Community Structure Characteristics Of Benthic Algae And Assessment Of Water Eutrophication In Chishui River Basin

Algae are the main primary producers of river ecosystems.They have the characteristics of wide distribution,abundant species,and a short life cycle.Therefore,algae are commonly used as indicators for water environment monitoring and evaluation.Chishui River is a tributary of the upper reaches of the Yangtze River.With the rapid economic development,there are problems such as eutrophication,and the decrease of aquatic biomass and biodiversity.The water environment was the basis of maintaining river biodiversity and good ecological function development.These were of practical significance for the eutrophication assessment of the watershed water ecosystem.Therefore,benthic algae in the Chishui River Basin were selected as research objects in spring,summer,autumn and winter from September 2019 to March 2021.The physicochemical indexes and benthic algal biomass of 36 representative sites in the Chishui River basin were determined,followed by morphological identification and e DNA metabarcoding to study the community structure characteristics of benthic algae.Firstly,the key environmental factors affecting the community composition,ecological distribution,and seasonal succession of benthic diatoms in the Chishui River basin were determined.Then,a suitable biodiversity index and diatom index were selected for evaluation in the Chishui River basin.Finally,the critical indicator species of benthic diatom were determined.The main purposes were to control the total amount of pollutants,improve the stability of the ecosystem,and establish the index system of early warning threshold of key environmental factors for a scientific basin.The main results were as follows:1.The results showed that the environmental factors such as water temperature,electrical conductivity,flow velocity,total nitrogen,and total phosphorus were significantly different in different seasons and different river sections(P<0.05).The total nitrogen concentrations in autumn were the highest,and it was significantly higher in the upstream than that in the middle and downstream(P<0.05),especially in the tributaries of the upper reaches of the Tongche River.The total phosphorus concentration in summer was significantly higher than that in other seasons(P<0.05).There also significant differences in total phosphorus concentrations in different river sections(P<0.05).The total phosphorus concentration in the middle and upper reaches was significantly higher than those in the lower reaches,and the total phosphorus concentration in the Zhaxi River and the Yanjin River tributaries was higher.The concentration of Chla of benthic algae was highest in winter and lowest in summer.The results showed that the concentration of Chla of benthic algae was negatively correlated with the flow velocity,and positively correlated with water temperature,electrical conductivity and total phosphorus.There were 258 species of benthic diatoms in 41 genera,17 families,7orders and 2 classes in the Chishui River basin during the experiment.Cocconeis placentula var.euglypta was dominant species in spring,summer and autumn.The abundance of Melosira varians in the middle reaches was 51% in spring.Achnanthes minutissima was the dominant species in the middle and downstream sites.The results of the canonical correspondence analysis(CCA)indicated that the main environmental factors affecting the composition of benthic diatom communities were water temperature and total dissolved nitrogen in spring and autumn.While total dissolved phosphorus,soluble reactive phosphorus and total nitrogen were the main environmental factors affecting the distribution of benthic diatom communities during summer and winter.2.A total of 470 OTU of benthic algae were obtained by e DNA metabarcoding,which identified 64 species of benthic diatoms in 48 genera,28 families,11 orders and 4 classes.Cocconeis and Navicula were the main dominant genera in the upstream sample and middle sites.The richness,Chao1 and Shannon indexes indicated that the diversity of the benthic algae community in spring and winter was significantly higher than that in autumn.Morphological identification and e DNA metabarcoding were two different for benthic diatom data technology method.Morphological identification has a greater advantage in species level identification,although e DNA metabarcoding has good repeatability for environmental samples,it can only detect the spatio-temporal changes in the relative abundance of known benthic diatom at family and genus levels.3.The richness index and Shannon-Wiener index of benthic diatoms in winter were significantly higher than those in other seasons(P<0.05).The analysis showed that the biodiversity of benthic diatom community was the highest in winter.The stability of benthic diatom community in Tongche River and Daoliu River with higher nitrogen and phosphorus concentrations were lower than that in other river basins.The evaluation results of Trophic Diatom Indiex(TDI)and Commission for Economical Community metric(CEE)indicated that the overall water quality grade of Chishui River Basin was " moderate ",in which the water quality of headstream and the main stream of the middle and lower reaches is better.The water quality of tongzi River,Datong River and Xishui River is better than that of Tongche River,Yanjin River and Gulin River.4.Quantitative analysis of benthic diatom community and water nitrogen and phosphorus concentration showed that the maximum threshold of total nitrogen and total phosphorus concentration of benthic diatom community was 0.64 and 0.037 mg/L,respectively.When the concentrations of both exceeded 1.72 and 0.044 mg/L,the number of pollution tolerant species decreased and the community composition changed significantly.Meanwhile,9 total nitrogen and 13 TP benthic diatom critical indicator species were identified.The total nitrogen threshold of Achnanthes minutissima was the highest(1.57 mg/L)among the 5 negatively responsive species.Total nitrogen threshold of Cocconeis placentula var.euglypta was 1.72mg/L,which was the highest among the four positive response species.In the 6 negative and7 positive species of total phosphorus critical indicator,Achnanthes lauenburgiana(negative response)and Nitzschia palea(positive response)had the highest thresholds,which were0.048mg/L and 0.150 mg/L,respectively.The concentrations of total nitrogen were higher than the maximum critical threshold of benthic diatoms in the upper reaches and Tongche River.The concentrations of total phosphorus in Yanjin River was higher than the maximum critical threshold of benthic diatom community.The above shows that these basins are disturbed to a certain extent,and it is necessary to control the input of nitrogen and phosphorus in the river.