Study On The Variation Characteristics Of Diatoms And The Driving Factors Of Eutrophication In Xian?nv Lake Over The Past 60 Years

As a national 4A scenic area and the main source of drinking water in Xinyu City,Xian?nv Lake is faced with eutrophication and degradation of water quality,which will threaten the normal water supply of residents.Xian'nv Lake was selected for research objects in this paper.In order to reconstruct the eutrophication process of Xian?nv Lake in the past 60 years,the key driving factors and response patterns of diatom community changes were revealed.The sedimentary nutritional indicators?i.e.,total organic carbon?TOC?,total nitrogen?TN?,total phosphorus?TP?,C/N?TOC/TN??and biological indicators?i.e.,sedimentary diatoms and modern diatoms?planktonic diatoms and benthic diatoms??of S1 in the sedimentary core of Xian?nv Lake were analyzed by palaeomagnology,combined with the sedimentary dept-chronological time series framework established by ²¹⁰Pb and ¹³⁷Cs dating method and watershed historical data,Then regression analysis,principal component analysis?PCA?,redundancy analysis?RDA?and other analysis methods were used to reconstruct the eutrophication process of Xian?nv Lake in recent 60 years,and to reveal the response of sedimentary diatoms to eutrophication,as well as the key driving factors and response patterns of diatom community changes.The main results are as follows:?1?According to the calculation of ²¹⁰Pb and ¹³⁷Cs dating CRS method,the total length of the sedimentary column sample of 37cm represented the deposition information of 155 years.The full text only analyzed the deposition samples with an effective depth from 0 to 23cm,corresponding to the period of 1955-2018.?2?142 species?including varieties?of diatoms belonging to 2 classes,5 orders,10families,24 generawere identified in the analysis of 23 samples of S1 core,9 samples of benthic diatoms and 12 samples of planktonic diatoms.The dominant species include Aulacoseira alpigena,Aulacoseira granulate,Cyclostephanos duius,Cyclotella meneghiniana,Cyclotella bodanica Grunow,Cyclotella radiosa,Cocconeis placentula?Fragilaria ulna?Achnanthidium minutissimum.The assemblage of columnar sedimentary diatoms can well represent and reflect the environment of the whole lake area.?3?The sediment S1 core was divided into four diatom assemblage zones from bottom to top by constrained cluster analysis?CONISS?combined with diatom abundance.Zone I?23-20cm:1955-1961AD?was characterized by the dominance of benthic diatoms.But Aulacoseira granulata,a planktonic diatom species,was dominant,and the overall low nutrient level.It represents the typical river type of rivers before the construction of the reservoir.Zone II?20-15cm:1961-1983AD?was characterized by the total content of planktonic diatoms increased significantly,but it was still dominated by benthic diatoms.The eutrophic species?e.g.Cyclostephaus duius and Cyclotella meneghiniana?were gradually increased,and the overall nutrient level increased.This period represents represented the evolution of river trunks to the characteristics of typical lakes at the beginning of the construction of the reservoir.Zone III?15-5cm:1983-2009AD?was characterized by the advantages of planktonic diatoms gradually become more prominent.The eutrophic species?e.g.Cyclostephaus duius,Cyclotella meneghiniana,and Achnanthes minutissina?showed an overall growth trend,and reached the maximum,and had a typical lake features.Zone IV?5-0cm:2009-2018AD?was characterized by number of diatom species increased,and the number of nutrient-tolerant species?e.g.Cyclostephaus duius,Cyclotella meneghiniana,and Achnanthes minutissina?decreased as a whole.Compared with combination III,the overall nutrient level decreased and tended to be stable.Diatom community in Xian?nv Lake changed from benthic diatom to planktonic diatom,and the eutrophic species was the most abundant from 1983 to 2009?Zone III?.?4?According to the analysis of the combination characteristics of diatom assemblage,ecological habits of dominant species,TDI values and nutrient indexes,the nutrient level of Xian?nv Lake continued to rise after the construction of the reservoir,and reached the highest level from 1983 to 2009.After that,the nutrient level decreased somewhat,but it could not be ignored.?5?Sediment TP,TN and temperature were the three significant variables affecting diatomite assemblage succession,respectively accounting for 11%,6.3%and 5.3%of diatomite assemblage variance.Eutrophication of Xian?nv Lake and climate change have had an impact on diatom species diversity and the driving patterns were similar,showing a single peak pattern.