Response Of Phytoplankton Community Structure To Water Nutrient Status In Harbin Section Of Songhua River

River ecosystem is one of the most important types of water ecosystems.It is the carrier of material flow and energy cycle.It has very important ecological functions and can provide basic conditions for the growth and reproduction space,food and habitat of organisms.As primary producers in river ecosystems,phytoplankton can respond quickly to changes in the surrounding environment,so they are often used for water environment monitoring.The Harbin section of the Songhua River is located in the north temperate monsoon climate zone and flows through the main urban area of Harbin.It is a typical river ecosystem and is also a key resource and environmental carrier.Therefore,it has important ecological significance to study the characteristics of phytoplankton community structure in the Harbin section of the Songhua River.In this study,samples were collected from 21 sampling points in the Harbin section of the Songhua River in September,October,and November in the fall of 2020,in April and May in the spring of 2021,and in June,July,and August in the summer of 2021.The phytoplankton samples were processed,and 9 physical and chemical indicators of the water body at each sampling point were determined.The temporal and spatial changes of the phytoplankton community structure and diversity in the Harbin section of the Songhua River were analyzed,and the succession characteristics of the dominant phytoplankton species were discussed.Using principal component analysis（PCA）and redundancy analysis（RDA）,the relationship between the phytoplankton community and water environmental factors during the survey period was studied,and the key factors driving the occurrence and succession of phytoplankton were identified.The single-factor evaluation method was used to evaluate the water quality of the Harbin section of the Songhua River,and the P-IBI evaluation standard was constructed to evaluate the health of the river,providing basic data for ecological protection and sustainable development in the Songhua River Basin.The main findings are as follows:（1）During the study period,in the Harbin section of the Songhua River,WT variation ranged from 0.1 to 32.7°C;DO variation ranged from 0.35 to 19.54 mg/L;p H variation ranged from 5.46 to 8.48;Sp Cond.variation ranged from 113.80 to 950.00μS/cm;Tur.variation ranged from 7.70 to 200.00 NTU;TN variation ranged from 0.30to 2.39 mg/L;TP variation ranged from 0.07 to 1.98 mg/L;COD_Mn variation ranged from1.48 to 13.85 mg/L;BOD₅ variation ranged from 0.19 to 14.05 mg/L.The results of principal component analysis showed that the physical and chemical indicators of the water body in the Harbin section of the Songhua River were mainly affected by the changes in the nutrient content in the water body.（2）During the investigation of the Harbin section of the Songhua River from 2020to 2021,a total of 230 taxonomic units of phytoplankton were identified,belonging to 7phyla,10 classes,23 orders,42 families and 100 genera.The average abundance of phytoplankton in autumn of 2020 was 15.06×10~6 ind./L;the average abundance of phytoplankton in spring of 2021 was 68.70×10~6 ind./L;the average abundance of phytoplankton in summer of 2021 was 42.50×10~6 ind./L.The community composition and abundance of phytoplankton changed significantly in both time and space,and the phytoplankton community structure was diatom-green algae type.During the study period,a total of 47 dominant species were identified,among which Bacillariophyta had the most species（16 species）,followed by Chlorophyta（9 species）,Cyanophyta（7species）and Euglenophyta（7 species）.Chrysophyta（3 species）,Cryptophyta（3 species）and Dinophyta（2 species）were few in number,and the dominant species were heterogeneous in different regions.（3）During the study period,the Margalef diversity index（H）of each sampling point in the Harbin section of the Songhua River variation ranged from 0.51 to 6.10;the Shannon-Weaver diversity index（H’）variation ranged from 0.33 to 4.86;the Simpson ecological dominance index（D）variation ranged from 0.07 to 0.95;Pielou evenness index（J）variation ranged from 0.07 to 1.19;diversity threshold（Dv）variation ranged from 2.58 to 6.17.The comprehensive analysis of various indicators showed that the individual distribution of phytoplankton in the Harbin section of the Songhua River was relatively uniform in summer and autumn,and the community structure was relatively stable.The spatial variation of phytoplankton diversity was not obvious.（4）The phytoplankton community pattern in the Harbin section of the Songhua River is closely related to water environmental factors.RDA analysis shows that Tur.,DO,TN,TP,and WT are the main environmental factors affecting the succession of phytoplankton in the Harbin section of the Songhua River from 2020 to 2021;WT,TP,TN,DO in autumn 2020,TN in spring 2021,and Sp Cond.,TN,TP,Tur.,WT,p H,and DO in summer 2021 drive the phytoplankton community succession in each season respectively major environmental factors.According to Pearson correlation analysis,there is an interaction relationship between physical and chemical factors and phytoplankton abundance in the Harbin section of the Songhua River.The environmental factors related to the total abundance of phytoplankton in different seasons were slightly different.The abundance of Cyanophyta and Euglenophyta were positively correlated with WT,Sp Cond.,TN,and TP,and negatively correlated with DO.Chlorophyta and Tur.was negatively correlated.（5）During the investigation period from 2020 to 2021,according to the evaluation results of DO and TN surface water environmental standards,the water quality status of the Harbin section of the Songhua River is preliminarily evaluated as good;according to the evaluation results of TP and BOD₅ surface water environmental standards,the water quality status is preliminarily evaluated It is moderately polluted;according to the evaluation results of the COD_Mn surface water environmental standards,the preliminary assessment of the water quality is mild pollution.According to Margalef diversity index and Shannon-Weaver diversity index,the evaluation result isβ-moderate pollution state;Pielou uniformity index evaluation result is light pollution.According to the P-IBI assessment,the river health of the Harbin section of the Songhua River is "average".