Source,Structural Characteristics And Ecological Indication Of Sediment DOM In The Primary Tributaries Of Dagu River Downstream

This study aims to analyze the contents of dissolved organic matter?DOM?,stable isotope(?¹³C),C/N ratio,three-dimensional fluorescence spectra,and potential ecological risk?heavy metal,nitrogen and phosphorus?based on the sediments of Dagu River downstream in Jiaozhou Bay.Carbon stable isotope is applied to calculate the relative contribution of five land uses to the sediment DOM of each tributary.Meanwhile,UV-Vis spectrophotometry and three-dimensional fluorescence spectroscopy are used to study the contents and structural characteristics of DOM in sediments,respectively.The correlation between DOM content and different forms of nitrogen and phosphorus is analyzed;and the potential ecological risks of heavy metals are also revealed.The results supplement and improve the carbon cycle of Dagu River downstream.It can provide a basic information for reasonable arrangement of land use and human activities in Dagu River and the prediction of potential risk of eutrophication in watershed.Meanwhile,it can reveal the spatial distribution of heavy metal risk in the study area for risk management.The specific conclusions are as follows:?1?In the sediments at the exports of the primary tributaries in the lower reaches of Dagu River,DOM content decreases from surface sediment to the bottom.The value of?¹³C increases with depth.In the land use around the river,DOM content,?¹³C and C/N ratios have shown obvious spatial differences.Among them,the content of DOM is related to the degree of human disturbance.The value of?¹³C in forest is close to C3plants,and the value of?13C in farmland is between C3 and C4 plants.The highest C/N ratio of forest is closely related to the high carbon content of vegetation litter;while the lower C/N ratio of other land use is mainly because it's affected by the high anthropogenic nitrogen application.?2?The results of the relative contribution of land use in the sediment DOM of Dagu River downstream are as follows:In Yunxi River,the allochthonous sources contributing most to the sediment DOM are industrial and rural area.The allochthonous sources with the largest relative contribution to the sediment DOM of Taoyuan River are forest and aquaculture pond.To the sediment DOM of the South Jiaolai River,the industrial sources contribute the most.?3?In the Dagu River downstream,the FI average value of sediment DOM is close to1.9,which indicates that the humus components of DOM are mainly from endogenous source of biodegradation.The HIX value increases with depth and the degree of humification of DOM is low.The BIX value decreases with the increase of sampling depth.In addition,five types of fluorescence peaks appear in the three-dimensional fluorescence spectra of sediment DOM.The fluorescence peaks of DOM in sediments from surface to bottom are similar,but the fluorescence intensity decreases gradually.?4?The contents of TN,NH₄⁺-N,NO₃^--N and A-P in sediments decrease with the sedimentary depth in the three watersheds.The DOM contents and humus-like components in sediments are strongly correlated with the contents of TN,NH₄⁺-N,NO₃^--N and A-P.Therefore,the release risk of nitrogen and phosphorus in sediment can be predicted by DOM contents and structural characteristics.?5?The potential ecological risk values of heavy metals in Dagu River downstream at 0-2 cm,2-5 cm and 5-10 cm are in the same order;which is Yunxi River>the South Jiaolai River>Taoyuan River.The potential ecological risk of heavy metals at 10-20 cm of sediment profile are different from the first three layers.The order is Yunxi River>Taoyuan River>the South Jiaolai River.This is mainly because the increase of forest and the decrease of industry in the source contribution of sediment DOM of the South Jiaolai River 50 years ago.