Study On The Succession Of Dominant Species Of Phytoplankton In Dianshan Lake And Its Influencing Factors

Dianshan Lake is the largest natural freshwater lake in Shanghai,it is also an important water source and ecological reserve in Shanghai.Nowadays,the water quality of Dianshan Lake is not optimistic,the algae of eutrophication is increasing,and the water environment security is seriously threatened.From January 2016 to December 2017,this study conducted a comprehensive survey of phytoplankton in Dianshan Lake,twelve sampling sites were set up in Dianshan Lake and samples were taken in the middle of each month for a total of 24 samples.Combined with the results of this laboratory's investigation since 2014,the composition of phytoplankton,the amount of biomass,the temporal and spatial changes of dominant species in Dianshan Lake were studied.At the same time,the key factors affecting the evolution of phytoplankton dominant population were studied by analyzing the factors such as environmental factors,meteorology and hydrology,which provided a theoretical basis and data support for phytoplankton related research,water monitoring and water quality assessment in Dianshan Lake.The main results are as follows:1.From January 2014 to December 2017,154 species of phytoplankton were identified,belonging to 7 phylum 93 genera.The species of Chlorophyta was the largest,with 41 genera of 81 species,accounting for 53% of the total;the second were Bacillariophyta with 22 genera 30 species,accounting for 18% of the total;Cyanophyta were found 15 genera 21 species,accounting for 14% of the total;Euglenophyta were found 5 genera and 9 species,accounting for 6% of the total;rest of the identified species include: Chrysophyta were found 5 genera and 6 species;Dinophyta were found 3 genera 4 species;Cryptophyta were found 2 genera and 3species.2.The annual average biomass of phytoplankton varies greatly: the highest amount of biomass in 2015 was 1886 ten thousand ind/L;the lowest amount of biomass in 2017 was 963 ten thousand ind/L.The number of phytoplankton biomass was the highest in summer and the proportion of Cyanophyta was the largest,but the composition of dominant species of Cyanophyta was different in different years.The main advantage species in the summer of 2014-2015 is Merismopedia minima,The main dominant species in the summer of 2016-2017 is Microcystis spp..3.Dianshan Lake phytoplankton seasonal succession of main dominantpopulation change is as followed: Bacillariophyta-Chlorophyta type in spring,Cyanobacteria leading in summer,Cyanobacteria-Bacillariophyta type in autumn,Cryptophyta-Bacillariophyta type in winter.The main dominant species in spring are Chroomonas acuta,Skeletonema potamos and Stephanodiscus hantzschii f.tenuis;the main dominant species are Microcystis spp.and Merismopedia minima in summer;the main dominant species in autumn are Chroomonas acuta and Skeletonema potamos;the main dominant species are Chroomonas acuta and Stephanodiscus hantzschii f.tenuis.4.The main factors affecting the change of dominance of phytoplankton are as follows: The key factors influencing the succession of Microcystis spp.and Merismopedia minima are temperature,TP,rainfall,light and wind speed.Water temperature is more positively correlated with Microcystis spp.,while TP is more positively correlated with Merismopedia minima.Sustained high temperature and sufficient light are conducive to the large-scale proliferation of Microcystis spp.;low rainfall,low wind speed(grade 1-2)is conducive to the formation of blooms.There was a significant positive correlation between COD and the temporal variation of Skeletonema potamos,Stephanodiscus hantzschii f.tenuis and Chroomonas acuta appears in every month,the correlation between they and environmental factors is not obvious.5.The dominant species of phytoplankton in Dianshan Lake have obvious seasonal succession are mainly influenced by temperature in terms of time.The spatial distribution is mainly affected by nutrients,mainly in the intake area.The upstream water has a great influence on the formation of nutrients and dominant species in Dianshan Lake.The concentration of nutrients in Dianshan Lake is significantly higher than that of eutrophication,and it already has the material basis for outbreaks of blooms.The longest hydraulic retention time is summer,and once weather conditions are suitable,Dianshan Lake can easily bloom.The hydrometeorological factors are the important contents of the forecasting and early warning of the water bloom outbreak of Dianshan Lake and the establishment of risk emergency system,at the same time,eutrophication treatment should also be conducted to reduce the input of nutrients.