The Effect Of Particulate Organic Matter On Danjiangkou Reservoir Was Discussed From The Perspective Of Phytoplankton

The Danjiangkou Reservoir is the water source of the Middle Route of the South-to-North Water Diversion Project.With a total length of 1,432 km,the Danjiangkou Reservoir supplied water to north China to alleviate water shortage.Therefore,its quality played an important role in the safe operation of the water diversion project.In previous investigations and studies of the Danjiangkou Reservoir,most of them focused on the spatial scale of water surface,but few studies on the vertical distribution characteristics of phytoplankton and particulate organic matter.In this artical,five sampling sites in the Danjiangkou Reservoir were sampled quarterly in three layers from 2021 to 2022,and water environment investigation and isotopic source analysis of particulate organic matter were carried out simultaneously.The main results were as follows:1.The difference of water environmental factors of temporal variation was greater than that of spatial variation.PCA analysis showed that TOC,PO₄^3--P,DO and p H were the main influencing variables of vertical distribution of environmental factors.The variation of water environmental factors was related to seasonal variation,the input of exogenous nutrients and human disturbance.The appearance and disappearance of thermal stratification affect the variation of vertical section of reservoir.2.A total of 95 species（including varieties）of 62 genera and 7 phyla were identified in the Reservoir.In terms of species richness,Chlorophyta accounted for 34%of the total species.The Bacillariophyta and Cyanbacteria accounted for 32%and 17%of the total species,respectively.At the same time,the relative abundance of the dominant phyla in the surface,middle,and bottom strata was Cyanbacteria,Cyanbacteria-Bacillariophyta,and Bacillariophyta.In addition,the dominant species in the middle layer and bottom layer were Bacillariophyta,followed by Cyanbacteria-Chlorophyta.Shannon-Wiener diversity index analysis showed that H’in the surface was higher than that in the bottom of the Reservoir.RDA analysis showed that the main environmental factors affecting phytoplankton community change in the surface layer were DO,PO₄^3--P,pH,WT,TOC,COD_Mn,NH₃-N,and EC.The main environmental factor driving the succession of middle-level phytoplankton was PO₄^3--P.At the same time,p H,COD_Mn,TOC,EC,PO₄^3--P,NH₃-N,and DO were the main environmental factors driving the succession of the dominant species in the bottom phytoplankton.3.The Iso Source model showed that theδ¹³C source contribution of pico-POM and nano-POM was similar in the wet season,and both of them mainly came from phytoplankton and soil organic matter.The contribution rate of atmospheric dry and wet deposition sources to nitrogen in wet season was significantly higher than that in dry season.The contribution rates ofδ¹⁵N from POM with different particle sizes（except pico-POM）were the same during dry season,which were human and animal excreta>industrial and domestic sewage>aquatic plants and phytoplankton>terrigenous plants>soil nitrogen loss>fertilizer>atmospheric dry and wet deposition.Correlation analysis showed thatδ¹³C of POM in all particle size ranges was negatively correlated with PO₄^3--P（p<0.05）,whileδ¹⁵N of POM was significantly positively correlated with TOC（p<0.01）.