Study On Distribution And Interspecies Relationship Of Plankton In Northern Large Reservoirs During Open Water Period And Ice-covered Period

To better utilize water resources,adapt to the carrying capacity of the water ecosystem,and promote sustainable development of the economy and society through the sustainable utilization of water resources,the idea of"giving priority to water-saving,spatial balance,systematic management,and concerted efforts"needs to be implemented.Accurately grasping the real-time situation of water ecology,understanding the framework of water ecology,and exploring the driving factors and evolution rules of water ecology is crucial,especially since China has the largest number of reservoirs in the world.Due to the serious pollution of rivers,an increasing number of reservoirs are being used as drinking water sources.These sources are not only the red line of ecological protection but also the lifeline of human beings.The Xiquanyan Reservoir,located in Heilongjiang Province in northeast China,is an essential surface drinking water source.Its hydro-ecological services play an irreplaceable role in supporting economic and social development.There is a gap in the comprehensive analysis of the water ecosystem services of reservoirs in China,as well as research on the ecosystem services function and value of reservoirs,especially as an important drinking water source.Although the water quality of Xiquanyan Reservoir is good,the potential impact of long-term exposure to environmental and trace pollutants on human health cannot be ruled out.In this regard,we carried out a study to explore the service function of Xiquanyan Reservoir and the factors affecting its water quality,to develop a long-term plan for the sustainable operation of the reservoir and assist the reservoir management department in carrying out targeted management to better serve human beings.A total of seven sampling surveys were conducted in May,July,and September during the open water period,and in December,January,February,and March during the ice-covered period in Xiquanyan Reservoir.The results showed that 160 species of phytoplankton were identified during the open water period,belonging to 64 genera and 8 phyla.The biodiversity index showed light pollution in May and September and moderate pollution in July.The upper,middle,and lower water layers were all lightly polluted.Phytoplankton can be divided into 19 functional groups.The relative biomass of functional group D was the highest in all layers.The relative biomass of functional group C was the highest in May,while that of functional group D was the highest in July and September 2021.A total of 90 species belonging to 4 classes of zooplankton were identified during the open-water period.The biodiversity index showed moderate pollution in May,July,and September.The upper,middle,and lower water layers were all moderately polluted.The highest relative biomass was group LCF（Large copepods filter feeders）in the upper and lower water layers and group MCF（Middle copepods and cladocera filter feeders）in the middle water layer.The relative biomass of functional group RF（Rotifers filter feeders）was the highest in May,and the relative biomass of functional group MCF was the highest in July and September.A total of 99 phytoplankton species belonging to 62 genera and 8 phyla were identified in Xiquanyan Reservoir during the ice-covered period.The diversity index showed that water quality was lightly polluted in December,January,February,and March.The upper,middle,and lower water layers were all lightly polluted.Phytoplankton can be divided into 18 functional groups,among which the number of species in functional group J was the largest,followed by groups MP and Y.The horizontal,vertical,and temporal distributions of phytoplankton functional group biomass were significantly different.A total of 72 zooplankton species belonging to 4classes were identified during the ice-covered period.The diversity index showed that all water layers were at a lightly polluted status during the study period.Zooplankton can be divided into 7 functional groups.Based on the horizontal,temporal,and vertical distributions of the relative biomass of functional groups,RF showed the highest biomass in all water layers.The analysis of the trophic level index（TLI）revealed the presence of three trophic levels in Xiquanyan Reservoir during the study period,namely mesotrophication,mild eutrophication,and moderate eutrophication.The open-water period showed moderate eutrophication,while the ice-covered period exhibited mild eutrophication,with a significant difference between the two periods（p<0.01）.Water temperature（WT）,water depth（D）,p H,ammonia nitrogen（NH₃-N）,chlorophyll（Chl-a）,chloride ion（Cl-）,total nitrogen（TN）,total phosphorus（TP）,chemical oxygen demand(COD_Mn),and dissolved oxygen（DO）exhibited significant correlation with phytoplankton functional groups during the open water period.Additionally,WT,turbidity（NTU）,COD_Mn,and TN showed a significant correlation with zooplankton functional groups.Chl-a exhibited a significant correlation with all functional groups across all layers during the ice-covered period.In the middle water layer,p H exhibited a significant correlation with all functional groups,while in the lower water layer,D and WT exhibited a significant correlation with all functional groups.It can be seen from the plankton ecological network diagram that the plankton interaction network in the Xiquanyan reservoir has a clear modular structure（MD>0.4）,and the plankton interaction is mainly positive and mutually beneficial.In the upper,middle and lower water bodies of the ice-covered period,the boundary relationship（%）was as follows:phytoplankton-zooplankton>phytoplankton-phytoplankton>zooplankton-zooplankton,indicating that the relationship between phytoplankton and zooplankton was closer in the ice-covered period.In conclusion,the ice-covered period decreased the phytoplankton relationship in Xiquanyan Reservoir,but increased the interaction between phytoplankton and zooplankton.In summary,the characteristics of plankton community structure in Xiquanyan Reservoir are different in spatial and temporal distribution,and have obvious stratification phenomenon during the open water period and the ice-covered period.This difference is mainly caused by water environment factors,and the change of water environment factors also changes the closeness of plankton connections in Xiquanyan reservoir.This paper deepens the understanding of the structure of biological communities in reservoirs at different periods,and also deepens the understanding of the influence of water environmental conditions on the interspecific interactions between primary producers and consumers.