Characteristics Of Phytoplankton Community Structure And Functional Groups And The Relationships With Environmental Factors In The Mainstream Of The Yangtze River

The Yangtze River is China's"Mother River".It ranks first in China and third in the world.The Yangtze River basin is an important birthplace of Chinese civilization.However,with the influence of human activities and rapid economic development in recent years,more and more serious ecological problems occur to the Yangtze River.In order to clarify the characteristics and driving factors of plankton community structure in different regions,seventy-six sampling sites were selected in the mainstream of Yangtze River during the survey of plankton from July and September of 2019.Meanwhile,we analyzed the relationships between aquatic environmental factors and phytoplankton functional groups and evaluatedused the water quality of mainstream in the Yangtze River by using the diversity index and QR index of phytoplankton functional group in order to provide theoretical basis for water resource utilization and aquatic ecological protection in the Yangtze River.The main results were shown as follows:During the study period,a total of 112 species of phytoplankton were found in the mainstream of the Yangtze River,belonging to 6 phyla and 53 genera.Among them,the diatoms had the most species(53 species),accounting for 47.32%of the total number of species.Chlorophyta followed with 37 species,accounting for33.04%of the total species.There were 14 species of Cyanophyta,accounting for12.50%of the total species,and the remaining three phyla accounted for 7.14%of the total species.The spatial variation of phytoplankton species was shown as the middle reach>the Jinsha River>the lower reach>the three Gorges Reservoir>the source region.The dominant species at each region were almost diatoms,e.g.,Melosira granulata var.angutissima and Pseudoanabaena sp.appeared the most frequently.The averaged density and biomass of phytoplankton were 1.38×10~6 ind./L and 1.53 mg/L,respectively.The spatial variation was shown as the middle reach>the lower reach>the three Gorges Reservoir>the Jinsha River>the source region.Phytoplankton in the mainstream of the Yangtze River were divided into 24functional groups,including A,B,C,D,E,F,G,H1,J,Lo,M,MP,N,P,S1,S2,SN,T,TB,Tc,W2,X1,X3 and Y,among which D,MP and P functional groups are dominant groups.The regional change in the number of functional groups was shown by the middle reach>the lower reach>the three Gorges Reservoir=the Jinsha River>the source region,and the regional succession of important functional groups was A+D+Lo+MP?A+D+G+MP?D?MP+P+T?MP+P from upstream to downstream.Pearson correlation analysis was conducted on the biomass of phytoplankton functional groups and environmental factors in the mainstream of the Yangtze River.The results showed that functional group D was positively correlated with water temperature,dissolved oxygen and nitrate nitrogen.There was no significant correlation between MP functional group and environmental factors.The P functional group was positively correlated with water temperature,and negatively correlated with dissolved oxygen,p H and total phosphorus.T function group was positively correlated with water temperature.RDA analysis of phytoplankton functional groups and environmental factors in five regions of the Yangtze River mainstream showed that water temperature,dissolved oxygen,p H and nitrogen and phosphorus nutrients were the main environmental factors affecting the phytoplankton functional groups in the mainstream of Yangtze River.The means of diversity indices(H?),abundance(d)and evenness(J)of phytoplankton were 2.22,1.61 and 0.68,respectively.The mean of Qr was 2.40.Based on the biodiversity indices and phytoplankton functional group QR index showed that water quality in the mainstream of the Yangtze River was at low-to-medium pollution level.