Seasonal Variation Characteristics And Sources Of N-alkanes In Water Column Of Fuxian Lake

n-alkanes in lake sediments have been widely used for palaeo-environmental reconstruction.The n-alkanes are mainly derived from plant waxes,which is the first contact surface between plants and the external environment.When the external environmental factors such as degree,humidity and illumination change,plant waxes change their physiological functions in order to adapt to the changes of environment and climate.Therefore,n-alkanes contain the information of environment and climate of abundance.In addition,due to the high carbon-carbon bond in the n-alkane molecules,the structure is relatively stable,and has a strong resistance to the degradation of the surrounding microorganisms.The n-alkane is not easy to be degraded,so the n-alkane in the sediments can still truly reflect the regional environment and climate change in the historical period.However,the source of n-alkanes is complex in different environments.The distribution and source of n-alkanes in the transport and sedimentation process in water will be disturbed by microorganism modification and secondary compounds generated.Therefore,using n-alkanes to reconstruct the palaeo-environment becomes more complex,and it is necessary to study the source of n-alkanes in suspended particulate matter of water column and the contribution of n-alkanes to sediments.In this study,the distribution characteristics,biological sources and seasonal variations of suspended par ticulate matter in water column of Fuxian Lake from April 2016 to March 2017 were studied.The results are as bellow:1.The n-alkanes with carbon numbers ranging from C₁₇to C₃₃,showing a single peak distribution pattern with dominant peaks C₂₇ and C₂₉.The vertical distribution of n-alkanes demonstrate the relative abundance of n-alkanes above the thermocline is higher than that below the thermocline.The maximum concentration of n-alkanes occurs in?20 m depth.The relative abundance of n-alkanes in water columns is weakly affected by sediment resuspension.The obvious seasonal fluctuation in n-alkanes with the highest value in October and April can be found.2.The seasonal variation trends of C₂₅?C₂₇?C₂₉ n-alkanes are consistent with that of phytoplankton biomass,which might proves that the phytoplankton is the main source of n-alkanes in water column of Fuxian Lake.Water temperature is one of the key factors to limit the spatial and temporal distribution of n-alkanes in water column of Fuxian Lake.3.The contribution of n-alkanes in suspended particulate matter with different carbon numbers to the n-alkanes in surface sediments is different,which leads to the obvious difference in the distribution patterns of n-alkanes.The distribution of n-alkanes in the surface sediments of Fuxian Lake showed a bimodal pattern,which indicates that the sources of n-alkanes belong to the mixed sources of endogenous and exogenous sources.The high values of C₃₁ and C₂₃ n-alkanes in sediments extremely correspond to terrestrial higher plants in Fuxian Lake,which indicates n-alkanes(C₃₁ and C₂₃)in sediments could be a dependable index for palaeo-environmental reconstruction.But the sources of high abundance of long-chain n-alkanes C₂₇ and C₂₉contain part of the contribution of phytoplanktons,macrophytes and terrestrial plants.so it is not suitable to be used in the reconstruction of palaeo-environment in the sediments of Fuxian Lake.