Phytoplankton Community Characteristics And Risk Assessment Of Urban Lakes In Cold Regions

Nanhai Lake located in the lnner Mongolia-Xinjiang Plateauarea and in the cold zone,which has a unique geographical location,climatic characteristics and hydrological conditions,and plays an ecological balancing role for the wetland ecosystem in the Baotou section of the Yellow River Basin.In this paper,based on the data of phytoplankton community structural characteristics and environmental factors obtained by continuous sampling in Nanhai Lake from November 2019 to October 2020,further research is conducted on the ecological characteristics of phytoplankton community and its relationship with environmental factors,phytoplankton diversity and stability,and a risk evaluation system based on the structural characteristics of phytoplankton communities and environmental factors to better propose response strategies to improve water quality in Nanhai Lake.The main conclusions are as follows:(1)A total of 182 species of phytoplankton in 7 phyla and 84 genera were detected in Nanhai Lake,the community structure was mainly composed of Chlorophyta,Cyanophyta and Bacillariophyta.The average abundance of phytoplankton was 56.39×10⁶cells/L and the average biomass was 15.46 mg/L,both of which basically showed a trend of decreasing radially from the center of the lake to the surrounding area in all seasons.Among them,the dominant species such as Synedra affinis,Aphanizomenon flosaquae,Scenedesmus quadricauda,Fragilaria crotomensis,and Diatoma vulgare,and there is seasonal succession.Through redundancy analysis(RDA),WT?p H?TN and COD_crwere found to be the main factors affecting the abundance and biomass of dominant phytoplankton species.(2)The results of the two-way ANOVA showed that,the Margalef index is seasonally dominated,while the Shannon-Wiener and Peilou indices are influenced by both seasonal and regional changes.When studying the relationship with temporal stability,it was found that as the Margalef and Shannon-Wiener indices of phytoplankton increased,the TSI values of phytoplankton showed a linear increasing trend with both of them,but the former showed a larger variation and the latter a relatively flat one,while the TSI values showed a concave curve relationship with the Pielou index.(3)The temporal and spatial distribution of the phytoplankton temporal stability index is significantly different,seasonal fluctuations are characterized by the highest in summer and the lowest in spring for all seasons,and spatial fluctuations are characterized by the core area of the lake being higher than the surrounding water bodies.When studying the relationship between phytoplankton species diversity and temporal stability,it was found that the TSI values of phytoplankton communities,Chlorophyta and Bacillariophyta all showed a linear relationship with the number of their species,while the TSI values of Cyanophyta showed a weaker single-peaked curve relationship with the number of their species.This is due to the fact that the stability of each phylum,Chlorophyta and Cyanophyta,is more determined by the"overproduction effect"of their diversity,and the stability of the phytoplankton community is more influenced by biomass fluctuations than the former.However,the stability of the Bacillariophyta has no obvious correlation with the total biomass and its fluctuations.While WT,DO,Chl.a and COD_craffect the total biomass of phytoplankton,which indirectly causes the stability of the phytoplankton community change.(4)Trophic level index shows that the water bodies in all areas of Nanhai Lake are in heavy trophic state,showing a trend of high summer and low spring.Based on the principles of correlation analysis and the selection of assessment indexes,the following indexes were selected for the risk assessment system of Nanhai Lake:water transparency(SD),dissolved oxygen(DO),total nitrogen(TN),total phosphorus(TP),number of phytoplankton species(PHY-S),phytoplankton abundance(PHY-FD),Pielou index(Pielou index-J)and Shannon-Wiener index(Shannon-Wiener index-H).The results showed that the water bodies in the four areas of the lake were in the III-IV risk stage,and the water bodies were in the moderate to poor condition.The findings of this paper are important for understanding the structural characteristics,diversity and stability of phytoplankton communities in cold urban lakes,and play an important role in maintaining the health of water ecosystems and suppressing eutrophication in lakes.