Phytoplankton Community Structure And Its Influencing Factors In Changhu Lake

Changhu Lake is an important wetland ecosystem that regulates the ecological balance in the middle reaches of the Yangtze River.To understand the dynamics of phytoplankton communities in Changhu Lake and further explore the relationship between phytoplankton communities and environmental factors.This study conducted a 2-year continuous monthly survey of Changhu Lake from May 2020 to April 2022,analyzing the spatiotemporal changes of environmental factors in Changhu Lake.Based on the species composition,abundance,biomass,dominant species,niche,and diversity index of the phytoplankton community,the structure of the phytoplankton community in Changhu Lake was discussed.We aimed to provide the theoretical basis for the protection and management of water resources in Changhu Lake by revealing the dynamics of the cyanobacteria population in Changhu Lake,assessing the risk of cyanobacterial blooms in Changhu Lake,combining physical and chemical indicators with biological indicators,evaluating the nutritional status of the water body in Changhu Lake,and discussing the relationship between phytoplankton and environmental factors.The main research results are as follows:1.Spatiotemporal distribution characteristics of phytoplankton community structure in Changhu LakeDuring the investigation period,a total of 119 phytoplankton species(genera)from7 phyla were identified in Changhu Lake,with the dominant group of Chlorophyta(47.1%),followed by Bacillariophyta(18.5%)and Cyanophyta(16.0%).From the perspective of time variation,the variety range of phytoplankton species is 28 to 53species(genera),with lower values occurring mostly in winter and higher values occurring mostly from late spring to mid-autumn.From the perspective of spatial change,the number of phytoplankton species has a downward trend from east to west,with a varied range of 38 to 48 species(genera).Among them,the number of species in the Yuanboyuan lake zone is the highest,while the number of species in the Longkouzhai and Xikou lake zone is the lowest.From the perspective of time variation,the variation range of phytoplankton abundance in Changhu Lake is 4.33×10~6～65.24×10~6 ind./L,and the abundance of phytoplankton from late summer to early autumn in 2021 is much higher than that in other periods.In terms of abundance composition,Cyanophyta(63.92%)and Chlorophyta(18.83%)accounted for a relatively high proportion from the end of spring to the beginning of autumn in each year,while Cryptophyta(41.70%)and Chlorophyta(34.70%)accounted for a relatively high proportion in winter and early spring.From the perspective of spatial variation,there are significant differences in the abundance and composition of phytoplankton in various lake zones,with an overall"M"shaped distribution of abundance from east to west.Among them,the abundance of Chlorophyta in the Yuanboyuan and Songjiachong lake zone is the highest,while the abundance of Chlorophyta in the Chenditou,Longkouzhai,and Xikou lake zone is the highest.From the perspective of time variation,the variation range of phytoplankton biomass in Changhu Lake is 7.11～50.45 mg/L,with a trough in November 2020 and a peak in September 2021.In terms of biomass composition,the average biomass proportion of Bacillariophyta is the highest(35.96%),followed by Cryptophyta(19.90%)and Chlorophyta(17.57%).From the perspective of spatial variation,the biomass of phytoplankton generally shows a downward trend from east to west,with a varied range of 34.85 to 76.51 mg/L.The biomass of Bacillariophyta in each lake zone is relatively high,with an average proportion of 37.04%,followed by Cryptophyta and Chlorophyta,with an average proportion of 28.33%and 13.09%,respectively.The abundance and biomass of Chlorophyta populations fluctuate greatly with seasonal changes,both of which are lower from late autumn to mid-spring(the abundance variation range is 0.18×106～0.69×10~6 ind./L,biomass is 0.04～0.44 mg/L),reaching a peak in September 2021(abundance is 49.80×10~6 ind./L,biomass is 16.34mg/L).Microcystis wesenbergii and Chromococcus sp.were dominant in the abundance of cyanobacteria,with an average percentage of 71.32%and 15.01%,respectively.The biomass of Anabaenopsis circularis and Oscillatoria sp.is high,with an average proportion of 32.32%and 28.34%,respectively.There are 10 dominant species of phytoplankton in Changhu Lake,among which the dominance of Microcystis wesenbergii,Cryptomonas sp.,and Chlorella sp.are high,with 0.29,0.10,and 0.09,respectively.The niche widths of Chlorella sp.and Scenedesmus bijuga are high,with 18.69 and 17.76,respectively.There are different degrees of niche overlap between the dominant species(0.05～1.32),with the dominant species of Chlorophyta and Cyanophyta having a higher degree of niche overlap.The phytoplankton diversity in Changhu Lake fluctuates significantly over time,with the Shannon index ranging from 1.86 to 3.12,the Margalef index ranging from1.97 to 3.45,and the Pielou index ranging from 0.50 to 0.82.The highest values of the Shannon index and the Margalef index both appeared in May 2021,and the lowest values both appeared in June 2020.In terms of spatial variation,the Shannon index and Pielou index of Changhu Lake generally show a wavelike trend from east to west,while the Margalef index generally shows a downward trend.Among them,the diversity index value of the Yuanboyuan lake zone is high,while the diversity index value of the Xikou lake zone is low.2.Change characteristics of environmental factors in Changhu LakeDuring the survey period,environmental factors in Changhu Lake mostly fluctuated with seasonal changes.The rainfall in Changhu Lake varies from 4.6 to 489.6mm,with the lowest rainfall in winter.In July 2020,the rainfall in Changhu Lake was the most abnormal,higher than the historical monthly average rainfall in the past 40years by 300.2 mm.The water level varies from 30.02 m to 32.88 m,with high water levels occurring mostly in summer.The most significant water level fluctuations were occurred in July and August 2020,with water levels of 2.18 m and-1.39 m,respectively.The variation range of water temperature is 4.36～30.84℃,with the highest water temperature in summer and the lowest in winter.The variation range of dissolved oxygen content is 3.98～10.96 mg/L,with the low dissolved oxygen state of water bodies mostly occurs in summer.The variation range of total nitrogen content is0.53～6.90 mg/L,showing an upward trend from late summer to mid-winter in each year.The variation range of total phosphorus content is 0.006～0.142 mg/L,which is relatively low from mid-autumn to late winter in each year.The content of chlorophyll-a varies from 5.19 to 118.01μg/L,and low chlorophyll-a occurs mostly in late autumn and winter.In terms of spatial variation,there is no significant difference in the water temperature of Changhu Lake,with the overall water temperature in the eastern lake zone being slightly higher than that in the central and western lake zone.The overall trend of dissolved oxygen and total nitrogen distribution is"higher in the central lake zone and lower in both lake zones".The total phosphorus content changes from east to west in an overall"M"shape distribution.The overall trend of chlorophyll-a decline from east to west.3.Water quality change characteristics and risk assessment of cyanobacterial bloom in Changhu LakeWe used the evaluation criteria for surface water environmental quality,indicator species of phytoplankton,phytoplankton abundance,chlorophyll-a content,and diversity index to evaluate the nutritional status of the Changhu Lake water body.The results showed that the overall trend of the Changhu Lake water body was eutrophic,with relatively serious pollution in most months.Based on the comprehensive analysis of the local climate characteristics,water nutritional status,and quantitative indicators of cyanobacterial blooms,the risk of cyanobacterial blooms in Changhu Lake is higher in late spring and summer and autumn.4.Relationship between phytoplankton and environmental factors in Changhu LakePearson correlation analysis and Redundancy analysis(RDA)show that the main environmental factors affecting the phytoplankton community in Changhu Lake are water temperature,dissolved oxygen,water level,rainfall,and nutrients.Multiple linear regression analysis showed that increasing water temperature could promote the growth of Cyanophyta,but was not conducive to the growth of Cryptophyta.Dissolved oxygen has a significant positive correlation with the abundance of Chrysophyta(P<0.05),and significant negative with the abundance of Euglenophyta(P<0.05).There was a significant(P<0.05)negative correlation between water level and abundance of Euglenophyta,Pyrrophyta,Cryptophyta,and Bacillariophyta.Increased rainfall can inhibit the reproduction of Chlorophyta and Chrysophyta.The increase of total nitrogen can promote the growth of Euglenophyta and Pyrrophyta.The abundance of Chrysophyta was significant negatively correlated with total phosphorus(P<0.05).