Spatiotemporal Variation And Difference Of DOM Fluorescence Characteristics Based On Intra-annual And Inter-annual Scales In Tidal Reach

The tidal reach,influenced by the tide jacking,is mixed with seawater and freshwater.It has become the key research area for the complex and changeable hydrological regime.Dissolved organic matter（DOM）is widely found in various natural water,and its complex composition is closely related to the migration and transformation of pollutants.Now it is defined as an emerging indicator for the evaluation of water environment quality in recent years,and the analysis of the distribution and sources of DOM has become a research hotspot in water environment issues.At present,DOM studies under the influence of tidal fluctuation in tidal reach are rarely seen.Therefore,Minjiang tidal research was selected as the research area and nine cross-sections（i.e.S1-S9）were investigated.The spatiotemporal variations of DOM 3D fluorescence were systematically studied at the intra-annual scale（high-flow,mean-flow and low-flow periods,respectively）and the inter-annual scale of the low-flow period from 2018 to 2020 were investigated,respectively.In addition,the difference analysis of DOM fluorescence spectrum of the whole tidal reach was conducted by using the theory of the cosine similarity and histogram similarity,and the changes of the pollution sources in the research area were preliminarily analyzed.The main research results were as follows:The salt water intrusion phenomenon was obvious in Minjiang tidal reach.From the intra-annual scale,the salinity intrusion distance was the longest in the low-flow period,followed by the mean-flow period and the low-flow period successively.From inter-annual scale,the salt water during the low-flow period in 2019 and 2020 could intrude to the Chengmen cross-section.While in 2018 it intruded to the Langqi cross-section.The p H values in ebb tide showed a decreasing trend from low-flow period to mean-flow period and then to high-flow period in intra-annual scale.However,it increased year by year.From the perspective of interannual variation,DO showed a decreasing trend during ebb tide and an increasing trend during flood tide.From the intra-annual variation,phosphorus concentration in high-flow period was higher than that in low-flow period.While there were no same trend between nitrogen speciation.Finally,from the inter-annual scale,the concentrations of nitrogen and phosphorus decreased year by year from 2018 to 2020,which indicated that the water quality in the study area was gradually improved.Three-dimensional fluorescence spectroscopic parallel factor method（PARAFAC）was used to analyze the annual variation of DOM in the high-flow,mean-flow and low-flow periods of 2019.The results suggested that there were three kinds of DOM fluorescence component in the research area,namely,C1 component for soluble organic metabolites,C2 component for humic-like fluorescence and C3 component for tryptophan proteins.Tryptophan proteins were dominant in the reaches affected by human production activities,which indicated that DOM concentration and composition could be significantly changed by the urbanization process.Based on cosin and histogram similarity analysis,the fluorescence map similarity of upstream section of S1-S3 was high,showing that the upstream DOM was relatively stable.The C3 showed up in the cross-sections from S4 to S7 which indicated that the exogenous pollutants were discharged in the Fuzhou urban area.The main component was C1 at S8 and S9,indicating C1 was the main DOM in the estuarine areas.The annual concentration of fluorescent dissolved organic matter（FDOM）was higher in the low-flow period than that in the high-flow period,and higher than that in mean-flow period.DOM components at S1-S3 stations in three periods were similar.While the similarities decreased at the urban area from S4 to S6 station,which indicated that the presence of exogenous substances.Finally,DOM at S7 was characterized by seawater intrusion.The inter-annual analysis for DOM in the low-flow periods from 2018 to 2020 using the PARAFAC method showed that there were three fluorescent fractions in the research reach.The inter-annual spatial distribution of DOM is similar to the intra-annual pattern.Meanwhile,the concentration of FDOM showed a decreasing trend from 2018 to 2020,which was consistent with the decreasing trend of nitrogen and phosphorus concentration.In addition,from 2018 to 2020 the DOM composition similarities for S1-S3 stations were consistent,and S4-S6 stations were also similar.Besides,finally,the similarity of S7-S9 near the estuarine areas can directly determine the intrusion range of salt water.