| Water eutrophication is a major environmental problem around the world today.It always triggers the outbreak of algal blooms,which has become a challenge for many eutrophic water bodies.As a kind of water blooms,cyanobacteria bloom not only cause serious damage to the aquatic environment,reducing water availability,but also bring great harm to human health through the cyanobacterial toxins.Cylindrospermopsis raciborskii,a planktonic cyanobacterium in fresh water,is first discovered in the tropical Lake Java in Indonesia.Because of its high temperature requirements for growth and sensitivity to temperature changes,it is considered to be a tropical cyanobacterium.However,in recent years,C.raciborskii has gradually spread into subtropical and temperate regions,leading to blooms in various habitats.Therefore,it is considered to be an invasive cyanobacterium with high spreading ability.C.raciborskii can release a variety of toxins,including cylindrospermopsin,paralytic shellfish poisoning,etc,seriously threatening human and animal health.Therefore,C.raciborskii has increasingly become the research hotspot by many scholars at home and abroad.Though the construction and operation of the Three Gorges Dam bring economic benefits to people,also it also brings many negative effects.Especially in the tributary of reservoir area where the decrease of flow rate and airflow exchange and an increase in the frequency of blooms and water eutrophication have been intensified to directly affect the water safety of the reservoir area.Huaxi River is a first order tributary of Yangtze River in Chongqing with a national first-grade protected drinking water source,Nanpeng Reservoir,in the upstream.Its water quality not only affects the water quality of the Three Gorges reservoir area,but also affects the health and safety of local residents.However,with the economic development,Huaxi River has been seriously polluted by domestic sewage and industrial wastewater,because wastewater was stenchly released to the water body,resulting in a significant decrease of water quality,and even the occurrence of cyanobacterial blooms.To study the change of water environment and the mechanism of blooms in Huaxi River,twelve sampling sites were set from the upper reaches of Huaxi River to the entrance to Yangtze River.From June 2017,physicochemical parameters,such as total nitrogen,total phosphorus,electrical conductivity,water temperature,and phytoplankton at the twelve sampling points were determined monthly for one year,and a redundancy analysis between phytoplankton quantity and environmental factors was carried out.On this basis,Pearson correlation analysis between C.raciborskii and environmental factors was completed to investigate the spatial and temporal changes of phytoplankton and C.raciborskii and the eutrophication status in Huaxi River.The aim is to clarify the relationship between the occurrence of C.raciborskii bloom and environmental factors and to provide some theoretical support for the further study of C.raciborskii bloom.The main findings were as follows:(1)A total of 353 species,102 genera,and 10 species of algae were detected in Huaxi River,among which green algae were the most abundant,with 127 species of 40 genera,accounting for35.98%of the total species,followed by diatoms,Euglena,and cyanobacteria accounting for27.76%,18.41%and 11.05%,respectively,then other algae species only accounted for 6.80%.The number of algae species in winter was significantly fewer than that in other seasons,and a most abundant of algae species werefound in spring.During the whole investigation period,C.raciborskii was not found at the sample points,S0 and S1,and the density did not reach the value that forming water bloom during the whole investigation period,and even no C.raciborskii was detected at the sample points in the middle and downstream,while the density of C.raciborskii at S2 and S3 reached a value that can form water bloom from June to November and the density in August was the highest,about 1.07×10~8 cell/L,From July to September,cyanobacteria bloom was the most serious and the number of C.raciborskii accounted for 98%of the total number of algae.(2)During the investigation,the contents of total nitrogen and total phosphorus in all of the twelve sampling points reached an eutrophication value,with the total nitrogen content of0.99-15.26 mg/L and the total phosphorus content of 0.021-0.914 mg/L.The contents of nitrogen and phosphorus increased from upstream to downstream and at slightly higher value in winter than that in other seasons.The lowest and highest average contents of nitrogen and phosphorus were determined at S0 and S10,and a lower value of the nitrogen and phosphorus contents was also measured at S2 and S3 when compared with those at the sampling points of S4-S12.The temperature of water body varied significantly with the season,with the highest in July and the lowest in January.The conductivities at S2 and S3 for each month were the lowest.The permanganate contents in July,August,and September was significantly higher than those in other months.The water-blocking dams in many places of Huaxi River resulted in a great difference in the current velocity among the various points,ranging from 0 to 1.04 m/s.(3)Correlation analysis and RDA sequencing analysis between C.raciborskii and environmental factors at the reservoir sampling points(S2 and S3)found that the number of C.raciborskii had a negative correlation with the current velocity conductivity,total nitrogen,total phosphorus,nitrate nitrogen,and transparency(only the correlation with conductivity was very significant),while a positive correlation between the number of C.raciborskii and water temperature,light,pH,dissolved oxygen,and nitrogen/phosphorus ratio(only the correlation with water temperature was very significant).In the three months(July,August,and September)when the blooms were the most serious,correlation analysis and RDA sequencing analysis of C.raciborskii with environmental factors at S2-S11 found that C.raciborskii had a negative correlation with the current velocity,conductivity,total nitrogen,total phosphorus,nitrate nitrogen,and orthophosphate(only the correlations with conductivity,total phosphorus,and orthophosphate were significant),while it had a positive correlation with water temperature,light,pH,and dissolved oxygen(only the correlations with light and pH were very significant)in July.In August,C.raciborskii had a negative correlation with the current velocity,conductivity,total nitrogen,total phosphorus,nitrate nitrogen,and orthophosphate(only the correlations with total phosphorus and orthophosphate were significant,especially,the correlation with electrical conductivity were very significant),while it had a positive correlation with water temperature,light,pH,dissolved oxygen,and nitrogen/phosphorus ratio(only the correlations with dissolved oxygen,nitrogen/phosphorus ratio,and pH were very significant).In September,C.raciborskii had a negative correlation with current velocity,conductivity,ORP,total nitrogen,total phosphorus,nitrate nitrogen,and orthophosphate(only the correlations with total nitrogen and nitrate nitrogen were significant,especially,the correlations with conductivity and ORP potential were very significant),while it had a positive correlation with water temperature,light,pH,dissolved oxygen,and nitrogen/phosphorus ratio(only the correlations with water temperature and dissolved oxygen were significant,especially,the correlations with light and pH were very significant). |
PDF Full Text Request