Geochemical Characteristics Of Carbon,Nitrogen,and Phosphorus Of Surface Sediments In Dalongdong Karst Reservoir In Guangxi Province,China

Dalongdong reservoir is a typical karst groundwater-fed reservoir,which is located in Shanglin county,Guangxi Province,SW China.Based on the test of total organic carbon?TOC?,total nitrogen?TN?and total phosphorus?TP?and carbon and nitrogen stable isotopes(?¹³C??¹⁵N)of the surface sediments in 31 sampling points in the Dalongdong reservoir,spatial distribution and influencing factors of carbon,nitrogen and phosphorus in the surface sediments were explored.At the same time,the origins of organic matter in the sediments were analyzed by using the ratio of carbon to nitrogen and carbon and nitrogen stable isotopes.The main conclusions are as follows:?1?The ratio of carbon to nitrogen?C/N?in the surface sediments in Dalongdong reservoir are relatively consistent from upstream to downstream,which the ratio ranged from 7.47 to 8.83 with an average of 8.05;Dalongdong reservoir sediments stable carbon isotope(?¹³C)in the surface sediments varied from-32.65‰to-24.84‰with the average value of-28.98‰.In addition,the nitrogen isotope(?¹⁵N)value ranged from 5.98‰to 9.33‰with the average value was 8.20‰.According to the previous research,C/N,?¹³C and?¹⁵N of phytoplankton is generally located in the range of 4¹0,-35.0‰^-25.0‰and 8‰,respectively.Therefore,comprehensive analysis shows that planktonic algaes are main source of sediment organic matter.?2?The content of TOC in the surface sediments of Dalongdong reservoir increased from upstream to downstream.The main reason is that the main source of organic matter in sediments is phytoplankton in Dalongdong reservoir.Deeper water depth and bigger water surface area of reservoir in downstream could suitable for phytoplankton growth and assembly,which could result in more organic matter accumulation.Organic carbon deposited in upstream could re-suspend and flow with water downstream.Thus,some of the resuspended organic carbon could deposit in downstream again,which will lead to the organic carbon content of sediments was higher than that in the upstream.?3?The content of TN in surface sediments increased from upstream to downstream,which it was similar to the TOC distribution in Dalongdong reservoir.The main reason is that TN in sediments was mainly organic nitrogen originated from phytoplankton and the relationship of TOC and TN shows significantly correlation,indicating which result in the similar spatial distribution trend with TOC.In addition,particulate nitrogen in water will accumulate in downstream because it will move downstream with water flow,which could lead to the TN content is higher in downstream than in upstream.?4?The TP distribution trend in surface sediments is that the content of TP is higher near the outlet of the underground stream,where it is the water source than in the other parts in the reservoir.The main reason for this spatial distribution is that there are different sources of C,N and P in sediments.The source of carbon and nitrogen are mainly planktonic algae,while phosphorus is mainly exogenous input.Many phosphorus wastewaters from agricultural activities in the upstream of underground stream were directly discharged into reservoir,which result in P deposition in reservoir upstream sediments.We also found in the field survey that there is a large-scale cage-culture fishing farming in the upper reaches of Dalongdong reservoir,which will affect the content of phosphorus in the surface sediments in the breeding process because of fodder employ.?5?According to the organic index method and evaluation criteria of organic nitrogen,the status of sediment pollution was analyzed.Results showed that all the organic index of sediment exceeded the pollution.Except for two points?B3,D1-3?with clean status,the rests belong to the pollution level according to the content of organic nitrogen.It shows that surface sediments in Dalongdong reservoir have been polluted by organic pollutants.According to the ecological toxicity evaluation criteria released by Ministry of Environment and Energy,Ontario province in Canada,TOC,TN and TP in surface sediments of Dalongdong reservoir reached the lowest level of ecological toxicity effects,indicating that the sediment has a toxic effect on the reservoir ecosystem and will threat benthic and ecological environment,although not yet reached a serious level,but it still should be paid attention to.