| The Lancang River basin is an important hydroelectric base in southwest China.The establishment of Xiaowan Hydropower Station is a rational exploitation and effective utilization of hydroelectric resources,and it also has benefits such as water storage,fishery and shipping.The impoundment of Xiaowan Reservoir will affect the concentration and distribution of nitrogen and phosphorus in the reservoir water,and change the original water environment characteristics of the reservoir.Phytoplankton is the basic link of aquatic ecosystem,and its composition and abundance can reflect the stability of the entire aquatic ecosystem.Phytoplankton and its diversity index have been widely used as water quality monitoring and evaluation indexes.Therefore,the study of phytoplankton in the bay can reflect the current water environment,provide a theoretical basis for ecological evaluation of water quality and water environment monitoring,and provide a theoretical basis for understanding the spatial and temporal distribution of phytoplankton in the bay and the development of fishery in the reservoir area.In this study,a biological survey was conducted in Xiaowan Reservoir on Lancang River from 2017 to August 2019.Shannon-Wiener index,Pielou evenness index and Margalef index were used to evaluate the diversity of phytoplankton,and evaluation criteria such as comprehensive nutrient status index,phytoplankton algae abundance and Q index were applied to comprehensively evaluate the water quality of Xiaowan Reservoir area.The spatial distribution of phytoplankton and water quality factors and their relationship with each other were analyzed by sequencing and clustering methods,and the important indicators of the reservoir,such as the structure of phytoplankton community,spatial distribution primary productivity and fishing productivity,were comprehensively analyzed.The results showed.(1)71 species of phytoplankton,34 species of chlorophyta,12 species of diatoms and 9 species of cyanophyta were identified in this survey.The composition and abundance of phytoplankton in the reservoir area were different from year to year,among which Chlorella,Phyllostella cupris,Phyllostella sirlostella and Phyllostella amphibians appeared in three consecutive years.From 2017 to 2019,there were 12 dominant species in Xiaowan Reservoir area,and the species composition of each year was similar.Apophysium amphibium,Chlorella limnoglass,Chlorella brachyclata and Sclerotium brachyclata were the dominant species for two consecutive years,and the composition of the dominant species changed significantly,among which Chlorella(2017),Sielotheca(2018)and Sclerotium brachyclata(2019)were the absolute dominant species in this year.The weighted modified Levins niche width formula and niche overlap index can be used to describe the distribution of dominant phytoplankton species in Xiaowan Reservoir,and show the adaptability,resource utilization ability and interspecific competition degree of different dominant species,and analyze the selection of species to the ecological environment in Xiaowan Reservoir.(2)Physical and chemical factors and evaluation of water quality:from 2017 to 2019,the comprehensive nutrient index(TLI)of Xiaowan Reservoir area was between 42.98-69.34,and the phytoplankton abundance was between 1.46-16.71×106cells/L.Q index varied from 1.90 to 3.76,Shannon-Wiener index(H')varied from 0.49 to 2.29,Pielou index(J)varied from 0.43 to 0.81,and Margalef(D)index varied from 0.31 to 1.55.Based on the comprehensive analysis of the six index evaluation results,Xiaowan Reservoir is in the medium-eutrophic level,and the water quality is in the state of light pollution-?-moderate pollution.From the change trend of three consecutive years,there is heterogeneity of plankton in different reaches of the reservoir,which is related to the spatial heterogeneity of precipitation conditions and nutrient input in the basin.Plankton abundance increased year by year,and the results of each index evaluation were basically the same for Xiaowan Reservoir.(3)Relationship between phytoplankton and water quality factors:According to the CCA analysis results of 12 phytoplankton dominant species and environmental factors from 2017 to 2019,the dominant species in 2017 and 2018 were positively correlated with dissolved oxygen(DO),water temperature(WT),pH value and transparency(SD).The nutrient was the limiting factor,and the dominant species in 2019 were positively correlated with most water quality factors.The results of CCA analysis showed that transparency(SD)and total nitrogen(TN)in 2017,nitrite nitrogen(NO2--N)in 2018,water temperature(WT)and pH values in 2019 were significant influencing factors for the succession of dominant phytoplankton species in Xiaowan reservoir area(P<0.05).According to the RDA analysis of 11 phytoplankton dominant functional groups in Xiaowan Reservoir from 2017 to 2019,nitrate nitrogen(NO3----N),phosphate(PO43--P)were used in 2017;In 2018,ammonia nitrogen(NH3-N)and nitrate nitrogen(NO3---N),total suspended matter(TSS)and ammonia nitrogen(NH3-N)were the significant influencing factors of phytoplankton functional group succession in Xiaowan reservoir area.According to the habitat characteristics of the corresponding points of the dominant functional groups,Xiaowan reservoir area is eutrophic water body.Taking dominance degree greater than 0.2 as the evaluation index,the annual succession rule of important functional groups is X1(2017)?X1/MP(2018)?)D/T(2019).RDA analysis shows that nitrate nitrogen(NO3---N)and ammonia nitrogen(NH3-N)are the main limiting factors in Xiaowan reservoir area,and Xiaowan reservoir area is in a nitrogen limiting state.(4)Based on the assessment of the primary productivity of phytoplankton in Xiaowan reservoir for three consecutive years,the black-and-white bottle method,the estimation of chlorophyll a assimilation coefficient and the phytoplankton biomass can all evaluate the spatial and temporal changes of the primary productivity of phytoplankton in Xiaowan reservoir area.The analysis shows that the estimation results of chlorophyll a method are stable,and the black-and-white bottle method has the highest accuracy.The average value of phytoplankton primary productivity in Xiaowan reservoir area was significantly higher than that in Lancang River area,and the productivity in front of the reservoir area was relatively high.The horizontal distribution of phytoplankton primary productivity showed the change trend of Heihui River>Dam Front Area>Lancang River.The average biomass of phytoplankton in Xiaowan was 1.76mg/L,2.56mg/L and 3.03mg/L,respectively.The average zooplankton biomass was 4.32,3.77 and 2.54mg/L,respectively.The total productivity of silver carp,bighead and bighead were estimated to be 166.40kg/hm2,161.61kg/hm2 and 129.76kg/hm2,respectively,according to the plankton stock(plant,animal and organic debris).The total productivity of Xiaowan water area was 2959940.8kg/year,and the sustainable fishing amount was 1479970kg.In order to make full use of the biological resources of xiaowan,control algal blooms and maintain the ecological stability of xiaowan reservoir area,the optimum ratio of silver carp and bighead fish fry is 1:2.9 in heihui river reach.In the heihuijiang river reach,the optimum ratio of silver carp and bighead fish fry should be 1:2.9,so as to make full use of the biological resources and water production potential of xiaowan,control algal bloom,maintain the ecological stability of xiaowan reservoir area,and realize the sustainable development of fishery in xiaowan reservoir area. |
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