Episodic enhancement of phytoplankton stocks in New Zealand

subantarctic waters: Contribution of atmospheric and oceanic

iron supply

P. W. Boyd,1 G. McTainsh,2 V. Sherlock,3 K. Richardson,3 S. Nichol,3 M. Ellwood,4

and R. Frew5

Received 5 December 2002; revised 20 May 2003; accepted 30 October 2003; published 25 February 2004.

[1] Around 30% of oceanic waters are high nitrate low chlorophyll (HNLC) where lowiron levels limit algal growth. HNLC waters have mainly been studied using shipboardand lab experiments. Since 1997, remote-sensing of phytoplankton via SeaWiFSOcean Color has permitted monitoring of the constancy of this ‘‘HNLC condition,’’ i.e.,spatial homogeneity and low temporal variability of chlorophyll over annual cycles.These trends can be exploited, as episodic iron inputs should be conspicuous bysubsequent expression as iron-elevated algal stocks. Subantarctic (SA) waters near NewZealand are HNLC, and the proximity of the arid Australian landmass, and the iron-richSubtropical Front, provide natural laboratories to detect episodic atmospheric and oceaniciron supply, respectively. Two approaches were used: Oceanic supply was inferredfrom episodic increases in chlorophyll concentrations in SAwaters, detected using OceanColor archives. Additional archives were used to confirm the oceanic provenance of ironsupply, and identify supply mechanism(s). Atmospheric supply was assessed usingdata on source areas and loads for dust storms monitored in central Australia. Dusttransport and its fate was assessed using air mass forward trajectories and SeaWiFSOcean Color and Aerosol Optical Depth maps. During 1997–2001, episodic elevatedchlorophyll events occurred in SA waters southeast of New Zealand. There was noevidence of these events being mediated by atmospheric iron supply; however, neitherwind-driven lateral advection or vertical mixing alone could account for these episodes.Dust storms, over this period sent plumes either into high iron SubTropical (ST) waters orinto SA waters in early spring, when cells are probably light- rather than iron-limited. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere

interactions; 1050 Geochemistry: Marine geochemistry (4835, 4850); 1065 Geochemistry: Trace elements

(3670); 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); KEYWORDS: dust

storms, iron and phytoplankton, iron biogeochemistry

Citation: Boyd, P. W., G. McTainsh, V. Sherlock, K. Richardson, S. Nichol, M. Ellwood, and R. Frew (2004), Episodic enhancement

of phytoplankton stocks in New Zealand subantarctic waters: Contribution of atmospheric and oceanic iron supply, Global

Biogeochem. Cycles, 18, GB1029, doi:10.1029/2002GB002020.

1. Introduction

[2] The last decade has produced striking evidence of therole of iron in limiting phytoplankton growth in the open

and coastal ocean [Coale et al., 1996a; LaRoche et al., 1996;Behrenfeld and Kolber, 1999; Hutchins and Bruland, 1998;Boyd et al., 2000; Hutchins et al., 2002]. Together, theseiron-limited high nitrate low chlorophyll (HNLC) waterscomprise �30% of the World Ocean [de Baar and Boyd,2000]. Biogeochemical budgets of iron [Martin et al., 1989;de Baar et al., 1995] indicate that key sources are bothatmospheric via dust deposition [Duce and Tindale, 1991],and oceanic via upwelling [Measures and Vink, 2001] or byresuspension of sediments [Johnson et al., 1999; Blain etal., 2001]. Furthermore, initial iron budgets from HNLCprovinces point to regional differences in the relative con-tributions of ocean and atmosphere to iron supply [Fung etal., 2000], with the Southern Ocean being supplied mainlyby oceanic sources [de Baar et al., 1995; Measures andVink, 2001]. In contrast, the northeast subarctic Pacific is

GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 18, GB1029, doi:10.1029/2002GB002020, 2004

1National Institute of Water and Atmosphere Centre for Chemical andPhysical Oceanography, Department of Chemistry, University of Otago,Otago, Dunedin, New Zealand.

2Faculty of Environmental Sciences, Griffith University, Brisbane,Queensland, Australia.

3National Institute of Water and Atmosphere, Greta Point, Wellington,New Zealand.

4National Institute of Water and Atmosphere, Hillcrest, Hamilton, NewZealand.

5Department of Chemistry, University of Otago, Otago, Dunedin, NewZealand.

Copyright 2004 by the American Geophysical Union.0886-6236/04/2002GB002020

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supplied primarily by atmospheric sources [Martin et al.,1989] from Asia [Merrill, 1989] and/or Alaska [Boyd et al.,1998].[3] Biogeochemical iron budgets have been attempted for

surface waters of the subarctic Pacific [Price and Morel,1998] and the Southern Ocean [Bowie et al., 2001], butmuch further work is required to define a global iron budget[Fung et al., 2000; Hunter et al., 2001] due to insufficientoceanic iron data [de Baar and de Jong, 2001], and aerosoliron deposition rates [Jickells and Spokes, 2001]. Thus it isdifficult to assess the contribution to upper ocean ironinventories of episodic events relative to sustained ironsupply, such as from vertical diffusivity [Law et al.,2003]. Episodic events include dust storms [DiTullio andLaws, 1991; Husar et al., 2001], and offshore eddy trans-port of iron-rich coastal waters [Whitney and Robert, 2002].Such episodic supply has meant that the study of theseevents has mainly been opportunistic [DiTullio and Laws,1991].[4] Since 1997, the availability of the Sea-viewing Wide

Field of View Sensor (SeaWiFS) [Hooker et al., 1992] toremotely-sense surface chlorophyll concentration (a proxyfor phytoplankton stocks) in HNLC waters [Foley et al.,1997; Moore et al., 1999; Murphy et al., 2001] has shownthat these waters are, for much of the annual cycle,characterized by near constant levels of chlorophyll(�0.3 mg m�3) over large areas (100 km length scales).This characteristic low temporal and spatial variability isthought to be due to low iron supply rates in tandem withsustained microzooplankton herbivory [Strom et al., 2000],and may be exploited as a means to identify indirectlyepisodic iron supply to HNLC regions. Such supply will beconspicuous as it may result in a marked (episodic)increase in chlorophyll above background HNLC concen-trations, provided there is no simultaneous limitation ofalgal growth by other factors such as silicic acid [Hutchinset al., 2001]. Support for the use of elevated chlorophyllconcentrations as a proxy for elevated iron supply havebeen provided in HNLC regions from shipboard observa-tions in the Southern Ocean (island wake effects, Kergue-len [Blain et al., 2001]), the equatorial Pacific (variations inupwelling rate, Cromwell Undercurrent [Chavez et al.,1999]), from autonomous profilers in the northeast Pacific(a dust deposition event, Asia [Bishop et al., 2002]), and insitu mesoscale iron fertilizations [Coale et al., 1996a; Boydet al., 2000]. Thus this approach to detect episodic ironsupply may be used to identify both oceanic and atmo-spheric source mechanisms.[5] A second approach to detect episodic iron supply,

specifically due to atmospheric dust, is to track episodesdirectly using the proxy, aerosol optical depth at 865 nm(AOD) [Wang et al., 2000], derived from remotely senseddata such as the Total Ozone Mapping Spectrometer(TOMS) [Herman et al., 1997], A Very High ResolutionRadiometer (AVHRR) [Husar et al., 1997], and SeaWiFS[Husar et al., 2001]. Under ambient oceanic conditionsAOD is expected to be �0.1. In contrast, values�0.3 are reported for desert aerosol-influenced sites[Holben et al., 2001]. Furthermore, both approaches,monitoring chlorophyll concentrations in HNLC waters

and dust tracking using AOD, have been combined tocompare trends in dust supply and any correspondingincreases in chlorophyll levels (Coastal Zone Color Scan-ner (CZCS) [Stegmann and Tindale, 1999] and SeaWiFS[Gabric et al., 2002]).[6] Around 35% of New Zealand’s offshore waters are

HNLC and associated with the circumpolar Subantarctic(SA) water mass [Murphy et al., 2001] which comprises10% of the world’s oceans [Banse and English, 1997].Banse and English [1997] analyzed monthly mean chlo-rophyll data, reprocessed from CZCS, in SA waterssoutheast of New Zealand. They observed chlorophyllconcentrations similar to that of other HNLC regions, butreported some high pigment events. Banse and English[1997] hypothesized that these events may be due tomechanisms including eddies shed from the SubTropicalFront (STF), transports from the New Zealand shelfregion, or iron supply from Australian dust. None ofthese proposed mechanisms were tested explicitly byBanse and English [1997]. Thus, unlike much of thiscircumpolar SA Water Ring [Banse, 1996], SA watersnear New Zealand may be considered a natural labora-tory to study episodic iron supply due to their proximityto both a source of aerosol dust, the arid interior ofAustralia, and the iron-rich waters of the STF [Boyd etal., 1999].[7] The aim of this study was to use remote sensing to

assess the relative contributions of episodic atmospheric andoceanic iron supply to the enhancement of phytoplanktonstocks of local HNLC SA waters over the period 1997–2001. Consideration of the role of broader-scale ‘‘far-field’’processes [Moore et al., 2002] on such episodes is beyondthe scope of the present study. Here, two distinct approachesare used. Aerosol supply of iron was evaluated using anevent-based analysis: Dust storm events over Australia wereidentified and air mass forward trajectories were calculated.For events in which trajectory routes were indicative of dusttransport over New Zealand waters, and hence deposition ofdust was possible, SeaWiFS and Ocean Color and Temper-ature Scanner (OCTS) [Kawamura and the OCTS Team,1998; Saitoh, 1995] data sets were examined for evidence ofdetectable increases in chlorophyll, relative to backgroundconcentrations.[8] In contrast, the investigation of episodic oceanic iron

supply commenced with the characterization of conspicuousincreases in chlorophyll, relative to HNLC backgroundconditions, in SA waters around New Zealand. Recurrentelevated chlorophyll concentrations were identified in latesummer in SAwaters east of the South Island in the vicinityof the Bounty Trough (BT) (Figure 1). Surface waters in thisregion are strongly stratified during summer [Morris et al.,2001; Sutton, 2001], suggesting enhanced chlorophyll lev-els may be related to transport mechanisms likely toincrease iron concentrations such as enhanced wind-forcedvertical mixing or wind-driven lateral advection (Ekmantransport) in a mixed layer which is effectively decoupledfrom underlying frontal features and bathymetric barriersthrough stratification. These hypotheses were examinedexplicitly using archives of Sea Surface Temperature(SST), surface wind, and surface momentum fluxes in

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conjunction with data on dissolved iron concentrations inthe New Zealand region.

2. Study Site and Methods

2.1. Study Site

[9] New Zealand offshore waters are characterized by theHNLC SA water mass separated from the iron-rich Sub-Tropical (ST) water mass [Banse and English, 1997; Boydet al., 1999] by the STF. Significantly, east of New Zealandthis front is bathymetrically locked onto the Chatham Rise(Figure 1) [Sutton, 2001] and thus does not meander locally,as the STF (elsewhere) and other circumpolar fronts areobserved to [Orsi et al., 1995]. Thus, monthly compositesof Ocean Color generally reveal a sharp north-south gradi-ent in chlorophyll (Figure 2). Correspondingly sharp gra-dients in other phytoplankton properties such as iron stress(diatom flavodoxin levels increase north-south) or photo-synthetic competence (Fv/Fm, decreases north-south) havebeen observed in surveys in spring [Boyd et al., 1999],

summer, and autumn (P. Boyd, unpublished data, 2003).Analysis of chlorophyll concentrations in local SA watersreveals a high degree of spatial homogeneity and lowvariability about the mean (Figure 3) [Murphy et al.,2001] consistent with local bathymetric locking of theSTF, and suggesting that iron supply event(s) may beconspicuous by the subsequent elevation of chlorophyllrelative to HNLC (�0.3 mg m�3) levels. Furthermore, theSTF east of the South Island, referred to as the SouthlandCurrent (Figure 1) [Heath, 1975], is reported to act as astrong physical barrier to prevent island-wake effects suchas offshore transport of resuspended iron from shallowneritic waters [Croot and Hunter, 1998], as observeddownstream of Kerguelen [Blain et al., 2001] and theGalapagos Islands [Gordon et al., 1998].

2.2. Atmospheric Iron Supply: Event-Based Studies ofDust and Chlorophyll Relationships

[10] This approach involves three steps: First, wind ero-sion events in arid regions of Australia were mapped from

Figure 1. Map showing the main bathymetric (250-, 1000-, and 2000-m isobaths) features of the NewZealand region within a box 30�S–55�S, 160�E–170�W. SubTropical (ST) waters lie to the north of theChatham Rise, and are separated by the SubTropical Front (STF) from Subantarctic (SA) waters to thesouth of this feature. The BOUNTY box denotes the Bounty Trough (BT) region used for analyses inFigures 8b, 9, 10, and 11. The Southland Front marks the boundary between the Southland Current andSA waters. The Wairarapa Eddy/East Cape current system lie to the east of the lower and middle NorthIsland, respectively.

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meteorological records of dust storm occurrence in thedatabase of McTainsh [1998], using dust visibility reduction(DVR) as the measure of event intensity to identify signif-icant dust events and source areas. The transport route of

each storm through the atmosphere was then predicted usinga forward air mass trajectory analysis model (HYSPLIT-4)[Draxler and Hess, 1997, 1998]. Four-day forward trajec-tories were calculated for the identified dust event source

Figure 2. A monthly standard mapped image composite of SeaWiFS chlorophyll for October 1998(30�S–55�S, 160�E–170�W) showing the north-south gradient in chlorophyll, the spring bloom in STwaters, persistently elevated chlorophyll levels associated with the STF (bathymetrically locked ontoChatham Rise) east of New Zealand, and low chlorophyll levels in HNLC SA waters. The isobathsrepresent five equally spaced contours: 1429, 2858, 4286, 5716, 7143 m.

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areas, using National Centers for Environmental Prediction(NCEP) re-analysis 6-hourly wind fields [ftp://ncardata.ucar.edu/data sets/ds083.0/] for an initial (mid-plume) alti-tude of 300 m: Trajectories at 300 m are appropriate as themajority of Australian dust storms are Haboob-type eventsassociated with cold fronts in which the vertical dispersionof dust is initially constrained to <500 m by the front[Raupach et al., 1994]. Following trajectory analysis,9-km-resolution daily standard mapped images [Campbellet al., 1995] of AOD were generated for regions and timescorresponding to the predicted dust trajectories, and used todetermine whether there were elevated SeaWiFS predictedaerosol optical properties related to the dust trajectories.One caveat of this approach is that a component of satelliteAOD might not interact with the ocean, and/or be differentfor wet or dry deposition. Next, 8-day chlorophyll standardmapped images were produced for a period 5–15 days afterthe dust event in regions corresponding to the predictedtrajectory to investigate whether aerosol iron supply wascorrelated with an increase in algal stocks, as observed inshipboard iron enrichments after 5–10 days in SA waters[Boyd et al., 1999, 2001].

2.3. Event--Based Estimates of Atmospheric IronDeposition Rates

[11] A large proportion of the annual dust deposition intothe ocean is reported to occur over relatively short periodsof time (weeks, Midway Island [Prospero et al., 1989]).Data from each Australian storm on the dust load at sourcepotentially provide another approach to estimate aerosoliron supply to the ocean. Estimation of the dust load in a

storm event at source was after Raupach et al. [1994], whotake into account the dimensions of the dust plume mea-sured from the DVR maps, and the dust concentrationcalculated from visibility data using the empirical relation-ship measured at the dust source by Chepil and Woodruff[1957]. Plume heights can vary considerably dependingupon the weather system responsible for dust entrainmentand concurrent convective mixing. A conservative estimateof 500 m was used here.[12] Potential dust supply to SA waters from New Zea-

land, associated with northwesterly winds from the semi-arid Canterbury Plains (Figure 1) during summer, has beeninferred by Thiede [1979]. His study of late Pleistocenequartz patterns in Australasia revealed a region of possibledust-derived quartz off the east coast of the South Island.Hourly wind data from Christchurch airport (near Canter-bury Plains) were obtained for January 1997 to present, andsubsampled at synoptic hours (0000, 0600, 1200,1800 hours). The frequency of occurrence of winds fromthe northwest sector, and the mean wind speed, were thencalculated.

2.4. Oceanic Iron Supply: Evidence of ElevatedChlorophyll From SEAWIFS

[13] Archives of monthly composite surface chlorophyllconcentrations for the New Zealand region for September1997 to May 2000 [Murphy et al., 2001] were extended toJanuary 1997 and March 2002 using OCTS and SeaWiFSstandard mapped image data, respectively. Monthly compo-sites were then used to qualitatively identify high chloro-phyll events in SA waters. Such events were defined

Figure 3. Means and inter-quartile ranges for OCTS and SeaWiFS chlorophyll in ST and SA waters,based on analysis of a 6� � 6� box for each water mass (redrawn from Murphy et al. [2001] where theywere denoted by ‘‘STE’’ [37�S–41�S, 179�E–173�W] and ‘‘SAC’’ [48�S–54�S, 168�E–175�E]). Notethat the SeaWiFS OC4V4 algorithm has been successfully validated for local case I waters [Richardson etal., 2003].

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arbitrarily as >0.8 mg chla m�3 (compare ambient HNLClevels of 0.3 mg chla m�3) based on iron-elevated chlo-rophyll concentrations reported from shipboard iron-enrichments of 6 to 8 days in SA waters [Boyd et al.,1999]. After initial identification of such events frommonthly Ocean Color composites, shorter timescale com-posites (such as 8-day) were used to constrain the time-scale of these high chlorophyll episodes.

2.5. Identification of Oceanic Mechanisms toIncrease Phytoplankton Stocks

[14] Upon qualitative identification of high chlorophyllepisodes, several approaches were used to identify thedynamical process(es) responsible for increasing algalstocks. There is an important distinction between physicalmechanisms which result in elevated iron supply, subse-quently leading to iron-elevated algal growth and accumu-lation of stocks (hereinafter referred to as local iron-elevatedalgal growth), and mechanisms which cause increases inphytoplankton stocks which are not related to changes iniron supply. Banse and English [1997] point to advectionand mixing/local iron-elevated algal growth as the mainalternatives to elevate chlorophyll in HNLC waters. Oceaniciron supply mechanisms include island wake effects [Blainet al., 2001], enhanced upwelling or vertical mixing [Barberet al., 1996], water mass interactions with large topographicfeatures [Moore and Abbott, 2002], and lateral advection ofiron-rich waters [Moore et al., 1999]. Oceanic chlorophyllsupply mechanisms include convergence and/or lateraladvection of high chlorophyll waters into HNLC waters[Turner and Owens, 1995]. Wind-driven vertical mixing,and lateral advection and/or mixing, are considered specif-ically here.[15] In a New Zealand context, iron supply mechanisms

include the mixing of ST or STF (high iron concentrations,Figure 4a) with SA (highmacronutrient concentrations [Boydet al., 1999], but low iron concentrations (Figure 4b)) watersto yield local iron-elevated algal growth. Chlorophyll supplymechanisms may include the transport of elevated chloro-phyll waters from the ST or STF (Figure 2) into HNLC SAwaters.[16] To estimate the volume of ST or STF waters required

for local iron-elevated algal growth, DFe concentrations inST and SA waters, and the threshold DFe concentration forincreased algal growth in SA waters must be known. DFeconcentrations in SA waters up- and down-stream of NewZealand are �0.1 nM in the surface waters in summer[Measures and Vink, 2001; Sedwick et al., 1997], andlocally 0.2 nM [Boyd et al., 1999] or <0.1 nM [Figure 4b].In contrast, DFe concentrations in adjacent ST and STFwaters are generally >0.2 nM (Figure 4a) [Boyd et al., 1999]with no evidence of algal iron stress [Boyd et al., 1999; P.Boyd, unpublished data, 2003]. Physiological studies reportthat DFe in HNLC waters must be elevated to >0.2 nM topermit local iron-elevated algal growth, and hence forchlorophyll concentrations �0.3 mg m�3 to accumulate[Coale et al., 1996b; Boyd and Abraham, 2001; Maldonadoet al., 2001].[17] Alternatively, enhanced chlorophyll in SA waters

may be due to iron supplied by vertical mixing, the

magnitude of which is dependent on the vertical gradientsof DFe in the upper ocean. Vertical profiles of DFe inSA waters indicate weak vertical gradients across theseasonal thermocline (Figure 4b, winter excepted). Simi-larly, Sedwick et al. [1997] report increases with depthfrom 0.1 nM DFe in the mixed layer to 0.15 nM at 75 m,and elevated concentrations of 0.2 to 0.3 nM at 200 mduring summer.

2.6. Vertical Mixing

[18] Dissolved iron concentrations in surface waters maybe increased by wind-driven vertical mixing and exchangewith deeper waters characterized by higher DFe concen-trations. Surface wind stress [X] and surface heat fluxes[Q] generate turbulent mixing in the upper ocean. Predic-tion of the depth of this turbulent mixing is complex;however, scale analysis suggests the mixed layer depth isproportional to X/Q1/2 [Price et al., 1986]. In this study, itwas assumed that vertical mixing depends principally onwind stress. The mean square surface wind speed, which isproportional to the magnitude of surface wind stress[Trenberth et al., 1990], was used as a proxy for mixedlayer depth. A qualitative analysis of the correlationbetween mean square wind speed and chlorophyll concen-tration was undertaken for the BT region (Figure 1) usingdaily 2.5� � 2.5� resolution surface wind data sets fromthe National Centers for Environmental Prediction ClimateData Assimilation System (NCEP CDAS) ReanalysisArchive (see http://ingrid.ldgo.columbia.edu/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1/).

2.7. Lateral Advection

[19] Lateral advection may enhance local chlorophyllconcentrations through either direct advection of highchlorophyll waters, or by advection (and subsequent mix-ing) of high iron waters leading to local iron-elevated algalgrowth. Ocean Color imagery and SST fields were ana-lyzed for evidence of direct advective transport of warmST or STF high chlorophyll waters into the SA watermass. SST data were extracted from a 1.1 km resolutionregional data set derived from AVHRR [Uddstrom andOien, 1999].[20] The iron-rich STF (Figure 4a) and Southland Current

[Croot and Hunter, 1998] flows are adjacent to the northernextent of SA waters (Figures 1 and 2), but represent strongbarriers to lateral exchange. Although the surface expres-sion (in SST) of the Southland Front persists over summer,there is a weakening of the corresponding surface expres-sion of the STF [Uddstrom and Oien, 1999], and stratifica-tion of the upper water column is maximum [Sutton, 2001].Under these conditions, wind-driven Ekman transports ofiron-rich ST or STF water south across Chatham Rise mayoccur.[21] The stationary Ekman mass flux south across the

transect [42.85�S, 174�E–178�E], was calculated based ondaily 2.5� � 2.5� resolution NCEP CDAS surface momen-tum flux data. Corresponding volume transports were cal-culated assuming a constant mass flux over the day; that is,V = EkXzS where Ek is the southward Ekman transport(integrated over the Ekman layer) (m2 s�1), Xz is the west-

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east boundary length (meters) and S denotes the duration ofthe transport in seconds.[22] Total southward Ekman volume transports were then

compared with order of magnitude estimates of the volumeof water required: (1) for direct advection of high chloro-phyll STF water: Vdir = A0.8DML, where A0.8 is the arealextent of waters with chlorophyll >0.8 mg m�3 and DML isthe mixed layer depth and (2) for advection of ST water andsubsequent mixing and local iron-elevated algal growth,Vgro, in SA waters,

Vgro ¼ Vdir= 1þ VSA

VST

� �; ð1Þ

where VSA/VST is the ratio of the volumes of the two watermasses which must mix to yield the threshold DFeconcentration, DFeth,

VSA

VST

¼ DFeST � DFeth

DFeth � DFeSA:

Calculations here assume a mixed layer of 30 m [Sutton,2001; M. Hadfield, personal communication, 2003],DFeSA = 0.1 nM (Figure 4b), DFeST = 1.0 nM [Boyd etal., 1999], DFeth = 0.2 or 0.25 [Boyd and Abraham, 2001;Maldonado et al., 2001] giving a required mixing ratio VSA/VST = 8 or 5, respectively. The volume required to elevatechlorophyll >0.8 mg m�3 (either by direct advection of high

Figure 4. Spatial and temporal trends in dissolved iron concentrations east of New Zealand. (a) Ameridional survey along 178�300E in October 2001 with samples taken at 3 m depth using a towed‘‘clean’’ fish [Bowie et al., 2001]; the error bars denote the standard error of the mean of three replicates,and the exceptionally low DFe concentration around 41�S denotes a sample from the waters within theWairarapa Eddy (see Figure 1). (b) Vertical profiles in the upper water column in SA waters (46�S,178�300E) during austral autumn, winter, spring, and summer. The standard error of the mean of threereplicates in each case was <0.03. Dissolved iron was measured following analytical methods outlined byBoyd et al. [1999].

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chlorophyll waters or by mixing high iron low macronu-trient waters (ST or STF) with low iron HNLC (SA) waters)was assumed to be sufficient to maintain the event for days/weeks [Boyd et al., 2001].[23] The volume requirement estimates for local iron-

elevated algal growth are sensitive to the assumed DFeconcentrations and threshold values. In the absence of morecomplete spatiotemporal sampling of DFe concentrations inthe region, values have been used which provide a realisticlower bound for this estimated volume requirement. Owingto this sensitivity issue, the volume requirement to permitlocal iron-elevated algal growth could increase by up tofourfold (i.e., an upper bound) if summertime DFe concen-trations in the STF were indeed comparable with thosemeasured southwest of New Zealand by Sedwick et al.[1997], or the lower bound in Figure 4a. However, indirectevidence of elevated iron concentrations in the STF year-round is provided by persistently high chlorophyll in theSTF from monthly SeaWiFS composites [Murphy et al.,2001], in conjunction with elevated photosynthetic compe-tence (Fv/Fm > 0.45) throughout the STF during shipboardsurveys along the 178�300E meridian during spring, sum-mer, and autumn (P. Boyd, unpublished data, 2003).

3. Results

3.1. Australian Dust-Event-Based Analysis

[24] Six significant dust storm events were identifiedbetween 1997 and 2000 in the arid central region ofAustralia; the earliest was in March 1999 and the latestduring September 2000 (Figure 5). Most events had multi-ple source areas, each of which was used as a start point forair mass forward trajectories. These trajectories track thehypothetical motion of a single air parcel, but may also beconsidered to follow the center of mass of a ‘‘cloud’’ thatresults from the horizontal and vertical dispersion due tomixing processes [Gorden, 1986]. Thus the trajectories fromthe multiple source areas predicted similar routes within thatevent (Table 1; Figure 6).[25] A further event was considered (4 February 2000), as

there was anecdotal evidence of ‘‘red dust’’ deposition onthe Southern Alps (Figure 1) (H. A. McGowan et al.,Provenancing of long travelled Australian dust, submittedto Geomorphology, 2004) (hereinafter referred to asMcGowan et al., submitted manuscript, 2004). Three dustsource areas, separated by a distance of �750 km, wereidentified for this event. The forward trajectory for two ofthese areas predicted airflow over the Tasman Sea, with onepassing over the Southern Alps, and the other the southof the South Island (not shown); the trajectories for theother source area predicted airflow over central Australia(Figure 6c). The different airflows during this event are dueto the passage of a front; westerlies associated with thefront and post-frontal southerlies [Raupach et al., 1994].The forward air mass trajectories for the seven events,classed as marine and/or terrestrial (Table 1), demonstratethat a wide range of routes, with deposition likely eitherinto the ocean or on to land, was possible (Figure 6).[26] Eight-day SeaWiFS images of chlorophyll and daily

images of SeaWiFS AOD were obtained for the four eventswhere trajectories indicated the potential fate of the dust

was either southwest of New Zealand or eastward acrossthe South Island (Figure 7). The mid-September 1999 duststorm occurred around the time of the spring bloom in theST iron-rich waters of the Tasman Sea with markedincreases in chlorophyll being observed along the east coastof Tasmania and Australia, and the west coast of NewZealand during September 1999 (Figure 7b). Similarremarks apply to the 6 September 2000 event (Figure 7d).In contrast, the March 1999 and April 2000 events passedsouthwest of New Zealand over HNLC waters and thuswere potentially iron supply events (Figures 7a and 7c).There was no strong evidence of elevated chlorophyll levelsassociated with the passage of these events (Figure 7).[27] Daily aerosol products (SeaWiFS) for 23–25 March

1999 were heavily cloud-contaminated and thus preventedconclusions about changes in AOD (Figure 7a). However,there was a suggestion of increased AOD associated with atrajectory southeast of Tasmania on 23 March (Figure 7a),although in several images, there appeared to be elevatedAODassociatedwith cloud edges,whichmay be an artifact ofthe SeaWiFS cloud screening procedure. Cloud cover wasgreater during 14–16 September 1999, but AOD images for6–8 September 2000 were less impacted by cloud, and therewas a patch of increased aerosol about 35�S (mid-TasmanSea) near the predicted trajectory on 7 September (Figure 7d).Unfortunately, cloud cover was again problematic during28–30 April 2000 (Figure 7c), with only a suggestion ofincreased AOD near the northern trajectory.[28] Estimated dust loads varied tenfold, from 3 �

105 tonnes to >5 � 106 tonnes (Table 2). The dustconcentration from a single source area, rather than themean dust load for each storm event, must be used in dust/iron deposition calculations; for example, the mid-Septem-ber 1999 event comprised three different dust source areas(Table 2), and the plumes from at least two of these sourceareas had very different trajectories, and thus different fates(Figure 7b). The April 2000 event was the most appropriateto assess oceanic deposition of dust as it had high dustconcentrations (154,465 mg m3 at Kimba, Table 2), and fromtrajectory analysis its probable fate was into HNLC waters.Owing to a large number of uncertainties about dustdeposition processes, and the limited number of large dustevents during the 1997–2000 monitoring period, it has notbeen possible to make robust event-based estimates of ironsupply to SA HNLC waters (see section 4.1.1).

3.2. Oceanic Iron Supply and Episodic Events ofEnhanced Chlorophyll

[29] Qualitative analysis of monthly OCTS and SeaWiFScomposites from November 1996 to May 2002 revealedseveral high chlorophyll episodes in SA waters. Examplesfor February (Figure 8a) display localized increases inchlorophyll to >0.8 mg m�3 near the Auckland Islands(Figure 1), the Campbell Island (Figure 1), and the PukakiRise (Figure 1). Similar trends were also noted in spring forthese regions (data not shown). High chlorophyll events, ofgreater areal extent, occur in the BT region (Figure 1) inFebruary/March 1997, 1998, 1999, and, in particular, 2001(Figure 8a), and exhibit variability in magnitude, location,and extent. Eight-day OCTS and SeaWiFS composites over

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Figure 5. Major dust storm events during the period 1997 to 2000. Solid circles denote the location ofdust monitoring stations, and shaded areas denote the location of dust source regions derived from DVRmaps.

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this period suggested that these high chlorophyll eventspersisted on a timescale of weeks (data not shown).[30] The mean and quartiles of chlorophyll concentrations

within the BT region (Figure 8b) were compared to anequivalent analysis for SA waters (Figure 3). With theexception of November 1998, chlorophyll levels in theBT region were similar to those of HNLC SAwaters duringautumn, winter, and spring. In contrast, in February/March1998, 1999, and 2001, chlorophyll in the BT region waselevated to �0.8 mg m�3, comparable to those levels at theSTF, and during the ST spring bloom (Figures 2 and 3). InFebruary/March of both 2000 and 2002, chlorophyll con-centrations in the BT region were similar to those observedin SAwaters (Figure 3). Thus, elevated chlorophyll levels inthe BT region exhibited a strong seasonal signature, occur-ring in late summer, and were characterized by marked

interannual variability in magnitude and spatial extent. Thefrequency, persistence, and large areal extent of the highchlorophyll feature in the BT region relative to the localizedchlorophyll patches associated with the SA Islands made itthe main focus of the oceanic component of this study.[31] The elevated chlorophyll feature in the BT region

was considered to be driven by oceanic supply since therewas no evidence from Australian dust storms and theirtransport routes of deposition in this region. There was alsono compelling evidence, from analysis of wind data fromChristchurch airport (Figure 1, data not shown), that localdust transport might account for observed interannual var-iability of chlorophyll concentrations in the BT region.[32] The BT high chlorophyll feature occurred in a region

that is, in principle, separated from ST waters by theSouthland Front (Figure 1) and STF [Heath, 1975]. Theseasonality of the BT high chlorophyll events, which occurwhen stratification of surface waters is maximum [Sutton,2001], suggest that elevated chlorophyll levels might berelated to wind-forced enhanced vertical mixing, or alter-natively, to wind-driven lateral advection (Ekman transport)in a mixed layer which is decoupled from underlying frontalfeatures and bathymetric barriers through stratification.Hypotheses concerning wind-driven circulation were there-fore tested explicitly.

3.3. Oceanic Supply Mechanisms

3.3.1. Vertical Mixing[33] A time series of chlorophyll concentrations and mean

square wind speeds for the BT region prior to and during the

Table 1. Summary of Australian Dust Events and Associated

4-Day Forward Trajectory End Pointsa

DateEndpoint ofthe Trajectory

TrajectoryType

24 March 1999 SSW NZ marine15 Sept. 1999 SW NZ marine4 Feb. 2000 NWAu terrestrial4 Feb. 2000 E NZ marine17 March 2000 SE Au terrestrial29 April 2000 ESE NZ marine1 Sept. 2000 NW NZ marine7 Sept. 2000 W NZ marine

aThe HYSPLIT-4 model was used [Draxler and Hess, 1997, 1998]. Audenotes Australia; NZ denotes New Zealand.

Figure 6. Forward air mass trajectories for seven dust storm events, (a) 24March 1999, (b) 15 September1999, (c) 4 February 2000 (based on anecdotal evidence only), (d) 17 March 2000, (e) 29 April 2000, (f) 1September 2000, and (g) 7 September 2000. The upper part of each panel denotes the trajectory route, andthe lower part shows the predicted trajectory height over time; hPa is millibars.

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February 2001 high chlorophyll event are presented inFigure 9. A period of low wind speeds in early January,was followed by one of high wind speeds (enhanced verticalmixing, potentially increasing DFe in surface waters) which

preceded the high chlorophyll event. Low wind speeds(reduced vertical mixing) then prevailed over most ofFebruary and March. Monthly mean square wind speedsfor this region were analyzed (Figure 10a) to assess if the

Figure 7. SeaWiFS 1-day AOD (865 nm) and 8-day chlorophyll composites before, during, and afterdust events with southeast or east routes in (a) March 1999, (b) September 1999, (c) April 2000, and(d) September 2000. Air mass forward trajectories presented in Figure 6 have been superimposed onto theAOD images.

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trends observed in early 2001 applied to other years for theBT region. These data provide no strong evidence for highwind stress and hence enhanced vertical mixing precedingthe high chlorophyll events in this region (Figure 10). Indeed,the weakest mean wind stresses occurred prior to the BTelevated chlorophyll event in summer 1999. Furthermore,there was no marked temporal variation in the magnitude ofthe wind stress in this region: If vertical mixing was thedominant iron supply process, these wind speed dataindicated that elevated chlorophyll levels should also be

observed in spring and early summer. Wind-driven verticalmixing will occur on horizontal scales associated withsynoptic weather systems (>100 km). Therefore, a corre-spondingly diffuse and spatially homogeneous increase inchlorophyll concentrations might be expected, whereasareas of elevated chlorophyll in the BT region weredelimited by strong spatial gradients. Moreover, verticalprofiles of DFe for SA waters [Sedwick et al., 1997] implymixing must occur to �150 m depth (i.e., fivefold greaterthan summer mixed layer depth in the region [Hadfield and

Figure 7. (continued)

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Sharples, 1996]), to elevate DFe concentrations to >0.2 nM,the threshold for local iron-elevated algal growth. Thus themajority of these data do not support the hypothesis thatincreased chlorophyll concentrations in this region resultdirectly from enhanced vertical mixing.3.3.2. Lateral Advection[34] Examples of 8-day composite SST and its temporal

variance are provided in Figure 11. SST fields exhibitedcharacteristic signatures of surface flows in the vicinity ofthe BT region, including marked eddy activity and SST

variability to the north of the Chatham Rise, variability inthe Southland Current and flows through the Mernoo Gap,and evidence of mixing southeast of Banks Peninsula (seeFigure 1). However, SST data sets provided no clearevidence of a dominant transport process for warmer waterswith a characteristic high-chlorophyll signal into the BTregion.[35] Similarly, monthly composite SST anomalies in the

BT region showed no correlation with enhanced chlorophyllconcentrations (data not shown) during summer months on

Figure 7. (continued)

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interannual timescales, suggesting neither direct advection(of warm, high chlorophyll waters) nor wind-driven verticalmixing (which tends to cool surface waters) was thedominant causal mechanism for observed high chlorophyllepisodes. However, the surface expression of the STF inSST (i.e., meridional gradients at the northern and southernbounds of the STF over Chatham Rise [Sutton, 2001]) wasgenerally weaker in summer months when the high chloro-phyll events were observed in the BT region (particularlyfor summer 1998, data not shown).

[36] Daily cumulative southward Ekman transports andvolume requirements to permit local iron-elevated algalgrowth (i.e., elevation of DFe to >0.2 nM) were plottedfor the high chlorophyll event in summer 2001 (Figure 9c).Cumulative southward Ekman transport over 90 days wasof the order of 25 � 1010 m3, i.e., five- to eight-fold lessthan the volume requirement for direct advection of highchlorophyll ST waters into the BT region (from equation(1)). This Ekman transport was comparable to the totalestimated volume requirement for local iron-elevated algal

Figure 7. (continued)

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growth (i.e., mixing of SA and STF waters) during the 2001event; however, the timing of the southward Ekman trans-ports was not consistent with that of the BT high chlorophyllevent. For example, to maintain chlorophyll at >0.8 mg m�3

from year day 50 until day 58, the required southward volumetransport was at least 20 � 1010 m3 as compared with anestimated upper bound on cumulative transport of 12 �1010 m3 by day 58 (Figure 9c).[37] This analysis for the February 2001 event was ex-

tended to monthly cumulative southward Ekman transport(January 1997 to March 2002), and compared to estimatesof the volume requirements to permit local iron-elevatedalgal growth (Figure 10b). Ekman transports were less thanthe estimated volume requirements in summer 1998 and 2001when high chlorophyll events were observed, while highEkman transports (i.e., greater than the estimated volumerequirements) occurred in January/February 2002, andMarch 2000, when no significant increases in chlorophyllwere noted. The latter cases may indicate that the assump-tions underlying the volume requirement estimates may beviolated; however, volume requirements may be significantlyunderestimated (see section 2.7), and do not take into accountthat iron is not always the only factor limiting phytoplanktongrowth in this region (see section 3.3.3).[38] Ekman transport may have contributed to the trans-

port of ST waters south in summer 1997 and 1999, but datado not suggest that southward Ekman transport of STwaterswas the dominant mechanism for iron supply required tosupport these high chlorophyll episodes. However, becausethe present estimates of volume requirements for local iron-elevated algal growth and Ekman transports are oftencomparable in magnitude, improved spatially- and tempo-rally-resolved DFe data sets in the region are required tostrengthen a comparison of Ekman transports and volumerequirements.

3.3.3. Iron Supply, Mixed Layer Depth, and SilicicAcid Supply[39] The BT high chlorophyll feature was typically ob-

served in late summer, when silicic acid levels of <1 mM[Boyd et al., 1999; M. Ellwood, unpublished data, 2003],and mixed layer depths of 30–35 m are often reported(P. Boyd, unpublished data, 2003). Such mixed layershoaling will elevate mean underwater irradiances,resulting potentially in reduced algal iron quotas [Sundaand Huntsman, 1997] and hence elevated chlorophyllconcentrations. As such, it might explain the observed BThigh chlorophyll episodes. However, the mean monthlymixed layer depth (CARS climatology, M. Hadfield, per-sonal communication, 2003; http://www.marine.csiro.au/�dunn/eez_data/atlas.html) for this region is <35 m forDecember until March, and incident irradiances are highestin December/January [Bishop and Rossow, 1991]. Thus it isunlikely that a shoaled mixed layer would elevate chloro-phyll concentrations only in February/March, and solely inthe BT region.[40] Sub-micromolar silicic acid reported in SA waters in

summer [Banse and English, 1997; Boyd et al., 1999] mayresult in the resident diatoms being simultaneously limitedby both iron and silicic acid supply [Boyd, 2002]. Iron-elevated chlorophyll concentrations are mainly driven byincreases in diatom stocks in both shipboard and in situexperiments [De Baar and Boyd, 2000], and thus observedhigh chlorophyll episodes in the BT region may require bothadditional iron and silicic acid supply. Slightly higher silicicacid levels are observed in summer in ST waters (>1 mM)compared with SA waters (M. Ellwood, unpublished data,2003). However, it is problematic to calculate the silicic-acid concentration that will limit phytoplankton growth (ascarried out for iron), since resident diatoms in SA watersexhibit a low degree of silicification in shipboard ironenrichments [Hutchins et al., 2001], and thus may be ableto increase in biomass despite relatively low silicic acidlevels.

4. Discussion

4.1. Atmospheric Supply of Iron

[41] The seven largest dust events during 1997–2000were selected using event-based analysis (six events), andanecdotal evidence. This analysis recorded dust storms inSeptember (two), February (one) March (two) and April(one), which is similar to the temporal trends of Gabric etal. [2002] between 1997 and 1999 for the coastline west ofTasmania. Stegmann and Tindale [1999] studied the zonaldistribution of aerosols (using CZCS aerosol radianceintensities) over the ocean, and reported an elevated bandof aerosol radiance in the SA Water Ring from spring(October) to late summer (February), as previously ob-served using AVHRR [Husar et al., 1997]. Stegmann andTindale [1999] stated that the exact source for these elevatedaerosol intensities was unclear. In each of these studies, thetiming of observed events coincides with the dust stormseason in eastern Australia (September to March: McTainshand Leys [1993]), although Husar et al. [1997] concludedthat dust sources in Australia had no discernible effect onAOD.

Table 2. Summary Information on the Major Australian Dust

Events Between 1997 and 2000a

Event(dd/mm/yy) Source Area

DustConcentration,

mg/m3

EventDuration(hh:mm)

DustLoad,Tonnes

24/03/1999 Walpeup 39,123 01:30 415,83124/03/1999 Kyancutta 74,086 5,188,768Total 56,605 average 5,604,59915/09/1999 Bedourie 74,086 02:00 1,657,37115/09/1999 Broken Hill 8,686 284,58715/09/1999 Andamooka 1,154,893 1,158,604Total 412,555 average 3,100,56217/03/2000 Kimba 74,086 01:30 521,573Total 74.086 average 521,57329/04/2000 Warracknabeal 122,985 03:30 646,28629/04/2000 Kimba 154,465 193,08129/04/2000 Mount Barker 64,944 174,863Total 114,131 average 1,014,23001/09/2000 Windorah 85,838 04:00 3,411,398Total 85,838 average 3,411,39807/09/2000 Windorah 8,686 01:00 168,19607/09/2000 Thargomindah 64,944 162,361Total 36,815 average 330,557

aIncluding date, source areas (often an event had more than one sourcearea; see Figure 5), dust concentrations (based upon visibility-dustconcentration relationship), event duration, and dust loads (based upondust concentrations and the estimated dimensions of the dust storm).

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[42] Previous investigations of aerosol iron supply, specificto the New Zealand region, have relied on anecdotal evidenceof dust deposition, such as ‘‘mud showers’’ over NewZealand [Kidson, 1930], ‘‘red snow’’ inNewZealand glaciers[Knight et al., 1995; McGowan et al., submitted manuscript,2004], or trends from air mass back-trajectories [Sturman etal., 1997; Meder et al., 1991]. In the present study, air masstrajectory analysis suggests that one event resulted in terres-trial deposition, four in oceanic deposition, and one resulted

in both.McGowan et al. [2000] concluded that regional dusttransport pathways in the vicinity of Australia and NewZealand are more diverse than the previously thought pre-dominately west-east route. The trends from trajectory anal-ysis in the present study are consistent withMcGowan et al.’sconclusions, with several trajectories passing over HNLCwaters south and south west of New Zealand.[43] In the September 1999 and 2000 events dust trans-

port occurs over high-iron ST waters in the Tasman Sea

Figure 8. (a) Monthly standard mapped image composites of SeaWiFS chlorophyll for February 1997to February 2002 (box size 30�S–55�S, 160�E–170�W). Note the elevated chlorophyll concentrations inthe Bounty Trough (2000 and 2002 excepted). The Ocean Color scale is as applied to Figure 2. (b) Meanand inter-quartile range for monthly observations (from 1997 to 2002) of OCTS and SeaWiFSchlorophyll for the Bounty Trough region (denoted by BOUNTY in Figure 1).

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during early spring, i.e., a region not characterized byHNLC conditions [Boyd et al., 1999], so the approachadopted here of using conspicuous increases in AOD orchlorophyll as a proxy for iron supply events cannot beapplied. Events that potentially resulted in dust depositioninto HNLC waters (March 1999 and April 2000) yielded noevidence of a marked increase in chlorophyll levels. Neitherof these events may have carried a large iron load as theEyre Peninsula, the main source area of both, has relativelyless iron-rich soils than other dust source areas in the Mallee(i.e., southwest New South Wales, and northwest Victoria)and in southwest Queensland (Figure 5a). The lack of adiscernible increase in algal stocks may also have been dueto the distances over which Australian dust must be trans-ported to reach HNLC waters (>1500 km). The dust load‘‘half-decrease distance’’ of 500–2000 km reported byProspero et al. [1989] in the drier North Pacific Oceanmay be overestimates of the dust transport distances in themore humid Southern Ocean. The higher frequency ofrainfall may result in rainout of the dust before it reachesHNLC waters, resulting in low iron supply rates to HNLCwaters (see section 4.1.1). The timing of the events, inautumn when light limitation of algal growth is evident[Boyd et al., 2001], may also explain the lack of evidence ofincrease algal stocks.

[44] The lack of compelling evidence of dust-mediatedlocalized increases in chlorophyll concentrations in local SAwaters is at odds with other studies. Gabric et al. [2002]presented evidence of a direct correlation (i.e., no time lag)between SeaWiFS AOD (865 nm) and chlorophyll concen-trations for SA waters west of Tasmania (40�S–50�S,125�E–145�E). They argue that this correlation supportsthe hypothesis that atmospheric delivery of iron stimulatesphytoplankton growth. However, Gabric et al. acknowledgethat the introduction of a 3- to 5-day lag, to account for iron-elevated algal growth following transient dust supply, intothe correlative analysis between AOD and chlorophyllresults in a weaker relationship, but do not explore thisresult further. Their approach was based on a statisticalanalysis of remote-sensing data, while in the present study,specific major dust storms were targeted.[45] In the present study, unlike that reported by Gabric et

al. [2002], there was very limited evidence from SeaWiFSAOD of elevated aerosol levels along the predicted dusttransport trajectories. This may be due to dust loads beingbelow the detection limit of SeaWiFS, or the cloudiness ofthe study site and small sample size of dust events. It is alsonoted that correlative analysis between AOD and chloro-phyll, both products of the SeaWiFS sensor, must be donewith great care: The atmospheric correction process, in

Figure 9. Time series for the February 2001 high chlorophyll event for the BT region of (a) chlorophyllconcentrations, from LAC 8-d SeaWiFS composites; (b) daily mean square wind speeds; and (c) estimated(solid lines) and required (to raise DFe above threshold levels for local iron-elevated algal growth, dottedlines) volumes for southward cumulative Ekman transport across the transect 42.85�S, 174�E–178�E.

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which the atmospheric contribution to top of atmosphere(TOA) radiance is subtracted from the total radiance re-ceived by the sensor, will inevitably induce correlationsbetween atmospheric products (e.g., AOD) and oceanicproducts such as chlorophyll concentration. This effecthas been reported [Chomko and Gordon, 2001].4.1.1. Is Event-Based Estimation of Aerosol IronDeposition Possible?[46] Estimation of iron deposition into the ocean is prone

to uncertainties, such as solubility, wet versus dry deposi-tion, scavenging ratios, etc. [Duce et al., 1991; Jickells and

Spokes, 2001; Johnson et al., 2003]. Previous calculationsof aerosol iron fluxes have been based mainly on measure-ments of atmospheric dust at ground-based dust monitoringsites (e.g., SEAREX), whereas in the present study, datawere available on dust load during uplift and prior todispersion, which will add further uncertainties to thecalculations; Jickells and Spokes [2001] have pointed outthe difficulties in comparing ground-based network mea-surements with those from altitude such as from remote-sensing. Dust load data may be coupled with information onthe crustal abundance of iron for the Australian region

Figure 10. Time series (months) from 1997 to 2002 of (a) monthly mean square wind speeds for the BTregion and (b) estimated (lines) and required (to raise DFe above threshold levels of 0.25 nM, diamonds)volumes of monthly southward cumulative Ekman transport across the transect 42.85�S, 174�–178�E.

Figure 11. Maps of the waters east of the South Island of New Zealand, (a) 8-day SST composite(scaled 10�–20�C); (b) 8-day composite SST standard deviation (scaled 0�–2�C); and (c) SeaWiFS 8-daychlorophyll composite for 28 February to 7 March 2001. The Ocean Color scale is as applied to Figure 2.High temporal variability of SST (i.e., high standard deviations) is typically indicative of changes in thelocation of frontal regions (large spatial gradients in SST) over the 8-day period.

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[Gatehouse, 2002] to estimate the iron ‘‘load’’ and henceiron concentration associated with each dust event. How-ever, little is known about the dispersion of the dust plumefollowing uplift [McGowan et al., 2000]. Gorden [1986]reported that the horizontal dispersion parameter of a cloudin the boundary layer is �50 km d�1, and assuming aGaussian profile, the radius of a circle containing 95% ofthe cloud changes at the rate of �100 km d�1. Thus, in adust event of 3 days duration, a dispersion factor of 8 mightbe appropriate.[47] Additional uncertainties that must be addressed

before robust event-based iron deposition rates can becalculated include estimation of the ‘‘half-decrease’’distance for the dust load for the Australasian region(values range from 500 km (northwest Pacific [Prosperoet al., 1989] to 2000 km (SEAREX network [Turekian etal., 1989]), the relationship between half-decrease distancefor dust load and the predominance of wet deposition(episodic and high rates of deposition) versus dry deposi-tion (sustained, low rates of deposition). It is alsounknown whether such half-decrease distances for dustloads apply to the iron load of the dust plume; this maydepend on the relationship between particle size distribu-tion of a dust plume, and the predominant location of theiron within the dust size spectra; for example, if the iron ismainly in small particles, this may lead to a successiveenrichment of the iron load in the plume as large particlesare deposited during transit.[48] It may be possible to assess independently the

relationship between dust load and subsequent oceanic ironsupply by back-calculation. If remotely-sensed evidence ofan algal response to dust deposition (i.e., transient increasesin chlorophyll concentrations above HNLC levels) areobserved, they may be used in conjunction with algalFe:C [Sunda and Huntsman, 1997] and regional C:Chlaratios [Bradford-Grieve et al., 1999] to indirectly constrainthe iron supply required from a dust event; iron budgetarycalculations have previously been related to algal ironrequirements using Fe:C ratios [Jickells, 1999; Banse andEnglish, 1997; Measures and Vink, 2001].[49] Banse and English [1997] suggested that episodic

blooms in New Zealand SA waters may have been drivenby Australian dust, but the present study does not support thisconclusion for 1997–2000. However, since over most of thisstudy, eastern Australia experienced a La Nina (or humid)weather phase (i.e., the monthly Southern Oscillation Indexwas positive for 76% of this period), and there is a strongnegative relationship between dust activity and rainfall[McTainsh and Leys, 1993], the dust inputs to the oceanduring this period, and hence the chances of establishing adust supply/ocean productivity relationship, are likely to below. The annual average dust storm frequency during 1997–2000 was 43, which is low relative to the most active period(1961–1964) which had an average of 123 events. Further-more, there is evidence in October 2002 of major dust eventsoccurring in Australia as presented on DUSTLINE (J. M.Prospero, Global aerosol report website, http://www.nrlmry.navy.mil/aerosol/globaer_world_loop. htm, 2000; see D.Westphal’s website, http://www.nrlmry. navy.mil/aerosol/satellite/seawifs/australia/200210/2002).

4.1.2. Oceanic Supply of Iron[50] Between 1997 and 2001, elevated chlorophyll con-

centrations were observed near the SA Islands, the PukakiRise, and the BT region. In several cases, such as theAuckland Islands, these features were observed in each ofseveral years. The lack of evidence for significant dustdeposition from Australian or New Zealand sources intolocal SA waters points to oceanic iron supply as the mostprobable explanation for these episodes. Elevated chloro-phyll episodes around the SA Islands have previously beenreported by Banse and English [1997] using the CZCSarchive. They suggested that the high chlorophyll observedaround these islands was probably due to island-wakeeffects associated with the resuspension of iron-rich sedi-ments in relatively shallow waters as observed in otherHNLC regions [Blain et al., 2001; Gordon et al., 1998].[51] Episodes of elevated chlorophyll downstream of

seamounts near the Pukaki Rise observed in the presentstudy were also reported by Banse and English [1997]. Onepossible explanation is that the events in the BT region weredriven by iron supply from persistent upwelling associatedwith these sea mounts [Heath, 1975]. However, highchlorophyll episodes downstream of the sea mounts wereobserved only in 2000 and 2001 (and not during 1997–1999, when high chlorophyll events occurred in the BTregion) suggesting that sea mounts are not the principle ironsource for the BT events.[52] Several potential supply mechanisms for the BT high

chlorophyll episodes were investigated, including directadvection of warmer high chlorophyll waters of the STF,or advection of high iron ST or STF waters and subsequentmixing with SA waters. The duration of the elevatedchlorophyll events, comparative SST analyses, and Ekmantransport calculations rule out direct advection of highchlorophyll waters from the STF region southward intothe BT region as an explanation for high chlorophyllepisodes.[53] Identification of a plausible mechanism to permit

local iron-elevated algal growth was less clear. The occur-rence of high chlorophyll events in late summer, whenstratification in the upper ocean is maximal, suggests a rolefor wind-driven transport processes in the mixed layer.However, the analysis undertaken here provides no com-pelling evidence that either wind-forced vertical mixing orsouthward Ekman transports of iron-rich STF waters acrossthe Chatham Rise were the principal source of iron neededto stimulate high chlorophyll episodes: Neither of thesedynamical processes alone accounts for observed chloro-phyll concentrations or their interannual variability in thisregion.4.1.3. Other Potential Iron Supply Mechanisms[54] The waters of the Southland Current are a potential

source of elevated iron (�1–6 nM [Croot and Hunter,1998]). This supply mechanism was not initially consid-ered in analyses because the Southland Front was assumedto be a strong barrier to mixing, based on the persistentsurface expression of the front in SST [Uddstrom andOien, 1999] and the marked gradient in DFe observed intransects across the Southland Current [Croot and Hunter,1998]. However, analyses of SST in the present study

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suggested marked variability in the location of the front.For example, in Figure 11, to the southeast of BanksPeninsula the maximum SST variance was associated withthe region of high horizontal SST gradients, and this is ageneral climatological feature of this region [Uddstromand Oien, 1999, Plate 3]. Variation in the northwarddisplacement of SST isotherms (not shown) also suggestedthat the strength of northward flows through Mernoo Gapvaries.[55] The observed variability of the Southland Current (as

deduced from SST variance) may be wind driven (e.g.,modulation of the strength of the Southland Current throughwind-forcing [Chiswell, 1996]) and/or may be related toperiods of strong reversals in the predominantly northwardflows through Mernoo Gap [Greig and Gilmour, 1992]. Inthe latter case it is possible that iron-rich Southland Currentwaters mix with SAwaters in the region southeast of BanksPeninsula and along the Southern flanks of the ChathamRise; Uddstrom and Oien [1999] hypothesized such a flowwhen interpreting seasonal asymmetry in SST in the latterregion. Direct advection of iron-rich ST waters throughMernoo Gap into the region southeast of Banks Peninsulamay also occur.[56] Limited data are available on flows through Mernoo

Gap, and the physical mechanisms for flow reversal are notwell understood. Greig and Gilmour [1992] analyzed a6-month current meter deployment in Mernoo Gap, anddemonstrated that strong flow reversals were associatedwith an influx of warm water into the area immediatelynorth of Mernoo Gap caused by eddy activity associatedwith the Wairarapa Eddy/East Cape current system(Figure 1). Daily SST imagery for summer 2001 showededdy activity and warming events off the east coast of theSouth Island which, as in the 8-day composite presented inFigure 11, are qualitatively consistent with the flow con-ditions described by Greig and Gilmour. During the 2001elevated chlorophyll event, SST to the northeast of BanksPeninsula and within Mernoo Gap warmed by severaldegrees over year days 30 to 55 at the onset of thechlorophyll event, again consistent with an eddy driveninflux of iron-rich warm water.[57] Eastward Ekman transport across the Southland

Front may also contribute to mixing of Southland Currentand SA waters, and hence elevated chlorophyll concentra-tions. This cannot be explored fully at present as it requiressurface momentum flux estimates at high spatial resolution(see proximity of the Southland Current to the South Islandof New Zealand). Analysis of NCEP CDAS meridionalsurface momentum fluxes indicate comparable eastward andwestward Ekman volume transports of the order of 0.2 �1010 m3 per 100 km north-south transect per day at 44�S,174�E. The magnitude of these eastward volume transportsare insufficient for direct advection of high chlorophyllwaters across the Southland Front to account for the BThigh chlorophyll episodes, but the volume transport of iron-rich waters may be sufficient to permit local iron-elevatedalgal growth. However, eastward transports cannot accountfor the timing of the onset of the 2001 event, and no markedinterannual variability was observed in eastward volumetransports over this period.

[58] Clearly, more observational data and further study arerequired to determine if and how transport of SouthlandCurrent waters into the BT region takes place. If such atransport mechanism is demonstrated, then issues such asthe biological and biogeochemical implications of mixingneritic and oceanic waters must be considered carefully.Improved characterization of the water masses in the BTregion, through in situ measurements, such as DFe, silicicacid, algal species composition, at appropriate spatial andtemporal (seasonal and interannual) scales, are essential toelucidating the processes governing the observed episodesof high chlorophyll concentrations.4.1.4. Oceanic Versus Atmospheric Iron Supply toNew Zealand Waters, 1997––2001[59] During this period, the episodic supply of iron to

local HNLC waters was predominantly driven by oceanicsource mechanisms. Unlike much of the SAWater Ring, theproximity of local SA waters to landmasses and shallowerwaters points to a key role for the bathymetry in the supplyof iron, as concluded by others for the Southern Ocean[Comiso et al., 1993; Banse and English, 1997; Moore andAbbott, 2002]. Locally, the supply of iron due to entrain-ment, advection, and upwelling appears to be persistentwhen associated with SA islands, but episodic when drivenby a more complex mechanism that potentially requires theinterplay of factors including bathymetry, and eddy activityas in the case of the BT region. Modeling studies of theproportion of total iron input provided by atmosphericsources suggest >30% in the Tasman Sea and south ofAustralia, but less than 10% south of New Zealand [Mooreet al., 2002]. This would appear to be consistent with thepresent study, but note that 1997–2001 was characterizedby relatively low dust activity in central Australia.

4.2. Contribution of Episodic Iron Supply toPhytoplankton Stocks and Productivity

[60] The presence of elevated chlorophyll levels, thatpersist for up to 1 month, in local SA waters will influenceregional productivity, and probably alter the trophic struc-ture of the SA pelagic ecosystem. The areal extent of SAwaters within the New Zealand Exclusive Economic Zone(EEZ) was estimated as 1.78 ± 0.15 � 106 km2 [Blezard,1980]. Murphy et al. [2001] report that mean SeaWiFSchlorophyll concentrations for SA waters were 0.3 mg m�3.Episodic iron supply from oceanic mechanisms elevatedchlorophyll levels to >0.8 mg m�3 over an area of 4 �104 km2, and thus increased algal stocks in the SA waterswithin the EEZ by 4.8% for periods of up to 1 month.Application of rates of chlorophyll-normalized primaryproduction (Pb

opt) from SA waters and underwater irradian-ces for summer (P. Boyd, unpublished manuscript, 2003)yielded increases in column-integrated primary production(based on work by Behrenfeld and Falkowski [1997]) of�0.8 g C m�2 d�1 (i.e., from 0.6 g C to 1.4 g C m�2 d�1)over a 1-month period for SA waters. From shipboard ironenrichments, such episodic increases in chlorophyll in SAwaters are associated with a shift from the usually dominantpicophytoplankton to larger diatoms [Boyd et al., 1999,2001; Gall et al., 2001; Sedwick et al., 1999]. Such iron-induced floristic shifts may alter trophic pathways [see

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Bradford-Grieve et al., 1999], and the fate of algal carbon[Nodder et al., 2001] during and after such events, respec-tively. These episodic events will alter the ‘‘constancy’’ ofthe HNLC condition, and may be more akin to naturallyoccurring perturbations such as the spring bloom event,as observed in mesotrophic waters, in impacting food-webs and associated biogeochemical cycles [Legendreand Rassoulzadegan, 1995].

5. Conclusions

[61] 1. Analysis of Australian dust storms yielded nocompelling evidence of localized increases in SeaWiFSAOD or chlorophyll associated with their subsequent atmo-spheric pathways. However, only two of these trajectorieswere over HNLC waters, each may have had a low ironload, and as evidence of changes in SeaWiFS AOD andsubsequently chlorophyll concentrations were cloud-con-taminated this test of a relationship between dust storms,altered AOD and chlorophyll was inconclusive.[62] 2. There are many variables in the relationship

between dust storms, iron supply, and phytoplankton, andhigher resolution data are required to quantify them,including more accurate estimates of dust loads using: animproved regional visibility–dust concentration relation-ship, measured dust concentration profiles, dust ceilings,and downwind dust plume dispersion rates. Data are alsoneeded on rainfall patterns in relation to dust plumetrajectories to distinguish wet from dry deposition. Micro-wave and radar satellite sensors such as SSM/I and TRMMpermit rainfall to be estimated indirectly. It may be possibleto link such satellite-derived rainfall (see SSM/I http://gpcp-pspdc.gsfc.nasa.gov/Chang.Nov93.html; or theGlobal Precipitation Climatology Project http://precip.gsfc.nasa.gov/) with the timing of dust events, air massforward trajectories, etc. There is also a need for moreprecision in the timing of events and in defining the travelheight of the dust-carrying air parcel. Moreover, to con-sider a concept such as half-decrease distance for iron load,the relationship between dust size spectra and the parti-tioning of the iron load within it must be explored.[63] 3. Anomalous increases of SeaWiFS chlorophyll

have been observed in HNLC waters in the BountyTrough region southeast of New Zealand. Subsequentanalyses examined whether these events were due towind-driven circulation in the mixed layer decoupled fromunderlying dynamical (frontal) and bathymetric barriers toflow by seasonal stratification. Although these analyses aresimplistic, they tend to preclude some supply mechanisms(direct advection, vertical mixing), and will inform futuresampling requirements to elucidate fully others (Ekmantransports, Southland current flows). Development andapplication of more robust methods for identification ofanomalous high chlorophyll regions by temporal filteringand spatial decomposition of multiyear remotely sensedoceanographic data (as in the work of Uddstrom and Oien[1999]) are envisaged; however, establishing improvednutrient inventories, with adequate spatio-temporal sam-pling, is the single most important step toward improvingon current analyses.

[64] Acknowledgments. Thanks to Ken Tews for dust event dataanalysis, and to two anonymous reviewers for their comments and advice,which substantially improved this manuscript. We acknowledge the con-tribution of Melissa Bowen, Phil Sutton, Steve Reid, and Michael Udd-strom (NIWA) for constructive comments on aspects of the analysispresented here. We are grateful to Mark Hadfield for a personal commu-nication, and to Michael Ellwood for the use of unpublished data. Theauthors wish to thank the Agency of Japan (NASDA), the SIMBIOS andSeaWiFS Projects (Code 970.2), and the Goddard Earth Sciences Distrib-uted Active Archive Center (Code 902) at the National Aeronautics andSpace Administration Goddard Space Flight Centre, Greenbelt, Md., for theproduction and distribution of OCTS and SeaWiFS data. The HYSPLIT-4model was used to calculate air trajectories; the model is available fromwww.arl.noaa.gov/ss/models/hysplit.html. This work was funded in part bythe New Zealand PGSF, and the New Zealand Royal Society MarsdenFund. This is contribution 1 to the New Zealand SOLAS programme.

ReferencesBanse, K. (1996), Low seasonality of low concentrations of surface chlor-ophyll in the subantarctic water ring: Underwater irradiance, iron, orgrazing?, Prog. Oceanogr., 37, 241–291.

Banse, K., and D. C. English (1997), Near-surface phytoplankton pigmentfrom the Coastal Zone Color Scanner in the Subantarctic region southeastof New Zealand, Mar. Ecol. Prog. Ser., 156, 51–66.

Barber, R. T., M. P. Sanderson, S. T. Lindley, F. Chai, J. Newton, C. C.Trees, D. G. Foley, and F. P. Chavez (1996), Primary productivity and itsregulation in the equatorial Pacific during and following the 1991–1992El Nino, Deep Sea Res., Part II, 43, 933–970.

Behrenfeld, M. J., and P. G. Falkowski (1997), A consumer’s guide tophytoplankton primary productivity models, Limnol. Oceanogr., 42(7),1479–1491.

Behrenfeld, M. J., and Z. S. Kolber (1999), Widespread iron limitation ofphytoplankton in the South Pacific Ocean, Science, 283, 840–843.

Bishop, J. K. B., and W. B. Rossow (1991), Spatial and temporal variabilityof global surface solar irradiance, J. Geophys. Res., 96, 16,839–16,858.

Bishop, J. K., R. E. Davis, and J. T. Sherman (2002), Robotic observationsof dust storm enhancement of carbon biomass in the North Pacific,Science, 298, 817–821.

Blain, S., et al. (2001), A biogeochemical study of the island mass effect inthe context of the iron hypothesis: Kerguelen Islands, Southern Ocean,Deep Sea Res., Part I, 48, 163–187.

Blezard, R. H. (1980), Calculated sea area of the New Zealand 200 nauticalmile Exclusive Economic Zone, N. Z. J. Mar. Freshwater Res., 14, 137–138.

Bowie, A. R., M. T. Maldonado, R. D. Frew, P. L. Croot, E. P. Achterberg,R. F. C. Mantoura, P. J. Worsfold, C. S. Law, and P. W. Boyd (2001), Thefate of added iron during a mesoscale fertilisation in the Southern Ocean,Deep Sea Res., Part II, 48, 2703–2744.

Boyd, P. W. (2002), Environmental factors controlling phytoplankton pro-cesses in the Southern Ocean, J. Phycol., 38, 844–861.

Boyd, P. W., and E. R. Abraham (2001), Iron-mediated changes in phyto-plankton photosynthetic competence during SOIREE, Deep Sea Res.,Part II, 48, 2529–2550.

Boyd, P. W., C. S. Wong, J. Merrill, F. Whitney, J. Snow, P. J. Harrison, andJ. Gower (1998), Atmospheric iron supply and enhance vertical carbonflux in the northeast subarctic Pacific: Is there a connection?, GlobalBiogeochem. Cycles, 12, 429–441.

Boyd, P. W., J. LaRoche, M. Gall, R. Frew, and R. M. L. McKay (1999),The role of iron, light and silicate in controlling algal biomass in sub-Antarctic water southeast of New Zealand, J. Geophys. Res., 104,13,395–13,408.

Boyd, P. W., et al. (2000), A mesoscale phytoplankton bloom in the polarSouthern Ocean stimulated by iron fertilisation of waters, Nature, 407,695–702.

Boyd, P. W., A. C. Crossley, G. R. DiTullio, F. B. Griffiths, D. A. Hutchins,B. Queguiner, P. N. Sedwick, and T. W. Trull (2001), Effects of ironsupply and irradiance on phytoplankton processes in subantarctic waterssouth of Australia, J. Geophys. Res., 106, 31,573–31,584.

Bradford-Grieve, J. M., P. W. Boyd, F. H. Chang, S. Chiswell, M. Hadfield,J. A. Hall, M. R. James, S. D. Nodder, and E. A. Shushkina (1999),Pelagic ecosystem structure and functioning in the Subtropical Frontregion east of New Zealand in austral winter and spring 1993, J. PlanktonRes., 21, 405–428.

Campbell, J. W., J. M. Blaisdell, and M. Darzi (1995), Level-3 SeaWiFSdata products: Spatial and temporal binning algorithms, NASA Tech.Memo., 104566, 69 pp.

Chavez, F. P., P. G. Strutton, G. E. Friederich, R. A. Feely, G. C. Feldman,D. G. Foley, and M. J. McPhaden (1999), Biological and chemical

GB1029 BOYD ET AL.: IRON SUPPLY TO SUBANTARCTIC WATERS

21 of 23

GB1029

response of the equatorial Pacific Ocean to the 1997–98 El Nino,Science, 286, 2126–2131.

Chepil, W. S., and N. P. Woodruff (1957), Sedimentary characteristics ofDust Storms: II. Visibility and dust concentration, Am. J. Sci., 255, 104–114.

Chiswell, S. M. (1996), Variability in the Southland Current, New Zealand,N. Z. J. Freshwater Mar. Res., 30, 1–17.

Chomko, R. M., and H. R. Gordon (2001), Atmospheric correction of oceancolor imagery: Test of the spectral optimization algorithm with the Sea-viewing Wide Field-of-View Sensor, Appl. Opt., 40(18), 2973–2984.

Coale, K. H., et al. (1996a), A massive phytoplankton bloom induced by anecosystem-scale iron fertilization experiment in the equatorial PacificOcean, Nature, 383, 495–501.

Coale, K. H., S. E. Fitzwater, R. M. Gordon, K. S. Johnson, and R. T.Barber (1996b), Control of community growth and export production byupwelled iron in the equatorial Pacific Ocean, Nature, 379, 621–624.

Comiso, J. C., C. R. McClain, C. W. Sullivan, J. P. Ryan, and C. L. Leonard(1993), Coastal Zone Color Scanner pigment concentrations in the South-ern Ocean and relationships to geophysical features, J. Geophys. Res., 98,2419–2451.

Croot, P. L., and K. A. Hunter (1998), Trace metal distributions across thecontinental shelf near Otago Peninsula, New Zealand, Mar. Chem., 62,185–201.

de Baar, H. J. W., and P. W. Boyd (2000), The role of iron in planktonecology and carbon dioxide transfer of the global oceans, in The DynamicOcean Carbon Cycle: A Midterm Synthesis of the Joint Global OceanFlux Study, edited by R. B. Hanson, H. W. Ducklow, and J. G. Field,chap. 4, pp. 61–140, Cambridge Univ. Press, New York.

de Baar, H. J., and J. T. de Jong (2001), Distributions, sources and sinks ofiron in seawater, in The Biogeochemistry of Iron in Seawater, edited byD. Turner and K. A. Hunter, pp. 123–254, John Wiley, Hoboken, N. J.

de Baar, H. J., J. T. de Jong, D. C. Bakker, B. M. Loscher, C. Veth,U. Bathmann, and V. Smetacek (1995), Importance of iron for planktonblooms and carbon dioxide drawdown in the Southern Ocean,Nature, 373,412–415.

DiTullio, G. R., and E. A. Laws (1991), Impact of an atmospheric-oceanicdisturbance on phytoplankton community dynamics in the North PacificCentral Gyre, Deep Sea Res., 38, 1305–1329.

Draxler, R. R., and G. D. Hess (1997), Description of the HYPLIT_4modelling system, NOAA Tech. Memo. ERL ARL-224, 24 pp., Natl.Oceanic and Atmos. Admin., Silver Spring, Md.

Draxler, R. R., and G. D. Hess (1998), An overview of the HYPLIT_4modeling system for trajectories, dispersion and deposition, Aust.Meteorol. Mag., 47, 295–308.

Duce, R. A., and N. W. Tindale (1991), Chemistry and biology of iron andother trace metals, Limnol. Oceanogr., 36, 1715–1726.

Duce, R. A., et al. (1991), The atmospheric input of trace species to theworld ocean, Global Biogeochem. Cycles, 5, 193–259.

Foley, D. G., T. D. Dickey, M. J. McPhaden, R. R. Bidigare, M. R. Lewis,R. T. Barber, S. T. Lindley, C. Garside, D. V. Manov, and J. D. McNeill(1997), Longwaves and primary productivity variations in the equatorialPacific at 0�, 140�W, Deep Sea Res., Part II, 44, 1801–1826.

Fung, I., S. K. Meyn, I. Tegen, S. C. Doney, J. G. John, and K. B. Bishop(2000), Iron supply and demand in the upper ocean, Global Biogeochem.Cycles, 14, 281–295.

Gabric, A. J., R. Cropp, G. P. Ayers, G. McTainsh, and R. Braddock (2002),Coupling between cycles of phytoplankton biomass and aerosol opticaldepth as derived from SeaWiFS time series in the Subantarctic SouthernOcean, Geophys. Res. Lett., 29(7), 1112, doi:10.1029/2001GL013545.

Gall, M. P., P. W. Boyd, J. A. Hall, K. A. Safi, and H. Chang (2001),Phytoplankton processes: 1. Community structure during the SouthernOcean Iron RElease Experiment (SOIREE), Deep Sea Res., Part II, 48,2551–2570.

Gatehouse, R. D. (2002), The significance of dust deposition in Quaternarylandscape evolution in southeastern Australia, Ph.D. dissertation, 339 pp.,Res. School of Earth Sci., Aust. Natl. Univ., Canberra ACT, Australia.

Gorden, N. D. (1986), Computer-derived air trajectories, N. Z. Meteorol.Ser. Sci. Rep., 22, 42 pp.

Gordon, R. M., K. S. Johnson, and K. H. Coale (1998), The behaviour ofiron and other Trace elements during the IronEx-I and PlumEx experi-ments in the equatorial Pacific, Deep Sea Res., Part II, 45, 995–1041.

Greig, M. J., and A. E. Gilmour (1992), Flow through the Mernoo Saddle,New Zealand, N. Z. J. Freshwater Mar. Res., 34, 103–116.

Hadfield, M. G., and J. Sharples (1996), Modelling mixed layer depth andplankton biomass off the west coast of South Island, New Zealand, J. Mar.Syst., 8, 1–29.

Heath, R. A. (1975), Ocean circulation and hydrology off the Southern halfof South Island, New Zealand, N. Z. Oceanogr. Inst. Mem., 72, 1–36.

Herman, J. R., P. K. Bhartia, O. Torres, C. Hsu, C. Seftor, and E. Celarier(1997), Global distribution of UV-absorbing aerosols from Nimbus7/TOMS data, J. Geophys. Res., 102, 16,911–16,922.

Holben, B. N., et al. (2001), An emerging ground-based aerosol climatol-ogy: Aerosol optical depth from AERONET, J. Geophys. Res., 106,12,067–12,097.

Hooker, S. B., W. E. Esaias, G. C. Feldman, W. W. Gregg, and C. R.McClain (1992), An Overview of SeaWiFS and Ocean Colour, NASATech. Memo., 104566, 24 pp.

Hunter, K. A., et al. (2001), Summary and recommendations, in The Bio-geochemistry of Iron in Seawater, edited by D. Turner and K. A. Hunter,pp. 373–388, John Wiley, Hoboken, N. J.

Husar, R. B., J. M. Prospero, and L. L. Stowe (1997), Characterization oftropospheric aerosols over the oceans with the NOAA advanced veryhigh resolution radiometer optical thickness operational product, J. Geo-phys. Res., 102, 16,889–16,909.

Husar, R. B., et al. (1998), Asian dust events of April 1998, J. Geophys.Res, 106, 18,317–18,330.

Hutchins, D. A., and K. W. Bruland (1998), Iron limited diatom growthand Si: N uptake ratios in a coastal upwelling regime, Nature, 393,561–564.

Hutchins, D. A., G. R. DiTullio, P. W. Boyd, A. C. Crossley, F. B. Griffiths,B. Queguiner, T. Trull, and P. Sedwick (2001), Effects of iron and majornutrient availability on phytoplankton growth in the Australian sector ofthe Subantarctic Southern Ocean, J. Geophys. Res., 106, 31,559–31,572.

Hutchins, D. A., et al. (2002), Phytoplankton iron limitation in the Hum-boldt Current and Peru Upwelling, Limnol. Oceanogr., 47, 997–1011.

Jickells, T. D. (1999), Atmospheric inputs of metals and nutrients to theoceans: Their magnitude and effect, Mar. Chem., 48, 199–214.

Jickells, T. D., and L. J. Spokes (2001), Atmospheric iron inputs to theoceans, in The Biogeochemistry of Iron in Seawater, edited by D. Turnerand K. A. Hunter, pp. 85–122, John Wiley, Hoboken, N. J.

Johnson, K. S., F. P. Chavez, and G. E. Friedrich (1999), Continental-shelfsediment as a primary source of iron for coastal phytoplankton, Nature,398, 697–700.

Johnson, K. S., et al. (2003), Continental-shelf sediment as a primary sourceof iron for coastal phytoplankton, Global Biogeochem. Cycles, 17(2),1063, doi:10.1029/2002GB002004.

Kawamura, H., and the OCTS Team (1998), OCTS mission overview,J. Oceanogr., 54, 383–399.

Kidson, E. (1930), Australian origin of red rain in New Zealand, Nature,125, 410.

Knight, A. W., G. H. McTainsh, and R. W. Simpson (1996), Sediment loadsin an Australian dust storm: Implications for present and past dust pro-cesses, Catena, 24, 195–213.

LaRoche, J., P. W. Boyd, R. M. L. McKay, and R. J. Geider (1996),Flavodoxin as an in situ marker for iron stress in phytoplankton, Nature,382, 802–805.

Law, C. S., E. R. Abraham, A. J. Watson, and M. I. Liddicoat (2003),Vertical eddy diffusion and nutrient supply to the surface mixed layerof the Antarctic Circumpolar Current, J. Geophys. Res., 108(C8), 3272,doi:10.1029/2002JC001604.

Legendre, L., and F. Rassoulzadegan (1995), Plankton and nutrientdynamics in marine waters, Ophelia, 41, 153–172.

Maldonado, M. T., P. W. Boyd, J. LaRoche, R. Strzepek, A. Waite, A. R.Bowie, P. L. Croot, R. D. Frew, and N. M. Price (2001), Iron uptake andphysiological response of phytoplankton during a mesoscale SouthernOcean iron enrichment, Limnol. Oceanogr., 46, 1802–1808.

Martin, J. H., R. M. Gordon, S. E. Fitzwater, and W. W. Broenkow (1989),VERTEX: Phytoplankton/iron studies in the Gulf of Alaska, Deep SeaRes., 36, 649–680.

McGowan, H. A., G. H. McTainsh, and P. Zawar-Reza (2000), Identifyingregional dust transport pathways: Application of kinematic trajectorymodelling to a trans-Tasman case, Earth Surf. Processes Landforms,25,633–25,647.

McTainsh, G. H. (1998), Dust storm index, in Sustainable Agriculture:Assessing Australia’s Recent Performance, A Report of the National Col-laborative Project on Indicators for Sustainable Agriculture, SCARMTech. Rep. 70, pp. 65–72, Standing Comm. on Agric. and Resour. Man-age., Canberra, ACT, Australia.

McTainsh, G. H., and J. F. Leys (1993), Wind erosion, in Land DegradationProcesses In Australia, chap. 7, pp. 188–233, edited by G. H. McTainshand W. C. Boughton, Longman-Cheshire, Melbourne, Victoria, Australia.

Measures, C. I., and S. Vink (2001), Dissolved Fe in the upper waters of thePacific sector of the Southern Ocean, Deep Sea Res., Part II, 48, 3913–3942.

Meder, R., K. A. Hunter, and B. M. Peake (1991), Trans-Tasman tropo-spheric transport of acid rain, J. R. Soc. N. Z., 21, 137–142.

GB1029 BOYD ET AL.: IRON SUPPLY TO SUBANTARCTIC WATERS

22 of 23

GB1029

Merrill, J. T. (1989), Atmospheric long-range transport to the PacificOcean, in Chemical Oceanogr., vol. 10, edited by J. P. Riley, R. Chester,and R. A. Duce, pp. 15–50, Academic, San Diego, Calif.

Moore, J. K., and M. R. Abbott (2002), Surface chlorophyll concentrationsin relation to the Antarctic Polar Front: Seasonal and spatial patterns fromsatellite observations, J. Mar. Syst., 37, 69–86.

Moore, J. K., M. R. Abbott, J. G. Richman, W. O. Smith, T. J. Cowles, K. H.Coale, W. D. Gardner, and R. T. Barber (1999), SeaWiFS satellite oceancolor data from the Southern Ocean, Geophys. Res. Lett., 26, 1465–1468.

Moore, J. K., S. C. Doney, D. M. Glover, and I. Y. Fung (2002), Ironcycling and nutrient limitation patterns in surface waters of the WorldOcean, Deep Sea Res., Part II, 49, 463–507.

Morris, M., B. Stanton, and H. Neil (2001), Subantarctic oceanographyaround New Zealand: Preliminary results from an ongoing survey, N. Z.J. Mar. Freshwater Res., 35, 499–519.

Murphy, R. J., M. H. Pinkerton, K. M. Richardson, J. Grieve, and P. W.Boyd (2001), Phytoplankton distributions around New Zealand derivedfrom SeaWiFS data, N. Z. J. Mar. Freshwater Res., 35, 343–362.

Nodder, S. D., M. Charette, A. Waite, P. W. Boyd, T. Trull, J. Zeldis, andK. Buesseler (2001), Particle transformations and export flux during an insitu iron-stimulated algal bloom in the Southern Ocean, Geophys. Res.Lett., 28, 2409–2412.

Orsi, A. H., T. W. Whitworth III, and W. D. Nowlin Jr. (1995), On themeridional extent and fronts of the Antarctic Circumpolar Current, DeepSea Res., Part I, 42, 641–673.

Price, J. F., R. A. Weller, and R. Pinkel (1986), Diurnal cycling: Observa-tions and models of the upper ocean response to diurnal heating, coolingand wind mixing, J. Geophys. Res., 91, 8411–8427.

Price, N. M., and F. M. M. Morel (1998), Biological cycling of iron in theocean, in Metal Ions in Biological Systems, vol. 35, Iron Transport andStorage in Micro-organisms, Plants and Animals, pp. 1–36, MarcelDekker, New York.

Prospero, J. M., M. Uematsu, and D. L. Savoie (1989), Mineral aerosoltransport to the Pacific Ocean, in Chemical Oceanography, vol. 10, editedby J. P. Riley, R. Chester, and R. A. Duce, pp. 188–219, Academic, SanDiego, Calif.

Raupach, M. R., G. H. McTainsh, and J. F. Leys (1994), Estimates of dustmass in some recent major Australian dust storms, Aust. J. Soil WaterConserv., 7(3), 20–24.

Richardson, K. M., M. Pinkerton, P. W. Boyd, M. P. Gall, J. Zeldis, M. D.Oliver, and R. J. Murphy (2003), Validation of SeaWiFS data fromaround New Zealand, Adv. Space Res., in press.

Saitoh, S. (1995), OCTS on ADEOS, in Oceanographic Applications ofRemoteSensing, edited by M. Ikeda and F. W. Dobson, pp. 473–480,CRC, Boca Raton, Fla.

Sedwick, P. N., P. R. Edwards, D. J. Mackey, F. B. Griffiths, and J. S.Parslow (1997), Iron and manganese in surface waters of the AustralianSubantarctic region, Deep Sea Res., Part I, 44, 1239–1253.

Sedwick, P. N., G. R. DiTullio, D. A. Hutchins, P. W. Boyd, F. B. Griffiths,A. C. Crossley, T. W. Trull, and B. Queguiner (1999), Limitation of algalgrowth by iron deficiency in the Australian Subantarctic region, Geophys.Res. Lett., 26, 2865–2868.

Stegmann, P. M., and N. W. Tindale (1999), Global distribution of aerosolsover the open ocean as derived from the coastal zone color scanner,Global Biogeochem. Cycles, 13, 383–397.

Strom, S. L., C. B. Miller, and B. W. Frost (2000), What sets lower limits tophytoplankton stocks in high-nitrate, low-chlorophyll regions of the openocean?, Mar. Ecol. Prog. Ser., 193, 13–31.

Sturman, A. P., P. D. Dyson, and P. C. D’Abreton (1997), A preliminarystudy of the transport of air from Africa and Australia to New Zealand,J. R. Soc. N. Z., 27, 485–498.

Sunda,W. G., and S. A. Huntsman (1997), Interrelated influence of iron, lightand cell size on marine phytoplankton growth, Nature, 390, 389–392.

Sutton, P. (2001), Detailed structure of the Subtropical Front over ChathamRise, east of New Zealand, J. Geophys. Res., 106, 31,045–31,056.

Thiede, J. (1979), Wind regimes over the late Quaternary southwest PacificOcean, Geology, 7, 259–262.

Trenberth, K. E., W. G. Large, and J. G. Olson (1990), The mean annualcycle in the global ocean wind stress, J. Phys. Oceanogr., 20, 1742–1760.

Turekian, K. K., W. C. Graustein, and J. K. Cochran (1989), Lead-210 inthe SEAREX Program: An aerosol tracer across the Pacific, in ChemicalOceanography, vol. 10, edited by J. P. Riley, R. Chester, and R. A. Duce,pp. 51–83, Academic, San Diego, Calif.

Turner, D., and N. J. P. Owens (1995), A biogeochemical study in theBellingshausen Sea: Overview of the STERNA 1992 expedition, DeepSea Res., Part II, 42, 907–932.

Uddstrom, M. J., and N. A. Oien (1999), On the use of high-resolutionsatellite data to describe the spatial and temporal variability of sea surfacetemperatures in the New Zealand region, J. Geophys. Res., 104, 20,729–20,751.

Wang, W., S. Bailey, and C. R. McClain (2000), SeaWiFS provides uniqueglobal aerosol optical property data, Eos Trans. AGU, 81(18), 197.

Whitney, F., and M. Robert (2002), Structure of Haida eddies and theirtransport of nutrients from coastal margins into the NE Pacific Ocean,J. Phys. Oceanogr., 58, 717–732.

�������������������������P. W. Boyd, National Institute of Water and Atmosphere Centre for

Chemical and Physical Oceanography, Department of Chemistry, Uni-versity of Otago, Dunedin 9001, New Zealand. (pboyd@alkali.otgao.ac.nz)M. Ellwood, National Institute of Water and Atmosphere, Gate 10,

Silverdale Road, Hillcrest, Hamilton, New Zealand. (m.ellwood@niwa.co.nz)R. Frew, Department of Chemistry, University of Otago, Dunedin 9001,

New Zealand. (rfrew@alkali.otgao.ac.nz)G. McTainsh, Faculty of Environmental Sciences, Griffith University,

Nathan Campus, Queensland 4111, Australia. (g.mctainsh@griffith.edu.au)S. Nichol, K. Richardson, and V. Sherlock, National Institute of Water

and Atmosphere, 301 Evans Bay Parade, Greta Point, Wellington, NewZealand. (s.nichol@niwa.co.nz; k.richardson@niwa.co.nz; v.sherlock@niwa.co.nz)

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