Spatial And Temporal Characteristics And Drivers Of Phytoplankton Community Structure During Ice-covered Period In Xingkai Lake

Phytoplankton is an important primary producer in the lake ecosystem,playing an extremely important role in maintaining the energy conversion and material cycle in lakes.Moreover,phytoplankton community structure can reflect water quality in lakes,leading to an important ecological significance to study the phytoplankton community structure and its succession process for the lake ecosystem.Especially with the intensification of eutrophication in lakes,the outbreak of algae blooms has become a huge threat affecting the ecological health and safety of lakes.Studying the structure characteristics of phytoplankton community during the ice-covered period is of great practical significance for preventing algae outbreaks in spring and reducing the potential threat of summer blooms,due to the seasonal cascade characteristics of phytoplankton community succession.As a seasonal frozen lake in middle-high latitude,Xingkai Lake,the China-Russia border lake,consists of Big Xingkai Lake and Small Xingkai Lake,in which phytoplankton blooms during the ice-covered period have been reported.We took Xingkai Lake as the ideal study area to study the species composition,biodiversity and relationships between phytoplankton community and environmental factors at different stages of the ice-covered period in 2021 and 2022.The investigation during the ice-covered period of Xingkai Lake We found that there were 165 species and varieties belonging to 7 phylums,including Cyanophyta,Chlorophyta,Bacillariophyta,Pyrrophyta,Cryptophyta,Euglenophyta and Chrysophyta,of which the largest number of Chlorophyta species was 73,following by 34Bacillariophyta and 26 Cyanophyta.There are 22 species of Euglenophyta,4 species of Chrysophyta,5 species of Pyrrophyta,and only 1 species of Cryptophyta,.There were83,53,and 97 species in the freezing,frozen,and thawing periods,respectively.A total of 144 species of phytoplankton were found in Small Xingkai Lake,and 146species of phytoplankton were found in Big Xingkai Lake.The phytoplankton in Xingkai Lake during the ice-covered period is mainly composed of Chlorophyta-Bacillariophyta-Cyanophyta.Phytoplankton abundance differed significantly at different stages during the ice-covered period（p<0.05）,while the difference between different sampling sites was not significant（p>0.05）.The average abundance of phytoplankton increased along frozen period（11.95×10⁴ind./L）,freezing period（28.55×10⁴ind./L）,and thawing period（49.98×10⁴ind./L）.At all stages of the ice-covered period,the phytoplankton abundance in Big Xingkai Lake was higher than that in Small Xingkai Lake.The succession rule of the relative abundance of the dominant species of phytoplankton was that Cyanophyta-Chlorophyta during the freezing period;Chlorophyta-Bacillariophyta during the frozen period,Cyanophyta-Chlorophyta during the thawing stage.Phytoplankton with a dominance degree（Y）greater than 0.02 was identified as the dominant species,and a total of 16 dominant species were identified.Of which the dominant species during the frozen period were up to 9 species,and 5 species in either the freezing or thawing period.Microcystic aeruginosa（Y=0.253）,Melosira sp.（Y=0.156）,and Merismopedia minima（Y=0.089）during the freezing,frozen,and thawing periods,respectively.The indexes of Shannon-Wiener diversity,Pielou uniformity and Margalef richness showed significant temporal differences at different stages of the ice-covered period（p<0.05）,but there was no significant spatial difference between sampling sites（p>0.05）.Phytoplankton community increased along the freeze-thaw process for the Shannon-Wiener diversity and Pielou uniformity indexes,but decreased during the frezon period and incresed during the thawing period for Margalef richness index The biodiversity indexes varied greatly at different stages,indicating that the composition of phytoplankton communities was unstable during the ice-covered period,and phytoplankton were still active under ice.Result of eukaryotic phytoplankton high-throughput sequencing showed that the Simpson index first decreased during the frozen period and then increased during the thawing period.Eukaryotic phytoplankton diversity was dominated by diatoms during the freezing and frozen periods,and then decreased with the rapid recovery of cyanobacteria,leading to a decreasing diversity of eukaryotic phytoplankton during the thawing period,which reflected the active succession of phytoplankton communities during the ice-covered period.The spatial interpolation results showed that the phytoplankton diversity index in undisturbed areas was significantly higher than that in areas affected by human activities,and the phytoplankton biodiversity in the eastern part was higher than that in the western part in Xingkai Lake.The water quality of Xingkai Lake during the ice-covered period was assessed according to the physical and chemical variables and biological indicators in Xingkai Lake with the methods of single-factor index,the modified nutritional status index（TSI_M）,and the phytoplankton diversity index.The single-factor evaluation method showed that the water quality level during the ice-covered period was class IV～V in Xingkai Lake.The results of TSI_Mshowed that Xingkai Lake was in a state of mild to moderate eutrophication during the ice-covered period.The evaluation of phytoplankton diversity index showed that water quality during the freezing and frozen periods were light tomedium pollution,and there was no pollution or light pollution during the thawing period.In general,the three evaluation indices indicated that the water quality in Xingkai Lake gradually improved from the freezing period to the thawing period.Moreover,TSI_Mis better to indicate the water quality in Xingkai Lake during the ice-covered period,indicating that Xingkai Lake was in a moderate eutrophication state during the ice-covered period.Combining Pearson and RDA analyses,the main drivers of phytoplankton communities in Xingkai Lake during the ice-covered period were explored.The results showed that the diversity of phytoplankton community structure was mainly affected by Fe³⁺,temperature（T）,total nitrogen（TN）,total phosphorus（TP）and dissolved oxygen（DO）.Phytoplankton communities were mainly affected by Fe³⁺during the freezing period,by T,TN and TP during the frozen period,and by DO and T during the thawing period.The factors controlling phytoplankton abundance during the ice-covered period were Fe³⁺,TP,electrical conductivity（EC）,ammonia nitrogen（NH₄-N）,T and Chl-a.Fe³⁺had a great effect on phytoplankton abundance during the freezing period.TP,EC,NH₄-N and T mainly impacted phytoplankton abundance during the frozen period.Meanwhile,Fe³⁺,T and Chl-a were the main factors influencing phytoplankton abundance during the thawing period.This study is based on the change of lake state during the ice-covered period of Xingkai Lake from 2021 to 2022,to elucidate the changing characteristics of physicochemical factors and water ecological health in the water column during the ice-covered period,and to explore the changing characteristics of phytoplankton community structure and important environmental factors in different period of Xingkai Lake during the ice-covered period for the first time,so as to provide valuable reference materials for the comprehensive management of Xingkai Lake.It is of great significance to the management of eutrophication and the prevention and control of water wars.