外文翻译缓解湖泊富营养化放开氮控制并专注于磷的减排节选Word文档格式.docx
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外文翻译缓解湖泊富营养化放开氮控制并专注于磷的减排节选Word文档格式.docx
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原文
MitigationofLakeEutrophication:
LoosenNitrogenControlandFocusOnPhosphorusAbatement
HaijunWang,HongzhuWang
Abstract
Traditionally,nitrogencontrolisgenerallyconsideredanimportantcomponentofreducinglakeeutrophicationandcyanobacteriablooms.However,thisviewpointisrefutedrecentlybyresearchersinChinaandNorthAmerica.Inthepresentpaper,thetraditionalviewpointofnitrogencontrolispointedouttolackascientificbasis:
theN/Phypothesisisjustasubjectiveassumption;
bottlebioassayexperimentsfailtosimulatethenaturalprocessofnitrogenfixation.Ourmulti-yearcomparativeresearchinmorethan40Yangtzelakesindicatesthatphosphorusisthekeyfactordeterminingphytoplanktongrowthregardlessofnitrogenconcentrationsandthattotalphytoplanktonbiomassisdeterminedbytotalphosphorusandnotbytotalnitrogenconcentrations.Theseresultsimplythat,inthefield,nitrogencontrolwillnotdecreasephytoplanktonbiomass.Thisfindingissupportedbyalong-termwhole-lakeexperimentfromNorthAmerica.Theseoutcomescanbegeneralizedintermsthatareductioninnitrogenloadingmaynotdecreasethebiomassoftotalphytoplanktonasitcanstimulatebloomsofnitrogen-fixingcyanobacteria.Tomitigateeutrophication,itisnotnitrogenbutphosphorusthatshouldbereduced,unlessnitrogenconcentrationsaretoohightoinducedirecttoxicimpactsonhumanbeingsorotherorganisms.Finally,detailsareprovidedonhowtoreducecontrolsonnitrogenandhowtomitigateeutrophication.
KeywordsLakeeutrophication;
Cyanobacteriabloom
1.Introduction
Eutrophicationofwatersisthephenomenonofanecosystembecomingmoreproductivebynutrientenrichment,stimulatingprimaryproducers.Itisusuallycharacterizedbyalgalblooms,causingwaterqualitydeteriorationandfishkills.Itisbecomingaglobalenvironmentalcrisis.InChina,theproblemoflakeeutrophicationisextremelysevere,withfrequentcyanobacteriabloomsthreateningtheecologyofwaters,economicdevelopmentandsocietystability.ThemostrepresentativecaseisthecyanobacteriabloomthatoccurredinLakeTaihuin2007,resultinginashortageofdrinkingwateranddomesticwaterfor5millioncitizensinWuxi,JiangsuProvince.Therefore,mitigationoflakeeutrophicationandcyanobacterialbloomisofgreatimportanceinChinaandtheworld.
Tomitigateeutrophicationandcyanobacterialblooms,nutrientcontrolisafundamentalprocess.Traditionally,besidesphosphoruscontrol,nitrogencontrolisgenerallyconsideredanecessarypractice.Abundantfundshavebeenspentonnitrogenremovalduringwastewatertreatmentprocesses.However,recentresearchesinChinaandNorthAmericasuggestachangetothetraditionalpracticeofnitrogenremovalforinlandwaters:
tomitigateeutrophication,itisnotnitrogenbutphosphorusthatshouldbereduced.Thisfindingisofcriticalimportanceforthepracticeoflakerestoration.
Inthispaper,thetraditionalviewpointofnitrogencontrolispointedoutfirsttolackaconclusivescientificbasis.Then,newresearchesareintroducedtoelaborateageneralrule:
reductioninnitrogenloadingcannotdecreasetotalphytoplanktonbiomass,butstimulatesbloomsofnitrogen-fixingcyanobacteria.Furtheranalysesproceededtoelaborateonthesignificanceofthisgeneralruleforthestrategytocontrolthesourceofnutrients.Intheend,suggestionsonlakerestorationareproposedindetail.
2.Thetraditionalviewpointofnitrogencontrollacksconclusivescientificbases
ThetraditionalviewpointoflimitingnutrientcontrolonphytoplanktonbiomassisbasedmostlyonconsiderationsofN/P(ratiooftotalnitrogentototalphosphorus)andonbottlebioassayexperiments.However,thefollowinganalysesindicatethatboththehypothesisandtheexperimentarenotconclusive.
2.1.TheN/Phypothesisisasubjectiveassumption
Generally,lakeshavebeenregardedaslimitedbyphosphorusifTN/TPwasrelativelylarge(suchasTN/TP>
17),limitedbynitrogenifTN/TPwasrelativelysmall(suchasTN/TP<
10)andcolimitedbynitrogenandphosphoruswhenTN/TPwasintermediate(suchas10<
TN/TP<
17).However,thethresholdsofTN/TPtoindicatenutrientlimitationvarygreatlyintheliterature,beingforinstance10–17,10–30,and7–15.Thevariationitselfimpliesthatthismethodisnotreliable.SomelimnologistshaveusedtheratioC:
N:
PproposedbyRedfieldasacriteriontoassessnutrientlimitation.Again,theRedfieldratioisnotauniversaloptimumratio,butanaverageofspecies-specificratios.TheoptimumN/Pratiosvarygreatlyamongvariousfreshwaterphytoplanktonspecies,from4.1to133.3.Obviously,itisalmostimpossibletosetaspecific“cut-off”ratiotoidentifyalimitingnutrient(s)foramulti-speciescommunity.
ByanalyzingtheregressionsandscatterplotsofphytoplanktonChlaorophyllwithnitrogenandphosphorus,SakamotofirstproposedtheN/Phypothesis.However,whenprobingintotheevidence,wefindthathisconclusionisjustasubjectivejudgmentonthepatternsofthescatterplots.Inhisanalysis,henotedthreepointswithaN/Pratiolargerthan15–17deviating.HeconcludedthatphosphorusbecamethecriticallimitingfactoriftheN/Pratiowaslargerthan15–17.Infact,therearetenpointswiththislevelofN/Pratioandtheleftsevenpointsfellexactlyontheregressionline.Obviously,suchareasoningbasedonminorpointsisnotright.Similarly,heconcludedthatphytoplanktonwaslimitedbynitrogenwhenN/Pwaslessthan9–10basedontwooutlierpoints,whenanalyzingtheregressionrelationshipbetweenlog(TN)andlog(Chla).Therefore,thisearliestempiricalevidencedoesnotsupporttheN/Phypothesis.
ManyresearchershavefollowedtheprinciplesespousedbySakamototoanalyzethelimitingnutrientandvariationsunderdifferentN/Pratios.However,alltheanalyseshavebeenperformedwithoutstrictstatisticaltests,failingtoprovethatthedependenceofphytoplanktononnitrogenandphosphorusaresignificantlydifferentunderdifferentN/Pratios.
Inconclusion,throughlogicalreasoningandempiricalanalyses,theN/Phypothesisisfoundtobejustasubjectiveassumptionwithoutconclusiveevidence.
2.2.Bottlebioassayexperimentsaretoosmallinscaletosimulatethenaturalprocessofnitrogenfixation
Bottlebioassayexperimentshavealsolongbeenusedtodemonstratethelimitingnutrientofphytoplanktoninawaterbody.Theexperimentsareusuallyperformedinanenclosedcontainertodiscriminatethelimitingnutrientbyobservingthegrowthofphytoplanktonafternutrientaddition.Theygenerallylastaperiodbetweenseveralhoursto1week.Ifgrowthofphytoplanktonisstimulatedbytheadditionofsomenutrient,thisnutrientisconsideredtobethelimitingfactor.Duetotheirsmallscaleintimeandspace,theseexperimentsfailtosimulatesomeimportantprocessesinrealopensystemssuchasbiologicalnitrogenfixation.Therefore,bottlebioassayexperimentscannotprovethelong-termexistenceofnitrogenlimitationinthefield.
3.Newresearchesfindthatnitrogencontrolfailstorestraintotalphytoplankton,butstimulatesbloomsofnitrogen-fixingcyanobacteria
InMay2008,ourarticledisprovedtheN/Phypothesisbasedonregionalcomparativestudies.InAugust2008,anarticlebyresearchersfromNorthAmericaindicatedthatnitrogencontrolmightstimulatenitrogen-fixingcyanobacteriabasedonalong-termwhole-lakeexperiment.Accordingly,itappearsthatreductioninnitrogenloadingcannotdecreasethebiomassoftotalphytoplankton.Itisalsologicalwhencomparingthecyclingcharacteristicsofnitrogenandphosphorus.
3.1.RegionalresearchintheYangtzeBasin
TotesttheN/Phypothesisonalargescale,weanalyzedtherelationshipsofChlaorophyllawithtotalnitrogenandtotalphosphorusbasedonmulti-yearinvestigationsofmorethan40Yangtzelakes.AlmostalltheTP-ChlaregressionshadhigherR2valuesandlowerpercentagepredictiveerrorthanthoseofTN-Chlaregressions,indicatingthesuperiorityofTPoverTNasapredictor.
BecausedifferentthresholdsofTN/TPratioshavebeenproposedintheliterature,wefurthercomparedthedifferencesofR2valuesbetweenTP-ChlaandTN-ChlaregressionsoverthecompleteTN/TPspectrum.Theresultsshowedthat28R2valuesofTP-ChlaregressionswerehigherthanthoseofTN-ChlaregressionsovertheentireTN/TPspectrum.AccordingtothetraditionalN/Phypothesis,thesecasesshouldbelimitedbyphosphorusandhaverelativelyhighTN/TPratios.However,theywerenotconfinedtoanyspecificregionofhigherTN/TPratios,butdistributedevenlybetween5and50.ItfurtherdemonstratesthefailureofTN/TPtoindicatenutrientlimitation.
ForagivenamountofTP,ChlavariedregardlessofthechangesofTN.Thatis,withintheTN/TPrangeofthepresentresearch,Chla/TPtendstowardsaspecificvalue.However,foragivenamountofTN,ChlaincreasedrapidlywiththeincreaseinTP.Therefore,inthefield,thetotalphytoplanktonbiomassisnotdeterminedbyTNbutbyTP.
OurresearchdisprovedthetraditionalviewpointofusingN/Pasanindextodiscriminatenutrientlimitingphytoplankton.WeareoftheopinionthatTPistheprimaryfactorregulatingphytoplanktonChlaregardlessoftheconcentrationofTN.
3.2.Long-termwhole-lakeexperimentsinCanada
InLake227,asmalllakeintheExperimentalLakesArea(ELA)ofOntario,Canada,researchersfromCanadaandtheUSAhaveperformedwhole-lakefertilizationexperimentsfor37years.Thelakehasbeenfertilizedwithconstantphosphorusanddecreasingnitrogenaddition.Int
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