| East Taihu Lake is a representative macrophyte dominated shallow lake in middle and down-stream Yangtze river basin and also a very important drinking water source for Suzhou and Shanghai city.Under the co-stress of global-scale environmental changes and multiple human disturbances,East Taihu Lake presents an imbalance in the ecosystem and triggers a trend of turbulent changes in lake water quality and water ecology,and then signs of a transformation from grass-type to algae-type lakes.With the inclusion of Wujiang in the"Yangtze River Delta Ecological and Green Integrated Development Demonstration Zone",the concept of"ecology priority,green development"has been deeply rooted in the hearts of the people,and the national strategic level has put forward higher requirements for the refined management of the water ecology and water source protection of the East Taihu Lake.Phytoplankton is an important primary producer of lake ecosystems.The outbreak of algal blooms not only endangers the health of the water ecosystem,but also poses a huge threat to the safety of drinking water.Therefore,it has important practical significance and application value to accurately implement the prevention and control of algae blooms and ensure the safety of drinking water source water quality,and maintain the health of the East Taihu Lake water ecosystem,through continuous monitoring and comprehensive analysis,the community structure characteristics of the phytoplankton in East Taihu Lake are clarified.This paper takes the phytoplankton community in East Taihu Lake as the research object,focusing on the dynamic succession mechanism of the phytoplankton community in different functional areas and the response mechanism to various influencing factors,using a combination of on-site in-situ survey,remote sensing analysis and laboratory simulation,carried out continuous monitoring research for 3 years(April 2017-June2020).At the same time,a comprehensive assessment of the ecological health of East Taihu Lake based on the path of phytoplankton function groups was carried out in combination with mathematical statistical methods such as multivariate statistical analysis.The main research conclusions are as follows:1.Obvious temporal and spatial differentiation of nutrient and primary productivity changes in East Taihu Lake.During the study period,the average concentration of TN was 0.79±0.50 mg/L,the average concentration of TP was 0.080±0.041 mg/L,which were lower in wetland reservoir area than other areas(p<0.05);the average concentration of Chl-a was 31.98±17.30μg/L,which was similar to the spatial distribution characteristics of TN and TP concentrations,and the wetland reservoir area were significantly lower than other areas(p<0.01);In the summer of2017 and 2018,the concentration of Chl-a in the aquaculture area was significantly higher than that in other areas,and in the summer of 2019 after aquaculture activities were completely banned,the concentration of Chl-a in the aquaculture area was significantly reduced.Principal component analysis(PCA)results showed that the water environment of East Taihu Lake showed obvious temporal and spatial dynamic differentiation in the three dimensions of nitrogen and phosphorus availability,water turbidity and primary production.The average PC1 scores of the open water area and the flood channel were 0.36±0.77,which showed that the water body had high turbidity in spring and the water quality was poor;The average PC1 scores of the aquaculture area and the wetland reservoir area were-0.53±0.66 and-0.55±0.76,respectively,indicating that the water bodies in the above two areas had high transparency and low concentrations of suspended particles.the results of the analysis of variance also showed that the transparency in the East Taihu Lake aquaculture area and the wetland reservoir area was significantly higher than that of the flood discharge channel;the higher the PC2 score,the higher the concentration of directly available phosphorus(TDP).The PC2 scores of the open water area and the flood channel were0.30±0.94 and 0.36±1.14,respectively,and the aquaculture area and the wetland reservoir area had lower scores,respectively,the values are-0.22±0.83 and-0.43±0.81,reflecting that the competition between aquatic plants and phytoplankton for the use of TDP might be the cause of the low TDP concentration in dense aquatic plants.2.During the study period,a total of 204 species of 74 genera and 7 phyla of phytoplankton were detected.The average density of phytoplankton was 2.19×10~7cell/L,and the variation range of the density of planktonic algae was1.20×10~6-1.85×10~8 cell/L.In June 2018,the flood channel area reached the highest value(1.88×10~8 cell/L),and the lowest value appeared in December 2020 in the wetland reservoir area(2.73×10~6 cell/L).Based on the spatial ranking of phytoplankton density from high to low(flood channel area>open water area>aquaculture area>wetland reservoir area),and redundant analysis results showed that the the phytoplankton observed in the open water area and the flood channel area were mainly affected by the West Taihu Lake;the seasonal variation characteristics of phytoplankton density were as follows:summer>winter>autumn>spring,Affected by the warm winter in recent years,the phytoplankton in the East Taihu Lake showed an upward trend in winter.In 2018,the average algal density of the flood channel area and the wetland reservoir area reached 1.45×10~8 and 1.07×10~8 cell/L,which were the highest records in recent years.Phytoplankton biomass varies between 1.03-55.53mg/L,with an average value of 10.98±9.14 mg/L.Phytoplankton density in aquaculture areas was significantly related to nutrients(p<0.01);TN(p<0.01)and TSS(p<0.05)explained the change of algae density and were important influencing factors in wetland reservoir area.3.The phytoplankton functional group had seasonal changes.Based on the theory of phytoplankton functional groups,the dominant phytoplankton species in East Taihu Lake were divided into 13 dominant functional groups:C,D,P,MP,T,S1,SN,Y,G,H2,L0,M,W1.In winter and spring with lower temperature and light intensity,functional groups W1,Y and D were dominant in the entire lake area.In the summer of high temperature and high nutrient content,functional groups M,H2,MP and T were dominant.The canonical correspondence analysis(RDA)of phytoplankton functional groups and environmental factors found that water temperature,turbidity and and nutrients were the main environmental factors that affect the succession of phytoplankton functional groups.4.The top-down of the East Taihu Lake aquatic ecosystem was significantly higher than the bottom-up effect.Nutrients and light of top-down effect were important ways to affect the temporal and spatial differences of phytoplankton in East Taihu Lake.Regression analysis showed that dissolved nitrogen(R~2=0.43,p<0.01)and particulate phosphorus(R~2=0.29,p<0.01)were both related to algae biomass.Suspended particulate matter dominateed 59.84%of the spatial variability of water optical properties and 77.77%of the seasonal variation.The relative contribution of the quantitative model to the influence of different components on the optical properties of the water body was non-algae particles(53.56%)>algae particles(34.78%)>colored dissolved organic matter(10.88%),indicating that inorganic suspended particles were the limiting factor affecting the photosynthesis of algae.The bottom-up effect was mainly the predation of zooplankton on phytoplankton.In the bottom-up effect,the correlation between zooplankton density and phytoplankton was weak(R~2=0.15,p>0.05),indicating that zooplankton had a small grazing effect on phytoplankton in the East Taihu Lake.5.There are significant differences in the correlation between phytoplankton and aquatic plants in different functional areas.The results of correlation analysis between phytoplankton and aquatic plants showed that there were fewer aquatic plants in the open water area and the flood channel area,and the impact on phytoplankton was small(p>0.05);phytoplankton in wetland reservoir area increaseed significantly with the increase of aquatic plants decrease(R~2=0.15,p<0.01),while the same aquatic plant-rich aquaculture area showed a significant positive correlation in 2018(R~2=0.30,p<0.01),and it turned into a negative correlation in 2019 after the aquaculture activities were banned(R~2=0.14,p<0.01),indicating that the impact of aquaculture activities was greater than the impact of aquatic plants on phytoplankton.Compared with the area without aquatic plants,the TSS of the dense aquatic plant area was62.2%lower,the Chl-a was 16.6%lower,and the transparency was twice as high.The coverage of aquatic plants was significantly negatively correlated with TDN and particulate phosphorus(PP),which confirmed that the interaction between aquatic plants and transparency,TSS,and nutrients will indirectly affect the density and community structure of phytoplankton through top-down effect.The simulation results showed that the density of Anabaena curlicina and Microcystis aeruginosa decreased with the increase of Ceratophyllum demersum biomass(R~2=0.68 p<0.01,R~2=0.59 p<0.01).The experimental results of the effects of aquatic plants on phytoplankton under different nutrients showed that aquatic plants reduced the TSS of the water body by 57.4%.Under low nutrient conditions,the density of phytoplankton was 57.8%lower than that of the no-plant group.Under medium and high nutrient conditions,the role of aquatic plants was weakened,with the reduction rates being39.8%and 20.5%,respectively.Under moderate nutrient conditions,aquatic plants had a strong inhibitory effect on cyanobacteria,and the relative abundance of cyanobacteria was reduced by 31.79%,but there was no significant change in the no-plant group;under high nutrient levels,the growth of cyanobacteria was restricted,and the plant group and the no-plant group decrease by 71.63 and 61.43%,respectively.6.East Taihu Lake had hidden concerns about grass-algae conversion,and the recovery of aquatic plants was the key.The calculation results showed that the dry weight ratio of grass and algae in East Taihu Lake varieed between 6.6-31.3,and the TP/Chl-a ratio fluctuateed between 3:1 and 1:1,indicating that it was in a grass-algae transition state in East Taihu Lake,only the wetland reservoir area have TP in 2018and 2020,the ratio of TP/Chl-a was less than 1:1,indicating a grass-type steady state.Combining the Q index(3.15)and TSI(57.38±5.26)evaluation results,it showed that East Taihu Lake had changed from the originally very stable grass-type steady state to the algae-type steady state.By comparing the nutrient status of functional areas in different states,it was concluded that the TP threshold to maintain grass-type steady state might be lower than 0.07 mg/L,and the TN threshold might be lower than 0.8mg/L.When the threshold was exceeded,there was a risk of increased phytoplankton density and even formation of algal blooms.Therefore,while strengthening the source control of nutrients in the whole watershed,restoring aquatic plants in waters such as Dongjiaozui was still the key to maintaining the grass-type stability of East Taihu Lake.7.The water quality and aquatic ecology of the original aquaculture area has improved significantly.After the aquaculture activities in East Taihu Lake were banned,the coverage and diversity of aquatic plants were restored.TN and TP decreased by 54%and 20%,respectively.and 34%,respectively,while SD increased from 0.76 m to 0.95 m,an increase of 26%,Chl-a dropped by 34%.Both the density and biomass of phytoplankton showed a significant decline,from 3.60×10~7 cell/L(2018)before the prohibition of aquaculture to 1.58×10~7 cell/L(2019)after the prohibition,the diversity index Shannon-Wiener(H’)increased from 1.85 to 2.22.In addition,the prohibition of aquaculture had particularly significant inhibition of cyanobacteria,and the relative abundance was reduced by 84.4%. |
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