Temporal And Spatial Variations Of Sediment Phosphorus In Typical Lakes On The Yunnan-Guizhou Plateau

With the economic development of industry and agriculture and the discharge of pollutants,thephosphorus pollution of lakes around the world is becoming more and more serious,causing ecological and environmental problems such as eutrophication.However,limited by monitoring data,our understanding of the long-term changes in lake phosphorus pollution and its relationship with human activities is relatively limited.Sediment is an important carrier of changes nitrogen and phosphorus and other elements in lake.It records the information of lake environmental changes and human activities in the watershed,and studies the temporal and spatial changes of phosphorus accumulation in lake sediments under different watersheds and different human activity backgrounds.It has important scientific and practical significance to the relationship between human activities in the basin and the changes of lake ecological environment.In this paper,three typical lakes（Dianchi Lake,Qilu Lake and Lugu Lake）with different degrees of eutrophication in the Yunnan-Guizhou Plateau were selected.13,6 and 15 sedimentary short cores were collected in different lake areas,the typical core age series were established by ²¹⁰Pb and¹³⁷Cs dating method.The layers of Dianchi Lake cores in about 1970 and 2000,Qilu Lake cores in about 1985 and 2005,and Lugu Lake cores in about 1920 and 1985 were determined by event dating method.Then the temporal and spatial variation characteristics of phosphorus content and morphological composition in sedimentary cores are analyzed.The relative contribution of natural and man-made sources of phosphorus in sedimentary cores is identified with reference to the enrichment coefficient（EF）results.Combined with the analysis of sediment particle size,magnetic susceptibility and other indicators,the temporal and spatial variation characteristics of phosphorus accumulation in different lake sediments in recent 100 years and its relationship with human activities and lake ecological environment are discussed.The results show that:（1）The average phosphorus content in the sedimentary cores of Dianchi Lake,Qilu Lake and Lugu Lake is 1920 mg/kg,973 mg/kg and 1308 mg/kg,respectively.The vertical trend of phosphorus content in the sedimentary cores of all lakes is generally consistent,and can be divided into three stages:the lower part of the sedimentary core（before Dianchi Lake 1970,Qilu Lake1985 and Lugu Lake 1960）has low phosphorus content,which is a quasi natural sedimentary stage with weak impact of human activities.From 1970 to 2000,the total phosphorus content in the sediments of Dianchi Lake increased rapidly,and the total phosphorus content was relatively high and stable since 2000.The total phosphorus content in the sediments of Qilu Lake decreased slightly from 1985 to 2005,and increased rapidly since 2005.The total phosphorus content in the sediments of Lugu Lake increased slowly from 1960 to 2000,and the total phosphorus content increased rapidly since 2000.The main forms of phosphorus in the sediments of the three lakes are Na OH-P and HCl-P,and the contents of Na OH-P and Org-P in the sedimentary cores of Dianchi Lake and Lugu Lake increase with the decrease of depth,reflecting the contribution of anthropogenic sources.The content of HCl-P decreased with the decrease of depth,which may be related to the decrease of natural phosphorus input.The content of Na OH-P in the sedimentary core of Qilu Lake decreases with the decrease of depth,and the contents of HCl-P and Org-P increase with the decrease of depth.（2）The vertical variation trend of phosphorus enrichment factor（EF）in the sediments of the three lakes is generally similar,but there are certain differences in the variation stages and amplitudes of each lake.The phosphorus enrichment factor at the bottom of the sedimentary cores（before 1970 in Dianchi Lake,before 1985 in Qilu Lake,and before 1960 in Lugu Lake）was≤1,indicating that the phosphorus in sediments mainly came from natural sources such as watershed soil and rock weathered debris source.Since the 1970s and 1980s,the enrichment coefficient of phosphorus in the sediments of various lakes has gradually increased,and the contribution of anthropogenic sources has gradually increased;The enrichment coefficient of phosphorus has remained at a high level since about 2000.The average enrichment coefficient of phosphorus in Dianchi lake sediments is 1.9,which is equivalent to the proportion of phosphorus from man-made sources and natural sources;the average enrichment factor of phosphorus in Qilu Lake sediments is 2.5,and the contribution of anthropogenic phosphorus has exceeded that of natural sources.The average enrichment factor of phosphorus in Lugu Lake sediments is 1.7.The cumulative fluxes of total phosphorus and natural phosphorus in the sediments of Dianchi Lake and Qilu Lake peaked in the 1970s,which is related to soil erosion caused by human activities such as deforestation;Since about 2000,the cumulative flux of natural phosphorus in the Sedimentary Cores of the three lakes has shown a downward trend,while the cumulative flux of total phosphorus and man-made phosphorus has shown a peak or increasing trend,which is mainly related to domestic sewage discharge,agricultural non-point source pollution and tourism development.（3）The cumulative flux of total phosphorus and anthropogenic phosphorus in the sediments of Dianchi Lake was higher than that of the other two lakes,and the accumulation of phosphorus in the sediments of each lake showed a certain spatial difference.From 1970 to 2000 and from 2000to 2014,the average cumulative flux of total phosphorus in the sediments of Dianchi Lake was2067 and 2476 mg/（m²·a）,and the average cumulative flux of anthropogenic phosphorus was 656and 1221 mg/（m²·a）,the high-value areas are located in the northern and southwestern lake areas,which are related to the discharge of urban domestic sewage and the input of phosphate mining wastewater;the average cumulative flux of natural source phosphorus in the sediments of Dianchi Lake in the above two periods is 1411 and 1255 mg/（m²·a）,respectively,the high-value areas are located in the southern and central lake areas,which are consistent with the high-value areas of deposition rate.From 1985 to 2005 and 2005 to 2015,the average cumulative flux of total phosphorus in the sediments of Lake Qilu was 699 and 875 mg/（m²·a）,and the average cumulative flux of natural source phosphorus was 622 and 448 mg/（m²·a）,the high-value areas are mainly located in the eastern lake area;the average cumulative flux of anthropogenic phosphorus is 77and 427 mg/（m²·a）,and the high-value areas are located in the southwestern and northeastern lake areas,the South and east of the basin are the main vegetable planting bases,and there are rivers entering the lake.The input of non-point source pollution may be the main reason for the high accumulation flux of anthropogenic phosphorus in the sediments of these lake areas.From 1985to 2014,the average cumulative fluxes of total phosphorus and anthropogenic phosphorus in Lugu Lake were 759 mg/（m²·a）and 239 mg/（m²·a）,respectively,which were relatively high at the junction of the north-south lake area and the northwest lake area,affected by tourism activities and the input of domestic sewage;the average cumulative flux of natural sources of phosphorus is 520mg/（m²·a）.It is relatively high in the east of the southern Lake area,which is related to the phosphorus deposition caused by hydrodynamic factors such as the only River out of the lake and the interception of aquatic plants in the lake area.（4）The historical changes in the cumulative flux of total phosphorus and anthropogenic phosphorus in sediments were synchronized with the succession of aquatic species such as diatoms and chironomids,indicating the important impact of phosphorus input on changes in lake ecological environment.With the increase in the cumulative flux of total phosphorus and anthropogenic phosphorus,the diatoms that prefer oligotrophic state in the water body of Dianchi Lake have gradually disappeared since the 1970s,and have been replaced by diatoms with a higher degree of nutrition;since the 1960s,the proportion of diatoms and cladocerans that like poor nutrition in the water body of Qilu Lake has decreased rapidly,while the proportion of diatoms and cladocerans that like medium nutrition has gradually increased;Since the 1960s,the diatoms in Lugu Lake that like poor nutrition have been gradually replaced by diatoms with potential eutrophication,and the growth of chironomid community has changed synchronously with phosphorus accumulation.