Sedimentary Records Of Water Environmental Evolution And Carbon Burial Of The Xin'an Reservoir

The Xin'an Reservoir,the large reservoir in China,is an important water supply source and water conservation area for the Qiantang River.However,after the occurrence of the two algae blooms in 1998 and 1999,the safety of water quality,as well as eutrophication in the reservoir,has been put into question.In this dissertation,the upstream area,the transitional area and the central reservoir area of the Xin'an Reservoir were chosen as the research objects.Based on accurate chronologies by radionuclide dating(¹³⁷Cs and ²¹⁰Pb),we investigated the change of nutrient input flux by TP and TN in sediments,and used C/N,carbon and nitrogen isotopic composition to judge the sources of organic matter in sediments.Furthermore,the study also analyzed the anthropogenic influence on environmental change.Based on the above analysis,we preliminarily discussed the control of algae boom and carbon burial in the Xin'an Reservoir.The main conclusions are as follows:?1?.According to ¹³⁷Cs result of central reservoir,we found identifiable peak of 1964in the mass depth of 11.93g/cm²,which was identical to the CRS model result of ²¹⁰Pb.For the upstream and transitional area,because of the failure of radionuclide dating,sedimentary accidents dating method corrected with radionuclide dating of central reservior have been used.The grain size of three cores can all reflect flood from 1994to 1999;the flood discharge in 1966 was observed in transitional area and central area.Based on these results,we calculated sedimentation rate of reservoir in historical period.?2?.The nutrient content of different region appeared obvious difference in the Xin'an Reservoir.In the upstream area,TP content ranged from 0.55 g/kg to 0.89 g/kg,TN content ranged from 0.12 to 0.25%,both of them showed a similar tendency with the local precipitation.Combined with local water dynamic feature,we judged that the local nutrient was originated from non-point input.In transitional area,the contents of TP and TN totally presented variation of“decrease-constant-increase”.Prior to 1997,the nutrient content of transitional area was low and rarely fluctuant,which represented stable nutrient input in this period.Since 1997,an obviously increasing tendency of TP and TN indicated the appearance of new nutrient source.In central reservoir,the nutrients showed a distinctly exponential growth,TP and TN in surface sediment even got up to 1.64g/kg and 0.7%,which demonstrated the growth of external nutrients in recent years.According to fish and grain output of Chun'an county,aquaculture might be primary reason which led to the nutrient increasing in transitional area and central reservoir.?3?.In upstream area,TOC,BSi and IC not only showed a similar variation with nutrients,but also presented the high correlation with TOC.It represented that organic matter and nutrient had same source,which were influenced by precipitation and runoff.The high C/N and?¹³C?-25.1‰—-26.0‰?further indicated that the terrestrial input was dominated organic matter source.In transitional area,unlike TOC variation,BSi and IC showed a similar change with nutrient.The low correlation about TOC-BSi and TOC-IC demonstrated that primary productivity was not the only organic source,which was further implied by the value of?¹³C?-25.8‰—-26.9‰?.In central reservoir,TOC,BSi and IC also presented a parallel change with nutrient.Prior to 1997,the stable content of TOC and the value of?¹³C?-26‰—-27‰?demonstrated that the organic matter was influenced by terrestrial input and primary productivity in this period.Since 1997,the value of?¹³C?-29.09‰—-26.2‰?and obviously increasing of TOC content indicated that the organic matter was mainly originated from primary productivity.?4?.Dam had a large influence on the eutrophication and carbon burial of the Xin'an Reservoir.According to calculated results,not only the nutrient content reached dangerous level,but also the bio-available N and P accounted for 95%and 80%of TN and TP in central reservoir.However,these nutrients rarely converted to endogenous pollution sources due to its deep depth and aquatic environment protection-oriented fishery policy.Moreover,the carbon burial flux was 2.49×10¹⁰ g.yr^-1 in the Xin'an Reservoir,and surpassed the reservoir carbon emission 1.4×10¹⁰ g.yr^-1,which represented that reservoir not only was a carbon sink,but also played an important role in the regional carbon cycle.