Hydrogeochemicalcontrolsontheorganicmatterandbacte

WaterResearch36(2002)2561–2570HydrogeochemicalcontrolsontheorganicmatterandbacterialecologyofasmallfreshwaterwetlandintheNewJerseyPineBarrensPatriciaA.Mauricea,*,LauraG.LeffbaDepartmentofCivilEngineeringandGeologicalSciences,UniversityofNotreDame,NotreDame,IN46556,USAbDepartmentofBiologicalSciences,KentStateUniversity,Kent,OH44242,USAReceived1July2001;receivedinrevisedform1September2001;accepted1September2001AbstractThisstudyinvestigatedtheeffectsofvariableground-water/surface-waterexchangeandphotoinducedprocessesonlongitudinalpatternsindissolvedorganicmatter(DOM)andbacterialcommunitiesinasmallfirst-orderstreamintheNewJerseyPineBarrens.DOMconcentration,alongwithDOMweightaveragemolecularweight(Mw)andabsorptivity(e280;anestimatorofaromaticity),andbacterialcellcountsalldecreasedfromthestreamandhyporheiczoneintotheshallowaquiferinaground-waterrechargezone.Furtherdownstream,influxofgroundwaterintothestreamresultedinalowerMwDOMpoolandwasaccompaniedbydecreasedcellcounts.Theobservedeffectofthisground-waterdischargeonbacterialnumbersmaybedirect,ifdischargetemporarilydilutescellcounts,orindirect,ifchangesinDOMconcentrationandpropertiescontrolthebacterialcommunity.Ineithercase,thisstudysuggestedtheimportanceofconsideringground-water–surface-waterexchangeinstudiesoflongitudinalchangesinthebacterialcommunitiesofstreams.r2002ElsevierScienceLtd.Allrightsreserved.Keywords:Bacterialecology;DOC;Streamwater;Wetlands;Organicmatter1.IntroductionDissolvedorganicmatter(DOM)isanimportantsourceoforganiccarbonformicrobialcommunities,andDOMisthusanintegralcomponentofthecarboncycle.StudieshavesuggestedthatitisnotthequantityofDOMpersebutratherthequalityandbioavailabilitythataffectmicrobialassemblages(e.g.,[1,2]).Therefore,researchhasfocusedondetermininghowthebioavail-abilityofDOMvarieswithpropertiessuchasmolecularweight(e.g.[3])andelementalcomposition(e.g.,[4–6]),orwithprocessessuchasphotochemicaltransforma-tions[7].Todate,nodefinitiveunderstandingofthecontrolsonDOMbioavailabilityhasemerged,duetothecomplexvariabilityofDOMalongwiththedynamicandvariedcapabilitiesofmicroorganisms.Forexample,althoughhighmolecularweightorganiccompoundshavelongbeenconsideredtoberecalcitrant(e.g.,[8,9]),thisviewhasbeencontradictedbyanumberofstudiessuggestingthatsomehighmolecularweightcompoundsareutilizedreadilybybacteria,particularlyinoceanicenvironmentsbutalsoinstreamwater(e.g.,[3,10–15]).Mostresearchonmicrobial/DOMassociationshasfocusedonbiogeochemicalinteractions;therehasbeenlessemphasisonintegratingmicrobiologywithmole-cular-scaleDOMcharacterizationandhydrologiccon-trols.Yet,accordingtoGilbertetal.[16],localinteractionsbetweensurfaceandgroundwatersplayimportantrolesinstreamecosystemstructureandfunction.Theresearchreportedherecombinedhydro-logicinvestigationswithmolecular-basedgeochemicalanalysisandbacterialcommunitycharacterizationto*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](P.A.Maurice).0043-1354/02/$-seefrontmatterr2002ElsevierScienceLtd.Allrightsreserved.PII:S0043-1354(01)00465-1determinethehydrogeochemicalcontrolsonDOMcharacteristicsandmicrobialabundanceinasmallfirst-orderstream(fenwetland),McDonaldsBranchbasinintheNewJerseyPineBarrens.McDonaldsBranchisanexcellent‘naturalobserva-tory’forstudyingDOM/microbiologic/hydrologicrela-tionships.ThestreamhasbeenaUSGeologicalSurveyhydrologicbenchmarkstationformorethan40yr.Thebasinhasorganic-richsurface-andshallowgroundwatersthatlendthemselveswelltoDOMsamplingandcharacterization.ThebasinispartofthePinelandsNationalReserveandisundeveloped.Moreover,naturalgradientsinhydrologicconditionswithinthestreamfromground-waterrechargetodischargezonesallowforobservationsofhowvaryingground-water–surface-waterinteractionsaffectDOMpropertiesandstreammicrobiology.Thestreamchannelisshadedinallbutonearea;hence,photo-inducedtransformationsoftheDOMalsowereinvestigateduponpassingthroughthisexposedarea.Watershed-scaleresearchofhydro-geochemistryinthebasinthroughthe1980sand1990sprovidedbackgroundforthecurrentresearch(e.g.,[17–19]).Meyer[20]identifiedseveralfactorslikelytoaffectbacteriainsurface-waterenvironments:quantityandqualityoflabileDOM,inorganicnutrientconcentra-tions,andpredationbyprotozoa.Inthisstudy,wefocusedonDOMconcentration(referredtohereinafterasDOC,inmgC/L)alongwithtwopropertiesoftheDOM,Mw(weightaveragemolecularweight)ande280(absorptivityat280nmnormalizedtomolesC).WedidnotinvestigatehowpropertiessuchasC:Nratioinfluencebioavailability(e.g.[4]),nordidweinvestigatepotentialeffectsofinorganicnutrientsandpredators.Wealsodidnotcharacterizemicrobialcommunitystructure.Nevertheless,asdescribedbelow,ourstudydemonstratedthathydrologicconditionsandphoto-chemicalprocessesneedtobeconsideredinlongitudinalstudiesofDOMpropertiesandmicrobialecologyinstreams.2.Methods2.1.DescriptionofthestudysiteMcDonaldsBranchbasinisasmall(6km2)forestedwatershedlocatedinLebanonStateForestinthePineBarrensregionoftheNewJerseycoastalplain(Fig.1).McDonaldsBranchisafirst-orderstream(fenwetland)thatflowswestwardfromitshea