Spatio-temporal Variation Characteristics Of Phytoplankton Community Structure And Evaluation Of Water Quality In The Upper Reaches Of Yongding River

Yongding River is an important water conservation area,ecological barrier and ecological corridor in Beijing-Tianjin-Hebei region,also the“mother river”of Beijing.The quality of its water environment and the health of its water ecosystem are of vital importance.To study the structural features of phytoplankton community and the current quality of water in the upper reaches of Yongding River,we collected water samples from 17 sampling points(in the upper reaches)of Yongding River in April(spring),July(summer)and October(autumn),and determined the temperature,velocity,p H and other indicators of the river on spot respectively.A classification analysis was carried out on the collected phytoplankton samples in the laboratory,and the physicochemical indicators of the water samples from 17 sampling points,including TN,TP,COD,NH₄⁺-N,NO₃^--N and NO₂^--N,were determined respectively.According to the results of analysis and determination,the biodiversity and water pollution indicators in the upper reaches of Yongding River were evaluated ecologically and then the correlation between phytoplankton and various environmental physicochemical factors in water were discussed.The results were as follows:1.The phytoplankton composition(in the upper reaches)of Yongding River was bacillariophyta-chlorophyta algae.According to our survey,there were altogether 226species in 8 phylaof phytoplankton.Among them,bacillariophyta had the most species(95),accounting for 42.04%of the total species,followed by chlorophyta,accounting for 27.88%of the total species.The annual density and biomass of phytoplankton were 71.97±9.62×10⁴ind/L and 1.78±0.49 mg/L respectively.2.There was no significant difference between sectors in terms of species composition of phytoplankton,but significant difference between seasons.According to our survey,there were 9 dominant species in different seasons,among which the main dominant species were Aphanizomenon flosaquae(L.)Ralf.,Synedra acus Kütz.,Nitzschialinears W.Hust.,Navicula simplex Krassk.and Cyclotella meneghiniana Kütz..A two-way ANOVA showed that there was no significant difference in density and biomass between seasons,but significant difference between sectors,but no significant interaction was observed.The density of phytoplankton exhibited obvious seasonal fluctuation,and the density of phytoplankton in summer was significantly higher than that in spring and autumn,probably because the influx of nutrients into the watershed introduced by summer rainfall resulted in a dramatic increase in the density of phytoplankton.In terms of spatial distribution,the density of phytoplankton presented an obvious longitudinal gradient distribution from the upstream inlet to the downstream Guanting Reservoir,and showed a progressive upward trend.The transitional area of reservoir was most conducive to the growth of phytoplankton,probably because it had a high nutrient concentrationand a low water velocity.3.A cluster analysis showed that phytoplankton community can be divided into two types:spring community and summer-autumncommunity.The main characteristic species in spring community were Aphanizomenon flosaquae(L.)Ralf.,Cyclotella meneghiniana Kütz.,and Scenedesmus bijuba Turp.,while the main characteristic species in summer-autumn community were Aphanizomenon flosaquae(L.)Ralf.,Scenedesmus quadricanda Breb.and Scenedesmus bijuba Turp..4.A correlation analysis indicated that density,Shannon's diversity index and richness indexof phytoplankton were positively correlated with COD,while density,biomass,Shannon's diversity index and richness index were all negatively correlated with NO₃^--N.The results of redundancy analysis showed that potassium permanganate index and nitrate nitrogen were key factors affecting the distribution of phytoplankton community in the upper reaches of Yongding River.The density of Cyclotella meneghiniana Kütz.,Aphanizomenon flosaquae(L.)Ralf.and Tribonema minus(Will.)Haz.were all positively correlated with COD and negatively correlated with nitrate nitrogen,probably because there was aconcentration ceiling for the promoting effect of nutrients on the growth of algae.Once this ceiling was exceeded,nutrients were no longer a limiting condition for the growth of algae.The results of biological integrity have shown that the Yongding River(upstream section)is in a healthy or sub-healthy.5.Single-factor evaluation,biotic index evaluation and Principal Component Analysis were used to evaluate and analyze the water quality.The results suggested that the water bodies in the upper reaches of Yongding River is relatively clean,which is class II,Yang River and Sanggan River are class III,and the section of Guanting reservoir is class IV,indicating that the water quality of the basin has been effectively improved.