Diatom records in sediments for eutrophication process of Lake Xian'nv, China since the mid-20th century

To well understand the eutrophication process for inland lakes where more lands had been used for agriculture and industry without adequate environmental protections, 9 surface sediments and 1 core sediment in Lake Xian’nv which was located in the middle of Jiangxi Province were chosen. Combined with 210Pb and 137Cs geochronology, nutrient index including total nitrogen (TN), total phosphorus (TP), total organic carbon (TOC), Carbon nitrogen ratio (C/N), diatoms assembles and TDI index of eutrophic diatoms were compared quantitatively and conducted to elucidate the eutrophication process using statistical methods. The results showed a total of 155 years from 1863 to 2018 were recorded from the entire sediment core. The nutrient level in 23 centimeters below the surface of the sediment core where was synchronized with reservoir construction indicated TN, TP, TOC, and Carbon nitrogen ratio had been gradually increased since the construction of Dam. In surface sediments, the three diatoms with higher average relative abundance were Achnanthidium minutissimum, Navicula Bory, and Aulacoseira granulata with a value of 54.2 %, 29.4 %, and 15.3 % respectively. Significant changes of the diatom communities from 1955 to 2009 had been recorded, where diatom communities could be classified into four distinct periods. In the first few years after the reservoir was built, the dominant diatoms were epiphytic and benthic diatom communities represented by Navicula cincta、Fragilaria ulna, and Cocconeis placentula in zone I on the bottom floor covered from 1955 to 1961. However, planktonic diatom communities were dominant after 1983 which were mainly constituted by Aulacoseira alpigena、Cyclostephanos dubius、Cyclotella meneghiniana, and Cyclotella radiosa . Both TDI index of eutrophic diatoms and Principal Component Analysis (PCA) revealed the succession from benthic diatoms to planktonic diatoms and changes in nutrient levels.