Isotopic Characteristics And Sources Of Organic Carbon In Water And Sediments In Lake Wuliangsuhai

Different forms of organic carbon in water and sediments are the main components of lake organic matter,and their stable isotope research is of great significance for effectively identifying lake eutrophication.We select Lake Wuliangsuhai as our research area to measure and analyze the indicator,such as δ¹³C,C/N and organic carbon,nitrogen content of organic matter in water and surface sediments of the lake area and the inflow canal in April（melting period）,July（summer irrigation period）,October（autumn irrigation period）in 2019 and January（freezing period）in 2020.We used δ¹³C and C/N in combination with isotope multivariate mixed model to investigate the sources and contributions of the lake organic carbon and the influence of environmental factors were analyzed.This study estimates the sources of organic carbon in the water dissolved,suspended particulates and bottom sediments in the submersible grass lake in the northern semi-arid area lake affected by the inflow canal,and provides an in-depth understanding for improving the lake pollution and studying organic carbon sources.The main conclusions are as follows:（1）The concentration of DOC in the Lake area and the inflow canal is between 1.97～17.63 mg/L.The seasonal variation trend of DOC in the lake area is:April>January>July>October.The δ¹³C_DOC ranged from -35.48‰ to -25.58‰,and the average values of δ¹³C_DOC in the lake area and the inflow canal were -28.27‰ and -27.54‰,respectively.The relative average endogenous contribution rate of DOC in Lake area is 40.25%,while the average exogenous contribution rate is 59.75%.There is no significant correlation between DOC, δ¹³C_DOC and the physicochemical indicators of the water,indicating that the δ¹³C_DOC is greatly affected by exogenous sources.（2）The POC of the Lake Wuliangsuhai is 0.31%～26.65%,and the average value of the lake area（2.96%）is higher than inflow canal（1.88%）.The POC/PON in Lake area fluctuates in the range of 1.68～17.41,showing obvious temporal and spatial heterogeneity.The range of δ¹³C_POC in the lake area is -35.6～-23.68‰,with an average value of -29.48‰.POC mainly comes from phytoplankton and silt,and the contribution of phytoplankton is the most important,at 73.29%,followed by silt（17.44%）. δ¹³C_POC has a significant positive correlation with POC/PON,TP,SPM,and has no significant correlation with other physicochemical indicators.（3）The sediment TOC range of study area is 0.06～10.77%,and the TOC content is:the lake area（4.60%）>the inflow canal（0.38%）.The C/N ratio of the lake area between 2.60～25.58,with an average value of 12.77.It first decreases and then increases from into the lake area to out of the lake area.The average value of δ¹³C_TOCin the lake area is -23.57‰,with a seasonal trend of July>October>January>April.The spatial distribution is as follows:into the lake area is heavier,the lake center area and out of the lake area is light.TOC mainly comes from silt（44.60%）and aquatic macrophytes（38.23%）,and phytoplankton（17.18%）also has a relatively small contribution.C/N and Chl-a have a very significant positive correlation.Similarly,TOC has a very significant positive correlation with C/N and δ¹³C_TOC.