Temperature And Lake Ecosystem Evolution History Recorded By Algae DNA From Lake Nanyi During The Holocene And Their Driving Factors

Climate change and lake ecosystem transformation are closely related to human human living conditions and social development,and they have become one of the hot issues in academia and the public.Since the late 1980s,Chinese scholars have used a variety of geological carriers and proxies to reconstruct numerous lake and climate environment evolution sequences since the Holocene.However,previous paleoclimate studies are mainly concentrated in North China,and there are very few records in South China,especially in the middle and lower reaches of the Yangtze River,and it is difficult to meet the requirements for regional comparison.In addition,the life cycle of algae is very short with the result that the biomass and community composition and succession is very sensitive to changes in the external environment.So,it is an intuitive manifestation of lake environmental changes and can be used as lake biological monitoring and an important indicator of climate environmental changes.However,the traditional methods for identification and classification of algae are mainly based on morphology.Moreover,it's difficult for some closely related species to show differences in external morphology,and many plankton microorganisms die without leaving any fossil diagnostic features,which are excluded from traditional micropaleontology observations.Fortunately,researches have indicated that the ancient DNA of algae in lake sediments can overcome the difficulties of long-term monitoring data and make up for the lack of morphological identification.This provides an additional perspective to the study of changes in the structure of the algae community in the past,and can reveal the succession of the algae community and biodiversity changes in the past in more detail.In summary,we chose Lake Nanyi,located in the middle and lower reaches of the Yangtze River,as the research object,and extracted the total DNA from the lake core(NYH1),and then combined with AMS¹⁴C dating and inorganic chemical elements to reconstruct the evolution history of the algae community structure in Lake Nanyi since the Holocene.In addition,based on the ecological habits of the dominant algae in Lake Nanyi,we reconstructed the temperature change trend in the area since the Holocene,and briefly analyzed its possible driving factors and their relationship with the East Asian summer monsoon changes.We also discussed the ecological changes of Lake Nanyi since the Holocene,especially the changes in algae diversity and its driving factors.Based on the above related work,this research found that:1.The algae in the sediments of Lake Nanyi are well preserved.A total of 50genera were detected,belonging to 5 phyla and 13 classes.The algae assemblages in Lake Nanyi are dominated by chlorobacteria and cyanobacteria,and the dominant genera mainly include Lobosphaera,Coelastrella and Synechococcus.2.The algae-based reconstructed summer temperature change trend(W/C)from Lake Nanyi shows that the temperature change in this area since 13.0 cal ka BP can be roughly divided into three stages:Although the temperature in Lake Nanyi was relatively low at 13.0-8.0 cal ka BP,but it still showed a fluctuating rising trend;The temperature reached its maximum during the mid-Holocene from 8.0-3.0 cal ka BP;The temperature at 3.0-1.0 cal ka BP has decreased,which was lower than that of the Middle Holocene.In addition,we compared the algae-based reconstructed temperature of Lake Nanyi with other paleotemperature records and found that its change trend is roughly the same,showing a pattern of consistent temperature changes in north and south China since the Holocene.At the same time,the feasibility and reliability of algal ancient DNA as temperature proxy in palaeoclimatology were further confirmed.Importantly,we think that the northern hemisphere summer solar radiation played a significant role in driving the temperature change pattern in Lake Nanyi,but glacier,greenhouse gas concentration changes and vegetation changes caused by precipitation may also have contributed to the temperature change pattern.Notably,many studies have shown that the enhanced East Asian summer monsoon in the Mid-Holocene will cause the rain belt to move north China and reduce precipitation in the middle and lower reaches of the Yangtze River.In addition,at this time,Lake Nanyi had the highest temperature during the Holocene,and the evaporation was vigorous,which intensified the arid climate in the middle and lower reaches of the Yangtze River during the Mid-Holocene,and further contributed to the precipitation mode of"Northern Flood-Southern Drought".3.The Simpson,Shannon,Richness and Pielou indices calculated based on the algae data in Lake Nanyi showed that the algae diversity was the most abundant and the lake ecosystem was the most stable at 8.5-5.5 cal ka BP.In addition,the change of algae diversity may be mainly controlled by climate-driven lake nutrient changes.The specific performance is as follows:In the early Holocene,there was there was a large amount of exogenous clastic inputs and nutrients into the lake which was response to the high precipitations,and the nutrition in Lake Nanyi can satisfy most of the algae growth and reproduction,which leads to an increase in algae diversity at the same time.However,the decrease of precipitation in the Mid-Holocene resulted in the decrease of exogenous clastic inputs and nutrient flowing into the lake,which in turn led to a decline of algae biodiversity,but it still maintained a high level at 8.5-5.5 cal ka BP.Since the late Holocene,the precipitation and lake nutrients increased again,and the algae diversity also showed an increasing trend.