hutchins, d. a., et al. phytoplankton iron limitation …kbruland/manuscripts/hutchins/hutch...997...

997

Limnol. Oceanogr., 47(4), 2002, 997–1011q 2002, by the American Society of Limnology and Oceanography, Inc.

Phytoplankton iron limitation in the Humboldt Current and Peru Upwelling

D. A. Hutchins,1 C. E. Hare, R. S. Weaver, Y. Zhang, and G. F. FirmeCollege of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, Delaware 19958

G. R. DiTullio, M. B. Alm, S. F. Riseman, J. M. Maucher, and M. E. GeeseyGrice Marine Laboratory, University of Charleston, Charleston, South Carolina 29412

C. G. TrickDepartment of Plant Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada

G. J. Smith, E. L. Rue, J. Conn, and K. W. BrulandInstitute of Marine Sciences, University of California, Santa Cruz, Santa Cruz, California 95064

Abstract

We investigated phytoplankton Fe limitation using shipboard incubation experiments in the high-nutrient SouthAmerican eastern boundary current regime. Low ambient Fe concentrations (;0.1 nM) in water collected from theHumboldt and Peru Currents were supplemented with a range of added Fe levels up to 2.5 nM. Phytoplanktonchlorophyll a, photosystem II photosynthetic efficiency, and nitrate and phosphate drawdown increased in proportionto the amount of Fe added. The Humboldt Current algal community after Fe additions included colonial andflagellated Phaeocystis globosa and large pennate diatoms, whereas the Peru Upwelling assemblage was dominatedby coccolithophorids and small pennate diatoms. Apparent half-saturation constants for growth of the two com-munities were 0.17 nM Fe (Humboldt Current) and 0.26 nM Fe (Peru Upwelling). Net molar dissolved Si(OH)4 :NO drawdown ratios were low in both experiments (;0.2–0.7), but net particulate silica to nitrogen production2

3

ratios were higher. Fe limitation decreased net NO : PO utilization ratios in the Humboldt Current incubation to2 323 4

well below Redfield values. Phytoplankton community sinking rates were decreased by Fe additions in the PeruUpwelling, suggesting potential Fe effects on carbon export. Our results confirm that primary producers in the PeruUpwelling/Humboldt Current system can be limited by Fe, but the biological and biogeochemical consequences ofFe limitation differ from Fe-limited California coastal upwelling waters that have been previously studied.

The first evidence for phytoplankton Fe limitation wasfrom shipboard growout experiments in the subarctic Pacific,the equatorial Pacific, and the Southern Ocean (de Baar etal. 1990; Martin et al. 1991). These results were later con-firmed by mesoscale open ocean Fe fertilization experimentsin two of these oceanic high-nutrient, low-chlorophyll(HNLC) areas (Martin et al. 1994; Coale et al. 1996a; Boydet al. 2000). The isolation of these regions from terrestrialFe sources and the scarcity of Fe relative to major nutrientsin upwelled water play a key role in maintaining the HNLCcondition. The biogeochemical effects of Fe limitation are,however, not limited only to these three HNLC regimes. Feavailability limits growth or photosynthetic efficiency of di-atoms (DiTullio et al. 1993), as well as cyanobacteria suchas Trichodesmium (Rueter et al. 1992) and Synechococcus(Behrenfeld and Falkowski 1999) in the oligotrophic centralgyres.

The most recent additions to the list of Fe-limited regimesare coastal upwelling areas. These small areas along the

1 Corresponding author ([email protected]).

AcknowledgmentsWe thank the captain and crew of the R/V Melville and the gov-

ernments of Ecuador and Peru. Support was provided by NSF OCE9811062 (D.A.H.), OCE 9811114 and CEBIC (K.W.B.), OCE9907931 (G.R.D.), and CEBIC (C.G.T.).

western coasts of the Americas and Africa cover only ;0.5–1% of the ocean’s surface, but support about 11% of totalglobal new production, totaling 0.8 Pg C yr21. This newproduction is more than that estimated for all of the sub-tropical gyres combined (0.5 Pg C yr21) and is equivalent toabout 73% of the annual new production over the entireSouthern Ocean (1.1 Pg C yr21, Chavez and Toggweiler1995). The importance of coastal upwelling areas in the car-bon cycle is also shown by estimates that organic carbonburial rates are an order of magnitude higher (0.3–0.6 3 108

tons yr21) than in all oceanic upwelling areas combined(0.03–0.09 3 108 tons yr21, Brink et al. 1995). Because oftheir short, productive food chains and ease of accessibility,they also provide much of the biological resources that hu-mans harvest from the sea (Ryther 1969).

In parts of the California coastal upwelling system, Feaddition allows nitrate drawdown and produces blooms oflarge, heavily silicified chain-forming diatoms such as Chae-toceros (Hutchins et al. 1998; Bruland et al. 2001). Fe avail-ability also regulates major nutrient biogeochemistry in thisregion. Silicic acid to nitrate (Si : N) molar utilization ratiosin low-Fe coastal California waters are 2–3, but fall to anutrient-replete diatom ratio of ;1 (Brzezinski 1985) whenFe is supplied (Hutchins and Bruland 1998; Firme et al. inpress). There are only minimal riverine inputs along most ofthe California coast during the summer upwelling season, so

998 Hutchins et al.

Fig. 1. Stations where near-surface water was collected for theHumboldt Current and Peru Upwelling Fe addition experiments,with bathymetry shown to illustrate proximity to shelf sources ofFe.

the primary source of Fe to the euphotic zone is the inter-action of upwelled water with shallow shelf sediments (John-son et al. 1999; Bruland et al. 2001). The continental shelfalong much of California is very narrow, and it is in theseareas that phytoplankton Fe limitation is most severe.

Fe limitation is likely to occur in other coastal regimes aswell when high upwelled major nutrient fluxes are combinedwith low Fe inputs from rivers and continental shelf sedi-ments. In particular, the world’s largest and most productivecoastal upwelling system off the coast of Peru and Chilemeets these criteria. This area is bordered by the AtacamaDesert, so riverine inputs are negligible. Like California, thecontinental shelf along much of Peru is very narrow, whichshould greatly reduce the supply of sedimentary Fe to phy-toplankton communities. This hypothesis is indirectly sup-ported by prior observations of conditions off western SouthAmerica, such as sporadic HNLC conditions and silicic aciddepletion relative to nitrate (Minas and Minas 1992; Dugdaleet al. 1995), which we now know to be diagnostic of Felimitation in California waters.

We report here on two shipboard Fe addition experiments,one carried out in the Peru Current upwelling region and onein the offshore northern Humboldt (Peru/Chile) Current, atransitional zone between the coastal upwelling and the oce-anic equatorial Pacific HNLC area. We examined the effectsof adding a range of Fe concentrations on nutrient biogeo-chemistry, phytoplankton growth, and taxonomic composi-tion. The results of these experiments allowed us to comparethe biological and biogeochemical consequences of Fe lim-itation off Peru with the results of previous investigations inthe California coastal HNLC area.

Methods

Water collections and shipboard incubations—ShipboardFe addition experiments began with the collection of phy-toplankton communities on 7 September 2000 from theHumboldt Current (2817.59S, 87830.79W) and on 15 Septem-ber 2000 from northern Peruvian coastal waters (9839.09S,81823.39W) (Fig. 1). Sea surface temperatures and salinitiesat these locations were 20.28C and 34.53 (Humboldt Current)and 18.38C and 35.13 (Peru Current), respectively. Waterwas collected from 8–10 m deep using a trace metal–cleanTeflon pump system and was dispensed into acid-washed2.7-liter polycarbonate bottles for 4- or 5-day deckboard in-cubations at ;40% of ambient light level, as described inHutchins et al. (1998). Experiments used triplicate unamend-ed controls and triplicate bottles supplemented with Fe ad-ditions (FeCl3 in 0.01 N HCl) of 0.1, 0.5, and 2.5 nM (Hum-boldt Current) and 0.2, 0.4, 0.8, and 2.5 nM (PeruUpwelling).

Dissolved and particulate nutrients and Fe—Dissolvednutrients (nitrate 1 nitrite, phosphate, and silicic acid) wereanalyzed daily at sea on 0.2-mm filtered samples using aLachat Quick Chem autoanalyzer. Particulate organic carbon(POC) and nitrogen (PON) were measured in the lab usingCHN analyses of GF/F-filtered samples (Hutchins et al.1998), and biogenic silica (BSi) analyses were performed on0.6-mm filtered samples according to Brzezinski and Nelson

(1995). Electrochemical total dissolved Fe measurementswere made at sea using cathodic stripping voltammetry on0.2-mm filtered, ultraviolet (UV)-oxidized seawater samples(Rue and Bruland 1997). Total dissolved Fe concentrationsat both stations were extremely low at ;0.1 nM (HumboldtCurrent) and ;0.08 nM (Peru Upwelling).

Phytoplankton parameters—In all incubation experi-ments, daily fast repetition rate fluorometry analyses (FRRF,Behrenfeld and Falkowski 1999) were used to estimate Feeffects on photosynthetic efficiency (Fv/Fm) and photosystemII cross-sectional areas (sPSII ). Samples were dark-adaptedfor 30 min and analyses were performed at the same timeof day to prevent diel periodicity bias. Chlorophyll a (Chla) size distributions were monitored daily using WhatmanGF/F (total Chl a) and 5.0-mm polycarbonate filters (.5.0mm) in acetone extracts using the nonacidified fluorometrictechnique (Welschmeyer 1994). Live flow cytometry sam-ples were analyzed within 1 h of sampling using a BectonDickinson FACSCalibur flow cytometer (15 mW argon ionlaser, 488 nm excitation) to count large, strongly red auto-fluorescent phytoplankton (e.g., diatoms), smaller red auto-fluorescent nanoplankton (e.g., eukaryotic flagellates), andorange and red autofluorescent picophytoplankton (e.g., Sy-nechococcus and Prochlorococcus, respectively) (Durandand Olson 1996). Epifluorescence microscopy cell countswere carried out on final samples from the experiments using50-ml glutaraldehyde-preserved samples (Hutchins et al.1998, 2001). Phytoplankton abundance in initial sampleswas found to be too low to obtain reliable visual counts. Inthe Peru Upwelling study only, we used the SETCOL meth-od (Bienfang 1981) to measure phytoplankton communitysinking rates at the end of the 5-d incubations.

999Phytoplankton Fe limitation off Peru

Fig. 2. Photosynthetic parameters in the 4-d Humboldt Currentincubations measured with fast repetition rate fluorometry (FRRF).(A) Photosynthetic efficiency assessed as variable over maximumfluorescence, Fv/Fm. (B) The cross-sectional area of photosystem II,sPSII. Symbols and error bars represent the means and standard de-viations of measurements on triplicate bottles. Controls, closed cir-cles; 0.1-nM Fe additions, open circles; 0.5-nM Fe additions, closedtriangles; 2.5-nM Fe additions, open triangles.

Fig. 3. Fluorometrically determined Chl a concentrations in theHumboldt Current incubations. (A) Total Chl a; (B) .5 mm Chl a.Symbols and error bars are as in Fig. 2.

Taxon-specific phytoplankton accessory pigments weremeasured using high-performance liquid chromatography(HPLC) to estimate the abundance of major phytoplanktongroups (DiTullio et al. 1993, in press). Because of the largesample volume required (1–2 liters), pigments were mea-sured only in the initial and final incubation samples. Sam-ples were filtered (Whatman GF/F) and frozen in liquid ni-trogen until analysis. Pigment concentration data wereanalyzed using the ChemTax program (Mackey et al. 1996)to estimate the abundance of specific taxonomic groups ofalgae relative to the total Chl a present. Details of the initialpigment ratios used in the ChemTax analysis will be pre-sented elsewhere.

Results

Humboldt Current—Phytoplankton response: The re-sponse of phytoplankton photosynthetic efficiency (Fv/Fm)was directly related to the amount of added Fe (Fig. 2A).

Fv/Fm in initial samples was low (0.17) but doubled by thefirst day in both 0.5- and 2.5-nM treatments and continuedto increase to a maximum of 0.47 on day 2 in the 2.5-nMbottles. Day 1 values in the control and 0.1-nM Fe additionwere 0.21 and 0.25, respectively, and Fv/Fm in these twotreatments was low throughout the incubation. Fv/Fm in thetwo highest Fe addition treatments decreased on days 3 and4, perhaps indicating the onset of nutrient or Fe limitationresulting from increased biomass.

The functional sPSII (Falkowski and Kolber 1995) showedan inverse dependence on added Fe levels and was nearly amirror image of the pattern observed for Fv/Fm (Fig. 2B).sPSII decreased rapidly over the first 2 d at the two highestFe levels then increased again as Fv/Fm decreased at the endof these incubations. In contrast, sPSII was relatively constantin the control and 0.1-nM Fe addition bottles for the first 3d of the experiment, followed by a small increase on day 4.

Fluorometrically determined, size-fractionated Chl ashowed a strong dependence on the amount of added Fe(Fig. 3). Although total Chl a in the controls approximatelydoubled over the 4-d incubation (Fig. 3A), total Chl a in-creases in the 0.1-nM Fe addition were larger. Incrementally

1000 Hutchins et al.

Tabl

e1.

Taxo

n-sp

ecifi

cph

ytop

lank

ton

pigm

ent

and

phae

opig

men

tco

ncen

trat

ions

(ng

L2

1 )at

the

init

ial

tim

epoi

ntan

dat

the

end

ofth

ein

cuba

tion

s.V

alue

sar

eth

em

eans

and

erro

rsar

eth

est

anda

rdde

viat

ions

ofm

easu

rem

ents

from

dupl

icat

e(H

umbo

ldt

Cur

rent

)or

trip

lica

te(P

eru

Upw

elli

ng)

bott

les.

Hum

bold

tC

urre

ntP

eru

Upw

elli

ng

Init

ial

Con

trol

fina

l

Fe

addi

tion

(nM

),fi

nal

0.1

0.5

2.5

Init

ial

Con

trol

fina

l

Fe

addi

tion

(nM

),fi

nal

0.2

0.4

0.8

2.5

Div

inyl

Chl

a29

61

00

00

146

10

00

00

Zea

xant

hin

296

121

63

226

316

622

386

759

65

596

2196

69

136

617

140

632

165

635

Chl

c 31,

274

636

155

614

731

634

1,18

46

114

1,51

36

9930

66

2024

56

4953

86

7054

96

1557

56

100

599

639

Fuc

oxan

thin

836

734

86

1447

36

5361

36

2874

56

6315

76

739

26

61,

762

653

62,

240

659

02,

067

615

12,

060

628

619

-hex

131

63

334

611

234

631

548

46

5148

66

771

36

4734

06

6579

66

106

684

634

567

615

551

36

146

19-b

ut48

65

836

390

60

916

1793

67

656

315

36

4934

16

730

66

3322

06

2621

36

38C

hlb

976

537

63

536

075

625

526

895

61

386

971

611

856

1478

624

966

19A

llox

anth

in15

60

506

552

67

806

1981

610

946

2761

621

936

7118

06

5816

16

5218

26

80P

erdi

nin

116

100

00

030

621

00

036

631

496

20P

haeo

phyt

inb

66

015

26

923

86

3328

46

4322

56

1597

66

746

1412

86

1110

26

4210

36

2198

614

Pha

eoph

ytin

a54

62

124

611

152

624

189

620

264

632

137

63

163

634

801

624

496

26

5896

66

641,

028

610

3P

haeo

phor

bide

216

423

76

1333

86

7534

16

4318

06

5229

66

699

631

71,

068

610

498

36

267

940

613

21,

329

628

9greater increases were observed in the 0.5- and 2.5-nM Feaddition. The lag period before exponential growth occurredwas only 1 d, shorter than in many previous growout ex-periments from other regions (de Baar et al. 1990; Boyd etal. 1996; Hutchins et al. 2001). Initially, about a third of thetotal Chl a was in the large size class (.5 mm, Fig. 3B).Throughout the incubations, Chl a increases were dominatedby the large size class, which accounted for one-half tothree-quarters of the total Chl a at the final timepoint.

Initially, the water had low concentrations of all taxon-specific accessory pigments except Chl c3 (Table 1).ChemTax analysis (Table 2) indicated that the sample wasinitially dominated by haptophytes. A 199-hexanoyloxyfu-coxanthin (19-hex) to fucoxanthin ratio of 1.6 confirmed thedominance of Type 4 haptophytes such as Phaeocystis. Thesmall amount of fucoxanthin indicated that diatoms were aminor component of the initial sample, as were divinyl Chla–containing prochlorophytes. Somewhat larger concentra-tions of Chl b and alloxanthin indicated the presence of pra-sinophytes and cryptophytes, respectively. Concentrations ofzeaxanthin (primarily found in cyanobacteria) and peridinin(dinoflagellates) were low. Only a minor fraction of the totalChl a was present as chlorophyll degradation products(phaeophytins a and b and phaeophorbide, Table 1).

The increases in HPLC-determined Chl a concentrationsduring the incubations were similar to those observed byfluorometry. Concentrations of divinyl Chl a, Chl b, peri-dinin, and zeaxanthin decreased from the initial values (Ta-ble 1). Chl c3 decreased in the controls, but final concentra-tions in 1Fe bottles were 4–103 those in controls.Fucoxanthin increases were large in Fe-amended bottles andwere similar in magnitude to the increases seen in Chl a;199-butanoyloxyfucoxanthin (19-but) nearly doubled in all1Fe treatments and controls, and 19-hex increased to 2–43the initial concentrations, with the largest increases in thetwo highest Fe treatments. In the control and 0.1-nM Fetreatment, alloxanthin increased to more than triple the initialconcentration, and in the 0.5- and 2.5-nM Fe treatments itincreased .53. In general, there was a moderate increasein algae that contain Chl c3 and 19-hex (pelagophytes andhaptophytes) and a large increase in algae that contain fu-coxanthin only (diatoms) or alloxanthin (cryptophytes).

ChemTax pigment data analysis indicated that phytoplank-ton taxonomic composition in all treatments and controlsshifted from that initially observed. Although there were bio-mass differences between treatments, the relative algal com-position was similar between treatments (Table 2). Diatomabundance increased dramatically until they were codomi-nant with type 4 haptophytes in all treatments. Cryptophyteswere of tertiary importance in these treatments.

Type 3 haptophytes (e.g., Emiliania huxleyi), pelagophy-tes, prasinophytes, and cyanobacteria were of minor abun-dance in every treatment, never containing more than a smallfraction of the total Chl a (Table 2). Prochlorophytes anddinoflagellates disappeared from the incubations. As a per-centage of total Chl a, phaeopigments increased from theinitial value of 16% to 41–46% in the control and the twolower Fe addition treatments and to 25% in the high-Fe treat-ment (data not shown).

Flow cytometry data indicated that cyanobacteria (Syne-


Table 2. Phytoplankton assemblage composition based on ChemTax results at the initial timepoint and at the end of the incubations.Values give the total Chl a content (mg L21) in the sample and the percentage of total Chl a in each algal group in the sample. Forprochlorophytes, the value is for divinyl Chl a.

Humboldt Current Peru Upwelling

InitialControl

final

Fe addition (nM), final

0.1 0.5 2.5 InitialControl

final

Fe addition (nM), final

0.2 0.4 0.8 2.5

Total Chl a (mg L21) 0.5 1.2 1.6 2.0 2.7 1.1 1.1 3.9 4.7 4.5 4.8

Percentage of total Chl a in each algal groupDiatomsDinoflagellatesCryptophytesPelagophytesType 3 HaptophytesType 4 HaptophytesPrasinophytesChlorophytesCyanobacteriaProchlorophytes

83

161

102818

046

270

244

1028

3040

300

1923

393030

300

2312

383120

370

2127

273040

02

223

3817

60

101

300

181619

040

120

5509

1115

12060

580

131010

02070

571

129742070

562

138642080

chococcus and Prochlorococcus) were numerically dominantin the initial collected water, although their actual abundancewas relatively low (;3,800 cells ml21), and they did notcontain a major fraction of the total Chl a (Table 2). Cya-nobacteria peaked on day 2 of the experiments and thendeclined to slightly below initial levels, and Fe additions hadlittle or no apparent effect on their abundance (Fig. 4A). Theabundance of eukaryotic nanoplankton also showed no clearresponse to the Fe additions and remained relatively constant(400–600 cells ml21) throughout the incubation (Fig. 4B).After an initial lag period, larger eukaryotic phytoplanktonwere much more abundant at the end of the experiment inthe three Fe addition treatments than in the control (Fig. 4C).

Microscopic cell counts from the end of the incubationshow that cell numbers of all diatoms in the 1Fe incubationswere roughly 3–43 higher than in controls (Table 3). Countsin all treatments were dominated by small unicellular pen-nate diatoms, especially several species of Navicula andPseudo-nitzschia. However, numerically rarer but larger spe-cies of chain-forming pennate diatoms (Nitzschia spp.) likelycontributed a substantial share of total diatom biomass in theFe addition treatments. The abundance of large unicellularcentric diatoms (Coscinodiscaceae) increased dramaticallywith increasing Fe availability, especially in the 2.5-nM Feaddition (123 the controls). Likewise, the large diatom gen-era Guinardia and Rhizosolenia were only observed in thesamples amended with 0.5- and 2.5-nM Fe, and Chaetoceroswas seen only in the latter treatment.

Numbers of individual flagellated Phaeocystis globosacells showed no strong trend among treatments (Table 3).However 1Fe bottles were visibly dominated at the end ofthe incubation by numerous macroscopic P. globosa colo-nies, whereas few were observed in the controls. Colonieswere also occasionally observed in the preserved micros-copy samples, but only in the bottles amended with 0.5 or2.5 nM Fe. Because of the difficulty in quantifying cell num-bers in P. globosa colonies, colony counts are not presented.As a result, the P. globosa cell numbers reported undoubt-

edly represent an underestimate of the true contribution fromthis group. Dinoflagellates were present in low abundanceand were highly variable among replicates (see SDs, Table3), as were cyanobacteria. Surprisingly in light of the pig-ment and flow cytometry results, nanoflagellates (other thanP. globosa) were not observed in the microscopy samples,although preservation of small cells (including cyanobacte-ria) was relatively poor in this experiment, similar to otherstudies (e.g., Gieskes and Kraay 1983).

Nutrient biogeochemistry: Nitrate in control bottles de-clined by only 16% over the 4-d incubation (Fig. 5A; Table4). In contrast, nitrate was nearly depleted by the third dayin the highest Fe addition (2.5 nM), and by the fourth dayin the 0.5 nM Fe treatment. The lowest Fe addition (0.1 nM)used about two-thirds of the initial nitrate by the end of theexperiment.

Phosphate utilization was similar to that of nitrate, withamounts used ranging from one-quarter (controls) to two-thirds (2.5-nM Fe addition) of the initial concentration (Fig.5B; Table 4). Compared to nitrate and phosphate, Fe addi-tions had much less effect on silicic acid drawdown in thisexperiment (Fig. 5C; Table 4). The controls and 0.1-nM Fetreatments consumed about a third of the original 6.6 mMSi(OH)4, and the two higher Fe additions used about half.

Net community Si(OH)4 : NO utilization ratios in the23

Humboldt Current experiment were 2–33 higher in controlsthan in any of the Fe addition bottles (Table 4). Notablehowever were the low absolute dissolved Si : N drawdownratios in all incubations (0.71, controls; ;0.2–0.3, 1Fe bot-tles) in this regime in comparison to previous results fromCalifornia (2–3, controls; ;1, 1Fe treatments, Hutchins etal. 1998). Fe also appeared to have a major effect on NO :2

3

PO utilization ratios in this tropical Pacific experiment.324

Although final N : P utilization ratios were at or above Red-field values in all three 1Fe treatments (;16–20), Fe limi-tation in the controls resulted in a much lower value (;9).

POC and PON production increased with added Fe. Bio-


Fig. 4. Results of flow cytometry analyses of phytoplanktonabundance during the Humboldt Current experiment. (A) Cyanobac-teria. (B) Eukaryotic nanoplankton. (C) Large eukaryotic phytoplank-ton, chiefly diatoms. Symbols and error bars are as in Fig. 2.

Tabl

e3.

Mic

rosc

opic

cell

coun

ts(c

ells

ml2

1 )fr

omth

efi

nal

day

ofth

eH

umbo

ldt

Cur

rent

(day

5)an

dP

eru

Upw

elli

ng(d

ay4)

Fe

addi

tion

expe

rim

ents

.E

rror

sar

eth

est

anda

rdde

viat

ions

oftr

ipli

cate

bott

les.

no,

not

obse

rved

.

Hum

bold

tC

urre

ntP

eru

Upw

elli

ng

Con

trol

Fe

addi

tion

(nM

)

0.1

0.5

2.5

Con

trol

Fe

addi

tion

(nM

)

0.2

0.4

0.8

2.5

Tota

ldi

atom

s40

76

312

1,34

56

320

1,06

86

262

1,79

16

362

2,34

06

1,18

010

,660

67,

280

16,1

806

3,30

012

,220

61,

040

61,2

006

8,24

0P

enna

tes

400

631

21,

328

632

61,

040

625

61,

670

634

02,

340

61,

180

10,6

606

7,28

016

,180

63,

300

12,2

206

1,04

061

,200

68,

240

Cos

cino

disc

acea

76

717

66

186

983

610

nono

nono

noC

haet

ocer

osno

nono

246

2no

nono

nono

Gui

nard

iano

no8

68

86

8no

nono

nono

Rhi

zoso

leni

ano

no2

60

66

1no

nono

nono

Pha

eocy

stis

(uni

cell

ular

)64

66

270

144

649

748

655

254

06

388

3,84

06

270

10,7

406

2,88

08,

280

61,

870

3,96

06

3,22

010

,660

61,

230

Nan

oflag

ella

tes

nono

nono

7,88

06

330

16,0

806

2,48

09,

560

612

,550

16,3

406

6,48

017

,340

613

0D

inofl

agel

late

s28

637

216

537

616

166

16no

3,12

06

2,50

03,

000

62,

800

2,36

06

2,00

07,

180

69,

800

Cya

noba

cter

ia33

06

901,

348

662

691

66

804

1,08

66

1,88

219

,620

62,

900

25,2

006

10,3

6041

,000

66,

640

22,2

006

13,4

2039

,000

628

,600

genic silica (BSi) production, however, was approximatelyequal in all of the treatments (Table 4). Net BSi : POC andBSi : PON production ratios and nitrate : silicate drawdownratios suggested that the phytoplankton biomass produced incontrols was considerably more silicified than in the 1Febottles. Except for the 2.5-nM treatment, though, BSi : PONproduction ratios were substantially higher (1.1–2.23) thanthe values calculated from dissolved N and Si drawdown.

Peru Upwelling—Phytoplankton response: The responsesof Fv/Fm and sPSII (Fig. 6A,B) and fluorometrically deter-


Fig. 5. Major nutrient concentrations during the Humboldt Cur-rent experiment. (A) Nitrate. (B) Phosphate. (C) Silicic acid. Sym-bols and error bars are as in Fig. 2.

mined total Chl a (Fig. 7A) in the Peru experiment werebroadly similar to those observed in the Humboldt Current.However, the size distribution of community Chl a was quitedifferent. Relatively little of the Chl a biomass in the Peruincubation was .5 mm (Fig. 7B), ranging from 16% in theinitial samples to 16–29% in the day 4 samples (chosen forcomparability with the 4-d Humboldt Current incubation).Although large phytoplankton size class Chl a did increaseslightly in response to the added Fe in the Peru incubations,no major shift toward a large cell-dominated community was

seen, and most of the Chl a in all treatments throughout theexperiment was found in the ,5-mm size class.

Initial samples had moderate to high concentrations of thepigments Chl b, Chl c3, 19-but, fucoxanthin, 19-hex, allox-anthin, and zeaxanthin (all .50 ng L21, Table 1). The highconcentration of Chl c3 and a 19-hex : fucoxanthin ratio of4.5 indicated dominance by type 3 haptophytes, likely coc-colithophorids. Of secondary importance were algae con-taining Chl b (prasinophytes), alloxanthin (cryptophytes),and zeaxanthin (cyanobacteria). The results from theChemTax analysis confirm these observations (Table 2).Type 3 and type 4 haptophytes and cryptophytes were themost abundant members of this assemblage, and cyanobac-teria and prasinophytes were secondary components. Pro-chlorophytes, pelagophytes, dinoflagellates, chlorophytes,and diatoms were negligible or minor contributors to totalcommunity Chl a. Total phaeopigments averaged about one-quarter of the total Chl a in the initial samples.

Relative changes in HPLC-determined Chl a concentra-tions were similar to fluorometrically determined values (Ta-ble 2). Differences in absolute values are due to secondarilyfluorescing compounds such as other chlorophylls and chlo-rophyll degradation products (DiTullio and Geesey 2002).Chl a concentration was nearly unchanged in the controlbottles but increased to about 3–43 the initial concentrationin the Fe addition treatments.

In the control bottles, 19-but and fucoxanthin increased tomore than double the initial concentrations while all otherpigments decreased or remained the same (Table 1). The 19-hex decreased in the controls but remained fairly constant inthe Fe treatments. In 1Fe bottles, the greatest increases overinitial values were observed for 19-but (3–53) and fuco-xanthin (11–143). In the Fe addition treatments, Chl b con-centrations decreased slightly and Chl c3 concentrations in-creased slightly compared to the initial values. Alloxanthinand zeaxanthin increased by similar amounts (1–33). Peri-dinin increased slightly only in the two highest Fe additiontreatments. Phaeopigments increased in all bottles, especiallythe Fe addition treatments. As a percentage of the total Chla present, though, they were greater in the control than inthe four Fe additions.

The ChemTax analyses revealed a more diverse phyto-plankton assemblage in the control treatment than in the Feaddition treatments (Table 2). In the control bottles, diatomswere the most abundant group, accounting for slightly lessthan a third of the Chl a, followed by type 3 haptophytes,cryptophytes, pelagophytes, cyanobacteria, and prasino-phytes. In the Fe addition treatments, diatoms accounted forover half of the total Chl a, with correspondingly less con-tributions by the other algal groups. All of these treatments,however, are quite different from the initial phytoplanktonassemblage.

Flow cytometry results (Fig. 8) also suggested that theresponse to the added Fe was mostly by eukaryotic phyto-plankton, presumably diatoms. Cyanobacterial abundancepeaked during the middle of the incubation; although num-bers were highest in the 0.8-nM Fe addition, there was noclear stimulation by Fe (Fig. 8A). Eukaryotic nanoplanktonexhibited a similar trend (Fig. 8B). Larger eukaryotes weredetected as two groups by flow cytometry: a more abundant


Tabl

e4.

Dis

solv

ednu

trie

nt(N

O,S

iOH

4,P

O)

and

part

icul

ate

(PO

C,P

ON

,BS

i)co

ncen

trat

ions

from

the

init

ial

and

fina

lda

ysof

the

Hum

bold

tC

urre

ntan

dP

eru

Upw

elli

ng2

323

4

Fe

addi

tion

expe

rim

ents

and

net

mol

arpa

rtic

ulat

epr

oduc

tion

(BS

i:P

ON

,B

Si:

PO

C)

and

diss

olve

dnu

trie

ntut

iliz

atio

n(S

iOH

4:N

O,

NO

:PO

)ra

tios

duri

ngth

etw

oex

per-

22

323

34

imen

ts.

Val

ues

are

the

mea

nsan

der

rors

are

the

stan

dard

devi

atio

nsof

trip

lica

tebo

ttle

s.

Hum

bold

tC

urre

ntP

eru

Upw

elli

ng

Init

ial

Con

trol

fina

l

Fe

addi

tion

(nM

)

0.1

0.5

2.5

Init

ial

Con

trol

fina

l

Fe

addi

tion

(nM

)

0.2

0.4

0.8

2.5

PO

C(m

M)

12.3

62.

542

.66

1.8

44.4

611

.259

.86

11.5

95.7

631

.59.

46

3.9

26.1

64.

044

.86

5.2

60.9

69.

564

.66

13.6

70.4

64.

8P

ON

( mM

)2.

06

1.7

5.1

60.

06.

26

0.8

8.2

62.

213

.36

0.6

0.8

60.

33.

26

0.2

7.5

60.

77.

06

1.3

7.0

61.

28.

06

0.8

BS

i( m

M)

0.3

60.

12.

86

0.9

2.0

60.

52.

76

0.5

2.7

60.

60.

16

02.

46

0.7

4.4

60.

65.

56

0.3

5.6

60.

16.

36

0.9

BS

i:P

ON

prod

uced

(mol

:mol

)—

0.81

0.40

0.39

0.21

—0.

960.

640.

870.

890.

86B

Si:

PO

Cpr

oduc

ed(m

ol:m

ol)

—0.

080.

050.

050.

03—

0.14

0.12

0.10

0.10

0.10

NO

( mM

)2 3

8.9

61.

37.

56

0.0

2.8

60.

71.

26

0.1

1.3

60.

016

.36

0.2

8.9

61.

72.

06

1.5

0.2

60.

00.

46

0.1

0.2

60.

0S

iOH

4(m

M)

5.6

61.

04.

66

0.6

4.5

60.

03.

56

0.1

3.0

60.

17.

86

0.0

6.5

60.

64.

16

1.1

2.6

60.

12.

16

0.2

0.9

60.

9P

O( m

M)

32 40.

636

0.06

0.47

60.

120.

326

0.08

0.17

60.

130.

146

0.03

1.2

60.

050.

696

0.11

0.32

61.

30.

136

0.01

0.09

60.

030.

076

0.0

SiO

H4:N

O2 3

used

(mol

:mol

)—

0.71

0.18

0.27

0.34

—0.

180.

260.

320.

360.

43N

O;P

Ous

ed2

323

4

(mol

:mol

)—

8.8

19.7

16.7

15.5

—14

.516

.315

.014

.314

.2


Fig. 6. Photosynthetic parameters in the 5-d Peru Upwellingincubation measured with fast repetition rate fluorometry (FRRF).(A) Photosynthetic efficiency assessed as variable over maximumfluorescence, Fv/Fm. (B) The cross-sectional area of photosystem II,sPSII. Symbols and error bars represent the means and standard de-viations of measurements on triplicate bottles. Controls, closed cir-cles; 0.2-nM Fe additions, open circles; 0.4-nM Fe additions, closedtriangles; 0.8-nM Fe additions, open triangles; 2.5-nM Fe additions,closed squares.

Fig. 7. Fluorometrically determined Chl a concentrations in thePeru Upwelling incubations. (A) Total Chl a; (B) .5 mm Chl a.Symbols and error bars are as in Fig. 6.

Fig. 8. Results of flow cytometry analyses of phytoplanktonabundance during the Peru Upwelling experiment. (A) Cyanobac-teria. (B) Eukaryotic nanoplankton. (C) Large eukaryotic phyto-plankton, chiefly diatoms. (D) A separate group of very large eu-karyotic phytoplankton, likely diatoms. Symbols and error bars areas in Fig. 6.

group of somewhat smaller cells (Fig. 8C) and a separategroup of very large but rare cells (Fig. 8D). Both of thesegroups increased when Fe was added, although the formergrew at all added Fe concentrations, whereas the latter grouponly increased substantially in the highest (2.5 nM Fe) treat-ment.

All of the diatoms counted by microscopy were pennates,and most were a single species of small (;1 mm wide, ;10mm long) unicellular Nitzschia (Table 3). Average diatomcell size was much smaller in the Peru samples than in theHumboldt Current incubations, but total cell numbers werean order of magnitude higher, leading to Chl a and fucoxan-thin levels in the Peru incubations that were comparable toor higher than those in the Humboldt experiment (Tables 1,2). Final diatom numbers in the 2.5-nM Fe bottles were morethan 103 higher than control counts and were at least ;53higher in the other 1Fe samples than in the controls. Noneof the very large eukaryotes suggested by flow cytometry


Fig. 9. Sinking rates of the entire phytoplankton communitymeasured on the final day of the Peru Upwelling incubation usingthe SETCOL method. The horizontal line indicates neutral buoy-ancy. Values are the means and error bars are the standard devia-tions of triplicate bottles for each treatment.

Fig. 10. Major nutrient concentrations during the Peru Upwell-ing experiment. (A) Nitrate. (B) Phosphate. (C) Silicic acid. Sym-bols and error bars are as in Fig. 6.

were observed, perhaps because they were too rare to detecteasily by microscopy. Flagellated P. globosa cell numbersincreased in three out of the four Fe additions relative to thecontrols and were higher than in the offshore experiment,but no macroscopic colonies were observed in the Peru in-cubation. In contrast to the flow cytometry results, visualcell counts also suggested a similar case for nanoflagellates,dinoflagellates, and cyanobacteria. Their abundance was of-ten somewhat higher in 1Fe bottles than in controls, al-though there was large variability among replicates for thesegroups (Table 3).

Community sinking rates as assessed by the SETCOLmethod at the end of this incubation were greatly affectedby the availability of Fe. Control samples had relatively rap-id settling rates of ;0.6 m d21 (Fig. 9). In contrast, phyto-plankton in all four of the 1Fe treatments were close toneutrally buoyant.

Nutrient biogeochemistry: Nutrient drawdown rates de-pended on the amount of added Fe, with higher Fe additionsresulting in faster utilization. All four treatments with addedFe used most of the original 16.4 mM nitrate by the end ofthe 5-d incubation (Fig. 10A; Table 4), and the pattern wasvery similar for phosphate (Fig. 10B). In contrast, controlbottles used less than half of the nitrate and phosphate. Un-like the Humboldt Current incubation, increasing Fe addi-tions resulted in progressively greater silicic acid drawdown(Fig. 10C; Table 4).

This effect of Fe on Si drawdown during the Peru incu-bations was reflected in the net dissolved Si : N drawdownratios of the community (Table 4). Unlike the previous ex-periment, Si : N utilization ratios were not highest in the con-trol. Instead, dissolved Si : N drawdown increased with in-creasing Fe added, although absolute molar utilization ratioswere again all quite low (0.18–0.43). Unlike the offshoreexperiment, net N : P utilization ratios in all control and 1Fetreatments were near Redfield values (Table 4).

POC production was incrementally greater as Fe avail-ability increased (Table 4). PON production also increasedin the 1Fe treatments relative to the controls but was ap-

proximately equal at the final timepoint in all 1Fe samples.In keeping with the increased silicic acid drawdown at high-er Fe levels, BSi production also increased in the same man-ner. As in the Humboldt Current experiment, BSi : POC pro-duction ratios were highest in the controls, but the ratios inthe Peru experiment were roughly twice as high. Net BSi :PON production ratios were all much higher (;2–53) thanthose calculated from dissolved Si(OH)4 and NO drawdown2

3

and had an opposite trend, with the highest values (0.96) inthe controls (Table 4).

Fe concentration versus net specific growth rates—Chl anet specific growth rates during the exponential phase of


Fig. 11. Chl a net specific growth rates (m) versus total dis-solved Fe (nM, added plus ambient) in the Humboldt Current (tri-angles) and Peru Upwelling (circles) incubations. Curves wereforced through the origin by assuming zero growth at a hypotheticalzero-Fe concentration. In the Humboldt Current incubation, appar-ent half-saturation constants for growth were lower (K1/2 app., 0.17nM Fe) and apparent maximum net specific growth rates were high-er (mmax app., 0.75 d21) than in the Peru Upwelling community (0.26nM Fe and 0.64 d21, respectively). R2 values for data fitting to thenonlinear saturation curve regressions were 0.86 (Humboldt Cur-rent) and 0.99 (Peru Upwelling).

growth in the Humboldt Current and Peru Upwelling incu-bations could be expressed as saturating functions of totaldissolved Fe (ambient plus added) (Fig. 11). In the Hum-boldt Current incubation, apparent maximum growth rates(mmax app., d21) were ;1.23 and apparent half-saturation con-stants for growth (K1/2 app., nM Fe) were 0.653 those in thePeru Upwelling experiment (Fig. 11). These results suggestthat the offshore algal community was able to grow morerapidly at both high (saturating) and low (limiting) concen-trations of Fe.

Discussion

Our experiments demonstrate that, as in the California re-gime, Fe availability can limit primary producers in both thePeru Upwelling and the offshore Humboldt Current. We sug-gest that the world’s largest and most productive coastal up-welling regime is also characterized by widespread Fe-driv-en HNLC conditions, with all of the consequences forbiological productivity and nutrient biogeochemistry thatthis implies. The South American upwelling regime is likelyto be a regional ‘‘mosaic’’ of Fe-limited and Fe-replete areas,as is the case along the central California coast (Hutchins etal. 1998).

Peru and California compared—Despite many strong par-allels, there are also fundamental physical, biological, andbiogeochemical differences between the North and SouthAmerican upwellings. The ultimate source of Fe in Califor-nia surface waters is from deposition of continental sedi-

ments by wintertime riverine flood runoff, which are thenstored on the shelf to supply Fe during the following summerupwelling season (Bruland et al. 2001). This source of Fe isprobably not as important in the Peru Upwelling, since it isbordered by the world’s driest desert. Peru completely lacksthe larger river systems that supply Fe to parts of the Cali-fornia upwelling. Small rivers along the South Americanwest coast do supply snowmelt runoff from the Andes brieflyduring the austral summer but are essentially dry for mostof the year.

Instead, the shallow underlying suboxic waters that arefound throughout this region (Morales et al. 1999) may bethe major source of Fe to Peruvian surface waters. Fe ishighly soluble under low-oxygen conditions (Bruland et al.1991), and vertical advection and subsequent oxidation ofsubsurface dissolved Fe(II) during upwelling may supplyphytoplankton with much or most of the Fe that supportstheir growth. In California, upwelled Fe is entrained directlyfrom sediment sources where it has been sequestered afterwinter flood deposits (Bruland et al. 2001), but in the Peruregion, the major biologically available reservoir is likelyshelf-derived Fe that has been transported and stored in thesuboxic water column. If so, inputs of Fe off Peru may alsobe less episodic and seasonal than is the case off California.

Upwelling events are also less temporally variable offPeru than in California waters. In coastal California, pre-vailing winds are favorable for upwelling only during thesummer. Even then, the system is characterized by alternat-ing strong upwelling periods followed by extended relaxa-tion events where stratification occurs, allowing phytoplank-ton to bloom and draw down nutrients (Dugdale andWilkerson 1989). Because upwelling-derived Fe supplies arecurtailed during relaxation, primary producers become muchmore Fe-limited during these periods (Firme et al. in press).

In contrast, upwelling off Peru occurs to some extent vir-tually year-round (Minas and Minas 1992), and alternationbetween strong upwelling events and relaxation conditionsis not as commonly observed. The result may be a systemin which primary producers do not experience the large fluc-tuations in Fe availability and limitation effects found overseasonal and shorter time scales off California. However,upwelling intensity in the Peru system is subject to largeinterannual variability due to effects such as basin-wide ElNino/Southern Oscillation cycles (Codispoti et al. 1982).Changes in upwelled nutrient and Fe supplies undoubtedlyaffect primary production over longer time scales, just asthey do in the equatorial Pacific HNLC area (Barber et al.1996).

Another major difference between California and Peru isa result of the broader geographical context of the two sys-tems. Ekman-driven upwelling plumes off California areeventually advected into the oligotrophic waters of the Cal-ifornia Current, where any residual nutrients they contain areused by the offshore, low-nutrient–adapted, picoplankton-dominated communities typically found here (Bruland et al.2001). The northwesterly trend of the Humboldt Current offPeru instead feeds more or less directly into the offshoreequatorial Divergence. Thus, there is no clearly definedboundary between coastal and oceanic HNLC waters, and


phytoplankton are likely Fe-limited throughout this transi-tional area, as at our Humboldt Current station.

Phytoplankton community structure—These differences inthe physics and chemistry of the two systems are matchedby the intriguing differences in phytoplankton communitystructure we found in our experiments. Unlike California,initial phytoplankton communities in both of the experimentspresented here were dominated by haptophytes. Type 4 hap-tophytes (P. globosa) dominated at the offshore station,whereas Type 3 haptophytes (coccolithophorids) were dom-inant in the Peru Current.

In Fe-limited waters off California, adding Fe virtuallyalways results in massive blooms of large, heavily-silicifieddiatoms, especially the chain-forming centric genus Chae-toceros (Hutchins et al. 1998). In the experiments presentedhere, Fe additions instead promoted the establishment of acommunity composed about equally of pennate diatoms andhaptophytes. Fe additions allowed diatoms to grow rapidlyand become codominant with the haptophytes, whereas thelatter responded only slightly or not at all to increased Feavailability. In the Humboldt Current experiment, crypto-phytes also benefited greatly from the Fe additions, althoughthey were never dominant.

Surprisingly, it was only at the offshore Humboldt Currentstation that large diatoms (mostly chain-forming pennates)made a significant contribution to community biomass afterFe addition. This community most closely resembled onesthat have been described from the Peruvian shelf region (Ro-jas de Mendiola 1981; Sellner et al. 1983). In the Peru Up-welling experiment, virtually all of the diatom growth inresponse to added Fe was by an abundant but very smallspecies of unicellular pennate diatom. Similar small pennatediatom-dominated assemblages have been observed in thepast around the Galapagos Islands (Jiminez 1981) and inareas just off the South American shelf (Sellner et al. 1983).The reasons for these differences between Fe effects on di-atom community composition in the North and South Amer-ican ecosystems are unknown, but size structure differencescould have large implications for trophic dynamics, nutrientcycles, and, especially, vertical fluxes of elements (Boyd andNewton 1999) in the two regions.

Another novel result was the apparent stimulation of P.globosa colony growth by Fe in the Humboldt Current ex-periment. Type 4 haptophytes (largely P. globosa flagellatedcells) were prominent in the initial assemblage, but the co-lonial form was apparently favored by the added Fe. Majornutrient limitation may promote the flagellated form overcolonies (Escaravage et al. 1995), and it seems reasonableto suppose that low Fe availabilities might also limit colonialgrowth because the volume-normalized diffusional Fe fluxto a 4-mm-diameter colony is 106-fold lower than to a 4-mmflagellated cell at the same external concentration. The im-portance of Phaeocystis in global cycles of elements such assulfur and their ecological dominance in other regimes suchas the Ross Sea, Antarctica (DiTullio et al. 2000; Smith andAsper 2001), makes investigation of these results a priorityfor future research.

Phytoplankton growth rates as a function of Fe avail-ability—Coale et al. (1996b) plotted Chl a growth rates ina number of different equatorial Pacific growout experimentsversus Fe concentration in order to estimate Fe half-satura-tion constants for growth. We used a similar approach, ex-cept instead of plotting results from several different incu-bation experiments begun at different stations, we usedgrowth rate data from the same community incubated at arange of Fe concentrations in each of our two experiments.Differences between the growth responses to Fe availabilityof the phytoplankton communities are suggested by the sat-uration curves in Fig. 11. The growth parameters calculatedby this type of data analysis should not be interpreted as truephysiological constants because the communities are mixedassemblages (not unialgal cultures), and they are not grow-ing at steady state because biomass is increasing and Fe andnutrients are being depleted in batch cultures. Factors otherthan Fe availability may affect whole-community net max-imum specific growth rates. For instance, larger cells (in-cluding the bigger diatoms and Phaeocystis colonies in ourHumboldt Current experiment) may enjoy a degree of im-munity to grazing, resulting in higher apparent maximumgrowth rates.

In spite of these caveats, a saturation curve data treatmentsuch as that presented here yields useful practical informa-tion about the potential growth rates of phytoplankton com-munities following varying levels of Fe inputs. Differencesin apparent Fe half-saturation constants for growth (K1/2 app.)are unlikely to be affected by grazing, and should representactual physiological differences in the whole-community re-sponse to Fe availability. In our experiments, the HumboldtCurrent assemblage had a lower K1/2 app. (0.17 nM Fe) thanthe Peru Upwelling community (0.26 nM Fe). This differ-ence suggests that the offshore community was able to growfaster at lower Fe levels, despite a greater degree of domi-nance by larger diatoms and Phaeocystis colonies. Coale etal. (1996b) found a somewhat lower apparent half-saturationconstant for growth of 0.12 nM Fe in equatorial Pacific com-munities, suggesting that oceanic HNLC phytoplankton aremore efficient at growing at lower Fe concentrations thaneither of the communities in our experiments.

Nutrient biogeochemistry and elemental ratios—MolarSi : N utilization ratios in our South American experimentswere strikingly lower than in our previous work in the Cal-ifornia regime, perhaps because of the differences in phy-toplankton communities discussed above. Codominance bynonsiliceous haptophytes and lightly silicified small pennatediatoms likely results in very low utilization of Si relativeto N by the phytoplankton community. Similar very low Si :N utilization ratios (;0.2–0.3) have been documented insubantarctic Southern Ocean experiments where the assem-blage also was composed of nonsiliceous taxa and verysmall pennate diatoms (Hutchins et al. 2001). This is instrong contrast to California communities, which are typi-cally dominated by heavily silicified centric diatom speciessuch as Chaetoceros (Wilkerson et al. 2000).

Despite low absolute Si : N drawdown ratios in our off-shore Humboldt Current experiment, there was still a 2–33increase under Fe-limited growth conditions in the controls


relative to the Fe-replete treatments. In California, this is notdue to increased diatom Si uptake under Fe stress; rather, itresults from decreased nitrate assimilation while Si uptakeremains relatively constant (Firme et al. in press). However,dissolved Si : N utilization ratios actually increased some-what with increasing Fe concentrations in the Peru Upwell-ing incubation. This observation could also be due to speciescomposition differences. Little or no relationship betweenSi : N drawdown ratios and Fe availability was reported inthe nanodiatom- and nanoflagellate-dominated subantarcticexperiments mentioned above (Hutchins et al. 2001). It maybe that increased Si : N ratios under Fe limitation are a par-ticular feature of communities with large numbers of heavilysilicified, large diatom species. Such communities are typi-cally found in California waters and the Southern Oceansouth of the Polar Front, where a similar effect was seen forFragilariopsis-dominated communities during SOIREE(Boyd et al. 2000). In regimes where the dominant taxa areunsilicified phytoplankton groups, small lightly silicifiedpennate diatoms, or both, Si : N drawdown ratios may re-spond quite differently to changes in Fe levels.

In these experiments, BSi : PON production ratios weremuch higher than we would have expected from the draw-down ratios of dissolved Si(OH)4 and NO . These high ratios2

3

were not due to discrepancies in silicon uptake and partic-ulate production because mass balance was generally quitegood for Si (Table 4). Instead, most of this effect was at-tributable to the fact that only a portion of the nitrate usedwas present as PON at the end of the incubations. It is likelythat micrograzing was intense in these experiments, judgingfrom the high levels of phaeopigments produced during theincubations and the decreases in small cells like cyanobac-teria. The pennate diatoms and nanoflagellates that wereabundant in the bottles may have also been small enough tobe consumed by large protozoans. We suggest that this mayhave been an example of the grazer-driven ‘‘silicate pump’’of Dugdale et al. (1995). Much of the nitrate assimilated bygrazers may have been converted to ammonium or DON (notmeasured in our experiments), leaving behind the indigest-ible BSi. The net result is higher ratios of Si : N in particulatematter than was assimilated by the phytoplankton commu-nity. In California, Fe addition experiments are nearly al-ways dominated by very large chain-forming diatoms thatare not efficiently grazed in the bottles, and nitrate is usuallyquantitatively converted to equivalent molar amounts ofPON (Hutchins et al. 1998). This may not be the case how-ever in our South American experiments, with their muchgreater abundances of small phytoplankton species. Thissuggestion is supported by particulate production ratios thatwere closer to the dissolved drawdown ratios in the Hum-boldt Current experiment, with its greater numbers of largephytoplankton, than in the Peru incubation that was almostentirely dominated by small species.

We also saw Fe-driven changes in community N : P utili-zation ratios in one of our two experiments. N : P drawdownwas much lower in Fe-limited controls than in Fe additiontreatments, an observation we also made in many other in-cubation experiments conducted throughout this region (Fir-me and Hutchins unpubl. data). This suggests that Fe limi-tation might play an important role in driving non-Redfield

behavior of these two nutrients in the South Americanboundary current system. We suggest that as is the case withchanges in Si : N ratios, these shifts in N : P utilization areprobably caused by reduced N uptake rather than by in-creased P assimilation under Fe limitation.

Sinking rates of cultured diatoms and diatom-dominatednatural assemblages have previously been shown to be high-er during Fe-limited growth (Muggli et al. 1996; Boyd et al.2000), which should facilitate export of carbon through di-rect sedimentation of phytoplankton (Smetacek 1985). In ourPeru Upwelling experiment, even the lowest added Fe con-centration (0.2 nM) was sufficient to change communitysinking rates from relatively rapid rates (0.6 m d21) to nearzero. Thus, although phytoplankton carbon fixation is re-duced in Fe-limited HNLC regimes, it may be that this car-bon is more rapidly transported to underlying waters than inFe-replete areas.

We conclude that Fe limitation is a major constraint onphytoplankton growth along the South Pacific eastern bound-ary current, just as it is in the upwelling regime off Califor-nia. It is now evident that there is a potential for algal Felimitation anywhere in the ocean where physical processessupply large amounts of major nutrients to surface waters,but where accompanying inputs of Fe are low. Althoughthere are strong similarities in this respect between Califor-nia and Peru, the two regimes are quite distinct in manyother ways. Just as is the case with the three oceanic HNLCareas, these two coastal HNLC areas are very different phys-ical, chemical, and biological environments that share a com-mon limiting nutrient.

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Received: 10 October 2001Accepted: 29 January 2002

Amended: 13 March 2002

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