兴凯湖百余年营养演化历史及营养物基准

Nutrient history in the past century and its baseline of Xingkai Lake

兴凯湖是东北亚地区典型湖泊。20世纪末以来人类活动持续增强,该湖营养快速富集,水体生态状况发生转变,但其环境演化历史及营养本底等信息尚知之甚少。本研究基于对该湖多根沉积钻孔年代、元素及沉积硅藻等多指标的分析,重建了兴凯湖百余年来的环境变化历史。结果表明:兴凯湖历史上保持着良好的环境生态条件,21世纪第一个十年以前水体环境变化较小(硅藻种群弦距<0.0005),近20 a来,兴凯湖生态环境逐渐退化,硅藻从以低—中营养Aulacoseira granulata的优势组合逐渐转变为以Cyclostephanos dubius为代表的富营养组合,污染元素(Pb、Zn等)含量及沉积速率快速升高。基于种群相似性的对比发现2000 a前的硅藻种群变化较小,故采用1994—1998年的水体监测均值作为兴凯湖的营养基准,即水体总磷(TP)、总氮(TN)基准分别为35μg·L^(-1)及970μg·L^(-1);结合参比元素回归分析得到的大、小兴凯湖沉积物TP基准分别为360 mg·kg^(-1)、500 mg·kg^(-1),可作为该湖的沉积物TP浓度的治理基准。上述系列水土营养基准可为兴凯湖日益严重的湖泊富营养化治理提供重要的参考目标。

Background,aim,and scope Nutrient baseline can provide important information for lake eutrophication control and its restoration.The Xingkai Lake watershed has rich vegetation types and high biodiversity,which has essential ecological impact on the lake environment.Due to the continuous enhancement of human activities since the end of the 20th century,the nutrient level of the lake has increased rapidly,and its ecological status has changed.However,the information about its nutrient evolution history and natural background is still unknown.In this study,we reconstructed the history of environmental changes in the northern parts of the Xingkai Lake over the past 100 a by analyzing the chronology,elements and subfossil diatoms in three sediment cores.Materials and methods This study collected the basic information of Xingkai Lake (topography,hydrology and other characteristics) as well as socio-economic data (GDP,population,cultivated land area,etc.) and meteorological data (annual average temperature,annual precipitation and annual average wind speed,etc.) from its watershed.A gravity sampler was used to extract sedimentary cores from the eastern part of the Xiaoxingkai Lake and the middle of the north part of the Daxingkai Lake,naming as XK03,XK05 and XK07,respectively.The obtained sediment cores were sectioned at 1 cm intervals for experimental analysis (including chronology,loss on ignition (LOI),χ,geochemical elements,diatom,etc.).In this paper,diatom assemblages around 1850 AD are set as ecological reference,and the water environment conditions (such as TP,TN and sediment phosphorus) in 1994—1998 are set as physical and chemical reference for water environment management.Quantitative reconstruction of historical water environment is based on analogue matching approach.Square chord distance (SCD) is used to calculate the deviation of diatom community from its baseline in historical period.Baseline of sedimentary phosphorus in Xingkai Lake is determined by regression analysis of reference element Fe.Results Xingkai Lake maintained good environmental and ecological conditions in the past centuries.Before 2000s,the water environment changed little (the SCD of diatom population was <0.0005).In the past 20 a,the ecological environment of Xingkai Lake was degraded under the combined effects from both climate change and human activities.Due to the small changes in the diatom communities before 2000s,the water monitoring values from 1994 to 1998 can be used as the nutrient reference of Xingkai Lake,i.e.,the TP and TN reference values are 35 μg·L^(-1)and 970 μg·L^(-1),respectively.Combined with the regression analysis of reference elements,the TP baseline of the sediments from the Daxingkai and Xiaoxingkai lakes are 360 mg · kgand 500 mg · kgrespectively,which can be used as the control baseline of the total phosphorus concentration in the sediments of the lake.Discussion The sedimentary records of Xingkai Lake in the past 200 a show that large lake ecosystems often have great system resilience.The sediment indices reveal that during the 1950s,the lake watershed and water body had undergone great changes:the land use in the watershed was further strengthened,the total amount of nutrients and substances into the lake increased,the concentration of pollutants such as Pb was significantly higher than that in the historical period,and the water quality also decreased.However,the diatom community did not show synchronous changes until the end of the 20th century.The community shows considerable resilience,which is very similar to that of Taihu Lake,a large lake in the eastern China.It is also worth noting that the diatom community has low diversity,dominated by Aulacoseira granulata.The diatom community is very similar to that of Jingpo Lake near the Xingkai Lake.Based on the calculation results of chord distance from the 1850 reference environment,the ecological environment of Xingkai Lake is highly similar to the reference environment at the end of the 20th century,that is,before 1998,and the average level of actual monitoring concentration of TP in lake water before 1998 could be used as the reference of TP in Xingkai Lake.In this study,Fe is selected as the reference element,which can effectively screen samples mainly from natural sources,and then determine the natural background of TP in sediments of Daxingkai and Xiaoxingkai lakes.The average sedimentary TP concentration of Xiaoxingkai Lake is higher than that of Daxingkai Lake,which reflects the difference in the source and migration of P in the sediments of Xingkai Lake.Conclusions Xingkai Lake transitioned from medium trophic level to slight eutrophication,which might resulted from the increasing local human activities.Considering the feasibility and necessity of environmental governance,although the total phosphorus concentration in the water body of the Xiaoxingkai Lake will be higher than that of Daxingkai Lake for a long time,the baseline epilimnetic TP value in the two Xingkai lakes can be set to 35 μg·L^(-1),as a common water body governance goals.However,for the sediments,the sedimentary TP background values are set to 360 mg · kgand 500 mg · kgfor Daxingkai and Xiaoxingkai lakes,respectively.Recommendations and perspectives From the perspective of long-term environmental evolution history,this study established corresponding sediment and water nutrient benchmarks for typical large shallow lakes in the Northeast China,which can provide important reference targets for lake management.