Temporal And Spatial Variation Of Dissolved Organic Matter In Jinfo Mountain Surface River Based On Lipid Biomarkers

The karst dynamic system is highly open and sensitive to the changes of the external environment.The surface karst system constantly exchanges materials and energy with the outside world,and carbon elements can be actively exchanged in the process of water-rock-gas interaction.The weathering of carbonate rocks sends CO₂into the ocean in the form of HCO₃^-,which fixes about 600 million tons of carbon every year through karst action,thus affects the global climate.However,the carbon sink effect caused by karstification has not been widely recognized by the academic community.In recent years,through in-depth research on biological photosynthesis,it has been found that aquatic photosynthetic organisms can partially transform inorganic carbon into organic matter and form endogenous organic carbon by using dissolved inorganic carbon.A carbonate weathering carbon sink model considering water-rock-gas-generation interaction is gradually formed.DOM is an important organic carbon pool of the earth and a key link in the process of global carbon cycle.Strengthening the DOM study on karst surface rivers is of great significance for a deep understanding of the stability of karst carbon sinks.DOM is diverse in structure and complex in composition,while lipid biomarkers are stable in the process of evolution,which can maintain the carbon skeleton of the original biological components and record the information of the source and precursor.Lipid biomarkers can reproduce the transport and transformation of organic matter during its evolution.Therefore,the most stable n-alkanes and fatty acids are often used for source resolution of DOM.This study is based on the surface river in Jinfo mountain which located at Chongqing,measured by GC/MS.the temporal and spatial changes of n-alkanes and fatty acids in karst surface river are taken as the main line of the study.This paper aims to study the DOM content,composition,source,migration and change characteristics in shizhong river of jinfoshan.The results show that:?1?The concentration of n-alkanes ranges from 369 to 13719 ng·L^-11 in Shizhonghe River,and the concentration of fatty acids ranges from 2023 to 33773 ng·L^-1.The spanning ranges are large,and there are significant differences in temporal and spatial variations.In terms of time variation,the concentrations of n-alkanes and fatty acids in rainy season are higher than those in dry season.Temperature and precipitation are the main factors affecting the DOM concentration in surface rivers.In terms of spatial variation,the concentration differences of n-alkanes and fatty acids in the upper and lower reaches of the river in dry season are small,while the difference increases in rainy season,and the concentration in the lower reaches is higher than that in the upper reaches.The content of DOM responds positively to the change of heavy rainfall weather.High intensity precipitation can cause the concentration of n-alkanes and the sharply rising of fatty acids.Source analysis reveals that a large amount of input of polygenic higher plants and mature organic matter are obtained.?2?The contents of fatty acid components in Shizhonghe river are as follows:SSFA>MUFA>PUFA>BSFA.Spatial and temporal variations also occur in each component.SSFA has the highest proportion,BSFA has the smallest proportion,MUFA has the most significant seasonal variation and PUFA has the smallest annual variation.The proportion of SSFA and BSFA decrease slightly in the rainy season,the proportion of MUFA increase slightly in the rainy season,the proportion of PUFA rise slightly in the middle reaches and decrease slightly in the upper and lower reaches.Different sources of DOM contribute different components of fatty acids,bacteria,algae and other microorganisms and higher plants contribute different causes of DOM content and composition.Seasonal changes and different land use patterns in upstream and downstream affect the source of river water,thereby affecting the content and composition of DOM in river water.?3?Seasonal variations of temperature and precipitation affect the freshness of organic matter.As a whole,with the movement of rivers,organic matter appears to be adsorbed,decomposed and transformed,and degraded from top to bottom.The difference is that the effect is stronger in dry season and weaker in rainy season.In dry season,the precipitation is less and the input of exogenous substances is less,which has a weak influence on the disturbance of rivers.Natural migration is characterized by the degradation of degradable unsaturated fatty acids from top to bottom.In rainy season,a large amount of fresh organic matter is imported into rivers.The influence of external environment on rivers hinders the natural migration process of rivers and weakens the differences among rivers.?4?Source analysis of DOM indicated by n-alkanes and fatty acids using characteristic parameters shows that they have similar provenance indicators.In dry season,DOM in the whole reach of the river is obviously controlled by bacteria and phytoplankton.The proportion of DOM from bacterial sources is more than 50%,the maturity of organic matter is low,the source species is single,and the difference between upstream and downstream is small.With the arrival of the rainy season,especially after heavy rainfall in May,the input of higher plants increases,and the input of lower reaches is the largest.However,as a whole,higher plant-derived organic matter has not been dominant.In the rainy season,the maturity of organic matter also increases.The maturity of organic matter increases from upstream to downstream.The maturity in downstream areas with more frequent industrial and agricultural activities and more serious soil erosion is higher.Although the DOM of bacterial source in rainy season is affected by the dilution of precipitation,the proportion of bacterial source in each reach decreases,but it still occupies the dominant position.The DOM of phytoplankton source is similar to that of higher plant source.With the increasing of temperature and precipitation,the contribution increases,but the proportion is always low.The fungi in the lower reaches contributes more,and soil erosion is more serious.