Effects Of Flood Discharge On Physicochemical Properties And Nitrogen Transformation Of Lake Xingkai

Nitrogen cycle in the water body is one of the important chains for the circulation of material and energy in the ecosystem.The transformation of nitrogen is the key to the nitrogen cycle in the water body,and also an important index for evaluating the eutrophication of water body.Environmental factors are closely related to nitrification and denitrification.Dissolved organic matter is the main source of organic matter in natural aquatic ecosystem,it is also the main material used by the organism in the water body.Planktonic bacteria exist in freshwater ecosystem and are important components of bio-geochemical reactions in natural freshwater biochemical system.Changes of these two factors and other physical-chemical properties including Dissolved organic carbon（DOC）,Chemical Oxygen Demand(COD_Mn),Chlorophyll a（Chl-a）,pH,dissolved oxygen（DO）,tempertature（T）in water all exist and affect the cycling of nitrogen in aquatic ecosystems.Lake Xingkai is located in the southeast of Heilongjiang Province.Every year in flood season,when volume of the upper reaches or the reservoir water exceeds the warning line,water from Small Lake Xingkai will be introduced into Lake Xingkai through the floodgate.The floodgate operation is one of the major flood control measures for the development of the lake.However,less literatures have been reported on the relationship between the floodgate operation and nitrogen transformation.Therefore,in this study,the water samples from six sampling points in Lake Xingkai before（July）and after floodgate operation（August and September）were selected.Two-dimensional fluorescence spectra and three-dimensional fluorescence spectra were used to characterize the effect of floodgate operation on dissolved organic matter in Lake Xingkai.The composition and diversity of planktonic bacterial communities in flood season were analyzed by denaturing gradient gel electrophoresis.Combined with parallel factor analysis,residual analysis and structural equation model,the impact of physicochemical factors,dissolved organic matter and plankton community composition on nitrogen transformation in Lake Xingkai in flood season were studied.The main results were as follows:（1）DOC significantly increased after floodgate opened,indicating that the DOM was introduced into Small Lake Xingkai.Dissolved oxygen（DO）decreased after the floodgate opening and increased again in September,suggesting that DO participates in nitrogen transformation during floodgate operation process.（2）Three components of DOM of Lake Xingkai in flood season was obtained using paralleling factor PARAFAC analysis:C1,C2 and C3,in which component C1 increased after gate opening,and components C2 and C3 showed a positive correlation.Results suggested that gate opening impacted on the fluorescence characteristics of DOM in Lake Xingkai.（3）Results of denaturing gradient gel electrophoresis（DGGE）showed that the diversity of planktonic bacteria in Lake Xingkai in flood season was very rich,and the composition of community changed obviously with the opening floodgate.Results of 16S rDNA sequencing showed that the predominant population of Lake Xingkai was actinomycetes,α-Proteus,Acidobacter and Green Beans.（4）The concentration of TN,NH₄⁺-N,NO₃^--N and NO₂^--N in Lake Xingkai during the flood season were significantly affected by the operation of the sluice.TN and NO₃^--N concentrations decreased and then increased,NH₄⁺-N decreased slightly while NO₂^--N decreased significantly.Analysis showed that flood discharge affects the transformation of nitrogen transformation in Lake Xingkai.（5）Structural equation modeling and redundancy analysis showed that T,pH,COD_Mn,DO and Chl-a are related with each other.Microbial community was influenced with their changes before and after the gate operation.NMDS Axis 1 had an indirect negative impact on NO₂^-by affecting NH₄⁺.NMDS Axis 3（e.g.,Acidobacillus）showed a significant negative impact on NO₃^-.DO had a significant positive impact on NO₃^-on NO₂^-content.DOC showed a negative impact on NH₄⁺content.Meanwhile,there was a positive correlation between component C1 and each bacterial band,while a positive correlation between C2,NH₄⁺and NO₂^-also showed,and C3 had no effect on BCC,indicating that C1and C2 were easily utilized by bacterioplankton and played an important role during the process of nitrogen transformation.In summary,it is important to assess the impact of floodgates on downstream aquatic ecosystems prior to being discharged into downstream watersheds.