The Changes Of Phytoplankton Community Structure And Water Quality Assessment In Mingzhu Lake,Chongming Island,Shanghai

Located in the town of Lvhua in the west of Chongming Island,Mingzhu Lake is the largest natural freshwater lake on Chongming Island.It is a tributary of the Yangtze River basin.It is a lagoon built by artificial embankments in the 1970s.There are two intake gates now.Mingzhu Lake belongs to the lake connecting the Yangtze River.After the Mingzhu Lake developed into a tourist scenic spot,it has certain pollution to the water body.Many researchers conducted a lot of ecological research work on Mingzhu Lake in 2007-2009,and proposed corresponding measures for the restoration of Mingzhu Lake water body.At present,there are few reports on the phytoplankton community structure and water quality evaluation in Mingzhu Lake,and few researchers have carried out research on the ecological management of Mingzhu Lake.From March 2017 to February 2018,this paper studied the relationship between phytoplankton community structure,species diversity and environmental factors in the Mingzhu Lake,and compared with the data of ten years ago and the oligotrophic water body-Backhore Wetland of Expo Garden,Shanghai.Provides basic information and reference systems for assessing lake restoration and treatment effects,ecotourism and ecological island construction.At the same time,it provides basic data for the in-depth study of plankton ecology in Chongming Island and the entire Shanghai freshwater ecosystem.1.From March 2017 to February 2018,a total of 187 species,87 genera and 8phyla(excluding undetermined species)were identified in Mingzhu Lake.Among them,Cyanophyta(74 species)had the most species,followed by Bacillariophyta(51species)and Chlorophyta(36 species).The species composition was dominated by Cyanophyta,Bacillariophyta and Chlorophyta.The number of species of Mingzhu Lake showed seasonal changes:the number of species in summer was the most,followed by autumn,and the number of species in spring was the least.2.A total of 27 dominant species were identified in Mingzhu Lake,including 14species of Cyanophyta,2 species of Bacillariophyta,7 species of Chlorophyta,2species of Cryptophyta and 2 species of Pyrrophyta.The main dominant species include Merismopedia punctata,Microcystis incerta,Synedra acus,Chlorella vulgaris and Scenedesmus quadricauda.3.The annual mean values of phytoplankton density and biomass in Mingzhu Lake were 3259.13±947.50×10~4cells/L,6.61±0.94 mg/L,respectively.The annual changes of density and biomass were unimodal,and the peak of density and biomass appeared in August.The density of Cyanophyta was the highest,followed by Chlorophyta and Bacillariophyta.The biomass of Cyanophyta was the highest,followed by Bacillariophyta and Chlorophyta.4.The annual mean values of Mingzhu Lake diversity indexes H,D and J were2.04±0.25,2.10±0.37 and 0.59±0.07,respectively.Diversity index H and D both showed that summer and autumn seasons were higher than winter and spring seasons,while diversity index J show that winter and spring seasons were higher than summer and autumn seasons.5.From March 2017 to February 2018,the phytoplankton density and biomass of Mingzhu Lake were positively correlated with water temperature and negatively with with transparency.Density was negatively correlated with dissolved oxygen and total nitrogen,and positively correlated with total phosphorus and permanganate index.Biomass was positively correlated with pH and total phosphorus,and negatively with dissolved oxygen and total nitrogen.RDA plot analysis showed that water temperature,total nitrogen,total phosphorus and permanganate index were the main factors affecting phytoplankton community structure.6.Compared with 2007,the data of the survey of Mingzhu Lake in 2017-2018showed that the phytoplankton species increased by 24 genera and 67 species,the density and biomass decreased,and the diversity index tended to be stable.Water temperature,total nitrogen and total phosphorus were still important environmental factors affecting the phytoplankton community structure.The water quality of Mingzhu Lake is in the middle nutrition-mild eutrophication level,belonging to II-III water,which is better than the water quality in 2007,but worse than the Backhore Wetland of Expo Garden,Shanghai.