Source,content And Structure Characteristics Of DOM And Their Ecological Indications In Surface Sediments Of Dagu River Estuary

The estuary area is an important area connecting rivers,oceans and land.In recent years,with the rapid development of industrial and urbanization,the pollution in the estuary area has become increasingly serious.The dissolved organic matter?DOM?in the estuary area,which is closely related to the pollution problems,is attracting widespread attention.This study was conducted based on the DOM of surface sediments in Dagu River estuary.The sediment samples of the Dagu River from the downstream to the estuary and the soil samples of the surrounding main land use types were studied.The content was mainly to explore the source,content and structural characteristics of DOM in the surface sediments of the Dagu River estuary and its ecological indications?expressed by the potential ecological risks of heavy metals and eutrophication?.The relative contributions of DOM sources were calculated using a carbon-stable isotope mixture model(¹³C and C/N values);the spectral characteristics of DOM in sediments of the Dagu River estuary were determined by UV-Vis and three-dimensional fluorescence spectra.In addition,the potential ecological risks of heavy metal and N,P release were also analyzed and evaluated using analytic hierarchy process and in-situ measurement.Finally,the relationship between the source,content,structural characteristics of DOM and potential ecological risks?heavy metals,N and P?was analyzed.This study aims to provide new ideas for the study of DOM ecological indications and provide basic information for environmental management and land use planning of coastal small watersheds.The main research conclusions are as follows:?1?The main sources of sediment DOM in Dagu River estuary area are terrestrial sources,sea sources,and spontaneous sources.The surface sediments of the Dagu estuary contain a variety of terrestrial sources.This is because large amounts of organic matter carried by the Dagu River and its tributaries is deposited at the estuary;the relative contribution rate of the DOM in the sediments of the estuary area in the rural areas and aquaculture,up to 75%and 35.4%,respectively.The reason is that human activities have great impacts on the material circulation of the estuary;the contribution rate of farmland and forest land in the basin to the DOM of sediments at all points is generally small.At the farthest point from the sea,the industry contributes 93%to the DOM,which is related to the discharge of a large amount of industrial wastewater from nearby industrial areas;?2?DOM content of sediment in estuarine area is high at both ends and low in middle.The DOM content of downstream river is higher than that at the estuary.The order of DOM content of different terrestrial soils in the watershed is:aquaculture pond>rural area>industrial land>farmland>forest land;The C/N value of industrial land is the largest and the C/N value of culture pond is the lowest;the?¹³C value?-21.03‰?of the breeding pond is the highest,while the?¹³C value of the woodland is the lowest,which is close to the C3 plant?¹³C value range.?3?DOM in downstream rivers sediments has higher aromaticity,humification degree,content of hydrophobic components and molecular weight of DOM than that in estuary;there are five different peaks of fluorescence in the fluorescence spectrum of the sediment DOM.From the downstream river to the estuary of the Dagu River,the degree of humification of the DOM decreases;the fluorescence index?FI?value is between 1.61and 1.99?the average is 1.74?,and the recent autochthonous source index?BIX?value is between 0.59 and 2.73?the average is 1.31?,indicating that the DOM in the sediments of the Dagu River estuary is affected by both external input and biological metabolism.?4?The DOM content and structural characteristics reflect that the potential ecological risks of single and comprehensive heavy metals in downstream river are higher than those in the estuary.Moreover,the middle zone has the highest potential for heavy metal risk;the potential risk of eutrophication in the downstream river is low,because humic-like substances of DOM will prevent the release of excessive nutrients;when the relative contribution rate in rural areas is high,the proportion of protein-like substances in the sediment DOM will also be high.While there is no significant relationship between farmland,forest and the composition characteristics of DOM.Specifically,rural areas are associated with high nitrogen ecological risks and high heavy metal risks.Industry is associated with low nitrogen ecological risks and heavy metal risks,and aquaculture may be associated with high nitrogen ecological risks.