Changes Of Water Quality In Pengxi River And The Relationship Between Plankton Succession And Environmental Factors During Bloom

The impounded of the dam has changed the hydrological conditions of river and its tributaries.As a result,many tributaries are changing from the original lotic system to the static water system,which accelerated the accumulation of nutrients in the water and provided a stable and favorable environment for the massive growth of algae,thus causing the outbreak of algal blooms.In this study,Pengxi River,a primary tributary of Three Gorges Reservoir,which outbreak water blooms frequently,was taken as example.We monitored the changes of nutrients,water quality parameters,and the plankton community during bloom period from May 2018 to October 2019.This paper study on the influence factors of algal bloom and the dynamic changes of the plankton community during the bloom to provide a scientific basis for further study on the outbreak mechanism of the bloom in the tributary backwater of Three Gorges Reservoir and the prevention and control of eutrophication.From November 2018 to October 2019,this study monitored the water quality of the Pengxi River(water temperature,dissolved oxygen(DO),conductivity,chlorophyll a(chl a)and nutrient salts,etc.).The results show:(1)Pengxi River experienced serious algal blooms in spring.The outbreak center was in the Gaoyang lake backwater area,with the average chlorophyll a content as high as 108 ?g·L-1.According to the revised Carlson index,The results showed that the risk of eutrophication in the upstream of Pengxi River was higher than that in the downstream and the water quality changed from spring to winter.(2)According to the correlation analysis,the nitrogen and phosphorus were the influencing factors of algal bloom,and the proportion of dissolved nutrients(DTP and DTN)varies seasonally,which was generally higher in autumn and winter than in spring and summer.DTP and orthophosphate(PO43-)were negatively correlated with chl a contens.Nitrate(NO3-)had a negative relationship with chl a,and ammonium nitrogen(NH4+)was significantly positive correlation with chl a(r=0.820*).The water content of NH4+ in spring and summer was significantly higher than which in autumn and winter,which suggested NH4+was conducive to the growth of algae and was one of the cause of the outbreak of the spring.(3)Affected by the main stream,seasonal stratification along with the regulation of water level in Pengxi River.There was stratification in spring,summer and autumn,but there is no stratification near the estuary in autumn,and the whole river was mixed in winter.There was a shallow mixed layer of water in spring and summer(<0.01m),and which increased to about 14 m in autumn.The water temperature was significantly positively correlated with the level of chl a(r=0.378*),which caused the stratification and provided a stable environment for the growth of algae.Twice daily samples were taken from May 16 to 23 in 2018 and from May 5 to 15 in 2019 in Gaoyang Lake.The sampling time is 8:30 am every morning,and the water sampler took by plexiglass at 0.5m depth,and repeated sampling was carried out at three sampling points at 3-meter intervals.D During the sampling period,the changes of water quality parameters and phytoplankton community structure were monitored.The community of plankton was analyzed by means of microscopic examination and high-throughput sequencing.The results show:(1)The algal bloom in Gaoyang Lake gradually changed to harmful bloom.Inferior 5th-degree blooms broke out in Gaoyang Lake form May 16 to 23 in 2018.and May 5 to 15 in 2019.In 2018,the bloom was dominated by cyanobacteria(algal density proportion: 10%-27%),green algae(algal density proportion: 26%-84%),and chl a concentration ranged 13.71 ?g·L-1 to 219.48?g·L-1.And the peak of bloom occurred on May 20,2018.Bloom in 2019 was dominated by cyanobacteria(algal density proportion>99%),with chlorophyll level ranging from 44.28 ?g·L-1 to 413.17?g·L-1.There were three outbreak points on May 9th,May 12 rd and May 15.Microcystis marginatae occupied an absolute advantage,the dominance index Y rang 0.11 to 0.29.In the RDA analysis,the correlation between cyanobacteria and nutrients was higher than that of other algae,which was related to the nutrient uptake and utilization capacity of microcystis.TN was significantly correlated with the algal density of Anabaena oscillarioides Bory and Anabaena circinalis,both of which had heterocyclic cells and are able to perform nitrogen fixation.(2)During the bloom period,the water formed stable mixed layer because the stratification,sufficient nutrients,appropriate temperature(21.86 ± 0.3?-25.92 ± 0.0?)and plenty of light(1113 lux-7300 lux)led to the large increase of algae in Pengxi River in spring in bottom-up model.Phytoplankton growth also changes the parameters of the water environment.When the parameters become the limiting factor of algae growth,the community structure of the planktonic ecosystem will also be affected by the "bottm-up" model.Both the RDA and correlation analysis were characterized by euphotic layer(Zeu)on algae growth is greater than the influence of ratio of the euphotic layer depth to the mixing layer depth(Zeu/Zmix).The prediction effect of Zeu/Zmix on algae growth trend needs to consider comprehensive factors such as nutrients.(3)The occurrence of algal bloom is spatially and temporally different,and the community structure varies greatly from day to day during the algal bloom(P<0.01).In 2018,microzooplankton was the dominate group,which may limit the occurrence of bloom in a top-down model.However,copepods' DNA proportion was higher than that of other microzooplankton in 2019,which may be a biological factor promoting microcystis bloom.(4)The results of absolute quantitative suquencing of 16 S plankton show Heterotrophic bacteria contribute more to the prokaryotic plankton DNA pool than photosynthetic bacteria,and react quickly with bloom.During high period of bloom,the DNA copy number of heterotrophic bacteria increased nearly 20 times,which were mainly Flavobacterium and Porphyrobacter.The classification and ecological function of aquatic fungi also need more attention.The ecological relationship between fungi and zooplankton is complex,which needs to be further studied.In 2018,the DNA proportion of fungi fluctuates greatly,and have shift trend of microzooplankton.