Structure And Dynamics Of The Phytoplankton Community And The Water Quality Assessment In Zhushuqiao Reservoir,Hunan Province

The Zhushuqiao Reservoir the second largest drinking water sources for Changsha City,which is the capital of Hunan province in central China.Thus,the ecosystem health and water quality of the Zhushuqiao reservoir are very important.To study spatial and temporal variations of eutrophication characteristics and conduct phytoplankton community-based ecosystem health asessment in Zhushuqiao reservoir,stratified water samples for abiotic and biotic variables were collected bimonthly from April 2016 to February 2017.The main results were as follows:1.The environmental factors had clear spatiotemporal variation characteristics in the Zhushuqiao Reservoir.The PCA revealed that pH and light intensity were the primary explanatory variables.The Pearson correlation analysis showed environmental factors were closely related,where chlorophyll a was positively correlated with the concentrations of water temperature,nitrogen and phosphorous.The appearance and disappearance of thermal stratification and human disturbance were the main reasons for the variation of limnology parameters in the vertical section of water column.The Zhushuqiao Reservoir was very soft water?Ca:0.81?1.39mg/L,Mg:1.32?1.75mg/L?,and had low concentrations of organic pollutants,inorganic pollutants and heavy metals?0?2.62?g/L?.2.A total of 171 phytoplankton species and 7 phyla were identified during the study period,which were dominated by Chlorophyta,Cyanobacteria and Bacillariophyta in the community.The community succession of Phytoplankton showed distinct seasonal change and vertical variations,with the lowest the density and biomass of phytoplankton in December and the highest in August.From the surface to the 10-m water layer,the density and biomass of phytoplankton were higher than the deeper layers.The spatial and temporal variations of phytoplankton communities were related to the changes of water temperature,thermal stratification,illumination intensity and the distribution of nutrients.Due to differences in the geographical positions,as well as the hydrological and environmental conditions,upstream phytoplankton-community structures were different from those in the downstream.3.The main influencing factors of Cyanophyta and Chlorophyta were similar.Cyanobacteria were more affected by temperature,while the growth of Chlorophyta were related to the growth of PO₄^3--P.Oocystis,Chlorella,Scenedesmus bijuba,Synedra,Cyclotella,Cryptomonas,Chroomonas acuta Uterm were the annual dominant taxa groups.The dominant species were different in different periods.Chlorophyta and Bacillariophyta had the widest niche breadth,the positive value of niche overlap,that indicated they had strong adaptability and competitiveness.In the Zhushuqiao reservoir,we may need to pay more attention to high water temperature,and the number of phytoplankton that increased dramatically,such as Synedra,Cryptophyta,and Peridinium,which could have the potential to form algal blooms.4.The phytoplankton taxa recorded during the study phases were sorted into 24functional groups.The phytoplankton functional groups had clear seasonal and cyclical succession characteristics.The succession dynamics of phytoplankton functional groups from upstream to downstream were X2+Y+F+Lo?April??D+J+Lo?June??F+X1+P?August??F+X1+Lo?October??C+Y+X2?December??Y+X2+Lo?February?.Corresponding growth strategies were as follows:C+CRS+CS+S?S+R?CS+R?CS+S?CR+CRS+C?CRS+C+S.Seasonal succession of phytoplankton and environment factors in two adjacent sites had strong similarities.The CCA of phytoplankton functional groups and RDA of growth strategies revealed that the key factors of driven seasonal phytoplankton succession were water temperature,pH,TP and DIN.5.All three indices indicated that upper reservoir water quality and ecological condition were worse than those in the lower reservoir.Dominant phytoplankton species in the upper reservoir differed significantly from those in the lower reservoir.The reservoir was eutrophic from June to October,especially in June.During the other months it was mesotrophic,with the best water quality in winter.Water temperature was the main driver of seasonal changes in both biotic and abiotic indicators by RDA.6.The nutrient enrichment bioassays?NEB?showed that the change of phytoplankton community was affected by the concentrations of nitrogen and phosphorous.During the same period,the response time of phytoplankton to the input of external nutrients was similar.The growth rates of phytoplankton were not persistent.This is related to the differences in environmental conditions,dominant species composition and phytoplankton growth strategies.Different dominant species also had different response mechanisms to nutrient addition.The growth effect of phytoplankton in the preliminary experiment in the laboratory was more significant than that in the field experiment.Water temperature and light were also important environmental factors affecting the growth of phytoplankton.Multivariate analysis of variance?ANOVA?showed that phosphorus addition alone had a more significant stimulation on phytoplankton growth than that of nitrogen,especially for Synedra.But addition of both nitrogen and phosphorus had the most stimulating effect on Cyanophyta and Chlorophyta.The phytoplankton had the maximum growth in TN:TP=19.87:1,phytoplankton growth was promoted with 0.9?13.59mg/L TN and0.02?0.684mg/L TP.When the total phosphorus in summer was greater than 0.04mg/L,the possibility of algal bloom will be greater.The linear regression fitted chlorophyll a and phosphorus well,and the regression equation was as follows:Log₁₀?Chl.a?=2.154+0.5103×Log₁₀?TP?.The binary regression equation of chlorophyll a and nitrogen and phosphorus was as follows:Chl.a=33.84+1.769TN+123.9TP.7.The nutrient concentration,climate and hydrological condition were main factors affecting eutrophication in Zhushuqiao reservoir.For the prevention and control of eutrophication,we need to pay attention to the outbreak of diatom bloom in late spring and early summer,and the storage flux of nitrogen and phosphorus nutrients should be reduced at the same time.What total nitrogen is lower than 0.9 mg/L or total phosphorus is lower than 0.02 mg/L can create the limitation effect on the diatom biomass in this reservoir.The water intake depth of the reservoir should be set at the water layer below 10m to avoid the water layer with high phytoplankton abundance.Reduce the impacts of human disturbances on water quality during the fishing harvest period?e.g.,artificial lowering of water level and fishing with seines?.Further,improve supervision and management of domestic sewage and household garbage in the upstream residential areas.