The Evolution Of The Ecological Environment And Its Sedimentary Records In The Small Watershed Of West Dongting Lake In The Past Century

In this study,the sediment core in the small watershed of West Dongting Lake was investigated.The ²¹⁰Pb activities were measured for dating by CRS model(¹³⁷Cs technology validation).The characteristics of n-alkanes and n-fatty acid in the profiles of the sediment core were analyzed and compared.From 1855 to 2019,the environment evolution,ecological risks and the influencing factors in the watershed were discussed by combining n-alkanes indexes,sedimentary age,nutrient indexes and ecological risk assessment method.The results provide a scientific basis for the ecological restoration,and the interception and regulation of organic matter in Dongting Lake.The main results are as follows:(1)The sediment core in the small watershed of West Dongting Lake was spanned from 1855 to 2019 with a sedimentary history of 164 years.The specific activities of²¹⁰Pb_ex and ¹³⁷Cs varied from 18.85 to 299.58 Bq kg^-1 and 0 to 26.76 Bq kg^-1,with an average of 119.26 and 7.55 Bq kg^-1,respectively.The deposition rate ranged from 0.09 to2.94 cm a^-1,with an average value of 0.85 cm a^-1.Since 1855,the deposition rate showed a fluctuating upward trend,fell back around 1944 and then began to rise again.In 2000,the deposition rate exceeded to 1 cm a^-1 in a rapid sediment stage,which may be attributed by the increased contribution of exogenous inputs with the intensification of human activities.The grain size analysis of the sediment core showed that the particle size compositions in the watershed were mainly clay and silty with weak transport capacity and great effects by exogenous input.(2)The contents of TOC ranged from 3.15 to 27.32 g kg^-1,and the average value is12.59 g kg^-1.The TOC contents increased from 1855 to 2000,and a sharp increase was found after 1992.Since 2000,the TOC level decreased rapidly and then recovered slowly after 2008.The concentrations of TN and TP varied between 0.31?4.19 g kg^-1(mean value,1.60 g kg^-1)and 0.18?0.8 g kg^-1(mean value,0.46 g kg^-1),respectively.The trend of TN variation was similar to TOC,showing a significant positive correlation.The TP concentrations in the sediments showed a fluctuating rising trend and the overall level was high.The single pollution index reflected that ecological risks of TN and TP occurred after1930 and increased significantly after 1992.The average C/N ratio is 12.89,showing that the input was dominated by terrestrial plants.(3)The characteristics of n-alkanes and their indexes indicated the effects of terrigenous and endogenous mixed sources on organic carbon deposition.In the sediment core,the range of n-alkanes is from C₁₂ to C₃₅,and most of the characteristics were bimodal distribution.The long-chain n-alkanes showed a significant odd to even dominance,indicating a mixed input from multiple sources.The bottom layer(1855?1949)was dominated by short-chain n-alkanes and C_max was C₁₂,which has no bias to odd carbon and indicates the endogenous input.From 1949 to 2014,there was a significant bimodal distribution.The odd to even dominance of long-chain gradually appeared and its proportion exceeded that of short-chain n-alkanes,indicating that the input from mixed sources and the input of terrestrial higher plants was the main part.The surface layer(2014?2019)was characterized by short-chain n-alkanes,and C₁₆ was the C_max,showing the characteristics of oil input.(4)The peak of n-fatty acids in the sediments was consistent with the meaning of n-alkanes.The range of n-fatty acids was C_8:0?C_34:0,and the medium and long-chain n-fatty acids C_12:0?C_30:0 showed a significant bias to even carbon.The even to odd predominance of n-fatty acids corresponds to the predominance of n-alkanes in sediments,which is mutually verified.C_16:0 is the main peak in short-chain n-fatty acids,indicating that lake microorganisms such as bacteria and phytoplankton were the main contributors to organic matter.Large amounts of C_16:1,C_18:1,and short-chain acids C_8:0?C_11:0 indicated the input of bacterial source.It is inferred that organic matter in the sediment is a mixed source of terrestrial higher plants and bacteria.(5)Human activities had a great impact on the evolution of the ecological environment in the small watershed.Low level of CPI_H,was deficient,high dry and wet climate index and lacking P_r,P_h indicating that a humid climate and bacteria played a significant role in this period.From 1948 to 1992,the rapid increase of CPI and OEP values indicated that the contribution of organic matter of terrestrial plants was dominant,which might be caused by the reduction of lake area influenced by human activities i.e.land reclamation and afforestation,and the decrease of hydrodynamic force,which resulted in the deposition of terrigenous debris.From 1992 to 2008,the concentrations of TOC and TN increased rapidly,indicating that a large number of nutrients and organic matter were imported into the ecological environment of the lake by husbandry and fisheries.Algae burst in the water body significantly improved the nutritional degree,and the ecological environment deteriorated sharply.The project of"forest removal and moisture return"was carried out around 2013 in the study area,and the significant increase of ACL_H values indicated that the vegetation types were mainly herbaceous plants,and the ecological environment of the watershed showed a relatively stable trend.However,the decrease of surface CPI value,C_max of C_16,and rich in P_r and P_h indicated that oil pollution occurred during this period.