming in Taiji Lake,respectively.There also existed strong differences in the pattern and timing of diatom community changes,with two shifts found in Xihu Lake but only one in Taiji Lake.Diatoms in Xihu Lake were dominated by both benthic taxa such as Cocconeis placentula and planktonic taxa(i.e.Cyclotella pseudostelligera),with little variation in community structure during the pre-1953 period.However,this was followed by an increase in the benthic genus of Fragilaria and the absence of Cocconeis placentula between the 1950 s and 1990 s.Thereafter,planktonic taxa(i.e.Cyclostephanos dubius)and benthic diatoms such as Navicula cryptotenella increased clearly,with the percentage of benthic diatoms decreasing sharply from ~ 90% to < 50%.In Taiji Lake,the diatom assemblage before the 1930 s was dominated by planktonic diatoms such as Aulacoseira alpigena,which slightly increased during two cold periods(i.e.,~1800-1850 s and 1900-1930s).Since the 1930 s,there was a clear decrease of Aulacoseira alpigena with a concurrent and continued increase in the percentage of benthic diatoms,which moderately fluctuated during the cold period between 1960 s and 1980 s.Xihu Lake serves as a national wetland park due to its abundant macrophyte coverage.To uncover the relationship between dominant aquatic plants and epiphytic diatoms,we found that different epiphytic diatoms had selective attachment to living macrophytes.Cocconeis placentula was most abundant on broadleaf plants such as Vallisneria natans,while But small Fragilaria species tended to attach to some thinleaf plants such as Ceratophyllum demersum.These results served as the evidence that the significant shift between dominant benthic diatoms in the 1950 s was probably attributed to the shift of dominant macrophytes.Therefore,the history of macrophyte changes could be reconstructed through examining the epiphytic diatom communities in shallow lakes.We further conducted the analysis of variance partitioning and the selection of minimum adequate models,aiming to identify key stressors and to quantify their strength in driving diatom assemblages for both lakes.The results showed that eutrophication was the predominant driver of diatom changes in Xihu Lake,followed by climate warming and hydrological fluctuation(as inferred by the sand proxy)to a lesser degree.In contrast,climate warming was the only signifiant driver and indepedently explained 23.8% of the total variance of diatom changes in Taiji Lake,which was higher than that in Xihu Lake(17.9%).Benthic diatoms such as Fragilaria exigua showed an increase corresponding to the long-term warming in Taiji,which likely reflected a longer ice-free season and increased benthic habitats.In addition,the decrease in water level and reduced water mixing could be induced by lake warming,cuasing the decrease of turbulence-tolerant planktonic taxa such as Aulacoseria alpigena.In the context of climate warming,eutrophication and increased water level could promote the growth of planktonic taxa(i.e.Cyclotella pseudostelligera)and eutrophic species such as Cyclostephanos dubius,leadign to an obvious decrease of benthic diatoms over the past few decades.Over the past two centuries,diatom assemblages in Taiji lake showed a clear shift from planktonic taxa to benthic diatoms,but to a less degree than that found in Xihu Lake,which had been heavily influenced by recent anthropogenic disturbances.For example,Xihu Lake experienced input of sewage wastes,shrinking surface area andintensive water regulation,in the conext of regional warming.All of these stressors have led to serious ecological consequences over time,including several shifts of diatom community,presence and dominant of new diatom species,and the exitinction of once dominant speices such as Cocconeis placentula.As a result,ecological stability and ecosystem resilience have decreased with incresing human impacts.Therefore,the type and strength of catchment development should be evaluated for effective protection and ecological restoration of impacted small and medium-sized alpine lakes in the context of climate warming.