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.Aquaculture1661998181–212ReviewNitrogenbiogeochemistryofaquacultureponds1JohnA.HargreavesMississippiStateUniÍersity,DepartmentofWildlifeandFisheries,Box9690,MississippiState,MS39762,USAAccepted6June1998Abstract.NitrogenNbiogeochemistryofaquaculturepondsisdominatedbybiologicaltransformationsofNaddedtopondsintheformofinorganicororganicfertilizersandformulatedfeeds.Nitrogenapplicationinexcessofpondassimilatorycapacitycanleadtothedeteriorationofwaterquality.throughtheaccumulationofnitrogenouscompoundse.g.,ammoniaandnitritewithtoxicitytofishorshrimp.Principalsourcesofammoniaincludefishexcretionandsedimentfluxderivedfromthemineralizationoforganicmatterandmoleculardiffusionfromreducedsediment,althoughcyanobacterialnitrogenfixationandatmosphericdepositionareoccasionallyimportant.Principalsinksforammoniaincludephytoplanktonuptakeandnitrification.Themagnitudeoflossesbyammoniavolatilizationandammoniumfixationtocationexchangesitesisminor,butunknown.InteractionsbetweenpondsedimentandwaterareimportantregulatorsofNbiogeo-chemistry.Sedimentrepresentsasourceofammoniaandasinkfornitriteandnitrate.ThelargevolumeofreducedsedimentsuggeststhatthepotentialforNremovalbydenitrificationishigh,althoughthemagnitudeofNremovalbythismechanismislowbecausenitrificationanddenitrificationaretightlycoupledinaquaticsedimentsandsedimentnitrificationislimitedbythedepthofsedimentoxygenpenetration.Nitrogenbiogeochemistryofaquaculturepondsisaffectedbyfeedsandfeedingpractices,waterexchangeandcirculation,aeration,ponddepthandothermanagementprocedures.OpportunitiesformanagementofNbiogeochemistryarelimitedandgoalsarebasedlargelyontheintensityoffishproduction.q1998PublishedbyElsevierScienceB.V.Allrightsreserved.Keywords:Ammonia;Nitrification;Denitrification;Sediment1ApprovedforpublicationasJournalArticleNo.J-9356oftheMississippiAgriculturalandForestryExperimentStation,MississippiStateUniversity.0044-8486r98r$19.00q1998PublishedbyElsevierScienceB.V.Allrightsreserved..PIIS0044-84869800298-1()J.A.HargreaÍesrAquaculture1661998181–2121821.Importanceofnitrogeninpondculturesystems.TheefficiencyoffishnitrogenNassimilationhasimportantimplicationsforwaterqualityandprofitabilityofpondaquaculture.Resultsfromavarietyofculturesystems.indicatethat,onaverage,about25%range:11to36%ofNaddedasfeedorother.nutrientinputisrecoveredbythetargetorganismTable1.ProteinsourcessuchasfishmealandsoybeanmealarethemostexpensivecomponentsofformulatedfeedsandimprovementintheefficiencyofNassimilationandutilizationwillthusimprovetheeconomicsoffishproduction.TheinherentefficiencyofnutrientutilizationbyfishimpliesthatNloadingofaquaculturepondsmaybelimitedbythecapacitytoassimilatenitrogenousexcretion,whichmayhaveadeleteriousimpactonwaterqualityandfishgrowth.Following.dissolvedoxygen,theaccumulationofdissolvedinorganicnitrogenDINisthefactor.mostlikelytolimitfeedingrateinaquaculturepondsKnud-Hansenetal.,1991b.Ammonia2isexcretedastheendproductofproteincatabolism,andmaybetoxicifallowedtoaccumulate.Ammoniatoxicityismanifestbyhyperactivity,convulsions,lossofequilibrium,lethargyandcoma.However,ammoniatoxicityinaquaculturepondsismostlikelyexpressedasthesublethalreductionoffishgrowthorsuppressionofimmunocompetence,ratherthanasacutetoxicityleadingtomortality.Coltand.Tchobanoglous1978demonstratedlinearreductionofchannelcatfishgrowthovertherange0.05to1.0mgly1NH–Nandacalculated50%reductionofgrowthatabout0.53mgly1NH–N.Themechanismsofammoniatoxicityhavenotbeenfirmlyestablished.3However,acombinationofplasmasodiumdepletionColtandTchobanoglous,1978;.Tomassoetal.,1980,biochemicaleffectsofKrebscyclesuppressionbydepletionof.a-ketoglutarateSousaandMeade,1977;Smart,1978resultinginreducedammonia.excretion,andotherfactorsMeade,1985areimplicated.Thetoxicityofun-ionizedammoniaisafunctionofpH,temperature,alkalinityandtotalammoniaconcentration.measuredatthegillsurfaceSzumskietal.,1982.AmmoniaismoretoxictofishatelevatedpHandtemperature,whichshiftstheionizationequilibriumtowardthetoxic,unionizedgaseousform.TheriskofelevatedpHandunionizedammoniaisgreaterin.poorlybufferedlowalkalinitypondsinthelateafternoon.ThecontributionofammoniaexcretiontoNflowinaquaculturepondsissubstantial.If25%ofinputNisretainedbyfish,then75%ofinputNisexcreted.Nitrogen..excretioncanbepartitionedintodissolved62%andparticulate13%fractionsFolke.y1y1andKautsky,1989.Forexample,atafeedingrateof100kghad32%protein.y2y1feed,ammoniaexcretionwouldbe317mgNmd.Alternately,usingafeed-basedy1.estimateofammoniaexcretionof30gNkgfeedColtandOrwicz,1991,then300mgNmy2dy1areexcreted.2AmmoniaexistsasacomponentofapH-andtemperature-dependentequilibriuminnaturalwaters..AcrosstherangeofpHmostcommonlyencounteredinnaturalwaters6.5to9.0,theequilibriumfavorstheq..aqueous,ionizedformNH,or‘ammonium’.ElevatedpH)9.3favorsthegaseous,unionizedform4.NH,or‘ammonia’.Theconventionadoptedherewillbetousetheterm‘ammonia’torefertounionized3q.ammoniaplusionizedammoniumNHqNHexpressedasnitrogenbymassunlessotherwiseexplicitly34indicated.()J.A.HargreaÍesrAquaculture1661998181–21