NASA Technical Reports Server (NTRS) 20140006592: Rates of Dinitrogen Fixation and the Abundance of Diazotrophs in North American Coastal Waters Between Cape Hatteras and Georges Bank

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by  NASA Technical Reports Server (NTRS)

Limnol.Oceanogr.,57(4),2012,1067–1083
E2012,bytheAssociationfortheSciencesofLimnologyandOceanography,Inc.
doi:10.4319/lo.2012.57.4.1067
Rates of dinitrogen fixation and the abundance of diazotrophs in North American
coastal waters between Cape Hatteras and Georges Bank
M. R. Mulholland,a,* P. W. Bernhardt,a J. L. Blanco-Garcia,b A. Mannino,c K. Hyde,d
E. Mondragon,e,1 K. Turk,e P. H. Moisander,e,2 and J. P. Zehre
aOld DominionUniversity, Departmentof Ocean,Earthand AtmosphericSciences, Norfolk, Virginia
bOld DominionUniversity, Center forCoastalPhysical Oceanography,Norfolk, Virginia
cNational AeronauticsandSpace Administration, GoddardSpace FlightCenter, Greenbelt, Maryland
dNational OceanicandAtmospheric Administration, Northeast Fisheries Science Center,Narragansett, RhodeIsland
eUniversity ofCalifornia Santa Cruz,Ocean SciencesDepartment, SantaCruz,California
Abstract
Wecoupleddinitrogen(N )fixationrateestimateswithmolecularbiologicalmethodstodeterminetheactivity
2
andabundanceofdiazotrophs incoastal watersalongthetemperate NorthAmerican Mid-Atlanticcontinental
shelfduringmultipleseasonsandcruises.VolumetricratesofN fixationwereashighas49.8nmolNL21d21
2
andarealratesashighas837.9mmolNm22d21inourstudyarea.OurresultssuggestthatN fixationoccursat
2
highratesincoastalshelfwatersthatwerepreviouslythoughttobeunimportantsitesofN fixationandsowere
2
excludedfromcalculationsofpelagicmarineN fixation.UnicellularN -fixinggroupAcyanobacteriawerethe
2 2
mostabundantdiazotrophsintheAtlanticcoastalwatersandtheirabundancewascomparableto,orhigherthan,
thatmeasuredinoceanicregimeswheretheywerediscovered.HighratesofN fixationandthehighabundance
2
of diazotrophs along the North American Mid-Atlantic continental shelf highlight the need to revise marine N
budgets to include coastal N fixation. Integrating areal rates of N fixation over the continental shelf area
2 2
between Cape Hatteras and Nova Scotia, the estimated N fixation in this temperate shelf system is about
2
0.02 Tmol N yr21, the amount previously calculatedfor the entire North Atlantic continental shelf. Additional
studiesshouldprovidespatially,temporally,andseasonallyresolvedrateestimatesfromcoastalsystemstobetter
constrainN inputsvia N fixationfrom the neritic zone.
2
Dinitrogen(N )fixationsuppliesnewnitrogen(N)tothe cyanobacteria thrive across a broader range of environ-
2
world’s oceans,thereby alleviatingNlimitation ofprimary mentsthanpreviouslythought(Moisanderetal.2010)and
productivity;however,mostpriormeasurementsofpelagic therefore global rates of N fixation may be underestimat-
2
marine N fixation rates are primarily from oligotrophic ed. Consistent with this observation, recent geochemical
2
regions, where N-depleted conditions are thought to favor models hint that global marine N fixation has been
2
growthofN -fixingmicroorganisms(diazotrophs).Despite underestimated by models and field sampling, and deter-
2
the recognition that biological N2 fixation ultimately miningsourcesofthismissingNmaybeanimportantstep
controls the input term of the marine N budget and that in resolving oceanic N budgets (Gruber and Sarmiento
diazotrophs contribute substantially to new production in 1997; Codispoti 2006; Deutsch et al. 2007).
oligotrophic gyres (Montoya et al. 2004; Capone et al. Previous reports suggest that planktonic N fixation is
2
2005; Zehr and Paerl 2008), little is known about the undetectable or insignificant in most estuarine and coastal
distribution of diazotrophs and their activity in most waters, including those along the North American Mid-
coastalregions,where21–30%ofthetotaloceanicprimary Atlantic shelf (Howarth et al. 1988; Zehr and Paerl 2008;
productivity occurs (Jahnke 2010), and where geochemical
Conley et al. 2009). However there are few rate measure-
andclimatologicalmodelsarepoorlyresolved(Gruberand
ments to support this assertion. The diversity of nifH, the
Sarmiento 1997; Deutsch et al. 2007). Rates of oceanic N
2 gene encoding the iron protein in the enzyme nitrogenase
fixation have been primarily reported from oligotrophic
thatmediatesN fixation,hasbeenshowntobefargreater
tropical and subtropical regions where nutrients (primarily 2
in estuarine environments such as the Chesapeake Bay
N)aredepletedandwarmsurfacetemperaturesarethought
(Zehretal.2003;Jenkinsetal.2004;Shortetal.2004)than
to be favorable for N fixation, at least by Trichodesmium,
2 in oceanic environments (Zehr et al. 2003), which suggests
the most commonly studied diazotroph (Carpenter and
thatthegeneticcapabilityforN fixationmaybegreaterin
Capone 2008; Zehr and Paerl 2008). However, evidence 2
estuarine-influenced coastal systems. However, the high
now suggests that other groups of oceanic diazotrophic
estuarine nifH diversity may also be due to contributions
from organisms transported into these systems from
*Corresponding author:[email protected]
sediments, soils, terrigenous material, and other allochtho-
nous inputs, rather than to populations of autochthonous
Presentaddresses:
diazotrophs actively fixing N in these systems. Along the
1NorthCoastLaboratories, Arcata,California 2
2Department of Biology, University of Massachusetts Dart- North American Mid-Atlantic shelf, inputs of microbes
mouth,NorthDartmouth, Massachusetts fromseverallargeestuaries,includingtheChesapeakeBay,
1067
1068 Mulholland et al.
can be augmented by the introduction of tropical and and depth sensor mounted to a sampling rosette. Water
subtropical diazotrophs that are advected into coastal samples were collected at 2–4 depths using Niskin bottles
watersfromtheGulfStream,whereN fixersareknownto mounted to the same rosette. Samples were collected from
2
be abundant (Capone et al. 2005; Carpenter and Capone near-surface waters in the upper 6 m and from near the
2008). Tropical marine N fixers commonly occur in Gulf bottom (when the shallow-water column was well-mixed)
2
Stream–influenced coastal waters south of Cape Hatteras or the fluorescence maximum at all stations. Additional
(Prufert-Bebout et al. 1993). water samples were collected between these depths or
Oligotrophic tropical and subtropical marine environ- when features were identified during vertical profiling.
ments depleted in dissolved inorganic nitrogen (DIN 5 Wholewaterwastransferreddirectlytoincubation bottles
(NO{ +NO{ +NHz)), and eutrophic freshand brackish during 2006 and transferred to clean 10-liter carboys
3 2 4
waterenvironmentsenrichedinphosphorus(P),havebeen during 2009. In 2009, water from the carboys was then
thought to be primary sites for N fixation in aquatic dispensed into incubation bottles. Samples destined for
2
systems(Howarthetal.1988;ZehrandPaerl2008;Conley nutrient analyses were immediately filtered through 0.2-
etal.2009).Consistentwiththisnotion,expressionofnifH mm Supor cartridge filters and filtrate was collected into
genes has been previously observed primarily in oligotro- sterile sample bottles and frozen. Frozen samples were
phic waters, including the temperate and subtropical transported to Old Dominion University (ODU) for
Pacific (Church et al. 2005; Needoba et al. 2007) and the analysis of DIN and dissolved inorganic phosphorus
subtropical and tropical Atlantic Ocean (Langlois et al. (DIP) as described below. Samples were also collected
2005). Despite the numerous sources of diazotrophic for analysis of chlorophyll a (Chl a). These samples were
organisms to temperate coastal systems, planktonic N filtered onto glass-fiber (Whatman GF/F) filters (0.7-mm
2
fixation has not been broadly examined in coastal marine pore size) and frozen in sterile centrifuge tubes, transport-
systems because nutrient inputs in these systems can be ed to the laboratory at ODU and analyzed using the
high and this is thought to be a condition that is method of Welschmeyer (1994) within 2 weeks of sample
unfavorable for active N fixation (Nixon et al. 1996; collectionduring2006andwithin18dofsamplecollection
2
Zehr et al. 2008; Conley et al. 2009). However, recent during 2009.
observations demonstrate that there is active N fixation Uptake experiments were initiated by adding tracer
2
in NO{-replete (, 10 mmol L21) waters below the upper additions (, 10%) of highly enriched (99%) 15N to gas-
3 2
mixed layer in the eastern tropical North Atlantic Ocean tight bottles filled with whole water (Montoya et al. 1996;
(5 stations; Voss et al. 2004), in surface waters in the Mulholland et al. 2006). We recognize that this method
English Channel (2 stations; Rees et al. 2009), and the may underestimate N fixation during short incubations
2
Mekong River plume in the South China Sea (22 stations (Mohr et al. 2010); all of the data reported here are from
over 2 seasons; Grosse et al. 2010). In addition, experiments incubated for 24 h in on-deck incubators and
diazotrophs and active N fixation have now been found so gas solubility was likely to be less problematic. If we
2
athigherlatitudesthanwaspreviouslythought(Reesetal. allowforequilibrationofthegasbubbleoverthefirst8hof
2009; Moisander et al. 2010). In this study, we further the incubation period, we estimate that N fixation would
2
broaden the domain of active oceanic N fixation to be underestimated by a factor of about 1.4. Incubation
2
include coastal ocean regions. bottles were placed in on-deck incubators equipped with
flow-through seawater to maintain near-ambient tempera-
Methods tures. After 24 h,incubations were terminatedby filtration
through precombusted (450uC for 2 h) GF/F filters.
WemeasuredN fixationratesanddiazotrophabundance Samples were placed into sterile microcentrifuge tubes,
2
in coastal waters along the North American Mid-Atlantic frozen, and transported to the laboratory for analysis. In
continental shelf on oceanographic cruises between 36.5uN thelaboratory,samplesweredried,pelletizedintotindisks,
and 39uN and 276uW to 274uW aboard the R/V Hugh and analyzed for total particulate N and carbon (C) and
Sharp during summer and autumn 2006, and from Cape isotopic enrichment on a Europa 20–20 mass spectrometer
Hatteras to Georges Bank (between 35uN to 43uN and equipped withanautomated nitrogen and carbon analyzer
276uWto265uW)duringacruiseaboardtheR/VDelaware preparationmodule.Ratesofuptakewerecalculatedusing
IIinAugust2009.Inordertounderstandthebiogeograph- a mixing model (Montoya et al. 1996; Orcutt et al. 2001).
ical and hydrographic constraints on the distribution of ArealratesofN fixationwerecalculatedbyintegrating
2
active N fixation and the diazotrophs mediating this our rate measurements from the surface mixed layer and
2
process, we simultaneously measured nutrient concentra- fluorescence maximum over the euphotic zone. This was
tions and hydrographic parameters at stations distributed defined as 1% of photosynthetic active radiation (PAR),
within our study area. Surface-water temperature and and was determined directly using a PAR sensor mounted
salinity mapping was also done using the ship’s underway tothesamplingrosette.ToestimateintegratedN fixation
2
samplingsystemduringtheR/VHughSharpcruisesin2006. rates over the euphotic zone, we multiplied the average of
In 2009, a weekly average of sea-surface temperature was rate measurements made in the surface mixed layer by its
constructed using Moderate Resolution Imaging Spectro- depthandtheratemeasurementsmadeatthefluorescence
radiometerTerraandAquasatellitedata. maximum and below the surface mixed layer by the
Atsamplingstations,verticalprofilesoftemperatureand remainder of the euphotic depth, and then added the two
salinity were measured using a conductivity, temperature, together. The upper mixed layer depth was calculated
Coastal dinitrogen fixation 1069
usamunseridInafnasgucnterthih.eterdealtcLaeobelvoopirrltaiuumtssoerstnyrigiitcmaratilatleO-ytDuacsnrUiindt,egrfDairoonInzPAeonsfctoof0irnl.it1care2aPn5tetarccawhitfaiaiocnsngntseuhtafwrrwioeeenrmdet ngJuly2006fluorescenceAtlanticshelfes.Standard n olmd)2221 83.748.8235.4204.9162.2216.4369.1152.7165.8158.8302.5370.2392.4153.1
ae0tn.0a1alyl.mzem1r9oa8lc4c)Lo.2rDd1,ientrgeectstopioemcntaivnlieumlfyia.tcstAuwmreemrresopn0ei.cu0i2mficmactmoiononclsenL(Pt2raa1rtsiaoonnndss shelfduriandattheatesMid-parenthes Nfixatio2 md)(m21 3)5)4)6)1)9)9)8)9)1)1)2)9)9)
pdfugap20wm0lrn0..rnouue70eoe0dSrdpor2tcue6meaherlop,ismermmoscmtessmimsaodzcspombdttreeeoliemeu)neuamal(dssscrSiyeLtnuaiefonerihtrf2ondolge2omeaaf10dmrv.(rtlao04tihzexqhf95mamaipue.t0mntnwaoiauBodiyusCulnfoeeseHnmucecnai1faduacrio)d9tlu.elgrlrtae6alysehSolpr9n2enuragt)doelumoehh,aausnf)srnucpig—wnatoGsluhchegilpiynynssFtetiauhsgthe/twleinrFshaelsSfoereaaftirtfgbinrecewilealaprtetdreeciinehsvmerrtorspeee,eatspmmonrexwnoidcroacdeehretfulodtii-ilroesshcdlitilcamidhenyinezhtarcpegaecszlttololaoloefyl2i(ciuerdtm00lhorrftOs.0roeelip2ooo6ttrDdp-hhsnzrmh(Uyaaieotiimtilnnnnonecf,l alongtheMid-Atlanticcontinentalnearthesurface(uppermixedlayer)overtheeuphoticzone.MASindicstations.Standarddeviationsarein DIPDIN:DIPmmolL)molarratio(nmolL2211 0.21(0.02)2.33.7(0.0.07(0.00)6.72.0(0.0.05(0.00)9.812.9(0.0.03(0.00)18.719.3(0.0.04(0.01)12.810.8(2.0.07(0.01)7.911.7(0.0.05(0.01)12.617.3(1.0.08(0.01)8.122.5(0.0.05(0.01)12.024.5(6.0.05(0.01)10.430.9(7.0.03(0.01)16.335.1(2.0.06(0.01)8.739.0(7.0.03(0.00)15.323.3(0.0.10(0.01)3.93.8(0.
ptRwC(Ppsw(a2sZaapp2d((wfwsUGFffuD2HPerreet0br0eoaraleJahearooeavtne2eareoFca00urtDha7lNsMrrlsneetmmeas.nyioatikd51ee9Aup5nqrsrufniBi.mbutnA;)aeuneo–6Mdrpuc.D0lraM2arcalec27rto2eFsai)easki.l2a2esnnstntS62h1nrEiie0nninnehn)iose.n0ganagkie.2t0t9i0giscsRgxa5e0tnszdspfsq.et5erasHwtetAy)om9ewtermpquaesret,daloouD,dtgaheairreeutaewsnrcgwefcaroibtfcddefn23osoeLehotdnretteerntfe0nh4clTfopae(anoirte2daSJrdteo0L.dcoagehtiodcsamwe7uwoa1stn7aehntlmdlupqvh,ilhnnsoad;lotae(gytdww.uerme2awo1etreernirrbner(iawvse1nenfF.ame2eireMav2snam83irteaue).drnt0ndieohsoi00n8.ixracgBesoCr90sgegoa0ssuum–Hitisjcavfuna8s7nhtiCoeiaci6rut3esattloeetsy)tsturcgatgrinhyr.rirpmdar.nnli9nertezaofrsdyemirngomwdaesgM8aih,ngploriTeise.dd(cpmicmieLnarn2idoqaatstdhe2ewmhdeoul2oshe5z2muPfeereg0tega0nobfqtChom.raoidgt1aC0ihd5t0nsicul-uhlelteehani4tLnpeuer(.6rrvRrstestna4n2abTCl)youoerehe2,nu.t.uneu)oCaH2waa-iaps1,eia8c1ssrAsasdmost0bnbdhd)ihl0aiesewuiciiecc.n(0nenddgulduteeobrtiTneeipaa7geCdagdgbrrosfumn2eesnrsa)aafattwree1xs(r0,ipcoaehaemdpocntpZgntis)aycsef0tedconrtrwe.oushersemrantoeri8eue-glleaTmoiqrpebebMhrawaoet)cpbret(oskrer.UusrhoytoraweCtezaroaianedeespaheeqerdolreltQnnaidtuehsien-riadrsenreuitfnshmjBuikbitt,ousnecFru(iotvmtaheii.SFcoomisenarodtataagpienJndetelchsnyhsyvesuannerlSh7eltiq.ha.areiiostisimQtd5tTaeCrrfnc-ulftintfPim2ryaais6pmytBetAagsueiunephtt0yPliann5adtrmtounnoraAsieep0aultotC7Cgggtniitdrmccgrsoi7nlindgia8eehebvroteaaeeeiehlRet)bdddonndlygy–aeeeeessrsf---ll.. Physicalandchemicalpropertiesofnear-surfacewatersamples(upper2m)collectedatstationsHughSharpfixationwereestimatedbasedonvolumetricrateestimatesmadeR/V.ArealratesofN2nearthebottom(Table2)atstationswithawell-mixedandshallow-watercolumnandintegratedSIindicatesGulfStreaminfluencedstations,andCBPdenotesChesapeakeBayPlumeinfluenced,of0.00indicatethatthestandarddeviationwas0.01. {{zNONONH+eLatitudeTemperature324mm(molL)(molL)2211am(N)Location(C)SalinityChl(gL()21uu 37.695MAS121.031.70.42(0.05)0.15(0.02)0.34(0.03)37.631MAS222.831.10.33(0.04)0.10(0.00)0.37(0.02)37.519MAS324.131.00.24(0.01)0.10(0.01)0.39(0.03)36.686GSI123.434.30.10(0.10)0.21(0.020.35(0.01)36.627GSI223.932.00.22(0.00)0.18(0.01)0.33(0.03)36.601GSI323.432.90.24(0.04)0.17(0.01)0.38(0.02)36.532GSI424.734.70.23(0.01)0.26(0.03)0.37(0.00)36.972CBP122.026.53.54(0.07)0.27(0.02)0.38(0.02)36.900CBP225.223.33.98(0.00)0.24(0.00)0.36(0.03)36.803CBP325.522.92.78(0.34)0.18(0.00)0.34(0.03)36.671CBP423.527.41.88(0.08)0.14(0.00)0.35(0.04)36.885CBP523.925.72.13(0.19)0.18(0.01)0.34(0.03)38.042MAS423.531.00.20(0.00)0.10(0.00)0.36(0.02)38.122MAS522.931.50.32(0.01)0.02(0.00)0.37(0.03)
((itn3iCof1Bln.u0sPe–w)n3–ce4ienr.d3ef)llu.boeywSnuectrrhefde(a0c.Ge1re0ugD–lif0oI.NSn4t2.remaAangmtdL,2sDhs1ue)IrPlaffanccdsoetasnaCtcileoihnnnlittsrayatawcinooandnscshetinwhgtoherareseer- Table1.aboardthemaximaorstations,Gdeviations Longitud(W)u 275.298275.152275.050274.683274.863275.058275.273276.019275.908275.857275.766275.719274.299274.384
1070 Mulholland et al.
Fig. 1. Areal rates of N fixation (mmol N m22 d21) in surface waters collected from the Mid-Atlantic shelf waters between
2
ChesapeakeandDelawarebaysduringcruisesin(A)Julyand(B)October–Novemberduring2006,superimposedupontheseasurface
temperature (SST)fieldsobserved during the respective fieldcampaigns.
uniform across the study area during July 2006, ranging fluorescence maximum. DIN:DIP ratios were , 16 at all
from 0.40 mmol L21 to 0.65 mmol L21 and from but 2 stations, one where DIP was near the limit of
0.03 mmol L21 to 0.21 mmol L21, respectively (Table 1). analytical detection (0.01 mmol P L21) and the other at a
{
The DIN:DIP ratio ranged from 2.3 to 18.7 and was deepstationwheretherewerehighconcentrationsofNO
{ 3
generally , 16, indicative of N limitation. Average surface + NO . Volumetric rates of N fixation were lower in
2 2
N fixation rates ranged from 2.0 to 39.0 nmol L21 d21 samples from the fluorescence maximum than in surface-
2
and depth integrated N fixation rates ranged from water samples, with average rates ranging from
2
48.8 mmol m22 d21 to 392.4 mmol m22 d21 over the study 1.0nmolL21d21to8.8nmolL21d21.Whenwecompared
area (Table 1; Fig. 1A). N fixation rates and temperature, we observed higher N
2 2
During the same cruise in July 2006, but in samples fixation rates at higher temperatures (Fig. 2A). However,
collected from the fluorescence maximum or near the the water column was stratified during this period, and so
bottom when the water column was shallow and well- all of the samples with water temperatures , 20uC were
mixed, water temperatures were lower (ranging from collected at the fluorescence maximum, where light levels
11.0uC to 21.9uC), salinity was higher (ranging from 29.4 were also lower.
to 35.4), and Chl a concentrations were higher (ranging During the October–November 2006 cruise, surface-
from 0.33 mg L21 to 4.20 mg L21) at all but the CBP- water temperatures were lower than in July, ranging from
influenced stations (Table 2). DIN concentrations were 15.2uC to 21.6uC (Table 3). However, surface salinity and
uniform, ranging from 0.46 to 0.64 mmol N L21 at water Chl a concentrations were similar to those measured in
{ {
depths , 40 m. Higher NO + NO concentrations were July, ranging from 23.5 mg Chl a L21 to 34.6 mg Chl a L21
3 2
observed at the two stations where samples were collected and from 0.46 mg Chl a L21 to 5.88 mg Chl a L21,
from.40m.DIPconcentrationsrangedfrom0.01mmolP respectively.Higherconcentrations ofChla wereobserved
L21 to 0.36 mmol P L21 in samples collected from the at shelf stations in November compared with July. DIN
Coastal dinitrogen fixation 1071
depthofsampleelfstations,GSIeviationsof0.00 fixationN2(nmolLd)2211 4.1(1.0)2.1(0.03)1.4(0.2)1.0(0.0)1.7(0.5)4.2(1.0)3.2(0.9)5.0(0.1)4.3(0.6)3.5(0.1)2.2(0.1)8.8(0.8)2.6(0.6)3.8(0.5)
ations(anticshndardd N:DIParratio 1.372.06.922.19.16.74.32.25.26.34.29.75.02.5
shallow-waterstndicatesMid-Atlparentheses.Sta DIPDImmolL)mol21 0.36(0.04)0.01(0.01)0.08(0.01)0.11(0.02)0.10(0.01)0.09(0.01)0.15(0.01)0.21(0.02)0.11(0.00)0.07(0.00)0.11(0.01)0.05(0.01)0.11(0.00)0.22(0.01)
orSiin (
mfMAare )21 3)4)0)1)2)1)3)2)2)1)1)2)3)3)
o s z L 00000000000000
thebotthSharp.deviation NH4m(mol 0.30(0.0.33(0.0.33(0.0.31(0.0.35(0.0.37(0.0.33(0.0.32(0.0.35(0.0.27(0.0.29(0.0.30(0.0.32(0.0.30(0.
nearHugdard {O2)21 2)1)2)5)1)2)3)1)2)2)1)0)2)2)
mum,ortheR/Vns.Stan {NON+3mmolL 0.16(0.00.21(0.00.19(0.02.06(0.10.55(0.00.21(0.00.31(0.00.14(0.00.20(0.00.19(0.00.19(0.00.21(0.00.21(0.00.24(0.0
fluorescencemaxigJuly2006aboardme–influencedstatio (amChl(gL)21 2.81(0.36)1.92(0.07)1.08(0.09)0.33(0.08)2.20(0.21)1.54(0.23)1.35(0.55)4.20(0.03)3.86(0.12)3.18(0.10)1.93(0.18)2.05(0.01)0.51(0.05)1.33(0.05)
en y
atthduriyPlu alinit 32.532.732.935.434.333.833.730.230.932.232.829.432.633.0
mplescollectedontinentalshelfChesapeakeBa emperature(C)Su 13.012.312.516.514.013.413.517.817.616.613.421.917.811.0
watersaAtlanticcPdenotes Tpth(m) 21242910042253613151418184840
alandchemicalpropertiesofd),atstationsalongtheMid-m–influencedstations,andCB,ndarddeviationwas0.01. atitude(N)LocationDeu 37.695MAS137.631MAS237.519MAS336.686GSI136.627GSI236.601GSI336.532GSI436.972CBP136.900CBP236.803CBP336.671CBP436.885CBP538.042MAS438.122MAS5
Table2.PhysicollectionisindicatendicatesGulfStreandicatethatthesta Longitude(W)Lu 275.298275.152275.050274.683274.863275.058275.273276.019275.908275.857275.766275.719274.299274.384 (tJwNnhuaomeltFyveso)iuraglrcn.sfLradau22mci.(es1Bpead)lseRn.2Osde1cp)laatooattoibntolhedenreds–thfeNolimupvooepvbrreeeermtstahcwtbeeueneresrceunedmivmunmorailsneuxargimmm(2epJu0tulrme0lisy6cd),cruaoaarntnliledndescgitaonecufdr(tNuCufimr2)soesfnsmiuxir(nafOnta(ieccoAaten–r-)
cii
1072 Mulholland et al.
continentalshelfduringOctober–rface(uppermixedlayer)andatthePlume,MASisMid-AtlanticShelf,,standarddeviationwas0.01. Nfixation2 (nmolNDIPmLratiod)(molmd)22221121 75.6(0.2)118.502.7(0.3)62.518.6(0.2)82.6815.7(0.4)233.4517.3(2.0)135.3620.3(5.9)253.416.5(0.6)121.124.0(0.1)59.697.2(6.6)179.102.0(0.2)65.852.3(0.3)70.021.4(1.1)40.931.3(0.1)51.764.1(4.3)228.524.5(1.2)133.304.1(1.2)91.7 atoshiprar2nhuaann0biegfontrr.ddsBle3fhiusgeosaeefeDreencrdcvsdniooemtanIec(bmPudfTeownrsssdldoceauu0earoLbumrts.tv0rftnelr2ateiae2riceh4nd1ctsen3mi0eegnodit;(m.ntns9T2wFruwcos1aaJaito.mlguaotbdtnDiL.etllmouyceer212Iner,sre5NB3in1srrn3r))a.ttci..g:a.ronD4noDanANglt0egsmuIiiuv.penoomP2medrtgnv6hfrorensaaaf-fmlmcrirgtnmwomeoiebwotmmeoewe2savrgrl2eawso0,rL1tal.ldee.u1o2ot3wrr23embw1esd1e,nsmeegwlermNdlrtmeer-reunvmstoi2rophcreeelfiildreainxNahxLcLngeltaia22ld2igttyvtth1hi1heof#eepiltdidnxyrsolru2a.u1ratt4rt1Tahm6inii.mtona1hgtieennno8geees-
AtlanticrthesukeBayhatthe DIN:molar 8.5.16.13.7.4.6.7.6.5.8.14.8.2.5.4. Os4i)mc.tioNlaburetrir–niNesnuotrvfecamocnebceaernndt2r0da0te6ieo,pnetsremwapnaedterarDtsuaIrNmep:aDlnesdIP(Tsaralabintlieiotsys3wwaeenrrdee
aat
atstationsalongthemid-etricrateestimatesmadeneawareBay,CBPisChesapedeviationsof0.00indicate zNHDIP4m(molL)21m(molL)21 0.86(0.10)0.22(0.02)0.73(0.08)0.23(0.02)1.17(0.07)0.26(0.02)0.86(0.09)0.21(0.01)0.45(0.03)0.19(0.02)0.21(0.03)0.15(0.02)0.40(0.01)0.19(0.02)0.72(0.01)0.19(0.00)0.48(0.02)0.11(0.00)0.21(0.03)0.09(0.01)0.73(0.07)0.10(0.01)0.07(0.01)0.02(0.01)0.11(0.02)0.05(0.00)0.01(0.00)0.05(0.01)0.07(0.01)0.14(0.01)0.22(0.04)0.13(0.03) acbtcaudw1hooul3usoesoulm.trtrol9utsiaeenopemclnslgmatlyiomenmrotdwe#hboidacleeslratfre1urrNarN6oipavus.mat2leetLutOud,tm2mfhnbtwi1heexdonea-aeutdivfhtncrl2oieurfifobr1nluldounau)sgierogseeteperenrhrstv,te,cacahhserevetnscedaedno,estclinehunsaesscuimnategasmmondhuetfdimtameroDrNrfxianaecitI2rmxcNseNreimfmuaicw:xwptmruDefeaeuaismrtrxiIte(iasoaePoretrtafnu.ohisnorN.rriiaggenWnHt2ehniohersfofeoiaaorseawxfmtrnraeewt0tsphvitt.ewolea2hharernn–eeeest,
collectedonvolumBPisDelStandard {{NO+2olL)21 6(0.06)2(0.07)1(0.26)5(0.49)6(0.26)8(0.14)5(0.06)4(0.14)8(0.07)4(0.00)2(0.00)1(0.06)0(0.07)2(0.00)6(0.09)0(0.07) lfloirkowemleyrthbteeemcsaupuresfraeactetuhraeensdwaatnhtdeerfnlcuoooldrueimsstcnienncwcteagsmro2wauxepilmli-nmugmixoe(fdFsiagdm.u2rpiBlne)gs,
sicalandchemicalpropertiesofnear-surfacewatersamples(upper2m)HughSharpboardtheR/V.ArealratesofNfixationwereestimatedbased2ma(belowtheuppermixedlayer)andintegratedovertheeuphoticzone.DGulfStream–influencedstations.Standarddeviationsareinparentheses. NOTemperature3m(mamLatitude(N)Location(C)SalinityChl(gL)21uu 38.933DBP15.531.22.89(0.30)1.036.900MAS117.430.62.17(0.18)0.436.967CBP115.223.63.89(0.71)3.036.884CBP215.223.55.88(0.18)2.036.800CBP315.424.83.99(0.69)0.936.667CBP417.830.93.24(0.36)0.436.417MAS2a18.130.52.96(0.06)0.736.417MAS2b18.030.72.24(0.13)0.636.434MAS318.331.22.01(0.04)0.236.450MAS418.832.30.91(0.13)0.236.467MAS519.232.60.73(0.03)0.136.483GSI121.634.50.46(0.04)0.236.484GSI221.434.60.93(0.00)0.337.417GSI319.633.50.55(0.02)0.137.517MAS617.933.60.64(0.04)0.637.700MAS717.032.21.53(0.03)0.3 (MflAtDBr1ctDafMs0tcPsNScoeoo0ihehuooo6.nuxoaum5Is2wI.riemiannlaDr9a1nsPpoNd1lgalifcceepslt2.fkenmflut-ay0uieeacricieApcomecsnx2tnnbtramoretect,ietenmamiatgttailecrhnnmrodvllrrnpamtttaai2aocaagmeniswauonbdlrlone0aenllottdacnardduyeoiintsnt0lleetaootdhiei,tG)Lddlt9enscnf.ctnnnreetr4Pus,2aLaiotuaaasshDoottrrgt1lutSnli3e2tsLeamrvlidfeireuhtcent5hdfoa1tesa2Duaohucrtre.isgnenorni7to2oirtgir1muloIngirgsfteaie4fg1nrotmtNreandednhgnnhah5e.wfsnmnMdgu0eadpwduseesotegurrcNtcphafhtasCrriaanerJsaerfwonneiaueisud0ittruoctusrngdiunohGaert.iaalmmoat2soeyafmdcngme5ulJnfnteae0lrDroryed.uuymdst2opcsuc9esowdrh,lrrt70Iere0iyamenmem4aeuDsPigtw-..rwrdse011htmw2taief,leoiIse,ehu2r.mieew0stoa2P3mearoCrowusraDm0n;eo91Bhttemlmfnr(9iewes.4brsletIfaoeFgae9r.uraprNunsodsLlaxoenlioOoNo(neeLgfostbf2k2tmrcrng:esb.nolNus.e2vaeta1Danie4giua2lil2gbsnetm1aavmos61nctChdI3ltaivoteD7owee1PvhtdLi3itplle)f0lomtnray.ehye.a.Iaru42lu0imlrct)NeiianrtsnWsudciMtn4ag,1esucgoChero,te.tthtate5aihruihaeeilmnehofl,ad4rfnein(aaltarlsersdonDTo-LaosndntelcGoAmsewdriihxnaGwe2dItmamat2gdie-ebytNgaeCe1mmet3wuluoiritclDaerlLtte.hDltrripausewaa1nufmtigtilI2Ntlnnu5omnthIdhoaooP,eeeCi1i)Pddngaeceersss2tfff.,,.
Table3.PhyNovember2006afluorescencemaxiandGSIdenotes Longitude(W)u 274.933275.700276.017275.900275.850275.750275.700275.700275.517275.250275.050274.833274.667274.400274.567275.034 tMfLohifx2eiaWd1aft-lnidaAuoa2tontel1ylrraiettnnistoccetasiem4ucln9rpcSd.fee8ahercmtaeneeltm-cafuwtxroibaioelmustneNtu.rwmtAseLhari2vtesmhe1arpcwdanolaeg2osise1nlern(rvsaTouonatarlgnufbeaadmldlceweesf5taarwr)oyl.iiacsmntiettrr1hyas.et0ehasclinogamhnoseegforltfNhoaNer2t
Coastal dinitrogen fixation 1073
ss0 )
w,well-mixedwatercolumnDBPisDelawareBay,CBPiStandarddeviationsof0.0 fixationDIN:DIPN2(nmolLmolarratiod2211 10.711.3(0.2)4.94.9(0.6)11.44.4(0.2)10.313.9(7.4)6.36.0(0.2)3.010.9(2.3)8.45.6(0.01)16.01.5(0.01)5.67.1(4.8)18.01.9(1.2)16.31.0(0.1)13.20.7(0.4)19.50.2(0.2)4.40.9(0.2)8.32.7(0.4)7.41.8(0.8) fG(smLfcwrrTraao2ooaealtlaiteolmm1xnbeesricirltmgtesot0o0yee,u.f.6sd390mD)Nr.3.nB5af.IrW2m1nNamoDgnfmmoami:eIkxltmDdNoeaNtrlwotIhicPftleoNhreLoonNemnfr2rmlaLcuepa1teL2oteindot2rr31tht2aseh01rets1ta.euwco7atterwinefoonel1rdatusnce.o2to9esre0D1a#rr.rme33nIas5Pmgcanc1n.omexe5g6mcdlnie.uoomcdofamlnAerultcofPnvmNermmeotn.hrLawmtLa8erxA2aga.2i4s1m0set1fi.lto1fuiuwvdonon6moo2ers2rllms1ul0es,arr-mms.umaam9lcornnoeeifpawxgglntanlreeecceeNiddddecesr
withshalloghSharp.arentheses. DIPmolL)21 20(0.02)22(0.01)27(0.04)26(0.03)22(0.02)16(0.02)19(0.01)15(0.01)11(0.01)11(0.02)11(0.02)03(0.02)20(0.01)06(0.01)11(0.00)14(0.02) ti1Tnh9ata.eb6nglermamt5th)eo,edlwNNriat2hnmgfi2texh2aetodiobh2nsi1gerhrtvoaeetset8ds3d7iine.n9pAtmhsumugrionufaltsectNge2ra0mwt0e29ad2tredNarns2.g1efD(diFxeaifgprt.iotohm3n-;
nsHunp m( 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0. rates measured at a station on Georges Bank. 2
bottomatstatioaboardtheR/Vdeviationsarei zNH4m(molL)21 0.99(0.06)0.64(0.06)1.70(0.14)1.41(0.08)0.67(0.07)0.22(0.02)0.90(0.09)1.51(0.02)0.38(0.04)0.63(0.04)0.09(0.01)0.06(0.03)0.03(0.01)0.03(0.01)0.07(0.01)0.79(0.00) csfolaebunmWsoteprrrhealvesetecsinodecnnwocstleehl(eamFccttoiaegmxdt.ihpm4ieaAnur,mehsCduig)r(FhfNaaeinc2gsedt.f4iwxrtCeaaa,mtttDieeoprs)ne(froFoarfriatguta.eNrlsel42Atwh(F,friBiteixhg)ea.actD4rinouBIdnNi,sDaestw)c,,otewhnrineee-
ncemaximum,orneartheOctober–November2006uencedstations.Standard {{NONO+32m(molL)21amhl(gL)21 3.81(0.06)1.15(0.09)2.49(0.16)0.45(0.06)3.80(0.27)1.33(0.00)3.28(0.35)1.21(0.00)3.80(0.29)0.69(0.06)4.79(0.01)0.24(0.00)2.52(0.39)0.69(0.15)2.21(0.08)0.93(0.06)2.16(0.17)0.24(0.00)1.85(0.10)1.37(0.32)1.67(0.08)1.75(0.16)0.63(0.00)0.27(0.06)0.42(0.05)3.86(0.82)0.87(0.01)0.21(0.06)1.97(0.01)0.80(0.18)1.31(0.01)0.24(0.00) pawmlnoroatoobaewutNostearseeeslseiiersrgfuvtdsoHnoreraeifdodfdnDiragNcdntaeIaaoi2Nnntn.teetrfmceticsxholcopaaaenoremttcrpirifoeeoapilnlenuntlasuetsotrshrirdaoewietupsnwsierocwrbhibenneeeneegtrfrtcwoeqweeiruenehemaDaedinnnngiaIvthyNDxNiiefdioismI2euftPdc;fautoihhlcmxneouoaccnstwcre(iircuopnneueinvgtsnir.eseataersrqt,snaiP0dootitn.hCinr0otess5Renrium)nes.rwpfwaaWateennhacrrddeeees-
plescollectedatthefluorescenticcontinentalshelfduringSIdenotesGulfStream–infl Temperature(C)SalinityCu 15.331.417.330.616.127.816.528.817.330.217.831.118.331.419.332.719.332.619.934.420.734.423.936.123.936.121.034.618.834.717.033.1 pdoMf3gdUlir.rarufio5aCoei–rgdmzvuiYH3eno-ipoAnNgtetrue1mAhto-Jls0eAaciplu7acyonhltyugynprtauliqoeiiucfnesn2upHsesos0eaidahcb0ndgcsae6atesopellicfotnpfraetdiciweeiecnmiierdataacdsieeztoatidirLeonpAo-(rt2piUnwsrtuerh1ossdgeCo,epr(uurbwYTehasrsetteinsaNfun,rrbgs2dgeo-eeTl0AeytmdaAr0sl)ai79sfucfrr)ows.hgr.eoaooauaUqmTmdmssudhnteptanuseihl2ncsmoere0aehtsit0inilmimuflqgc9guimpaohu.olbballeaGselorslatensetetcntaNhntitcndbfeheoiu2cudar-lRrcmonlfbueoifiudlxbcricpesgaotiheienehntmeresdogss--t.
ofwatersamtheMid-AtlaShelf,andG01. Depth(m) 966655101210182320—302725 gsoCNeeuhq2nteuefstiehaxnpeaccotesiiapntoukignedesoywBfearaererxyetetrarioinaebwfcvlsteiueetdherdvntDecbhdeyNedienArqxePtcfgherCiepooRtmnCio.BtnAwhPeolatsafsherosestluataoag,twtihtoi.honhensSisgnuwhiubdimtsreheanbqittneeuifrsetihenoodetff
emicalpropertiesindicated),alongSisMid-Atlantic,viationwas0. N)Locationu DBPMAS1CBP1CBP2CBP3CBP4MAS2aMAS2bMAS3MAS4MAS5GSI1GSI2GSI3MAS6MAS7 pDgerionAstecelsutohsbosfariuocogtnmherioaUli(gCFoiYtgr.No5p-)hA.icctryoapniocablacatnedriasu,batrnodpicaalamnadrince-
hsAe ( 3074077740734770
ysicalandccollectioniPlume,Mstandardd Latitude 38.9336.9036.9636.8836.8036.6636.4136.4136.4336.4536.4636.4836.4837.4137.5137.70 ebh(Hnraavvociewrkoaibnsrhemtehnweneattttsheaorld.ueeg1nph9vlet8itr8eot;donPmiabneeenrDltps2IrN0iem0n,8raia;rcnyChdeosdnietlueienstyropfeophtrhoaisclNp.h2f2ro0efr0siux9hsa),ta(iPwon)nde,
Table4.Ph(depthofsampleChesapeakeBayindicatethatthe Longitude(W)u 274.933275.700276.017275.900275.850275.750275.700275.700275.517275.250275.050274.833274.667274.400274.567275.034 ssT(frToyuaasutrabetfnbeasledmlceswesae1ewc2v,rt,aeeivnt34eoe,,rwfsNtaahe(nnne2rddrafenhi6x5gDi)ga)e.hItwiNIaon(annFcsd2oii0g0nn..a01ctt61e3h,;ntimtshwTrmeataeeotbimfflolloeLunpsuo2sen1rr1wdea–tst4eocetr)ehe5cnaa.moctn9ae0deNsatmmsa2tmuhlafraoimxatxlibamaLnltreiuii2fonmHi1nne;
1074 Mulholland et al.
entalshelfduringAugust2009uppermixedlayer,andatthed-AtlanticShelfbetween39Nune.StandarddeviationsareinPratio.Standarddeviationsof Nfixation2 mDIP(molN(nmolNatiod)md)L22221121 1.1(0.1)30.29.2(5.0)152.32.2(1.4)53.421.9(4.8)215.811.9(4.6)778.88.6(1.5)169.7—69.01.2(0.3)19.65.3(1.5)35.725.7(15.8)192.449.8(9.5)351.32.3(0.5)25.82.6(0.1)34.33.0(0.5)56.31.8(0.8)60.39.1(5.3)86.94.5(1.8)208.028.6(15.1)837.95.6(0.7)125.63.7(0.4)92.97.0(0.6)102.610.6(0.8)198.74.1(0.4)97.62.5(0.7)52.24.3(0.5)122.43.1(0.5)66.41.9(0.6)51.97.5(0.01)207.62.2(0.4)47.922.2(1.8)194.318.3(5.2)245.5
Mid-Atlanticcontinthesurface,intheN,MASNistheMiintheGulfofMaiculatetheDIN:DI DIPDIN:molL)molarr21 40(0.15)4.409(0.07)2.950(0.12)3.330(0.10)1.121(0.07)3.314(0.07)2.9BDL—BDL—18(0.02)1.803(0.01)36.3BDL—55(0.09)3.4——12(0.01)9.119(0.10)3.427(0.03)4.434(0.34)8.325(0.02)6.352(0.01)11.334(0.09)8.466(0.14)2.984(0.18)4.349(0.09)2.778(0.05)4.757(0.16)2.516(0.03)8.332(0.00)2.047(0.07)3.650(0.19)8.802(0.67)2.043(0.37)4.4
thenear39uandocal m( 0.0.0.0.0.0. 0.0. 0. 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.
collectedatstationsalongetricrateestimatesmadeetweenCapeHatterasand270WonGeorgesBankudetectionlimitwasusedt {{zONONH+324mmmolL)(molL)2211 1.13(0.87)0.63(0.43)0.24(0.35)BDL1.61(0.24)0.04(0.03)BDL0.30(0.38)0.28(0.31)0.41(0.01)0.04(0.00)0.37(0.16)0.48(0.16)1.00(0.66)0.34(0.04)0.50(0.04)BDL0.30(0.00)0.19(0.09)0.90(0.09)0.20(0.04)0.34(0.12)0.27(0.02)1.62(0.03)——0.20(0.25)0.89(0.07)0.11(0.12)0.54(0.25)0.74(0.11)0.45(0.06)0.50(0.29)2.31(0.47)0.16(0.20)1.42(0.03)5.44(0.09)0.46(0.01)1.39(0.43)1.47(0.19)0.02(0.00)1.91(0.03)0.10(0.11)3.54(0.12)BDL1.28(0.04)1.65(0.56)1.98(0.11)0.39(0.09)1.01(0.06)BDL1.30(0.10)0.29(0.24)0.35(0.04)0.13(0.16)1.55(0.17)0.08(0.09)4.30(0.16)BDL2.01(0.14)0.30(0.40)1.61(0.11)
samples(upper6m)atedbasedonvolumMid-AtlanticShelfbheshelfareaeastofmit,inwhichcasethe N(amyChl(gL)21 0.57(0.12)0.23(0.04)0.51(0.04)0.22(0.02)0.12(0.02)0.19(0.02)0.28(0.02)0.85(0.10)0.30(0.05)4.54(0.43)2.33(0.33)0.57(0.04)—0.52(0.03)0.89(0.08)0.21(0.05)0.22(0.01)0.24(0.03)1.74(0.12)0.57(0.03)4.05(0.21)3.95(0.72)1.57(0.04)1.92(0.10)2.47(0.57)0.49(0.08)1.39(0.14)0.94(0.12)3.11(0.38)0.84(0.06)0.31(0.07)
ewatereestimMASisnsontctionli Salinit 31.4—31.431.2—31.131.030.832.430.730.930.831.030.429.932.233.032.932.132.332.332.332.132.332.332.332.232.232.331.231.1
Physicalandchemicalpropertiesofnear-surfacDelawareIIR/V.ArealratesofNfixationwer2maximaandintegratedovertheeuphoticzone.2butwestof70W,andGBGMdenotesstatiouBLDindicatesthatthevaluewasbelowthedete,thatthestandarddeviationwas0.01. TemperatureW)Latitude(N)Location(C)uu 41.314MASN21.041.304MASN22.640.059MASN24.839.098MASN25.638.464MAS25.737.855MAS26.437.516MAS27.036.946MAS26.735.745MAS23.636.554MAS24.637.428MAS26.039.007MASN23.139.232MASN22.339.516MASN26.240.063MASN25.540.330MASN23.540.164GBGM23.140.641GBGM22.641.160GBGM11.440.607GBGM18.841.404GBGM17.341.268GBGM17.040.984GBGM18.041.572GBGM14.241.687GBGM15.442.057GBGM17.042.478GBGM16.141.995GBGM17.242.050GBGM16.242.181GBGM20.442.498GBGM22.1
Table5.aboardthefluorescenceand41.5Nuparentheses.0.00indicate Longitude(u 270.944271.811272.837273.344273.300274.642274.974275.180275.395275.706275.477274.515274.512273.880273.388271.740269.991269.912269.324268.529268.214267.822267.203266.955266.442265.736266.652266.797267.601269.282269.672
Coastal dinitrogen fixation 1075
edne
hofsamplHatterasanBankanditocalculat Nfixation2(nmolNLd)2211 1.1(0.1)1.8(0.6)1.0(0.1)1.9(0.1)20.3(10.5)2.7(2.0)2.8(0.1)0.7(0.8)1.3(0.3)10.9(2.3)9.1(0.6)1.8(0.0)2.1(0.5)2.0(0.1)2.4(0.4)1.2(0.6)0.3(0.1)5.5(1.0)2.1(0.4)4.5(0.2)4.7(0.5)12.2(1.2)5.6(1.0)1.2(0.0)5.0(1.9)1.0(0.2)3.2(0.2)5.0(0.3)1.3(0.3)2.0(0.0)3.3(0.3)
stations(deptetweenCapeWonGeorgeslimitwasused DIN:DIPmolarratio 0.30.53.64.51.46.78.42.41.81.313.30.7—2.42.75.816.22.614.01.68.32.31.93.64.43.34.11.91.60.95.0
um,ornearthebottomforshallow-waterDelawareII.MASisMid-AtlanticShelfb2otesstationsontheshelfareaeastof70udetectionlimit,inwhichcasethedetection {{zNONONH+DIP324mm(molL)(molL)2211m(molL)21 0.02(0.00)0.14(0.11)0.50(0.07)0.25(0.04)BLD0.56(0.01)1.02(0.00)BLD0.29(0.09)2.54(0.17)0.91(0.03)0.77(0.07)BLD0.28(0.36)0.21(0.00)0.44(0.00)0.03(0.01)0.07(0.00)0.23(0.05)0.61(0.30)0.10(0.00)0.85(0.06)0.47(0.06)0.56(0.05)0.14(0.16)0.42(0.00)0.32(0.06)0.31(0.09)0.25(0.02)0.44(0.00)0.14(0.16)0.26(0.01)0.03(0.02)0.23(0.12)0.29(0.07)0.79(0.04)———0.16(0.20)0.30(0.01)0.19(0.11)0.84(0.06)0.66(0.03)0.55(0.03)3.66(0.25)1.13(0.05)0.83(0.22)0.49(0.05)0.48(0.12)0.06(0.01)0.41(0.00)0.37(0.02)0.30(0.00)4.58(0.36)1.01(0.06)0.40(0.13)0.35(0.03)0.28(0.02)0.39(0.19)0.18(0.23)5.33(0.00)0.66(0.15)0.26(0.10)1.52(0.09)0.77(0.21)BLD1.00(0.08)0.53(0.06)1.59(0.02)1.25(0.01)0.78(0.11)2.22(0.67)0.60(0.06)0.64(0.00)0.13(0.15)0.39(0.07)0.16(0.04)0.64(0.16)0.50(0.00)0.28(0.08)0.54(0.49)0.46(0.01)0.53(0.00)BDL0.50(0.03)0.33(0.32)0.35(0.23)0.22(0.04)0.62(0.00)9.19(1.97)0.32(0.01)1.91(0.14)
ximR/Vdenthe )21
orescencemaaboardtheandGBGMuewasbelow,was0.01. amChl(gL 1.34(0.01)1.29(0.01)4.22(0.00)1.87(0.09)2.66(0.55)1.71(0.04)0.73(0.13)4.76(0.11)0.52(0.21)4.79(0.09)1.52(0.08)1.85(0.07)—0.52(0.06)2.32(0.05)1.50(0.07)1.62(0.26)4.95(0.68)1.69(0.16)3.08(0.58)4.17(0.14)4.39(0.81)4.39(0.74)1.85(0.09)0.97(0.07)1.34(0.02)1.50(0.01)1.63(0.03)3.38(0.20)2.03(0.22)0.81(0.08)
edatthefluAugust20092stof70W,usthatthevalarddeviation Salinity 31.432.932.833.035.532.933.133.133.332.732.431.631.632.732.533.433.832.732.132.332.332.332.232.332.532.430.732.332.332.232.3
ectngweatend re
ofwatersamplescollcontinentalshelfduri39Nand41.5Nbutuuarentheses.BLDindic0indicatethatthesta TemperatuDepth(m)(C)u 14.419.03119.830.59.8308.439.117.628.69.916.520.924.9—1318.87.118.310.319.411.517.1—15.018.813.724.911.225.713.927.318.722.512.08.911.322.316.617.517.317.416.910.516.01514.015.112.719.715.114.616.115.314.59.515.923.111.822.913.0
calpropertiesMid-AtlanticShelfbetweenationsareinpviationsof0.0 Location MASNMASNMASNMASNMASMASMASMASMASMASMASMASNMASNMASNMASNMASNGBGMGBGMGBGMGBGMGBGMGBGMGBGMGBGMGBGMGBGMGBGMGBGMGBGMGBGMGBGM
Physicalandchemiindicated),alongtheNistheMid-AtlanticMaine.StandarddeviIPratio.Standardde W)Latitude(N)uu 41.31441.30440.05939.09838.46437.85537.51636.94635.74536.55437.42839.00739.23239.51640.06340.33040.16440.64141.16040.60741.40441.26840.98441.57241.68742.05742.47841.99542.05042.18142.498
Table6.collectionis39N,MASutheGulfoftheDIN:D Longitude( 270.944271.811272.837273.344273.300274.642274.974275.180275.395275.706275.477274.515274.512273.880273.388271.740269.991269.912269.324268.529268.214267.822267.203266.955266.442265.736266.652266.797267.601269.282269.672
1076 Mulholland et al.
Fig. 3. Arealratesof N fixation(mmolN m22d21)insurface waterscollectedfromtheMid-AtlanticshelfwatersbetweenCape
2
Hatteras and Georges Bank in August 2009, superimposed upon a weekly averaged sea surface temperature (SST) field from satellite
observations madeduring the fieldcampaign.
genecopieswereabundant(Table 7)in(1)areasinfluenced fluorescence maximum, ranging from averages of 2.0 nmol
bytheChesapeakeBay(CBP)ortheDelawareBayplumes N L21 d21 to 39.0 nmol N L21 d21 during summer (July
(DBP), (2) Gulf Stream–influenced waters (GSI), and (3) 2006),1.3nmolNL21d21to20.3nmolNL21d21during
Mid-Atlanticshelfwaters(MAS).Duringourwidersurvey autumn (Oct–Nov 2006), and 1.0 nmol N L21 d21 to
in 2009, we found that high rates of N fixation (Fig. 3; 49.8 nmol N L21 d21 during August 2009 (the limit of
2
Tables 5 and 6) and abundant nifH gene copies (Table 7) analytic detection was , 0.1 nmol N L21 d21). This is
extended into coastal waters north (to almost 43uN) and consistent with the observation that cyanobacterial diazo-
east (to 265uW) ofthe Delaware Bay plume, includingthe trophs (and most cyanobacteria) are predominantly pho-
highly productive Georges Bank. Expression of nifH genes toautotrophic (Zehr et al. 2008). Rates reported here from
waspreviouslyobservedprimarilyinoligotrophicwatersin the euphotic zone are also within the range or higher than
the temperate and subtropical Pacific (Church et al. 2005; those reported previously for the tropical and subtropical
Needoba et al. 2007) and the subtropical and tropical Atlantic Ocean and other areas of the world’s oceans
Atlantic Ocean (Langlois et al. 2005). Similarly, rates of (Table 8), but not as high as some of those reported from
pelagic marine N fixation have been primarily reported tropical coastal waters north of Australia (Montoya et al.
2
from tropical and subtropical oligotrophic gyres (Table 8; 2004).
Mahaffey et al. 2005; Carpenter and Capone 2008; Temperature is thought to limit planktonic N fixation
2
Mulholland and Lomas 2008). bysomemarinecyanobacteria(Staaletal.2003;Breitbarth
N fixation rates in this study were generally higher in et al. 2007; Moisander et al. 2010). Consistent with this
2
the well-lit surface mixed layer than at the depth of the idea, during cruises on the continental shelf between the