GLOBAL BIOGEOCHEMICAL
CYCLES, VOL. 12, NO. 3, PAGES 429-441, SEPTEMBER 1998
Atmosphericiron supply and enhancedvertical carbon flux in the NE subarctic
Pacific:
Is there a connection?
P. W. Boyd,•'2C. S. Wong,3J. Merrill,4F. Whitney,3J. Snow,4P. J. Harrison,• and J. Gower 3
Abstract. Recentstudieshaveconfirmedthe relationshipbetweeniron supplyandphytoplankton growthratesin all threehigh-nitratelow-chlorophyll(HNLC) oceanicprovinces.However,there is little evidence,sofar, of the role of iron in alteringthe efficiencyof the biologicalpumpvia increaseddownwardexportof particulateorganiccarbon(POC). The NE subarcticPacificis uniqueamongHNLC regionsin that longtime seriespelagicobservations anddeep-moored sedimenttraprecordsexistwhichmay providethe bestopportunitythusfar to testaspectsof the
ironhypothesis. Episodic elevated levelsof chlorophyll a ( > 2.0 [tgL-l) wereobserved 6 times between1964 and 1976 at the former site of OceanStationPapa(OSP). In addition,between 1984 and 1990 on at leastthreeoccasions, concurrentpulsesof POC and biogenicsilicawere recordedin deep-mooredtrapsat OSP. Possibleexplanationsfor theseevents,suchas lateral advectionof more productivewaters,iron-mediatedblooms,or grazingby salpswarmsare discussed andtestedusingan existingdownwardPOC flux model. Owing to the episodicnature of suchevents,no availabledataare sufficientlycomprehensive to unequivocallyrule out any of
theseexplanations.Nevertheless, fromthe dataavailable,the occurrence of pelagicor deepwater pulses,approximately onceevery3 years,aremostconsistent with iron-mediated diatomblooms, andof the sinkingof POC andbiogenicsilica(from sucha bloom)to depth,respectively.A comparison of thetimingof theseiron-mediated pulseswith thatof thetransportprobabilities of atmospheric dustsupplyfrom Asia andAlaskaprovidesan opportunityto assess the likelihoodof a couplingbetweenthe atmosphere andthe ocean. 1. Introduction
Recentstudieshave confirmedaspectsof Martin's [1990] iron hypothesis, namely,the positiverelationship betweeniron supply and phytoplanktongrowth rate in the equatorial Pacific [Behrenfeld et al., 1996;Coaleet al., 1996a,b] andNE subarctic Pacific [Boyd et al., 1996], and between iron supply and phytoplanktonstocksin the SouthernOcean [de Baar et al., 1995]. While other studies/reviews have focusedmainly on the mechanisms which may controlthe biogeochemical cycling of iron [Hutchins,1995; Barbeauet al., 1996; Tortell et al., 1996], no studyhas so far attemptedto investigatethe role of iron in controllingthe export of carbonto depth. Specifically,does an increasein iron supplyto phytoplanktonresultin a strengthening of the "biologicalpump"[Longhurst,1996]? Suchstrengthening wouldresultin a significantiron-mediated sequestration of carbon • Department of EarthandOceanSciences, Universityof British Columbia,Vancouver,British Columbia, Canada. 2Now at NationalInstituteof Water andAtmosphericResearchCentre for ChemicalandPhysicalOceanography, Departmentof Chemistry, Universityof Otago,Dunedin,New Zealand. • OceanScienceand Productivity,Instituteof OceanSciences,Patricia Bay, Sidney,BritishColumbia,Canada.
4 Schoolof Atmospheric Chemistry, GraduateSchoolof Oceanography, Universityof Rhode Island,Narragansett.
to depth. Recently,Kumar et al. [1995] have provided,using paleoceanographic proxies for primary and export production, tantalizingevidenceof markedfluctuationsin theseparametersin the SouthernOceanin the geologicalpast,which lend supportto the importantinfluenceof iron supplyon the oceaniccarboncycle [seeMartin, 1990]. Contemporaryevidenceof the role of iron in altering the magnitude of primary/ export production would furtherconfirm aspectsof the iron hypothesis. The NE subarcticPacific is unique among high-nitrate lowchlorophyll(HNLC) regionsin that long time seriesof dataexist, albeit covering different time periods, for pelagic observations and deep-mooredsedimenttraps. Thesedataprovideperhapsthe best opportunity to begin to examine questionssuch as the following: Is there any contemporaryevidencefor iron-mediated increasesin the strengthof the biologicalpump?,If so, how often do theseepisodesoccur?, Is it possibleto use thesepresent-day observationsas models for events in the geologicalpast? In addition,on the basisof iron budgetsfor the NE subarcticPacific [Martin et al., 1987] and SouthernOcean [de Baar et al., 1995] and observationsin the equatorialPacific [Coale et al., 1996a], the NE Pacific appearsto be the only HNLC region where iron might currentlybe suppliedpredominantlyvia the atmosphere.It is perhapsthe most appropriateregion in which to examinethe potentiallink betweenatmosphericdust supply and increasesin the magnitudeof downward particulateorganic carbon (POC) flux in the modern ocean.
Since such dust/productivityevents appear to be episodic [DiTullio and Laws, 1991; Younget al., 1991], the establishment of a relationship between atmosphereand the ocean will be difficult, if not impossible,usingpresentlyavailabledata. With
Copyright1998by the AmericanGeophysicalUnion. Papernumber98GB00745. 0886-6236/98/98GB-00745512.00
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BOYD ET AL.'
ATMOSPHERIC
IRON SUPPLY AND ENHANCED
VERTICAL
CARBON FLUX
this in mind,the objectivesof this studyare,first, to establishif thereis evidenceof iron-elevated production/downward POC flux in theNE Pacificand,second, to ascertain if suchan atmosphericoceanic link can be adequatelyassessedusing the relatively comprehensive NE subarcticPacifictime seriesof data.
to predicthowthe pathwaysof C flow througha pelagicfoodweb may influencethe magnitudeof downwardPOC flux from the upperocean(Figure 1). BoydandNewton[1995] extrapolate this downwardPOC flux to depthby usingthe verticalcarbonflux algorithms of Benderet al. [1992] andMartin et al. [1987]. This approachhas beenusedsuccessfully to predictthe magnitudeof
2. Materials
the downward
and Methods
POC
flux
at 3300
m associated with
the NE
Atlantic spring bloom in 1989 and 1990 [Boyd and Newton, The data were obtained from the former site of Ocean Station 1995]. Note this modeling approachcannottake into account Papa (OSP) in the vicinity of 50øN, 145øW in the NE subarctic changesin the sinking speed of particles resulting from the Pacific. repackagingof material, such as by aggregationin the upper ocean.
2.1. Pelagic and Deep Water Measurements
2.3. Relationship Between the Timing of Surface and Deep The pelagicdata collatedfor this studyare for surfacewater Water Events chlorophyll a and column-integratedprimary production. The timing of surfaceeventsresponsible for deepwater trap Chlorophylla levelswere determinedapproximatelyonceevery events (such as the observedbiogenic flux peaks) was back 7 days for the period 1964-1976 using the spectrophotometric calculatedin two steps. First, the sinkingrate of particulate method[Stricklandand Parsons,1965]. The primaryproduction materialto 3800 m was estimatedfrom the timing of downward
rates(1984-1996)were measured usingthe 14C technique POC flux maximaand minimareachingsedimenttrapslocatedat [Steemann-Nielsen, 1952] and samples(with a few exceptions, seeFigure2 caption)were collectedandmanipulated usingclean techniques[Fitzwateret al., 1982]. Limited datacoverage(1984, 1988, and 1993-1996) of size-fractionated phytoplankton biomass/production was also available (see methodsgiven by Welschmeyeret al. [1993], and Boyd et al. [1995, 1996]). Supportingevidencein section4 has been drawn from studies conducted
in the 1980s and 1990s.
These studies should be
consulted for details of the methods used.
The downward fluxes of POC and biogenic silica were obtainedfrom PARFLUX particle interceptortraps (aspectratio
2.5,aperture of 0.5 or 1.2m2, andbaffles consisting of 25 mm cells [Honjo, 1984] moored at 3800 m in the vicinity of OSP (water depth of 4200 m). In all cases(1984 -1990), the traps were deployed in May for a 6 month period before being recoveredand redeployedin October. The samplinginterval of eachrotary collectorwas 12-17 days and was dependenton the numberof collectorson the trap turntable(i.e., 21-30 samplesper year). The traps were filled with a deep seawatersolution
enriched with5 g NaC1andl g NaN3 L-1. Uponrecovery ofthe traps,swimmerswere removedby hand or 1 mm sieve,and were not abundantin deepwatertrap samples. Sampleswere split into four andagaininto sixteenif therewasa largeamountof material. A split portion was then filtered onto a 0.8 [tm porosityfilter (silverfor carbonand nitrogen,andpolycarbonate for opal), dried at 50ø-60øC,and weighed. POC was analyzedon a Carlo Erba C440 elementalanalyser;sampleswere combustedat 950øC after an acid-leachstep to remove carbonates. Biogenic silica was analyzedfollowingDeMaster [ 1981].
2.2. Modeling the POC Flux to the Deep Ocean In the absenceof concurrentpelagicobservations to assistwith the interpretationof the deep trap time series, an existing modelingapproach[Boydand Newton, 1995] was used(1) as a tool to predict the downward POC flux based on published primary production/foodweb observations from OSP and (2) to explorethe permutations of the flows throughthe pelagicfood web/magnitude of primaryproductionrequiredto supplya POC flux similar in magnitudeto that observedat 3800 m at OSP. Boyd and Newton [1995] input pelagic data into a versionof a food web/verticalflux model [Michaelsand Silver, 1988] in order
1000and 3800 m depthat OSP [Takahashiet al., 1990] Second, a timeperiodof 12 dayswasassumed to represent the spanof an iron-mediatedphytoplanktonbloom from its onsetto its decline; in vitro iron-additionexperimentsat OSP indicatea decline in iron-elevated ratesof productionafter6 days [Boydet al., 1996] in responseto iron stress[La Rocheet al., 1996]. The error bars for the estimatedtiming of surfaceeventswere +__14 days; calculations were basedon sinkingrates[Takahashiet al., 1990] andthe samplingperiodof the trapcups. 2.4. Air Mass Back Trajectory Analysis
Iron is thoughtto be primarily suppliedto the offshoreNE Pacific from atmosphericsources[Martin and Gordon, 1988]. As dustsupplyis probablyepisodicandthoughtto be transported via the troposphere,such episodeswill be difficult to assess. However, it is possible to estimate the probability of dust transportfrom the continentsto OSP by meansof air massback trajectory calculations. This techniqueis used to track the movementof a hypotheticalparcelof air throughthe troposphere and can be appliedto specificperiods[Merrill et al., 1989] (see section4.6) andalsoclimatologically[Merrill, 1994]. The motion of the air parcelsis assumedto be adiabatic,and the trajectories arecalculatedfrom a speciallypreparedisentropicanalysisbased on wind and thermodynamic dataincludedin the globalanalysis of the
U.S.
National
Centers
for
Environmental
Prediction
(formerly the National MeteorologicalCenter). This is a set of griddedfields of dynamicalvariablesat a resolutionof 2.5ø of latitudeand longitudesavedat fixed isobariclevels at 0000 and 1200UTC eachday.Trajectories backin timealongthe isentropic surfacearecalculatedusingkinematictechniques; that is, only the quasi-horizontalwind field is used in the calculation.The time stepusedis 12 hours,andthe calculationis continuedbackup to 10 daysin time. Fourhypotheticalparcels,eachdisplacedby 0.5ø of latitudeand longitudefrom the site, are followed.This allows assessment of the diffiuence in the flow in the area of the site. The
uncertaintyin the estimatedposition of the hypotheticalair parcelsgrowsin time and can exceedhundredsof kilometersover
thelongtransporttimescontemplated here. Note backtrajectories onlyconsiderthe pathwayby whicha parcelof air hasmoved. Althoughtrajectoriesmay indicatethe sourceof the air, they do not provideinformationon the compositionof the materialthat
BOYD ET AL.: ATMOSPHERIC IRON SUPPLY AND ENHANCED VERTICAL CARBON FLUX
431
C SALPS
A
MZOO
HCIL
HFLAG
B
HBACT
PICOP
NANOP
MICROP
Figure1. Schematic of thefoodwebstructure used to estimate thedownward POCflux fromtheupper ocean. Thealgalfluxversion of theMichaels andSilver[1988]model, whichpermits 50%of largealgalcellsto sink ungrazed outof theupperocean is usedin thepresent studyasthestandard run. As algalbiomass at Ocean Station Papa (OSP)isobserved tovarylittleinmagnitude overtheannual cycle [Frost, 1993;Wong etal.,1995], themodel wasruninsteady state, thatisallinputted dailyprimary andbacterial production (seesection 2) iseither grazedand/orsinksto depth.Thefateof bacterial andprimaryproduction in thestandard runof themodelis represented byfoodwebflows(solidarrows).Thetrophic transfer efficiency of carbon ateachfoodweb flowisas
described byMichaels andSilver [1988].Theresulting downward particulate organic carbon (POC)fluxes inthe standard runarerepresented byarrows A andB whichdenote thefluxes resulting fromfoodwebinteractions and
direct algalsinking, respectively. Arrow C represents partof theoutput (asa downward POCflux)froma modified version ofthemodel which incorporates anadditional grazer compartment (salps). Inthesalp version (based ontheworkof Michaels andSilver [1988]), 50%ofeach carbon flowinthefoodweb(solid arrows) was
directed through thesalp compartment. Theother 50%ofeach carbon flowwasdirected through thepathways used inthestandard run.There isnodirect algalsinking permitted inthesalpversion; instead thedownward POC fluxisthesumofarrows A andC. HBACT, PICOP, NANOP, MICROP, HFLAG,HCIL,andMZOOdenote
heterotrophic bacteria, picophytoplankton, nanophytoplankton, microphytoplankton, heterotrophic flagellates,
heterotrophic ciliates, andmesozooplankton, respectively.
is beingtransported or deposited alongthetrajectory.Thusthe explain the observedpulses directly. In the absenceof distribution and variabilityof sourcestrengths and removal correspondinginformationon time variationfor Alaskan sources (deposition) processes must be evaluatedseparately. The (either mineraldust or volcanic),the trajectoryresultswere assessment of theseprocesses is beyondthescopeof thepresent analyzedin a climatologicalcontext,examiningthe variation studyin whichthe probability thathypothetical air parcels will throughthe year. reachthestudyareafrompotentialsource regionsis considered. Two potential source areas in Asia were consideredover the Theprobabilities arethefractionof timeduringeachmonththat period1984-1990:30ø-50øN,80ø-140øE,coveringthe arid areas air fromthesourceareaunderconsideration doespassoverthe of Chinaand industrialareasthereand in Japan,and 50ø-60øN, remotesite,basedonanensemble of trajectories. 80ø-160øE,includingsemiaridand industrialareasfarther north.
The trajectoryanalyses use event-specific meteorologicalFor Alaska, a box boundedby 55ø-70øN, 135ø-168øW was fields. However,analysisof trajectories backin time fromthe selectedfor this period. The locationof theseboxeswas suchas assumed surface eventperiods associated withtrappulses together to includethe mostlikely terrainfrom which sourceparticles withtheseasonality of long-range transport of mineraldustto the would be available. The boxes over Asia cover areas which are remotePacificOceandid not suggest that sourcesin Asia could
known to be significantsourcesof both naturalmineraldustand
432
BOYD ET AL.' ATMOSPHERIC
IRON SUPPLY AND ENHANCED
VERTICAL
CARBON FLUX
3
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
3
2
4 2
1972
1971 3
4
5
"? . ,:•...'' :7... 1973
1974
1975
1976
Figure2a. Chlorophyll a concentrations in surface waters(0-10 m) sampled fromtheweathershipsat OSPfor the period1964-1976[fromParslow,1981]. The numbers on individualplots(suchas 1964)represent chlorophyll a concentrations to the nearestintegervalue. Chlorophylla pulseswererecordedin June1964, July 1965, March 1969, andFebruary,July, and October1975.
pollutionaerosols[Duce et al., 1991]. Alaskahas not been identifiedas an importantsourceareafor atmospheric transport, but evidencepresentedhere suggestssuch a role. Where possible,publisheddatawereobtainedon the timingof mineral dustmaximaandon the seasonality of dustloading. 3. Results 3.1. Pelagic Data Sets Unlike other oceanicregions, in the NE Pacific, becauseof prevailing cloud cover, periods of elevated phytoplankton biomassare difficult to detectfrom oceancolor satelliteimages [Banse and English, 1994]. Discrete measurementsof chlorophylla (chl a) thereforeprovide the most comprehensive record of algal biomassat OSP. Measurementsobtainedfrom weathershipsstationedat OSP for 50% of eachyear showthat phytoplanktonbiomassis generallylow and constant(0.3-0.4 gg
1200
000-
800600 400 -
200 0 0
2
4
6
8
10
12
Months
Figure 2b. Plotsof column-integrated primaryproductionversus monthsof the year for the period1984-1996at OSP. The square representsthe mean value (n > 20) for late spring/latesummer (1984-1988) from the Subarctic Pacific EcosystemResearch et al., 1993]. The circlesrepresentdata chla L-l) for eachannualcyclebetween 1964and1976(Figure program[Welschmeyer 2a). However, during 5 yearsbetween1964 and 1976 there is from the period 1984-1990 (several data from 1984/1985 were evidenceof episodicperiodsof elevatedchlorophylla of up to 10 madeusingnoncleantechniques[Wonget aL, 1995,Table 1). The times ambientlevels (Figure 2a). In someyears,one event was trianglesdenotedata from 1992-1996(P.W. Boyd, unpublished recorded(see 1964 in Figure2a), in othersup to threeeventswere data, 1997). The magnitudeof productionover the year will be observed(see 1975 in Figure2a). Theredoesnot appearto be an influencedby solar forcing;however,the relationshipbetween obvioustrendregardingthe timing of theseeventsoverthe annual irradianceandcolumn-integrated productionat OSP is not clearly resolved[Welschmeyer et al., 1993]. cycleor interannually.
BOYD ET AL.: ATMOSPHERIC IRON SUPPLY AND ENHANCED VERTICAL CARBON FLUX
Table 1. Proportionof Phytoplankton BiomassandProductionat OSP Accountedfor by Cells < 5
Study
Biomass,
Production,
%
%
Booth etal. [1993]
50*
Welschmeyer etal. [1993] Boydet al. [1995]
68++
74+ 65++
Boyd etal.[1996]
705
755
In the caseof Welschmeyer et al. [1993] datawere expressed as < 3 gm and > 3 gm cells. It was assumedthat the proportionof < 3 gm cells approximated that of the < 5 gm cells. For the purposes of the foodweb model,thosecells> 3 gm were equallypartitionedinto the > 5 gm and> 20 gm sizeclassesbasedon observations by Boydet al. [1995, 1996]. *Denotesmean of C biomassfrom cell countsfrom six cruises(19841988). +Denotesmean of 91 observations(1984-1988). ++Denotes the meanof two February-Marchcruises(1993 and 1994).
$Denotes meanof twoMay cruises (1993and1994).
summer meanPOCandbiogenic silicafluxes were2.4mgC m-2 d-1 and16.6mgSim-2 d-1,respectively. In contrastto 1985, only one pronouncedbiogenicflux peak was observedduring 1988 (Figure 3b). The timing of the pulse was in mid-August,and as for 1985, the peak was coincidentfor both POC and biogenic silica. Despite the differencesin the numberof pulsesobservedin 1985 and 1988, the summermeans for POC flux were similar, while thosefor biogenicsilica were higher in 1988 than in 1985 (Table 2). The magnitudeof the 1985/1988 summerbiogenicfluxes (pulsesomitted) was 2 times higher than observed during the winter period. This was comparableto the annual range observedfor rates of primary
production (Figure2). Althoughthetraprecordswereincomplete for 1986, 1989, and 1990, therewas also evidenceof pronounced peaksin biogenicfluxes,relativeto the annualmeanfluxes,for February 1986 and August 1987. In each case,one concurrent POC/biogenicsilica episodewas observedper year. From back calculation,the surfaceeventsthat likely resultedin thesepulses took placeduring late spring/summer with the exceptionof 1985 and 1986, when the timing of the surfaceevent was in October and January,respectively(Table 2). As for 1985 and 1988, the mean annual downward
A limited numberof primary productionprofiles (and none fromperiodsof elevatedchlorophyll a), relativeto thechlorophyll
433
POC fluxes for 1984-1990
showed little
variation betweenyears (2.5+/-0.5, n=6); the samewas observed for biogenicsilica (18+/-6).
a data, are availablefrom 1964 to 1976 [Parslow, 1981]. These 3.3. Predicted Deep Water POC Fluxes dataarenot includedin the presentstudybecauseof uncertainities Size-fractionatedprimary production [Welschmeyeret al., regardingtheir validity as they were made using "dirty" 1993] and heterotrophicbacterialproduction[Kirchman et al., techniques (seeFitzwateret al. [1982] and discussion sectionof Wonget al. [1995]). However,primaryproductiondata from 1993] data from summer 1988 at OSP were used in conjunction with the Boyd and Newton [1995] approachto predictdownward SubarcticPacificEcosystemResearch(SUPER) [Welschmeyer et al., 1993],Instituteof OceanSciences Line P [Wonget al., 1995], POC fluxes. Although no winter productionrates at OSP were available for 1988, to permit a comparison of the range of and the JointGlobal OceanFlux Study-Canada(CJGOFS), (P.W. Boyd unpublisheddata, 1998) programsprovide sufficient predictedand observeddownwardPOC fluxes over the annual cycle, datawere usedfrom 1993/1994 [Boydet al., 1995]. It was information to examine general trends in the magnitude of assumedthat the winter ratesusedwould be comparableto those productionover the annualcycle. Productionratesfrom > 40 incubationsin the February-Octoberperiod between 1984 and of winter 1988: the magnitudeof winter ratesin the period 19921996 display an approximatelythree fold variation between 1997 is low and constant (P.W. Boyd and P.J. Harrison, winter and summervalues(Figure 2b). This narrow annualrange Phytoplanktondynamicsin the NE subarticPacific, submittedto In 1988, concurrent is probablya consequence of the relativelyshallowdepthof the Deep Sea Research, Part II, 1997). permanentpycnoclineat OSP (100 m [Boyd et al., 1995], observationson the residentpelagicgrazersat OSP indicatedthat compared to that of the NE Atlantic(400 m [Gloverand Brewer, microzooplankton [Booth et al., 1993] and crustacean 1988]), which permitsthe phytoplanktonto successfully subsist mesozooplankton[Mackas et al., 1993] were dominant. These overwinter [Boydet al., 1995]. Data on the partitioningof algal observationswere used to select the most likely pathways of biomassandproductioninto sizeclassesindicatethat cells< 5 lam carbontransferthrough the pelagic food web (Figure 1). The are responsiblefor 50-70% of community biomass and predictedPOC fluxes at 3800 m, basedon the mean production from1988,were1.4mgC m-2d-1forsummer (Figure4) productionover the annualcycle at OSP (Table 1). This trend rates (notshown).These predicted concurs with taxonomic studies which report that < 5 lam and 0.5mgC m-2d-1forwinter autotrophicflagellatesdominatephytoplanktonbiomassat OSP fluxes were of the same order as observed for winter and summer in 1985 and 1988, while those for summer were lower than the [Boothet al., 1993;Boydet al., 1995;Boydet al. 1996]. meanPOC valuesrecordedoverthe period 1984-1990 (Table 2). The vertical flux model was also used to predict what 3.2. Deep-MooredTraps permutationsin the magnitude of primary production, in the DownwardPOC and biogenicsilicaflux dataare presentedfor partitioningof productionbetweensmall and largecells,and/orin the 1985 and 1988 annual cycles(Figure 3). Observationsfor the nature of food web flows would simulate a POC flux of the magnitudeobservedduring the pulses. Several scenarioswere severalotheryearswere incompleteand are presentedin Table 2. run, such as a sustainedincreasein primary productionwith no In 1985, low POC and biogenic silica fluxes were recordedin winter, followed by an increasein these fluxes in late spring. changein the partitioningof productionbetweenthe algal size fractions(see Figure 4 caption). Only when column-integrated (Figure 3a). Two concurrentpeaks(peaksarbitrarilydefinedas 2 primary production wasincreased to levelsof 2 g C m'2 d'1 in times greaterthan the summermean flux, Table 2 footnote) in POC and biogenic silica in middle and late summer were conjunctionwith a floristic shift towardslarge cells, as observed superimposedon the seasonalcycle of biogenic fluxes. The duringin vitro iron enrichments at OSP [Boydet al., 1996], could
434
¸
BOYD ET AL.' ATMOSPHERIC IRON SUPPLY AND ENHANCED VERTICAL CARBON FLUX
,/
2 o
o
o
2
4
•
8
10
12
8o
t ?
60
B
40
•I
20
.?',, 0
2
4
;
8
10
12
o
2
Months
4
'6
',,% 8
10
12
Months
Figure 3a. DownwardPOC and biogenicsilicafluxesderived from deep-moored sedimenttrapsat 3800 m at OSP for 1985. The solidlinesrepresent themeansummerfluxes(calculated from
Figure 3b. DownwardPOC and biogenicsilicafluxesderived from deep-mooredsedimenttraps at 3800 m at OSP for 1988. The solidlinesrepresent the meansummerfluxes(calculatedfrom
May-Septemberinclusiveandexcludingpeaks).
May-Septemberinclusiveandexcludingpeaks).
Table 2. TimePeriodsWhenPOC andConcurrent BiogenicSilicaPeaksWereObserved at OSP Year
Pulse
PeakPOC,
PeakSilica,
Timing
mgC m-2 d-1
mgSi m-2 d-1
Summer
Summer
MeanPOC, MeanSilica, mgC m-2 mgSim-2 d-1
Timing of
Surface Event
d-1
1984
July 29
5.5
9
2.8
8.6
June25
1985
Nov. 20
5.3
37
2.4
16.6
Oct. 16
1986'
Feb. 18
6.8
50
na
na
Jan. 17
1987 1988
Aug. 18 Aug.24
8.8 11.1
42 70
3.5 1.9
26.0 25.1
July 16 July 21
1989' 1990*
May 01 May30
4.1 7.6
55 na
na 2.9
22 na
April01 April27
The medianpointof thesampling period(15 d) by thetrapcupis thetimeperiod.Peakswereidentifiedwhenboth POCandsilicavalueswereapproximately twotimesgreater thantheirrespective summer means(calculated fromMay to September inclusively butexcluding obvious peaks)andwhenPOCandconcurrent silicapeakswereobserved.The 1984pulsedataareincluded asit wascharacterized by highPOCbutrelativelylow silicaandassuchmayreflectsalp grazing. Thecomplete silicaandPOCrecords for 1985and1988areshownin Figure3; nadenotes notavailable. * Denotessomemissingdata.
BOYD ET AL.' ATMOSPHERICIRON SUPPLYAND ENHANCED VERTICAL CARBONFLUX '•'
14
-
E
E• 12
435
paradigm,thatof the ecumenicalironhypothesis, wherebothiron supply and grazing pressure control the magnitude of phytoplankton stocks[seeCullen, 1995]. On six occasions(1964, 1965, 1969, and 1975) chlorophylla
wasgreater than5 timesambient levels(> 2.0 [tg L-l). These E• 10
,.•
8
•
4
MODEL
RUN
Figure 4. PredictedPac verticalflux at 3800 m for various ecologicalscenarios usinga food web modeling/PaCalgorithm approach[Boydand Newton, 1995]. Model run A denotes communitystructurebasedon ambientsummerconditions, using meansummerprimaryproduction[.Welshmeyer et al., 1993] and phytoplankton sizespectra(production split60:30:10between< 5 [tm,5-20 [tm and> 20 [tm sizeclasses [Welschmeyer et al., 1993; Boyd et al., 1996]). Model B run denotescommunityhigh productivityconditions,usingthe highestobservedproduction rateat asp [Welschmeyer et al., 1993]andpartitioningasfor run A. Model C run wasbasedon salpgrazingconditions,as for run A but seeFigureI captionfor detailsof modifiedcarbonflows in the food web.
Model
run D was based on iron-mediated
productivityconditions,using column-integrated productionof
I g C m'2 d-1 in conjunction witha shiftin phytoplankton size spectratowardlargecells(10:10:80). Model run E wasbasedon identicalconditionsto modelrun D but with productionof 2 g C
m-2 d-1 [Boyd etal., 1996].
episodesrepresentphytoplanktonblooms and should not be confusedwith other reportsof bloomsbasedsolely on elevated diatom abundances[Clemonsand Miller, 1984]. Booth et al. [1993] arguedthat the bloomsreportedby Clemonsand Miller representedlittle increase in algal biomass. The sampling coveragebetween1964 and 1976 was not alwayscomprehensive (see 1970 in Figure2), so it is possiblethat episodesof elevated chlorophylla duringthisperiodmay havebeenmissed.Although suchepisodesof high chlorophylla at aSP havebeenpreviously reported [Miller et al., 1991], no explanationwas offered as to why considerablyhigher than ambientlevels were measuredin this HNLC region. Such high chlorophyll a levels cannot be explainedby the currentlow iron supply/highgrazing pressure ecosystemwhich maintainHNLC conditionsat aSP. The primaryproductiondata (1984-1996), althoughrelatively comprehensive for an openoceanarea,representpoorersampling resolutionthan availablefor chlorophylla (1964-1976) and deepmooredtraps (1984-1990) at asP. Productionrates displayed threefoldannualvariationsfrom a winter low to a summerhigh, with no evidenceof elevatedlevels as observedoccasionallyfor chlorophylla or biogenicfluxes time series. It is probablethat any such elevatedrates of productioncorrespondingto higher algal biomass(or biogenic flux peaks) may have been missed becauseof the poorer samplingcoverageover the annual cycle relativeto thosefor chlorophylla or traps. There appearto be only two feasible explanationsfor the episodicincreasesin chlorophylla: occasionaladvectionof a water mass characterizedby high chlorophylla into the aSP regionor episodiciron supplyto initiatean algal bloom at asP. The feasibilityof theseexplanations is considered. Assessment of the likelihood of episodiclateral advectionof high chlorophylla watersrequiresinformationon both current velocitiesat asp andthe biologicalcharacteristics of neighboring watermasses.In general,the prevailingcurrentflow is from west to east,the west wind drift. Tabata [1975], using a geostrophic current velocity section, estimatesrelatively weak upper water
the magnitudeof the observed1988 Pac pulse be mimicked column currents of 6 cms'1 (5 kmd-1), dropping steadily with (Figure4). If no shift toward large cells was permittedin the depth.Currentspeeds of thisorder,with an eastward flow for 400 simulations,column-integrated productionrates in excessof km eastof aSP, have been estimatedfrom an array of satellite-
4gCm -2 d'1 wouldbe required to mimicthe Pac pulse drogueddrifters(droguedepth100 m) releasedin the vicinityof recordedin 1988; the highestcolumn-integrated productionrate at aSP wasapproximately a third of this value [Welschmeyer et al., 1993]. Alterationof the partitioningof production,or a changein the food web structureto salpsalso resultedin predictedPac fluxescomparablein magnitudeto someof the observedfluxes. 4. Discussion
asp [Thomsonet al., 1990]. The aboveobservations suggest that the region is characterised with low horizontaladvective flows.
Surface waters at least 500 km west of aSP are characterized
by high nitrate levels [Andersonet al., 1969; F. Whitney, unpublisheddata, 1997]. In addition,comparablelevels of chlorophylla, anda similarflora wereobservedfor at least100 km west of
4.1. Pelagic Data Sets
and 400 km to the east of aSP (P.W. Boyd,
unpublished data, 1996). In May and September1995, surface waters east of asp for > 300 km were characterizedby low
The most comprehensive recordof phytoplanktonbiomassat aSP (50-60 measurements per year) was obtained between1964 and 1976 (Figure 2). The persistentobservationof low and constantphytoplanktonstocks over this 13 year period is supported by the findingsof the SUPER and CJGaFS programs in the 1980sand 1990s. This trend is explainedby the current
dissolved ironconcentrations (< 0.5nmolkg-1 ), chlorophyll a of 0.3 [tgL'l, anddiatompopulations whichexhibited ironstress [La Rocheet al., 1996]. Thusthe biologicalobservations to the west and the east of asp
indicate that the waters are HNLC
in
character. Althoughthe dataare not sufficientlycomprehensive to disprovethe role of lateraladvectionin explainingepisodic
436
BOYD ET AL.:
ATMOSPHERIC
IRON SUPPLY AND ENHANCED
pulsesof high chlorophylla, there is a substantialbody of evidencewhich suggests that it is unlikelyto explainsuchevents. The in vitro supplyof iron (to 2 nM Fe final concentration) to residentphytoplanktonin 25 L carboys at OSP resulted in elevateddiatom growth ratesand biomasslevels of > 2 •tg chl a
L-1 after5-6daysin May 1993andMay 1994[Boydetal., 1996] and in September1995 (P.W. Boyd, unpublisheddata, 1996). As healthymesozooplankton were presentat ambientabundances in the carboys,diatomgrowthratesmusthaveexceededzooplankton grazingrates[Boydet al., 1996]. Thus iron-mediatedincreases in chlorophylla in in vitro iron enrichmentswere comparablein magnitudeto the observedepisodicpulses(Figure 2). While there are insufficientdatato unequivocallyascribetheseelevated chlorophylla pulsesto increasediron supply,on the basisof what is knownabouthow cellsrespondat OSP to iron supplyandtheir ability to escapegrazingpressure,this is a feasibleexplanationfor such events.
4.2. Fate of Phytoplankton Pulses?
What is the fate of such observed episodic pulses in phytoplanktonstocks? Miller et al. [1991] speculatedthat iron supplyto OSP would resultin cellsblooming,in turn, exhausting the iron supplied,cell senescence, and finally cell sinking. A recentlab studyexaminedthe effect of iron statuson the sinking rateof a diatomActinocyclussp. isolatedfrom OSP andsuggested that after utilizing the iron from an initial enrichmentthe resulting iron-depleteddiatomswill sink 5 times faster than iron-replete diatoms [Muggli et al., 1996]. Presumably,after an episodic pulseof iron, diatomsbloom, becomeiron limited, then sink to depth without being grazed significantlybecauseof insufficent mesozooplankton grazingpressure[Boydet al., 1996]. 4.3. Deep Water Trap Data
The datarevealseveralpronounceddownwardPOC flux peaks during the period 1984-1990 (Figure 3 and Table 2), which contrast with the usual low and constant downward
POC fluxes
observedover this period. Significantly,the POC flux maxima were coincidentwith markedbiogenicsilicaflux maxima(Figure
VERTICAL
CARBON
FLUX
explorethe feasibilityof severalscenarios that mightexplainthe magnitudeof the observedPOC pulses. 4.4. Explanation for the ObservedDownward POC Pulses IncreaseddownwardPOC flux may result from a numberof scenarios:these include advectionof a more productivewater massor of particlesat depth associatedwith sucha water mass [Siegelet al., 1990], a changein planktoniccommunitystructure suchas from copepodsto salps[Michaelsand Silver, 1988], the sinking of large cells (ungrazed)from an iron-mediateddiatom bloom [Miller et al., 1991], or a changein the repackagingof POC in the upperwater columnsuchthat it sinksmore rapidly andconsequently is remineralizedto a lesserextent. When consideringthe link betweenthe surfaceand the deep ocean, lateral advection of material at depth must also be considered[Siegelet al., 1990;Siegeland Deuser,1997]. These studieshave demonstratedthat particlesfrom a surfacesource regionwith a lengthscaleof severalhundredkilometersmay end up in deep traps. Siegel and Deuser [1997] also report that in regionswith low currentspeeds, mesoscalevariability will have an increasinginfluenceon the propagationof materialfrom the surfaceoceanto depth. As observedfor the pelagicdatasets, it is unlikely that eastwardadvectionof HNLC waters(from west of OSP) would elevatethe downwardbiogenicfluxes at 3800 m sufficientlyto explain the magnitudeof the observedpulses. Furthermore,Takahashi(1986) reportedthat basedon the timing of flux maxima and minima and speciescomponents,traps at 1000 and 3800 m depthat OSP were cross-correlated. Takahashi et al. [1990] concludedthatthe oceanicflux provincesof two trap sites(OSP and anothersite 600 km to the east)were similar and that the processesresponsiblefor export productionwere also likely to be similar. Thus it is likely that advectionand/or mesoscaleeddy activity play a minor role in determiningthe magnitudeof the downwardPOC flux at OSP. Generalistfeeders,suchas salps,cangrazea wide rangeof cell sizes. Michaelsand Silver [1988] modeledthe effectof food web dynamicson exportproductionandreportedthat a salpdominated food web would resultin a large downwardPOC flux, relativeto a copepod-dominated food web. In addition, although not considered directly by Michaels and Silver [1988], the repackagingof small cells into salp faecescan sink at up to
3). The timingof thepeaksin theperiod1984-1990appeared to be episodic(Table 2). The magnitudeof the highestPOC and silicapeaksat OSPweresimilarto thosereportedto be associated 1000md-1 Such fastsinking particles willberemineralized toa with the sinking out of the spring diatom bloom in the NE lesserextent,resultingin the transferof a greaterproportionof Atlantic[Honjoand Manganini,1993;Newtonet al., 1994],but POC to depth. As salp swarmsare episodic[Bathmann,1988], occurredfor shorterperiods. The concurrentsilica peaks they may be missed by shipboardstudies,but the resulting indicatedthat the eventsat OSP are probablyof diatomaceous elevatedfluxes would be collectedby traps. At OSP, salp origin,and this is confirmedfrom analysisof the trap contents abundances of 20 individuals m-3 in daylight andup to 80 [Takahashi,1987a,b]. Indeed,the majorityof the POC measured individuals m-3 in darkness havebeenreported [Purcelland in the deeptrapsat thesetimesin 1985 appeared to havebeen Madin, 1991;Madin et al., 1998]. Purcelland Madin reportthat supplied by diatoms[Takahashi et al., 1990,Figure7]. salpsfed activelyat depths> 30 m by day and ceasedfeedingat The observedtrend of relatively low and constantfluxes night. The reportedabundances do not constitutea salpswarm
superimposed with episodicelevatedPOC and biogenicsilica [Bathmann,1988]. pulsesis consistent with the patterns observed for chlorophyll a In orderto estimatethe salp-mediateddownwardPOC flux, the between1964 and 1976(Figure2). Comparison of trap data pathwaysin the pelagicfood web modelwere altered,suchthat betweenyears suggeststhat there was little markedinterannual salpsgrazed50% of all cells (see Figure 1 caption);50% was variability in the magnitudeof the winter POC/silica flux selectedas salpsresidentat OSP havea grazingsizethresholdof (Figure 3). This seemsto be the casefor summeralso. When the 4 •tm [Madin and Purcell, 1992] and becausea largeproportion downward POC flux dataassociated with episodicpulsesare of the phytoplankton at OSP are smallcells(Table 1). A salpremovedfrom computation of the annualmean,the predicted dominatedecosystemresultedin an increaseddownward POC
POC fluxesfrom the modelprovidereasonable agreement with
flux of 5 mg C m-2 d-I at 3800m (Figure4), whichis
the annual mean POC fluxes.
comparable to the magnitudeof observedPOC pulsesin 1984,
The model was used as a tool to
BOYD ET AL.: ATMOSPHERIC
IRON SUPPLY AND ENHANCED
1985,and 1989 (Table2). However,while salpsmay grazeall cellsdownto a threshold of 4 lam,themajorityof thesecellsat OSPwouldbe nondiatomaceous [Boothet al., 1993]and would havea relativelylow silicacontent.Thus,whileinvokingsalp swarms mayaccountfor elevatedPOCflux to depth,it is unlikely thattheycanexplaintheconcurrence of pulsesof biogenic silica in mostyearsexamined.Saipscouldaccount onlyforthepulsein July1984,whichhada highPOC/Siratiorelativeto otheryears. It is possible thatsalpsswarms mightdevelopin response to ironmediated diatomblooms: a salpswarmdeveloped duringanalgal bloomin theNE Atlantic[Bathmann, 1988]. In
vitro
iron
enrichments at OSP
result in
elevated
phytoplanktonbiomassand production,and a floristic shift towardslargediatoms[Martinet al., 1987;Boydet al., 1996].
Themagnitude of phytoplankton production [Bishop,1989],and cell size [Michaelsand Silver, 1988] both exertcontrolon the downwardPOC flux. In addition,an iron-mediatedshift towards
VERTICAL
CARBON FLUX
437
POC fluxes at 3800 m, the often observedconcurrentpulse of biogenic silica can probably only be explainedby an ironmediateddiatom bloom sinking to depth directly. Thus the diatom bloom scenario appears to most consistentwith the observedpelagicanddeepwaterevents. A comparison of the frequencyandtimingof elevatedpulses of algal biomassduring each annual cycle from the pelagic observations(Figure 2) appearsto be consistentwith those for biogenicpulsesin the deepocean. Bothpelagicanddeepwater pulsesoccur approximatelyonce every 3 years:there were six chlorophylla pulsesin a 13 yearperiodandat leastthreedeep waterbiogenicflux pulsesin 7 years.As observed for thepelagic datasets,thereare yearsin whichone (1988, in Figure3) and several biogenic deep-waterpulses (1985, in Figure 3) are observed.In 1986and1975,pulsesof POC/Siandchlorophylla, respectively, areobserved in winter. As recent5 day in vitro iron enrichments at OSP have shownno increasein phytoplankton biomassin winter[Boydet aL, 1995],a timeperiodlongerthan5
diatomswill probablyelevatethe exportof biogenicsilicato depth. Thepredicted POCfluxto 3800m resulting fromaniron- daysis requiredto establishsuchiron-mediated bloomsduring mediated bloomwas similarto the magnitude of the highest thisseason (P.W. Boyd,unpublished data,1998). observed fluxes(Figure4 andTable2). It wasnotpossible to predicttheflux of biogenic silicafromthismodeling approach. 4.6. Supply of Iron However,unlessthere is a floristicshift towarddiatoms,it is
If suchpelagic/deep waterepisodes areiron-mediated, thenthe
difficultto invokeanother mechanism to concurrently increase supplyof iron is likely to be atmosphericratherthan marine bothPOCandbiogenic silicafluxes.Thisobservation strongly [Martin et al., 1989]. In orderfor sucha bloomto occur,four suggests the role of iron in mediatingincreases in the downward factorsareneeded: ironmustbetransported in sufficient quantity, biogenicfluxes.
mustbe depositedto the ocean,andmustbe in bioavailableform,
4.5. Case for Iron
andphytoplankton growthratesmustrespond to ironsupply(i.e., growthrate mustnot be ultimatelylimited/co-limited by some
The exploration of these scenariosto accountfor both the
other factor such as light [Raven, 1990; Sunda and Huntsman,
pelagicanddeepwaterobservations doesnotresolvethiscomplex issueunequivocally.The natureof episodiceventsmeansthatthe availabledata coverageis seldomadequate.Nevertheless, iron supplyis the most likely candidatemechanism for the high chlorophyll a pulses.In addition,whilethepredicted fluxesfrom
1997]). Herethe transport of dustusingtrajectoryprobability analysisandfrequencydistributions of duststormsareexamined.
Theresults of air massbacktrajectories indicate thattransport probabilities of up to 10% are observed for bothsourceregions overtheannualcycle(Figure5). Althoughit is possible thatthe both the salp and diatom bloom scenariosmatch the observed northAsiansource couldaccount for thelate1985pulse(Table2 18
-'
16
Box 1, S. Asia
---D---Box 2, N. Asia 14
- -.- - Box3,Alaska
•,12 •1o
D-I,
/
\
/
,,
.•, 6
Dec-Jan
Feb-Mar
Apr-May
Jun-Jul
Aug-Sep
Oct-Nov
Months
Figure5. Transport probabilities overtheannual cycleof tropospheric airmasses fromAsia andAlaskapassing through OSPduringtheperiod1984-1990.Box1 (triangles) is located overAsia;thisincludes desert regions. Box2 (squares) is locatedoverRussia. Box3 (circles)is locatedoverAlaska.
438
BaYD ET AL.: ATMOSPHERIC
IRON SUPPLY AND ENHANCED VERTICAL
CARBON FLUX
Figure 6a. Annotatedsatelliteimageshowinga streamof airbornedustbeingblownoffshorefrom the Copper RiverValley in Alaska,October21, 1996,NationalOceanicandAtmospheric Administration (NaAA) 14 band2, near-IR image.
1993; Watson,1997]. Althoughthe timing of theseNE Pacific events is late in the annual cycle and it is not possibleto quantitatively assess the dustcomposition/loading or trajectoryof eachplume, theseimagesdo provide the first direct evidenceof the potentialimportanceof this regionfor dustsupply,from two distinctsourcemechanisms, to the openocean. The relatively high and constant transport probabilities observedfor Alaskan sources in summer (Figure 5) when comparedwith the relatively few iron-mediatedepisodeseach summerin the pelagicand deep water recordssuggestthat the magnitudeof mineraldust transportedby theseeventsmay be from Asia. episodic.The episodicnatureof theseeventsis supported by the presented in Figure6. Further, In contrastto Asia, the probabilityof transportfrom Alaska is natureof the supplymechanisms lessvariablethroughthe year, with highervaluesin summerand datafromtheU.S. GeologicalSurvey[WrightandPierson,1992] fall monthsthanfor Asia (Figure5). While it hasbeensuggested indicatethat there have been > 50 eruptionsassociatedwith that glacial outwashand river deltasin this regionmay be volcanoeson the AlaskanPenisulain the last200 years. The data significantsourcesof dust [Pewe,1968], lessis knownabout on trajectory probability in conjunctionwith recent remote sources for aeoliantransportfrom Alaska,as comparedto Asia. sensingevidenceof airbornedust (Figure 6) suggestthat, in asan important However, satellite images from October and November 1996 additionto Asia,Alaskamustalsobe considered provideevidenceof sources of dust(Figures6a and6b). In the potentialsourceregionfor dustthat may initiateepisodicdiatom caseof the dustplumethat probablyoriginatedfrom the Copper blooms at aSP. River Valley, the dustis movingtowardthe coastalNE Pacific. In contrast,the volcanicdustplumefrom Mount Pavlofappearsto 5. Conclusions
and Figure 5), the majority of the elevatedbiomass/fluxevents observedin both the pelagic and deep-waterrecordsoccur over late spring/summer.Trajectory analysis shows that transport probabilitiesfrom Asia are low in the late spring and at a minimum during the summer(Figure 5). While this argues againstAsia as the operativesource,withoutinformationon the actual magnitude of the dust flux, this region cannot be discounted as a major source area. Although transport probabilitiesare high in the winter,the frequencyof duststorms are relativelylow [Merrill et al., 1989]. The eventswhich occur duringthe summerare thusnot easilyexplainedby dusttransport
be movingtowardsthe opensubarctic Pacific,in the directionof 1. Despite the availability of a relatively comprehensive asp (seeFigure 6c). In anotherHNLC region,the Southem Ocean,the depositionof iron-richvolanicashfrom the Pinatubo oceanographicdata set for the NE subarcticPacific, it is not eruptionhas been cited as being potentiallyresponsiblefor possibleto unequivocallyexplainthe causeof episodiceventsin transientincreasesin the productivityof this region [Sarmiento, the pelagic and time seriesrecords. However, on the basisof a
439
Figure 6b. Annotatedsatelliteimageshowinga volcanicdustplumemoving ESE from the eruptionof Mount Pavlof, November 30, 1996 (NOAA-14 band 3, mid-IR ).
45N
................................................................................................................................................................
165W
155W i
i145W
1135W
1•5.,.W Jt
Figure 6c. A map of the NE subarcticPacific showingthe locationsof thesetwo imagesof dustplume source areasin Alaska(box on the right sideof the mapis Figure6a) in relationto OSP.
440
BOYD ET AL.:
ATMOSPHERIC
IRON SUPPLY AND ENHANCED
substantialbody of circumstantialevidence,iron-mediatedbloom events appear to be most consistentwith the presenceof such pelagicand deepwater events. 2. If this is so, then the frequencyof both the pelagic and deepwater eventsappearsto be onceevery 3 years. However, on several occasionsfor both these sets of data, more than one event was observedduring an annualcycle. Thesetrendsare probably relatedto the frequencyof the air massespassingthroughOSP, the magnitudeof dust supply carried,the bioavailabilityof the irontransported, and its potentialto relievealgaliron stress. 3. On the basis of the timing of such episodes,the probabilityof dustsupplyfrom Alaskansourceregionsis higher
VERTICAL
CARBON
FLUX
hindcasting of seasurfacepCO2at weathership StationPapa, Prog. Oceanogr.,32, 319-351, 1993.
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theseair masses. While satelliteimagery is a powerful tool to identify dust events, more information is required on the seasonalityof mineral dustcompositionfrom Alaskanand Asian sourceregions. Such data may be used in conjunctionwith air massbacktrajectoryanalysis. 4. The biogeochemicalsignalfrom the bloomsin somecases was comparablein magnitude(if not duration),with respectto chlorophylla, deep water downwardPOC, and biogenicsilica fluxes,to that observedduringthe NE Atlantic springbloom. In the vicinity of OSP, this will likely result in a significant drawdownof pCO2 (estimatedto be 25-30 [tatm, using in vitro productiondatafrom a 6 day iron enrichment[Boydet al., 1996] and following Chipman et al. [1993]). The timing of these episodiceventsmaybe problematic regardingthe interpretation of data from this region obtainedduring legs of the global CO2 survey [see Murphy et al., 1998). It is desirableto use such present-dayobservations as modelsfor eventsin the geological
Effectson the chemistryof the oceaninterior, in Productivityof the Ocean:Presentand Past, editedby W.H. Berger,V.S. Smetacek,and G. Wefer, pp. 117-138, JohnWiley, New York, 1989. Booth,B.C., J. Lewin, andJ.R. Postel,Temporalvariationin the structure of autotrophicand heterotrophiccommunitiesin the subarcticPacific,
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Prog. Oceanogr.,32, 57-99, 1993. Boyd, P., and P. Newton, Evidence of the potential influence of planktoniccommunitystructureon the interannualvariability of particulatecarbonflux, Deep Sea Res.,Part I, 42, 619-639, 1995. Boyd,P.W., F. Whitney, C.S. Wong,andP.J.Harrison,The NE subarctic Pacificin winter,II, Biologicalrateprocesses, Mar. Ecol.Prog.Ser., 105, 21-32, 1995.
Boyd, P.W., D. Muggli, D. Varela, R.H. Goldblatt, R. Chretien, K.J. Orians,and P.J. Harrison,In vitro iron enrichmentexperimentsin the NE subarcticPacific, Mar. Ecol. Prog. Ser., 136, 179-193, 1996. Chipman,D.W., J. Marra, and T. Takahashi,Primary productionat 47øN 20øW in the North Atlantic Ocean: A comparisonbetweenthe 14C incubationmethodandthe mixed layer carbonbudget,Deep Sea Res., Part II, 40, 151-170, 1993.
subarcticPacific, Dee-Sea Res., Part A, 31, 85-95, 1984. 5. Recommendations for futurestudiesincludedeveloping/ deployingstand-alone instrumentation in boththe deepwaterand Coale, K.H., S.E. Fitzwater, R.M. Gordon, K.S. Johnson, and R.T. Barber, Control of community growth and export productionby pelagicrealmsto detectsuchepisodicevents. A suiteof sensors upwellediron in the equatorialPacific Ocean, Nature, 379, 621-624, including pCO2, fluorescence, passiveoceancolor,dissolved 0 2, 1996a. dissolved ironandnitrate,andacousticequipment configured for Coale K.H., et al., A massivephytoplanktonbloom inducedby an salp target sizeswould be required. In addition,currentmeter ecosytem-scale iron fertilizationexperimentin the equatorialPacific and drifter data are neededto link the pelagicand deepwater Ocean, Nature, 383, 495-501, 1996b.
events via the estimation of the size of surface water source
squares for theparticlesintercepted by sediment traps. Acknowledgments.We wish to thankJohnCullen,Tony Michaels and two anonymous reviewers for their constructivecomments which
helpedimprove thismanuscript. Thisresearch wassupported by funding fromanNSERCSpecialCollaborative grantto theCanadian JointGlobal
Cullen, J.J., Status of the iron hypothesisafter the Open-Ocean EnrichmentExperiment,Lirnnol.Oceanogr.,40, 1336-1343,1995. de Baar,H.J.W., J.T.M. de Jong,D.E.C. Bakker,B.M. Loscher,C. Veth, U. Bathmann,and V. Smetacek, Importanceof iron for plankton bloomsandcarbondioxidedrawdownin the SouthernOcean, Nature, 373, 412-415, 1995.
DeMaster,D.J., The supply and accumulationof silica in the marine environment,Geochirn.Cosrnochirn. Acta, 45, 1715-1732, 1981. OceanFlux Study. NE Pacificstudiesat the Instituteof OceanSciences are supported by Canada'sGreenPlan OceanClimateProgramandthe DiTu!lio, G.R., and E.A. Laws, Impact of an atmospheric-oceanic disturbanceon phytoplanktoncommunitydynamicsin the North Panelfor EnergyResearchand Development.At URI, supportwas PacificCentralGyre, Deep Sea Res.,Part A, 38, 1305-1329, 1991. provided by theNASA/MTPEGlobalTropospherical Chemistry program aspartof thePacificExploratory Missions.Thetrajectory andduststorm Duce, R.A., eta!., The atmosphericinput of tracespeciesto the world ocean,Global Biogeochern. Cycles,5, 193-259, 1991. analysesmade use of computingfacilitiesand meteorological data
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(e-mail:
[email protected]) J. Merrill andJ. Snow,Schoolof AtmosphericChemistry,Graduate Schoolof Oceanography, Universityof RhodeIsland,Narragansett, RI 02882-1197. (e-mail:jmerrill•boreas.gso.uri.edu, snow•gsosun1.gso.uri.edu) (ReceivedAugust5, 1997; revisedJanuary23, 1998' acceptedFebruary25, 1998.)
