| Bang Lake is a natural lake on the edge of Poyang Lake, annually seasonal highfluctuation water levels of Poyang Lake also has a great influence on Bang Lakewater levels. Bang Lake is near Gan River, Xiu River and Poyang Lake, and BangLake is separated from them as an independent closed lake because of natural levee indry season. In wet season,with the rise of water level of Gan River, Xiu River andPoyang Lake, water enters into Bang Lake over the dam and gradually comestogether. The water source of Bang Lake is different in different water levels, whichmakes the nutritive salt, anion, cation and trace elements differences. And theelements and isotopes of lake water body, sediment, aquatic plants respond toenvironment changes. Bang Lake is the research subject. The exchange informationof water body among Bang Lake,Gan River and Xiu River were analysed by studyingthe ion concentration of anion and cation, hydrogen and oxygen stable isotope ratiosand inorganic nitrogen stable isotope ratios. The influence of inorganic nitrogenstable isotopes and particle organic matter C and N stable isotopes of Bang Lake fromwater level changes were discussed, judging their origins and response on water levelchanges. Furthermore the characteristics of heavy metal and the isotope, and sourcesof inorganic nitrogen and organic matter between Bang Lake (nature lake) andElephant Lake(urban lake) were comparatively analyzed.(1) The differences of heavy metal pollution between different ecological typelakes: The mean concentrations of heavy metals of surface sediment were Cu> Zn>Pb> Cr> Cd in Bang Lake. The pollution degree was low. The average value of Cdwas at the Grade3of Soil environmental quality.Based on RSP value, Pb was belongto light pollution in Bang lake, the biological potential harm of other elements wasnot great. It had a close connection between the Cu content distribution yearly and theindustrial development of Poyang lake.The average concentrations of heavy metal of surface sediment in Elephant Lakewere Zn> Pb> Cu> Cr> As> Cd. The most severe pollution in Elephant Lake wasCd. It should be payed more attention for Pb pollution in city lake. The depth distribution of heavy metal concentrations was serrated in the two Lakes, andconcentration was lower with depth increases in Bang Lake but not obvious inElephant Lake.(2) The exchange information of water body of Bang Lake,Gan River andXiu River: In the water level rising period there were not obvious differences on theion concentrations of anion and cation, hydrogen and oxygen stable isotope ratiosamong Bang Lake,Gan River and Xiu River because Bang Lake received the waterbody from Gan River, Xiu River,and more from Poyang Lake. However in dryseason, there were obvious differences on those ion concentration or stable istopicratios among them,which showed that Bang Lake is a independent lake.Inorganic nitrogen stable isotope analysis also reflected the water exchangeinformation among Bang Lake and Gan River and Xiu River. The inorganic nitrogenstable isotopes in dry season were negative,but positive in Gan river and XiuRiver,which suggested that no water exchange happended among them. However inthe water level rising period isotopes were very close with positive amongthem,which indicated water exchange among Bang Lake and the two rivers.The ammonia nitrogen stable isotope of Bang Lake,Gan River and Xiu River inflooding period was negative, mainly came from the ammonia nitrogen source of rainwater of drainage basin.Nitrate nitrogen mainly came from the agricultural nitrogensource by rain runoff. In lowering period,nitrate nitrogen stable isotope of Bang Lakewas negative,which was opposite to the two rivers, showing that the water body ofBang Lake was the retention water of flooding period, the two rivers’ water may notenter the Bang lake, and the nitrification of Bang Lake was stronger.(3) Great influence on δ15N-NH4+、δ15N-NO3-from the water level changes ofBang Lake: There were significant differences for inorganic nitrogen stable isotopesof Bang Lake in the flooding, lowering, lentic and flowing periods, which reflectedthe different water sources in different water levels. The water sources of flowing andflooding periods were river water and rain water. The water sources of lowering andlentic periods were the retention water of flooding period.(4) C and N stable isotopic compositions of organic matter and C/Nindicating organic matter source and migration: Seasonal water level changes of Bang Lake were the main control factor of the C and N cycle, also affected C and Nstable isotopic compositions of organic matter.1) Suspended particulate organic matter (SPM): δ13C of SPM in flooding periodwas relative increase(-27.5‰~-23.1‰), relative decrease in lowering period (-29.7‰~-26.8‰) and lentic period (-31.7‰~-27.3‰). δ13C (35.4‰~30.8‰) inflowing period had significant difference with the other water levels. C/N of SPM hada slight upward trend when water level rose. C/N of most of the samples was greaterthan7. SPM in flooding period was from foreign soil organic matter and the debris ofaquatic plants. The detritus of aquatic plants of the lake in lowering period was themajor composition of SPM. Sediment OM resuspended caused by wind and wavewas important composition of SPM in lentic period.2) Surface sediment organic matter (SOM): The δ13C of SOM significantlychanged with the water levels and seasons, with-30‰~-25.5‰in winter andspring and-26.5‰~-24‰in summer and fall. δ13C was very high in May after thewater level rose. The C/N of SOM in flooding and lowering period was2~3.5,which was much lower than that of dry period (6.5~10.2) and flowing period (5.7~9.2). SOM in flooding period may be from foreign soil organic matter and selfbiogeochemical. degredation. In flowing period, foreign water source had flushingaction on surface of sediment, which made the δ13C increase than that in dryperiod,.and no contribution on SOM from soil organic matter and aquatic plantdebris.3) δ13C change of aquatic plant was mainly seasonal variation. δ13C ofaquatic plants increase in summer (-29.2‰~-23.2‰), while that of other seasonswas-29.5‰~-27‰. C/N of aquatic plants in spring and summer was high (9.6~16.4) but C/N was8~10in the fall and winter. The C and N cycing of aquatic plantswere mainly controlled by biogeochemical processes in lake. In different seasons,isotope fractionation happened due to the differences of light, temperature, plantphotosynthesis and respiration. The δ15N change of particulate organic matter (POM)was not big, mainly by seasonal changes.(5) The analysis of isotope differences and source of inorganic nitrogen andorganic matter between different ecological type lakes: According to δ15N of water, δ13C and δ15N and organic C/N of organic matter, sources of inorganic nitrogen andorganic matter of two lakes can be identified approximately. The water N content inBang Lake was low while high in Elephant Lake. δ15N of DIN in Bang Lake wasnegative, showed nitrogen pollutions from rain water and agricultural fertilizer.δ15N-NH4+in Elephant Lake had a big distribution range. δ15N-NH4+of twopollution sample points were+13.5‰and+25.4‰,showing input characteristics ofsewage.The change of δ15N-NO3-was mainly caused by fractionation of lakeinternally biogeochemical processes in Elephant Lake. That aquatic plant was themain source of organic matter of Bang Lake can be concluded by combined the δ13Cwith organic C/N. The distribution ranges of δ15N of surface sediment of ElephantLake and organic C/N were big, δ15N with+3.6‰~+8.3‰, organic C/N with2.6~10.8, which suggested that the sources of organic matter for urban lake were wide. |
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