Study On Spatial Distribution And Photosynthetic Characteristics Of Phytoplankton In Danjiangkou Reservoir

Danjiangkou Reservoir as the water source of the South-to-North Water Diversion Project,supplies water to more than 20 cities in China,and its water quality safety has been widely concerned.According to the investigation of water quality in the reservoir area in recent years,the water quality in the reservoir area is good and meets the standard of Class I-II water,but the high concentration of total nitrogen（TN）is still a problem that cannot be ignored.Nitrogen as one of the essential nutrients for phytoplankton growth and metabolism,different concentrations and forms of nitrogen nutrients are closely related to phytoplankton growth.As the most important primary producers in water bodies,changes in the community structure of phytoplankton are often used to monitor the environmental conditions of water bodies,their photosynthetic characteristics are important for understanding primary production and material cycling in aquatic ecosystems.Maximum quantum yield of PSII（F_v/F_m）,light utilization efficiencies（α）,maximum electron transfer rate（ETRmax）and semi-saturated light intensity（I_k）are the representative indexes in the study of photosynthetic activity of phytoplankton,which helps to explain the light adaptation mechanism of phytoplankton community in a certain living environment.However,little research has been reported on the distribution characteristics of phytoplankton community structure and photosynthetic characteristics of phytoplankton in deep-water.In this study,the community structure characteristics of phytoplankton between0.5 to 20m and photosynthetic characteristics of phytoplankton community at 0.5m water depth were systematically studied in seven different areas of the reservoir using a combination of high-throughput sequencing,dual-channel chlorophyll fluorometry,redundancy analysis and correlation analysis,and the main environmental factors affecting the phytoplankton community structure and photosynthetic capacity were analyzed.And the representative algae species of diatoms were selected for indoor simulation to analyze the physiological effects of different nitrogen and concentration on algal cells and chlorophyll fluorescence characteristics,the main results are as follows.（1）The results of the high-throughput sequencing showed that a total of 63genera of phytoplankton belong to 12 phyla were detected,and phytoplankton community were mainly composed of Dinophyceae（41.0%）,Bacillariophyta（35.3%）,Chlorophyta（13.3%）,Haptophyceae（7.5%）,Chrysophyceae（1.9%）and so on.The phytoplankton abundance index（Chao index varied from 89 to 735,Observed species index varied from 86 to 676）and diversity index（Simpson index varied from 0.09 to0.96）indicated that there were large differences in phytoplankton abundance and diversity between 0.5 to 20 m at each sampling site.In particular,significant differences in the diversity of phytoplankton communities on vertical distribution were found.The diversity and abundance of phytoplankton community in Songgang（SG）,Tumen（TM）,and Dangzikou（DZK）decreased with the increase of water depth,and the relationship between phytoplankton and water depth in other sampling sites was not obvious.（2）Pielou-evenness index and Shannon-Wiener diversity index were used to evaluate the water quality of the reservoir,and the results showed that the mean value of Pielou-evenness index was 0.45±0.06,and the water quality of Danjiangkou was judged to beβ-middle pollution zone.The mean value of Shannon-Wiener index was3.79±0.54,judging that the water quality of Danjiangkou Reservoir was light pollution.Ammonia(NH₄⁺-N,0.08-0.15mg·L^-1),p H（8.66-8.96）and conductivity(274-308μS·cm^-1)and other water quality physicochemical indicators show that the overall good water quality in the reservoir area meets Class I water standards,and the TN concentration was 0.68～1.15 mg·L^-1,which meets Class III-IV water standards,compared with 2019,the water quality in the reservoir area in 2021 has been improved.Redundancy analysis（RDA）showed that nitrate（NO₃^--N）was the most important environmental factor affecting the vertical distribution of phytoplankton in the reservoir,followed by water depth,dissolved oxygen（DO）,p H and water temperature（WT）.（3）The rapid light curves of phytoplankton at 0.5 m in summer in the Xichuan reservoir area of Danjiangkou reservoir included three stages of light limitation(0-200μmol·m^-2·s^-1),light saturation(200-500μmol·m^-2·s^-1)and photoinhibition(500-800μmol·m^-2·s^-1),phytoplankton in the reservoir area had a moderate photosynthetic physiological state（mean value of F_v/F_mwas 0.30）,growth potential（αvaried from 0.10 to 0.16）and high photosynthetic potential(mean value of I_kwas 237.5μmol·m^-2·s^-1).（4）Study on the representative species of diatoms（Aulacoseira granulata）in Danjiangkou Reservoir,found that different concentrations and forms of nitrogen had significant effects on chlorophyll fluorescence parameters of algal cells（p<0.05）,and the overall trend of F_v/F_m,I_kand ETRmax values showed an increase followed by a decrease,and the trend ofαvalues showed an increase followed by a decrease and then an increase.The results indicated that within a certain concentration range(0.5-2mg·L^-1),increasing the NH₄⁺-N concentration will promote the photochemical reaction of algal cells,higher concentration of ammonia nitrogen will inhibit the photosynthesis of algal cells,and the nitrogen-limited condition(0.5 mg·L^-1)of Aulacoseira granulata will improve the value ofαand thus improve the environmental adaptation.