JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 105, NO. C12, PAGES 28,709-28,722, DECEMBER
15, 2000
Phytoplankton chlorophyll distributions and primary
production in the Southern Ocean
J. Keith
Moore • and Mark
R. Abbott
College of Oceanic and AtmosphericSciences,Oregon State University,Corvallis
Abstract. Satellite oceancolor data from the Sea-viewingWide Field-of-view Sensor
(SeaWiFS)were usedto examinedistributionsof chlorophyllconcentrationwithin the
SouthernOcean for the period October 1997 throughSeptember1998. Over most of the
Southern
Ocean,meanchlorophyll
concentrations
remained
quitelow(<0.3-0.4mgm-3).
Phytoplankton
blooms
wherechlorophyll
concentration
exceeded
1.0mgm-3 were
observedin three general areas,which included coastal/shelfwaters, areas associatedwith
the seasonalsea ice retreat, and the vicinity of the major SouthernOcean fronts. These
chlorophylldistributionpatternsare consistentwith an iron-limited system.Mean
chlorophyllconcentrationsfrom SeaWiFS are comparedwith valuesfrom the coastalzone
color scanner(CZCS). The SeaWiFSglobalchlorophyllalgorithmworksbetter than the
CZCS in SouthernOceanwaters.Primary productionin the SouthernOcean was
estimatedwith the verticallygeneralizedproductionmodel of Behrenfeldand Falkowski
[1997].Annual primaryproductionin the SouthernOcean (>30øS) was estimatedto be
14.2Gt C yr-•, withmostproduction
(-80%) takingplaceat midlatitudes
from30øto
50øS.Primaryproduction
at latitudes
>50øSwasestimated
to be 2.9Gt C yr-•. Thisis
considerablyhigher than previousestimatesbasedon in situ data but lessthan some
recent estimatesbasedon CZCS data. Our estimatedprimaryproductionis sufficientto
accountfor the observedSouthernHemisphereseasonalcyclein atmospheric02
concentrations.
1.
Introduction
Phytoplanktonbloomdynamicsin the SouthernOceanhave
long representeda paradoxfor oceanographers.
Despite high
concentrationsof availablenitrate and phosphate,chlorophyll
concentrationsin open ocean waters remain low (typically
<0.5 mgm-3) [Tr•guer
andJacques,
1992;Comiso
etal., 1993;
Banse, 1996]. The SouthernOcean is thus the largesthighnutrient low-chlorophyll(HNLC) region in the world ocean
[Martin, 1990;Minas and Minas, 1992]. In recent years, substantial evidencehas accumulatedthat the availabilityof the
micronutrientiron playsa critical role in limiting phytoplankton biomassand productionwithin HNLC regions[Martin et
al., 1990a, 1990b;de Baar et al., 1995; Coale et al., 1996; Gordon
et al., 1997;van Leeuweet al., 1997;Takeda,1998].
There
have been several satellite-based
studies of Southern
Ocean phytoplanktondynamicsthat relied on data from the
coastalzonecolorscanner(CZCS).ArrigoandMcClain [1994]
used CZCS data to estimateprimary productionin the Ross
Sea.
Several
studies
combined
CZCS
data
with
satellite-
sociatedwith sea ice retreat, shallowwaters, areas of strong
upwelling,and regionsof high eddy kinetic energy (mainly
associated
with SouthernOceanfronts) [Comisoet al., 1993].
The strongestcorrelationwith pigmentswas a negativecorrelation betweenoceandepth and pigmentconcentrations,
possiblythe result of higher availableiron concentrationsin shallow water regions[Comisoet al., 1993].Sullivanet al. [1993]
also examined CZCS pigment data for the whole Southern
Ocean.They noted intensephytoplanktonbloomswithin and
downstreamof coastal/shelfareas,which they attributed to
elevatediron availability[Sullivanet al., 1993]. Silicic acid
availabilitymay limit diatom productionand biomassaccumulation in severalareas [Trdguerand Jacques,1992;Sullivanet
al., 1993].Mitchell and Holm-Hansen[1991] developeda regionalSouthernOceanpigment(SOP) retrievalalgorithmfor
the CZCS on the basisof the bio-opticalpropertiesof Antarctic Peninsulawaters. This regional pigment algorithm was
deemednecessary
becausethe globalpigment(GP) algorithm
for CZCS [Gordonet al., 1983] resultedin underestimates
of
surfacechlorophyllconcentrationsin Southern Ocean waters
[MitchellandHolm-Hansen,1991;Sullivanet al., 1993;Amigoet
al., 1994].Amigoet al. [1994] providedcorrectionfactorsfor
convertingGP pigmentestimatesto SOP pigmentvalues.The
SOP pigment values are higher than the GP estimatesby a
factor rangingfrom -2.1 to 2.5 for chlorophyllconcentrations
derivedice coverinformationto examineice edgephytoplankton bloomdynamics[Sullivanet al., 1988;Comisoet al., 1990].
Comisoet al. [1993] examinedCZCS pigmentdata for the
entire SouthernOcean and analyzedtheir relationshipto several geophysicalfeatures. They noted that phytoplankton
bloomsoccurprimarily in severalregionsincludingareas as- below1.5mgm-3 [Amigo
et al., 1994].
In this study, Sea-viewing Wide Field-of-view Sensors
•Nowat Advanced
Studies
Program,
NationalCentersfor Atmo- (SeaWiFS) satellite-basedestimatesof surfacechlorophyll
sphericResearch,Boulder,Colorado.
concentrationsare used to examine phytoplanktonbiomass
Copyright2000 by the American GeophysicalUnion.
distributionsand dynamicswithin the SouthernOcean.We use
a broad definitionfor the SouthernOcean encompassing
the
Paper number 1999JC000043.
0148-0227/00/1999JC000043509.00
area from the Antarctic
28,709
continent
north to 30øS. This northern
28,710
MOORE
AND ABBOTT:
SOUTHERN
OCEAN PHYTOPLANKTON
CHLOROPHYLL
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MOORE AND ABBOTT: SOUTHERN OCEAN PHYTOPLANKTON CHLOROPHYLL
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MOORE
AND ABBOTT: SOUTHERN
OCEAN PHYTOPLANKTON
boundaryis ---5ø of latitude north of the mean positionof the
North SubtropicalFront as defined by Belkin and Gordon
[1996]. Technically,the North SubtropicalFront marks the
northernborder of the SouthernOcean.The 30øSboundaryis
used to encompassnorthern meandersof this front. Mean
latitude in the Indian sectorof the North SubtropicalFront is
as far north as 32ø-33øS[Belkinand Gordon,1996].
Primaryproductionin the SouthernOceanis estimatedusing the verticallygeneralizedproductionmodel (VGPM) of
Behrenfeld
and Falkowski[1997].Satellite-derivedestimatesof
surfacechlorophyll,seasurfacetemperature,seaice cover,and
surfacephotosynthetically
availableradiationare usedto drive
thismodel.Our resultsare comparedwith severalrecentstudiesthat useddatafrom the CZCS to modelprimaryproduction
in SouthernOcean waters [Longhurstet al., 1995;Behrenfeld
and Falkowski,1997;Arrigo et al., 1998] as well as with estimatesextrapolatedfrom in situ measurements.
We also compare our estimatesof total primary productionwith estimates
of seasonalnet productionbasedon atmospheric02 variations
[Benderet al., 1996].
CHLOROPHYLL
ContinentalShelfZone of Tr•guerandJacques[1992].The SIZ
is definedas areaswith >5% ice cover duringAugust 1997
(maximumice extentprior to growingseason)andless<70%
icecoverduringFebruary1998(seasonal
minimumiceextent).
The MIZ
is a subset of the SIZ where there has been recent
meltingof seaice [Smithand Nelson,1986].We operationally
definethis ecologicallyimportant region as areaswith >50%
seaice coverthe precedingmonth and <50% ice coverin the
current month. Previously,we have used 70% ice cover to
defineMIZ [Mooreet al., 2000]. Here we use 50% to include
thoseareasthat begin the growingseasonat <70% ice cover.
A wide rangeof cutoffvaluescan be usedin determiningthe
MIZ (---30-70%) that givesimilarresultsbecauseseaicemelts
rapidlyin the SouthernOcean,typicallygoingfrom heavyto
little ice cover in less than a month. The PFR is defined as all
areaswithin 1ø of latitude of the mean path for the Antarctic
Polar Front (PF) as definedby Moore et al. [1999a].This
locationof the PF is basedon satelliteseasurfacetemperature
data and thusmarksthe surfaceexpressionof the front. The
PF hasboth surfaceand subsurface
expressions
[seeMooreet
al., 1999a].Arguably,the surfaceexpressionhas a stronger
influenceon the biota and is thus appropriatefor usehere. In
2.
Methods
and Materials
any casethe mean locationsof the front basedon surfaceand
SeaWiFS-derivedestimatesof surfacechlorophyllconcen- subsurface
definitionsare quite similarin mostregions[Moore
trationsfor the period October 1997 throughSeptember1998 et al., 1999a].The POOZ is the areanorthof the SIZ andsouth
were obtained from the Goddard SpaceFlight Center [Mc- of the PFR.
The SWR is defined as the area from the PFR north to the
Clain et al., 1998]. Daily level 3 standardmappedimagesof
chlorophyll concentration (version 2) were composite- SubtropicalFronts [Banse,1996].The northernborderof this
averagedto produceweekly, monthly, seasonal,and annual regionis setat 35øS,slightlynorth of the meanpositionof the
meansfor the SouthernOcean. We comparetheseSeaWiFS North SubtropicalFront as defined by Belkin and Gordon
estimateswith data from the CZCS. Monthly CZCS compos- [1996].North of thisregion,are the mid-oceangyresystems
of
ites processed
with the globalalgorithm(GP) [Gordonet al., the SouthernHemisphere.This area, 30ø-35øS,we term the
1983] over the 1979-1986 period were obtainedfrom NASA. MGR. To defineopenoceanpatternsbetterwe haveexcluded
We also derived monthly CZCS compositesbased on the areas shallower than 500 m from the MGR, SWR, PFR, and
SouthernOceanpigment(SOP) algorithm[MitchellandHolm- POOZ areasusingthe topographydata of Smithand Sandwell
Hansen, 1991] using the correctionfactors of Ardgo et al. [1994]. These shallowcoastaland shelf areas are collectively
referred to as the MCR.
[1994].
Monthly mean sea ice concentrations
(NASA Team algoThere is someoverlapbetweentheseecologicalregions.The
rithm) from the SpecialSensorMicrowaveImager (SSM/I) WRR and the MIZ are subsets of the SIZ. We have excluded
DMSP-F13 satellite sensorfor the years 1997-1998 were obtainedfrom the Earth ObservingSystemData and Information
System (EOSDIS) National Snow and Ice Data Center
(NSIDC) DistributedActive Archive Center, Universityof
Colorado,Boulder [NationalSnowand Ice Data Center,1998].
Ice concentrations
of <5% were consideredto be openwater
in our analysis.A minimum of 70% ice cover was used to
determinethe minimum(summer)sea ice extent.This value
waschosenbecausesignificantphytoplanktonbiomasscan accumulate
in the water
column
at ice concentrations
below
the WRR
areas from
the MIZ
and SIZ.
In the southwest
Pacificand throughDrake Passagethe SIZ and MIZ overlap
with the PFR. Theseareashavebeen includedonlywithin our
SIZ and MIZ regions,and thus are excludedfrom the PFR.
Chlorophylldistributionsare also comparedwith the mean
positionsof the major SouthernOcean fronts.We haveused
the meanpath of Mooreet al. [1999a]for the AntarcticPF. The
positionsof the Southern Antarctic Circumpolar Current
Front (SACCF) and the Subantarctic
Front (SAF) are from
the analysisof Orsi et al. [1995]. The locationsof the North
SubtropicalFront (NSTF) and South SubtropicalFront
(SSTF) and the AgulhasCurrentare fromBelkin[1993,1988]
and Belkinand Gordon [1996]. The regionfrom the SSTF to
the NSTF is termed the SubtropicalFrontal Zone (STFZ)
[Belkinand Gordon,1996].The terminologyand abbreviations
usedin definingthe ecologicalregionsand frontsare summa-
---70% [e.g., Smith and Gordon, 1997]. All sea ice data were
remappedto a 9 km equal anglegrid.
The satellitechlorophylldata were analyzedby ecological
region within the Southern Ocean. We divide the Southern
Ocean into severalecologicalregions,includingthe Midlatitude Gyre Region (MGR), the Midlatitude CoastalRegion
(MCR), the Subantarctic
Water Ring (SWR), the PolarFron- rized in Table 1.
tal Region (PFR), the Permanently Open Ocean Zone
(POOZ), the SeasonalIce Zone (SIZ), the MarginalIce Zone
(MIZ), and the Weddell-RossRegion(WRR). From the Ant- 3. Modeling Primary Production
arcticPolar Front southto Antarcticatheseecologicalregions
Primaryproductionover monthlytimescaleswasestimated
are similar to thoseproposedby Tr•guerand Jacques[1992]. usingthe VGPM [Behrenfeld
and Falkowski,1997].The inputs
The WRR consists of the waters of the Weddell and Ross Seas
to the VGPM are satellite-derivedsurfacechlorophyllconcenthat are southof 73øS.This regionis similarto the Coastaland trations, sea surface temperatures, and photosynthetically
MOORE AND ABBOTT: SOUTHERN
Table
1.
Abbreviations
Used
for the Various
OCEAN PHYTOPLANKTON
Southern
Definition
MIZ
EcologicalRegions
Weddell-RossRegion, high-latitudeWeddell and RossSeas
SeasonalIce Zone, >5% ice coverAugust 1997 and <70%
ice cover February 1998
Marginal Ice Zone, >50% ice coverin precedingmonth
POOZ
PermanentlyOpen Ocean Zone, north of SIZ and south
PFR
Polar Frontal Region, area within 1ø latitude of Antarctic
SWR
MGR
MCR
SubantarcticWater Ring, area north of PFR to 35øS
Midlatitude Gyre Region, area from 30ø to 35øS
Midlatitude Coastal Region, areasin MGR, SWR, PFR,
and POOZ, where depth <500 m
WRR
SIZ
and <50%
in current
mean location
Ocean Fronts
SACCF SouthernAntarctic CircumpolarCurrent Front
PF
SAF
Antarctic Polar Front
Subantarctic Front
SSTF
NSTF
AC
South SubtropicalFront
North SubtropicalFront
Aghulas Current
availableradiation (PAR) at the water's surface[Behrenfeld
and Falkowski,1997].Photoperiod,or daylength,is calculated
as a function of latitude and time of year. Monthly optimally
interpolatedsea surfacetemperature(SST) data at 1ø resolution were obtained
from the National
Centers for Environmen-
tal Predictionfor the October 1997 to September1998 period
[Reynolds
and Smith,1994].The SST datawere remappedto a
9 km equalanglegrid usinglinear interpolationbetweengrid
points.
The SST data are used to estimate
this reason, there are little or no SeaWiFS data south of --•45ø-
50% during austral winter. These gaps must be removed in
some manner to avoid underestimatingtotal primary production. There is high spatialvariabilityin chlorophyllconcentrations within
month
of PFR
Polar Front
the local maximum
28,713
smallnumber of pixelshad no data becauseof persistentcloud
coverfor the entire month.A secondlarger gap appearsin the
chlorophyll data at high latitudes seasonallybecauseof the
short day length and the inability of SeaWiFS to produce
accuratechlorophyllestimatesat very high solar angles.For
Ocean EcologicalRegionsand Ocean Fronts
Abbreviation
CHLOROPHYLL
rate
of primaryproduction
B in the modelaccording
(Popt)
to the
Poptversus
SSTregression
ofBehrenfeld
andFalkowski
[1997].
B
This empiricalfunctionisvalid onlyfor SSTvaluesfrom - 1øto
B
29øC.For areaswhereSSTwas<-1.0øC,Poptwassetto the
-I.0øC value(1.1mgC (mgChl)-• h-•), andfor SSTabove
B
29øC,
Poptwassetto the29.0øC
value(3.8mgC (mgChl)-•
h-•). In theSSTdataset,thereoftenwerenovalidSSTdata
at very highlatitudes,mainlyin the southernWeddell and Ross
Seas.An SSTvalue of - 1.0øCwasusedfor computingprimary
productionin theseareas.Examinationof the PathfinderProgram SST data set [Smithet al., 1996]revealedthat mean SST
values were below -I.0øC in these areas during our study
period.
Monthly surfacePAR data for the years 1983-1984 were
obtainedfrom the SeaWiFS Project, and the data from the 2
years were averaged to produce a monthly mean (cloudcorrected)surfacePAR, whichwasremappedto a 9 km equal
anglegrid with linear interpolationbetweengrid points.Total
water column chlorophyll was estimated from the satellite
chlorophylldata using the equationsof Morel and Berthon
[1989].Equation(3a) of Morel andBerthon[1989]wasusedfor
areasequatorwardof 50øS,andfor latitudes_>50øS
(well-mixed
waters)we usedMorel and Berthon[1989,equation(5)]. Total
water columnchlorophyllwas then used to estimateeuphotic
depthusingMorelandBerthon[1989,equations(la) and (lb)].
Two typesof gapswere presentin the monthly chlorophyll
compositesused to drive the primary production model. A
the Southern
Ocean.
For
this reason
we have
chosento smooththe data over time rather than space.We
treat the area equatorwardof 40% (weak seasonality)differentlythanthe areapolewardof 40% (strongseasonality).
Gaps
in the data equatorwardof 40% were filled with the annual
mean chlorophyllvalue over the October 1997 to September
1998 time period.
For the data gapsat latitudes _>40%we first averagedthe
monthlydata from March and September1998.The composite
of these 2 months was then used to fill data gaps. During
March and September(late fall and early spring)phytoplankton biomass(and chlorophyllconcentrations)
is low, and the
systemis likely light-limited. Using the compositeof these 2
months thus representsa conservativemethod to fill in chlorophyll gapsthat will not result in an overestimateof primary
production.Most of the gaps filled were the seasonaltype
describedabove and occurredduring australwinter. The few
remaininggapsafter this two-stepprocesswere filled by averagingall adjacentpixelswith valid chlorophylldata.To prevent
extendingthe chlorophylldata into areaswith heavy sea ice
cover(whereprimaryproductionis negligible),we compared
the smoothedmonthly chlorophylldata with the monthly sea
ice data and removedchlorophylldata in areaswhere sea ice
concentrationexceeded70%. The smoothedchlorophyllfields
were used only in the calculationsof primary production.All
other analysesare baseduponthe originalmonthlycomposites
of the level 3 chlorophylldata.
We estimate area-normalizedproductionusing two methods. In the first the mean production per square meter is
calculated for all productive waters in a given region over
monthly timescales.Productivewaters are those with <70%
seaice coverwith somelight available(duringwinter months)
for primary production.The method gives resultssimilar to
what repeated ship surveysto the region would find and is
probably the better value for cross-studycomparisons.In the
secondmethodwe dividethe total productionby the total area
of a givenregion.This total area includesboth productiveand
nonproductivewaters and thus results in lower estimatesof
area-normalizedproduction.
4.
Results
4.1.
Satellite-Based Estimates of Chlorophyll in the
Southern
Ocean
Several previousstudiesargued that the global processing
algorithm (GP) used for the CZCS consistentlyunderestimated surface chlorophyll concentrationsin the Southern
Ocean [Mitchelland Holm-Hansen,1991;Sullivanet al., 1993;
Arrigo et al., 1994]. An early comparisonof a limited in situ
data set from the Polar Frontal region near 170øW,60øSwith
SeaWiFSdata (N = 84) indicatesgenerallygoodagreement
betweensatelliteandin situmeasurements
(r2 = 0.72, from
a linear regressionof shipboardversussatellitedata) but also
that the SeaWiFSalgorithm(OC2) may underestimateabso-
28,714
MOORE
AND ABBOTT: SOUTHERN
OCEAN PHYTOPLANKTON
Table 2. Mean ChlorophyllConcentrationsfor Austral
Spring/Summer
Months (October-March) Over Several
Latitudinal
Bands a
SeaWiFS,
CZCS
(_G3P),
mgm-3
mgm
35ø-50øS
50ø-90øS
35ø-90øS
30ø-90øS
0.29
0.35
0.34
0.30
CHLOROPHYLL
that the SeaWiFS estimatesfor mean surfacechlorophyllconcentrations
over all latitudinal
bands lie between the CZCS
GP
andSOP estimates.Thesevaluescanbe comparedwith several
meanscompiledfrom in situ data. Banse [1996] compileda
CZCS
(SOP),number of in situ data sets and estimateda seasonalpeak
mgm-3
0.15(0.21)
0.28 (0.39)
0.22(0.31)
0.21 (0.29)
withintheSWRof -0.30 mgm-3. Thisisin goodagreement
•,•
3
0.37(0.52)
0.58 (0.81)
0.49(0.69)
0.45 (0.63)
with our mean October-March concentrationof 0.29 mg m-
for the 35ø-50øS
region(Table2) anda meanSWR December
valueof 0.30mgm-3 (Plate5).Fukuchi[1980]estimated
mean
aCZCS pigmentvalueswere computedwith the originalglobalchlorophyll algorithm(GP) [Gordonet al., 1983], and then the data were
correctedto reflect the SouthernOcean Pigment (SOP) algorithm
[MitchellandHolm-Hansen,1991]usingthe correctionfactorsofArrigo
et al. [1994]before averaging.CZCS mean pigmentvalues(shownin
parentheses)
were convertedto chlorophyllusinga meanchlorophyll/
phaeopigmentratio of 2.51 [Arrigoet al., 1994].All valuesexceeding
10.0mgm-3 wereexcluded
duringaveraging.
chlorophyllconcentrations
overthe latitudinalrange35ø-63øS
to be 0.38 (east Indian sectorDecember),0.23 (west Indian
sectorlate February/early
March),and 0.27 mg m-3 (east
Atlantic sector late February/earlyMarch). These values
bracketthe mean October-MarchSeaWiFSestimates(Table
2). The compilationof in situ data by Arrigo et al. [1994]
estimated
meanchlorophyll
southof 30øSto be 0.42mgm-3
(0.58mgm-3 totalpigments
anda chlorophyll/phaeopigment
ratio of 2.57). This value is higherthan the SeaWiFSestimate
lute Southern Ocean (SO) chlorophyllconcentrations,althoughto a lesserextentthan the CZCS [Mooreet al., 1999b].
The SO pigmentalgorithm(SOP) was developedfor CZCS
data and resultedin chlorophyllconcentrations
higherthan in
the GP algorithm[Mitchelland Holm-Hansen,1991].
To examine how well SeaWiFS is estimatingchlorophyll
concentrations
in the SO, we comparedmean chlorophyllconcentrationsfrom SeaWiFSwith the CZCS data processed
with
the GP algorithmandwith the SOP-corrected
values(Table2).
Monthly meanswere composite-averaged
over the Octoberto
March period. Mean chlorophyllconcentrationswere calculated for severallatitudinalbands(Table 2). CZCS meanpigment valueswere convertedto chlorophyllusinga mean chlorophyll/phaeopigmentratio of 2.57 from Arrigo et al. [1994],
which was calculated
from
an extensive
in situ SO data set
(N - 1070). As in previousaveragingof CZCS data by
Sullivanet al. [1993], we excludedareas of chlorophyll(or
pigment)concentration
>10.0mgm-3 (a smallpercentage
of
but lower than the CZCS
SOP estimate for latitudes
•30øS
(Table 2).
The irregular temporal and spatialcoverageof the in situ
data likely bias mean in situ chlorophyllconcentrationsupward. Since chlorophyllconcentrationstend to increasewith
latitude (Table 2), simple averagingover a latitudinal ship
transectwith regularspatialsamplingwouldunderestimatethe
total contributionfrom low-chlorophyll(Subantarctic)waters
while givingequal weight to watersfarther south(which are
muchsmallerin their spatialextent,seeTable 3). In addition,
manyin situ studieshave focusedon the biologicallyproductive marginal ice zone where chlorophyllconcentrationsare
much higher than SO averages(Plate 5 and Table 2). The
distributionof in situ samplesusedby Arrigo et al. [1994] is
weightedtoward samplesfrom >50øSin the Atlantic and Indiansectors,with fewer samplesfrom Subantarctic
waters(the
generallyhigh-chlorophyllwatersnear New Zealand are well
sampled)[seeArrigo et al., 1994, Figure 21]. This partially
accountsfor the highermeanchlorophyllconcentrationof 0.42
the total pixels,always<0.43%).
Several lines of evidenceargue that the SeaWiFS global mgm-3 [Arrigoet al., 1994].
By comparingthe in situ values with the estimatesfrom
chlorophyllalgorithmis doing a better job of estimatingSO
chlorophyllconcentrations
than wasthe CZCS. Table 2 shows Table 2 it is evidentthat the CZCS GP algorithmdid seriously
Table 3. Annual PrimaryProductionEstimatesUsing the VGPM Model of Behrenfeld
and Falkowski[1997]a
Productive
Total
Waters,
All Waters,
Total Area,
Production,
Region
gC m-2 yr-•
gC m-2 yr-•
millionkm2
Gt C yr-l
MGR
MCR
SWR
PFR
POOZ
SIZ
WRR
SouthernOcean
(30ø-90øS)
SouthernOcean
(50ø-90øS)
MIZ
145.3
528.8
161.2
82.0
62.3
50.8
156.2
148.5
145.3
505.8
159.7
73.2
58.5
29.1
58.4
131.0
15.3(14.1)
2.57 (2.4)
58.0 (53.6)
4.89 (4.5)
8.70 (8.0)
16.2(15.0)
0.95 (0.88)
108.2(100)
2.22(15.7)
1.30(9.2)
9.26(65.4)
0.385(2.7)
0.509(3.6)
0.472(3.3)
0.0555(0.39)
14.17(100)
82.2
62.4
45.7 (42.2)
2.85(20.1)
0-5.53 (0-5.1)
0.096(0.67)
54.2
aThepercentageof total SouthernOceanprimaryproductionand oceanarea accountedfor is shownin
parenthesesfor each region. Area-normalizedproductionis calculatedin two ways.The mean of all
productivewaters(<70% ice coverand somelight availablefor photosynthesis)
wascalculatedmonthly
andthen summed(column1). For calculatingcolumn2, total productionwasdividedby total area,which
includesnonproductive
waters(waterswith >70% ice coveror no light duringaustralwinter).
MOORE AND ABBOTT: SOUTHERN
OCEAN PHYTOPLANKTON
underestimate chlorophyll concentrationsin the Southern
Ocean [Mitchelland Holm-Hansen,1991;Sullivanet al., 1993;
Arrigo et al., 1994]. In contrast,the CZCS SOP algorithm
values are higher than the in situ means.There is reasonto
believethat the CZCS SOP algorithmmayhaveoverestimated
pigmentconcentrationat low pigmentconcentrations.
Arrigoet
al. [1994]and Sullivanet al. [1993]comparedCZCS dataprocessedwith the GP and SOP algorithmswith a large in situ
data set averagedover 1ø latitudinal bands.Both algorithms
explained71% of the varianceseenin the in situ data set,with
the SOP algorithmperformingmuchbetter at higherpigment
CHLOROPHYLL
28,715
locationof the Antarctic Divergence(AD), where strongupwellingoccursas a resultof wind-drivendivergenceof surface
waters[seeComisoet al., 1993].This upwellingis clearlyseen
in oxygenconcentrationsat 100 m depth [Gordonet al., 1986].
The recent upwelling of these waters may accountfor the
low-chlorophyllvalues observedin this area. Relatively low
chlorophyllconcentrationsare also seenin the Indian sector
---100ø-130øE
at similarlatitudes(Plate lb) in anotherregion
of high winds and strong upwellingduring austral summer
[Comisoet al., 1993].
Phytoplanktonbloomswith chlorophyllvaluesexceeding1.0
concentrations
(>-0.6 mgm-3) [Sullivan
et al., 1993].How- mgm-3 areseenin severalareas.The highest
valuesoccurin
ever, a regressionof the SOP algorithmestimatesof surface
pigment versusin situ data had the equationy - 0.16 +
--3
0.905x [Sullivanet al., 1993].The largey offsetof 0.16 mg m
impliesthat the SOP algorithmmaysubstantially
overestimate
pigmentconcentrations
at lower chlorophyll(pigment)values
coastal and shelf waters
associated with
Southern
Ocean
is-
lands and continents.Thus high-chlorophyllvalues are seen
surroundingand downstreamof Kerguelen Island (---50øS,
70øE),over the large shelf region on the east coastof South
America, and in shelfregionsassociated
with Africa, Australia,
(<---0.6mgm-3, i.e., by 50% wherechlorophyll
is 0.32mg and New Zealand. High-chlorophyllvaluesare alsoseenin the
m-3). SinceovermostoftheSOchlorophyll
concentrations
do
notexceed
0.6mgm-3, useof theSOPalgorithm
maysignificantly overestimatechlorophyllconcentrationover much of
the SouthernOcean.The SOP algorithmwas developedfrom
a data set from AntarcticPeninsulawaters,with no total pig-
southern Weddell
and Ross Seas and in other coastal Antarctic
waters.
Monthly percentageseaice coverfor the November1997 to
February 1998 period is shownin Plate 2. During November
the sea ice cover was still near its maximal winter extent over
mentvalues<0.5 mgm-3 [Mitchell
andHolm-Hansen,
1991]. mostof the SouthernOcean.There wasa rapid meltingof the
In additionto the relativelyhigh pigmentvaluesin Antarctic
Peninsulawaters,species-related
pigmentpackagingeffectsin
this regionmay not be representativeof the whole SO. Some
phytoplanktonspeciessuchasPhaeocystis
spp. are commonly
found near the Antarctic Peninsulabut typicallyare a minor
componentof the assemblages
farther north. Thus application
of this algorithm to all SO waters, particularly to lowchlorophyllSubantarcticwaters,may overestimatechlorophyll
concentrations.This highlightsthe problemswith regional
chlorophyllalgorithms:namely,overwhat area shouldthey be
seasonalseaice coverduringDecemberandJanuary(Plate 2).
Most of this seasonal retreat
of the sea ice cover occurs in the
Weddelland RossSeas(Plate2). By comparingPlates1 and2,
it can be seenthat large phytoplanktonbloomsoccurin areas
wherethe seaice hasretreatedin the Weddell Sea (---0-30øW,
below ---60øS).Similarphytoplanktonbloomsare apparentin
the Ross Sea ---150ø-175øW,below ---60øSduring December
and below ---65øSduring January and February (compare
Plates 1 and 2). It is important to note that phytoplankton
applied
andhowisthetransition
between
regions
•managed?bloomsdo not occurin all areasof sea ice retreat (compare
Plates1 and 2). For example,chlorophyllvaluesremainvery
In general,the SeaWiFS estimatesare in better agreement
low between ---10ø-60øEand 60ø-65øSdespitethe retreat of
with the in situ data than either CZCS data set. In part, this
likely reflectsthe better temporaland spatialcoverageby Sea- sea ice here over the November-Januaryperiod (compare
WiFS. The largest gaps in the CZCS coverageoccur in the Plates1 and 2). In other areaswhere ice coveris retreating,
low-chlorophyllwaters of the Pacific sector [Comisoet al., includingparts of the Weddell and Ross Seas, chlorophyll
1993].Large areaswith no valid chlorophylldata alsooccurin
the Indian sector(generally,a low-chlorophyllarea;seePlates
1 and 3) during most austral summer monthly composites
[Comisoet al., 1993].Thesegaps_may
partiallyaccountfor the
higher mean values in the CZCS SOP data set. The good
agreement between SeaWiFS and the in situ data argues
stronglythat the globalchlorophyllalgorithmisworkingwell in
the SO and, at least for Subantarcticwaters, a regionalalgorithm is not necessary.At higher latitudes,SeaWiFS may be
underestimatingchlorophyll concentrations,although to a
lesserextentthan did the CZCS (Table2) [Mooreetal., 1999b].
concentrations
alsoremainquitelow(<0.3 mgm-3). Possible
causesfor thesevariationsin ice edgebloom dynamicsinclude
variationsin wind speed(and thusmixedlayer depth),differential accumulationof atmosphericdust on the ice (which
affectsthe amountof iron releasedto the water columnduring
melting), and differences in Ekman-induced upwelling/
downwellingby the winds.
A largepolynyaopenedin the southernWeddell Sea during
earlyaustralspring(Figures2a and2b). This earlymeltingand
consequentlow summerice extent in the southernWeddell
Sea is very unusual[seeComisoand Gordon,1998] and had a
stronginfluenceon the biota. A strongphytoplanktonbloom
4.2. Chlorophyll Distributions in the Southern Ocean
developedwithin the polynya,which eventuallyencompassed
Monthly mean chlorophyll concentrationsduring austral the entiresouthernWeddellSea(at latitudes>70øSin the west
springandsummerare shownin Plate 1 (areasin blackhadno and >---73øSin the east;Plates 1 and 3). Analysisof weekly
valid chlorophylldata becauseof heavysea ice or persistent maps of chlorophyllconcentrationand sea ice cover in this
cloud cover). Over most of the SouthernOcean, mean chlo- region indicatesthat this phytoplanktonbloom developedas
rophyllconcentrations
remainlow despitethe availablenitrate soon as the polynyaopenedup and persisteduntil the ninth
andphosphate
in surface
waters(typically
<0.3-0.4 mgm-3, week of 1998 when heavy sea ice cover rapidly coveredthe
Plate 1). Very low chlorophyllvaluesare seennorth of--•35øS region. Comisoet al. [1990] also observedhigh chlorophyll
persistinginto March in the Weddell Seawith
within the subtropicalgyres,in the southern half of Drake concentrations
Passage(---80ø-60øW),and along---60ø-65øS
between20ø and the CZCS. Reflection of light from ice- and snow-covered
70øE.This last area of low chlorophyllalong60øSmarksthe areasmay bias the SeaWiFSdata near the coast(within
28,716
MOORE AND ABBOTT: SOUTHERN
OCEAN PHYTOPLANKTON
km). However, the large size of this bloom in the southern
Weddell Sea precludesits being an artifact of this type.
The remainingareas of high-chlorophyllvaluespresentin
the monthlycompositesshownin Plate 1 are associated
with
the major fronts of the SouthernOcean.This is illustratedin
Plate 3 wheremean chlorophyllfor the periodDecember1997
to February1998is shownalongwith the mean positionof the
major SouthernOcean fronts. Moving from high to low latitudes, the fronts depicted include the SACCF, the PF, the
SAF, and the SSTF and NSTF. Also depictedin the region
southof Africa is the AgulhasCurrent (AC).
Frontal-associated
phytoplanktonbloomsat higherlatitudes
often occur in regionswhere the fronts interact with large
topographicfeatures [Moore et al., 1999b]. Thus persistent
phytoplanktonbloomsare seenat the SACCF whereit follows
the SoutheastIndian and Pacific-AntarcticRidges(---160øE
145øW)and alongthe Mid-Atlantic Ridge (---5ø-30øE)(Plate
3). Likewise,bloomsat the PF can be seenalongthe PacificAntarctic Ridge (especially---145ø-160øW),at the Falkland
Plateau (---48ø-38øW),and along the Mid-Atlantic Ridge
(---20øW-30øE).
Mesoscalephysical-biological
interactionswhere Southern
Ocean fronts interact with topographycan stimulatephytoplanktonblooms[Mooreet al., 1999b].Two physicalprocesses
can result in an increased
nutrient
flux from subsurface waters
to the surfacelayer in theseregions.Vertical mixingmay be
increasedby eddy actionsas relative vorticity added to the
water columnby the large changesin oceandepthis dissipated
[Mooreet al., 1999b].Second,in theseregionsof strongtopographicinteractionand just downstream,mesoscalemeandering of the PF is intensified(at spatialscales<100 km) [Moore
et al., 1999a,1999b].Suchmeanderingleadsto localizedareas
of upwellingand downwelling,which can stronglyinfluence
oceanbiota [Flierland Davis, 1993]. In addition,cross-frontal
mixingcould stimulatephytoplanktongrowthif different nutrients are limiting growth north and south of a particular
front. For example,there is often a stronggradientin silicic
acid concentrationsacrossthe PF, resultingin silicalimitation
of diatomgrowthto the north and, perhaps,iron limitationto
the south [Trgguerand Jacques,1992; Comisoet al., 1993].
Likewise,in the subtropicalfrontal region,nitrate availability
likely limits phytoplanktongrowthrates in the gyreregionsto
the north, while iron limitation is more likely to the south
[Sedwicket al., 1997].
Phytoplanktonblooms at the SAF, SSTF, and NSTF are
apparentin the Atlantic and Indian sectorsbut are generally
lacking in the Pacificsector.Iron inputs at midlatitudesfrom
atmosphericdepositionare much lower in the Pacific comparedwith the Atlantic and Indian Oceans[Duceand Tindale,
1991].Phytoplanktonbloomsare apparentin the vicinityof the
SAF in the southeastPacificduringDecember1997(Plateslb
and 3). This is notablebecausephytoplanktonbloomswere not
observedin this regionwith the CZCS sensor[Sullivanet al.,
1993; Comisoet al., 1993].
Intense,persistentphytoplanktonbloomsare alsoassociated
with the AC and the SSTF in the African sector(---20ø-70øE)
(Plates1 and 3). The AC is influencedby topographic
features
in thisregionand interactswith, and at timesmergeswith, the
SSTF andthe SAF [seeBelkinand Gordon,1996].Comisoet al.
[1993] alsonoted high chlorophyllvaluesin this region.They
attributedthesephytoplanktonbloomsto the higheddykinetic
energyand possiblecurrent-inducedupwellingin this region
[Comisoet al., 1993].
CHLOROPHYLL
One way to quantify these chlorophylldistributionsis to
divide the SouthernOcean into ecologicalregions.The MIZ
covered
a largeareaduringDecember(5.5millionkm2) and
January
(4.3millionkm2).Thelocationandshapeof theMIZ
changesconsiderablyfrom month to month duringthe spring/
summerperiod (Plates4b and 4c). The size of the MIZ was
negligiblysmall over most of the rest of the year, reachinga
minimumduringMay 1998(<1800km2).
Monthly mean chlorophyllconcentrations
within theseecologicalregionsare displayedin Plate 5. Maximum chlorophyll
levels were observed in the southern Weddell and Ross Seas,
witha peakvalueof 5.5mgm-3 duringDecember
1997.High
chlorophyllvalueswere alsoobservedin the MCR and within
the MIZ. Mean chlorophyllvaluesremainedpersistently
low in
the otherregions,
typically
below0.3 mg m-3 (although
the
PFR reached
0.43mgm-3 duringDecember;
Plate5).
Mostregions
showed
a seasonal
peakin chlorophyll
concentration during December in phasewith the seasonalsolar radiation cycle.Exceptionsto this pattern include the MCR,
where little seasonalitywas observed,and the MGR, where
there was an inversepattern, with a minimum during austral
summer.The seasonalpattern within the MGR is similar to
that observed in the North Central Pacific Gyre at station
ALOHA [Letelieret al., 1993].In that regionthe seasonalcycle
is primarilydue to photoadaptation,with chlorophyllper cell
in the surfacelayerincreasingduringwinter months[LetelJer
et
al., 1993].Another possibilitywould be increasednutrientin-
put (and phytoplankton
biomass)duringwintermonthsin the
MGR becauseof deeperwind mixing.The SIZ and MIZ regionshad peak mean chlorophyllvaluesin Januaryand February,respectively
(Plate 5).
4.3. Primary Production in the Southern Ocean
Primary productionwithin the Southern Ocean, summed
over seasonal timescales, as estimated with the VGPM
is
shownin Plate 6. The highestproductionoccursduringaustral
summer,with maximum values in the coastal/shelfwaters off of
South America, Africa, Australia, and New Zealand and in the
southernWeddell and RossSeas(Plate 6b). Elevatedproduction is alsoseenin the STFZ during all seasonsin the Atlantic
and Indian sectors and, to a much lesser extent, in the Pacific
sector(comparePlates3 and6). This highprimaryproduction
reflects the generally elevated chlorophyllvalues within the
STFZ in the Atlantic and Pacific basins,combined with favor-
able SSTs(seediscussion
below) (Plates1, 2, and 6).
At high latitudes,heavy sea ice cover and the seasonalinsolation cycle lead to strong light limitation and negligible
primary productionbelow 60øSduring australwinter (Plate
6d). The effect of sea ice distributionis also apparentduring
australspring(comparePlates2a and 6a). Someproduction
occurswithin the seasonalice zone during australfall prior to
the seasonalseaice expansion(Plate 6c). North of thesehighlatitude areas,significantprimaryproductionoccursduringall
seasons(Plate 6).
We next examine seasonalprimary production patterns
within the ecologicalregionsdescribed.Productionper unit
area was calculatedusingmethod 1, averagingall pixelswith
someproductionin a givenregion (i.e., ice cover<70% and
somelight availableduringwinter months;Plate 7). Primary
productionper unit area washighestin the coastal/shelf
areas
of the WRR and MCR (Plate 7). All regionsexceptthe MGR
showmaximumproductionduring austral summer,with the
MOORE AND ABBOTT: SOUTHERN OCEAN PHYTOPLANKTON
amplitude of this seasonalcycle increasingsharplywith increasinglatitude (Plate 7).
SST has a strongimpact on the model estimatesof primary
productionby settingthe localmaximumrate of primaryproduction [Behrenfeldand Falkowski,1997]. Within the lowchlorophyllregions,there is a latitudinaltrend with the highlatitude (cold SST) regionshavinglower primaryproduction
valuesthan regionsfarther north (warmerSST values)(Plate
7). Thus, despitehaving higher biomassthe SIZ and MIZ
regionsgenerallyhavelowerarea-normalizedproductionrates
relativeto the other regions(comparePlates5 and 7). This
temperatureeffect alsoaccountsfor the moderatelyhigh productionvalueswithin the SWR despitegenerallylow chlorophyllconcentrations
(Plates5 and 7). Likewise,the warm SST
valuesassociatedwith the midlatitudegyrespartially make up
for the very low biomass/chlorophyll
concentrationsin this
region (Plates 5 and 7). Finally, comparingthe two coastal
regions,the warmer SST valuesin the MCR raiseproduction
values here to higher levels than in the cooler but higher
chlorophyllwatersof the WRR (Plates5 and 7).
If we examinetotal primaryproductionby ecologicalregion,
it is apparentthat the SWR accountsfor most of the production within the SO becauseof its large areal extentand moderatelyhighprimaryproductionduringall seasons
(Plate 8). A
weak seasonalcycleis seenin the MCR and MGR regions,and
a strong seasonalcycle is apparent with increasinglatitude.
Despite its high chlorophylllevels and primary production
values the total carbon fixed within the WRR
is small because
5.
CHLOROPHYLL
28,717
Discussion
The patternsin chlorophylldistributiondescribedhere are
qualitativelysimilarto thoseof previousstudiesof the SO with
the CZCS sensor[Sullivanet al., 1993; Comisoet al., 1993].
There are some notable differences,most likely due to the
sparseCZCS coveragein this region.Extensivephytoplankton
bloomswere observedin the southeastPacificsectorduring
December1997(Plate lb). No bloomswereseenin thisregion
in the CZCS data, and silica limitation was suggestedas a
possiblecause[Sullivanet al., 1993;Comisoet al., 1993].Silica
limitation or perhapsiron limitation may accountfor the lack
of bloomshere in Januaryand February(Plate 1). The plume
of elevatedchlorophyllvaluesextendingdownstreamfrom the
KerguelenPlateau(beginningat -70øE; Plate lb) waslarger
than observedpreviously[Sullivanet al., 1993; Comisoet al.,
1993].The bloomin the southernWeddellSea(Plate lb) was
much larger and extendedfarther west than was observedwith
the CZCS, likely becauseof the unusuallystrongsummerice
melt during this season[Sullivanet al., 1993; Comisoet al.,
1993;Comisoand Gordon, 1998].
At high latitudes(>50øS), maximumproductionwas esti-
matedwithintheWRR at 1.39gCm-2 d-• duringDecember
1997,withanannualproduction
of 156gCm-2yr-• (Table3).
Severalstudieshavemeasuredprimaryproductionon the Ross
Seashelfat rates> 1.0gCm-2 d- • [Smithetal., 1996;Bateset
al., 1998]. Nelson et al. [1996] combinedin situ data from
severalseasonsto estimatean annual primary productionon
theRossSeashelfof 142gCm-2 yr-•. Themodeling
studyof
of a relativelysmall spatialextent.
Amigoet al. [1998]usedCZCS chlorophyllconcentrations
(corAnnual primary productionestimatesfor the various eco- rectedwith the SOP algorithm)to estimateprimaryproduction
logical regionsof the SO are summarizedin Table 3. Also of 3.94and1.44gC m-2 d-• for the RossandWeddellshelf
shownin Table 3 are the percentages
of the total SO primary areas,respectively,during December. By combiningall shelf
productionand area accountedfor by each subregion.Note areasfor the monthsof DecemberandJanuary,theyestimated
that area values in Table 3 are the maximum total area before
the average
production
to be -1.5 gC m-2 d-• [Amigo
et al.,
correctingfor seaice coverandfor completelight limitationat
highlatitudesduringaustralwinter.For example,note that the
1998].
Weferand Fischer[1991]estimatedannualprimaryproduc-
total areabelow50øSis 45.7 millionkm2;however,the maxi- tionin thePolarFrontZoneto be83gCm-2 yr-• onthebasis
mum area over which the model was run was 41.3 million km 2
of sedimenttrap data in the Atlantic sector,similarto our PFR
(duringFebruary1998,minimumice extent),similarto previous studies[seeAmigoet al., 1998]. Annual area-normalized
productionwas calculatedby the two methodsdescribedin
section2. Total production(column3) wascalculatedby summing the monthlyproductionmaps.
Note that the effectivegrowingseasonfor phytoplankton
declinesfrom 12 months per year at low latitudes to <6
estimateof 82.0gC m-2 yr-• (Table3). For the combined
monthsper year at the highestlatitudes.Thus the WRR has
higher productionper unit area than the SWR over monthly
timescales,but total productionis similar over annual time-
POOZ and SIZ regionsthey estimatedannual primary pro-
ductionto be 21.5gCm-2 yr-• [Wefer
andFischer,
1991].This
is considerablylessthan our estimatesof 62.3, 54.2, and 50.8
gCm-2yr-• forthePOOZ,MIZ, andSIZ,respectively
(Table
3). If nonproductivewaters are includedin calculatingareanormalizedproduction(total productiondividedby maximum
SIZ extent;Table 2), mean SIZ productiondropsto 29.1 gC
m-2 yr-• (Table3).Amigoet al. [1998]estimated
annualprimaryproduction
tobe-130 gCm-2 inpelagic
waters(>50øS),
scales(Plate 7 andTable 3). High chlorophyll(phytoplankton -141 gCm-2 for theMIZ, and-184 gCm-2 for shelfwaters
biomass)andgrowthduringall seasons
(in mostregions)result (annualmean productioncalculatedfrom Amigoet al. [1998,
in a highmean annualproductionwithin the MCR of 528.8gC Table 3] usingour method1).
m2 yr-•, whichaccounts
for 9.2%of totalSO production
but
Comparisons
betweenour MIZ valuesandthoseofAmigoet
only 2.4% of the area (Plate 7 and Table 3). The bulk of al. [1998]are difficultbecauseof the differentdefinitionsused
primary productioncomesfrom the midlatitude regions,with in definingthis region.Different percentageice covercutoffs
areas poleward of 50øS accountingfor only 20.1% of total were used,and in addition,we excludedthe highbiomassareas
primary productionwhile occupying42.2% of the total area of the southernWeddell and RossSeas(the WRR) from our
(Table 3). This reflectsa shortergrowingseasonat high lati- MIZ region. The maximum estimatedMIZ production rates
tudes, which is a function of strong light limitation during duringJanuary
was0.336gC m-2 d-• (seePlate7). Thisis
winter months.Total primary productionin Antarctic Surface considerablylower than the rates estimatedby Amigo et al.
Waters, i.e., areas from the PF south to Antarctica, is 1.42 Gt
[1998] and the estimatesof Smithand Nelson[1986] of 0.571
C yr-• froma maximum
areaof 30.74millionkm2 (Table3: gCm-2 d-• fortheWeddellSeaand0.962gCm-2 d-• forthe
WRR + SIZ + POOZ + PFR).
Ross Sea based on in situ measurements.
Our
estimate
of
28,718
MOORE AND ABBOTT: SOUTHERN OCEAN PHYTOPLANKTON CHLOROPHYLL
lli lllll--'
I! •
•
i:
i ....
I ....
I.....
•'•IIi- i [
Ill Illl [
i
(•m/ lira)Ii.(qdo.lolqD
I
lJ• ......
cS
MOORE AND ABBOTT: SOUTHERN OCEAN PHYTOPLANKTON CHLOROPHYLL
\
f
I
ß
i
o
(:)3) uoD•npo•I
I
!
•
I "'i
!..........!
o
o
(qmom/•m/3•)
uoganpo.M
•m•. d
28,719
28,720
MOORE AND ABBOTT: SOUTHERN
OCEAN PHYTOPLANKTON
CHLOROPHYLL
annual total MIZ production(0.10 Gt C; Table 3) is also
considerablylessthan previousestimates(0.38 Gt C [Smith
andNelson,1986]and0.41 Gt C [Arrigoet al., 1998]).Our total
SIZ productionwas0.472 Gt C (Table 3).
Severalfactorscould accountfor the differing estimatesof
MIZ productivity.The differing definitionsfor the MIZ resulted in larger areal extent in both of these other studies.
MaximumMIZ extent(duringDecember)was6 [Arrigoet al.,
mated chlorophyllconcentrationsover much of the SO. If so,
estimatesbasedon thesechlorophyllconcentrations
would be
too high. The model of Arrigo et al. [1998] wasbasedon the
tion, which is below 30øS, increasesthe total to ---11.2 Gt C
from
yr-• [Longhurst
et al., 1995,Figure2, Table2].
-•6.598 x l0 •3 m2. Thus the net productionestimates
of
satellite data and the available
nitrate
and silicic acid but did
not allow for iron limitation of phytoplanktongrowth rates.
This likely leadsto overestimates
of production,particularlyin
open ocean waters [see Moore et al., 2000, and references
therein].
The satellite-basedestimatescan be comparedwith esti1998]and6.84millionkm2 [SmithandNelson,
1986],whileour
valuewas5.5 millionkm2. We excluded
the high-productivitymatesthat extrapolatefrom a limited numberof in situobserWRR from our MIZ areas. If WRR areas are not excluded, vations.This approachlacksthe high spatial coverageof the
our total MIZ productionis increasedto 0.114Gt C. Smithand satellitedata.Smith[1991]estimatedtotal productionsouthof
Nelson[1986]usedthe averageof the Weddelland RossSeain the PF at 1.23 Gt C. Priddleet al. [1998] estimatedproduction
situ measurements
to calculatetotal production.These areas southoftheSAFat ---1.7Gt Cyr-•, or ---30-40g C-2yr-•, on
had the strongestice edgebloomsduringthe 1997-1998season the basis of estimatesof carbon consumptionby top level
and muchhigherchlorophyllvaluesthan in other areasof ice predatorsand the productionneeded to supportit and on
retreat (Plates 1 and 2). Thus applyinga productionmean seasonal nutrient drawdown at several locations. These methcomputedfrom theseareasmay overestimateproductionelse- odsmay haveunderestimatedtotal production.Extrapolation
where. Mean MIZ productionduring Decemberin the Wed- from a limited numberof locationsis unlikely to be accurate
dell Sea (0-70øW, not excludingWRR areas)was 0.744 gC given the large degreeof regional and latitudinalvariability
m-2 d-•, comparable
to theSmithandNelson[1986]estimates demonstratedin thispaper.In their foodweb model,Priddleet
but more than twiceour averagefor the whole MIZ duringthis al. [1998]did not allowfor directsinkingof diatomsout of the
month.Mean MIZ productionin the RossSea(160øE-140øW, surfacelayer, which may be an important loss processand
notexcluding
WRR areas)was0.401gCm-2 d-•. Thusspatial would increaseproductionestimates.Estimatesbasedon nuand interannualvariabilitycouldbe importantfactors.A third trient drawdownmay underestimateproductionbecauseof
factor that may be relevantis the use of SST to estimatelocal recyclingwithin the surfacelayer.
Seasonalvariations of atmosphericoxygenconcentrations
maximumproductionrates in the VGPM. The regressionof
B
Poptversustemperature
of Behrenfeld
andFalkowski
[1997] can be used to estimatelarge-scaleoceanicproductionand
may underestimatemaximumproductionratesin coldice edge global-scalecarbonsinks[Keelinget al., 1993;Benderet al.,
surfacewatersof the Antarctic as samplesof thesewatersare 1996].Our total productionestimates(scaledby appropriate
f
minimallyrepresentedin their in situ data set.
ratios) can be comparedwith these atmospheric-based
estiOur estimatefor total primaryproductionat latitudes>30øS matesof seasonalnet production.Benderet al. [1996] examwas14.2Gt C yr-• (Table3).Longhurst
etal. [1995]estimated ined seasonalO2/N2 ratios in the SouthernHemisphereand
primaryproductionwithin differentregionsof the globalocean argued that a seasonalminimal net oceanic production of
for the
usingCZCS data (GP algorithm)combinedwith a primary ---2.6-3.6molCm-2 yr-• wasrequiredto account
productionmodel. The total productionwithin the four SO seasonalatmospheric02 variationsbasedon the modelresults
regionswas8.2 Gt C yr-• [Longhurst
et al., 1995].Thistotal of Keelinget al. [1993].This representsa seasonalnet produclatitudes
of ---31.2-43.2
gCm-2, witha mean
does not include coastalproductionor the productionfrom tionin temperate
et al., 1993;Bender
et al., 1996].The
mid-oceangyre regionsbelow 30øS.Adding productionfrom of ---36gC m-2 [Keeling
theseother regions,after multiplyingby the approximatefrac- temperateband of Keelinget al. [1993] consistedof latitudes
Our estimatedtotal primary productionfor all areasat lat-
23.6 ø to 44.4øS. Total
ocean
area
over
this band
is
Benderet al. [1996] representa total seasonalnet production
itudes>-50øS was2.9 Gt C yr-• (Table3). Longhurst
et al. overthisbandof (6.598x 10•3) x 36 = 2.37x 10•sgC,or2.37
[1995]estimated
totalproduction
of 2.24Gt C yr-• for their Gt C. This is a roughestimatethat ignoreslatitudinalmixingin
Antarctic and Antarctic Polar regions.These regionsaccount the atmosphereacrossbands.However,suchmixingis limited,
for most of the area below 50øS.If primary productionfrom as evidencedby the muchweaker seasonalcyclein oxygenat
half of their Subantarctic
regionis added(approximateportion lowlatitudesand the oppositephasingof the seasonalcyclesin
below50øS),
thetotalisincreased
to 4.01Gt C yr-• [Longhurstthe Northern and SouthernHemispheres[Keelinget al., 1998].
The mean Southern Hemisphere seasonalcycle in atmoet al., 1995].Arrigoet al. [1998] estimatedproductionfor all
waters>50øSat 4.4 Gt C yr-• on the basisof totalpigments spheric 02 increasesover the 4 month period October(with the SOP correctionto CZCS data),with lowerestimates January,with an amplitudeof ---70per meg, and is largelythe
of 3.24and3.95Gt C yr- • aftercorrecting
forphaeopigments,result of oceanicprocessesbelow 30øS[Keelinget al., 1998].
which were not distinguishablefrom chlorophyllwith the Approximately81% of the seasonalcycleat midlatitudesis due
CZCS. A recent run of the VGPM model using the SOP to net oceanicproduction[Keelinget al., 1993].A similarseaalgorithmcorrectionsfor the CZCS data and correctingfor sea sonalamplitudein oxygenis seenat the SouthPole [Keeling
et
ice covergavean annualproduction
of 2.9 Gt C yr-• (M. al., 1998].Of the 3 yearsexaminedby Benderet al. [1996]the
1993-1994seasonmostcloselyresembledthe long-termmean
Behrenfeld,personalcommunication,1999).
These model estimatesare heavilydependenton the satel- cycleat southernmidlatitudes[seeKeelinget al., 1998].The
lite surfacechlorophyllconcentrationsused. If SeaWiFS un- 1997-1998seasonalatmosphericoxygencyclewasnearlyidenderestimatessurfacechlorophyllconcentrations,our estimate tical to the meanpatternof Keelinget al. [1998](R. Keeling,
of primaryproductionwould be too low. Alternatively,as dis- personalcommunication,1999). The 1993-1994estimatefor
cussedin section4.1, the SOP algorithm may have overesti- minimalnet oceanicproductionby Benderet al. [1993]was36
MOORE AND ABBOTT: SOUTHERN
OCEAN PHYTOPLANKTON
gCm-2, which,ascalculated
above,wouldimplya seasonal
net
CHLOROPHYLL
28,721
bution of CO2 species,estimatesof net communityproduction,and
air-seaCO2 exchangein the RossSeapolynya,J. Geophys.
Res.,103,
oceanicproductionof 2.37 Gt C. On the basisof severalcon2883-2896, 1998.
siderations,Benderet al. [1993] arguedthat actualoceanicnet Behrenfeld, M. J., and P. G. Falkowski,Photosyntheticrates derived
productionwas higher than their minimum estimatesby a
from satellite-basedchlorophyllconcentration,Limnol. Oceanogr.,
42, 1-20, 1997.
factor of--• 1.5.This net productionwould increasethe total net
productionestimateto 3.56 Gt C. Can our estimatedprimary Belkin, I. M., Main hydrologicalfeaturesof the central South Pacific,
in Pacific SubantarcticEcosystems
(in Russian),edited by M. E.
productionat latitudesbelow 30øSaccountfor a seasonalnet
Vinogradov and M. V. Flint, pp. 21-28, Nauka, Moscow, 1988.
productionof 2.37-3.56 Gt C overthe October 1997to January
(English translation,pp. 12-17, N. Z. Transl. Cent., Wellington,
1998 period?
1997.)
Our estimated total primary production for the area 30ø- Belkin, I. M., Frontal structureof the SouthAtlantic, in Pelagicheskie
EkosistemyYuzhnogoOkeano(in Russian),edited by N.M. Voro60øSover the 4 month period October 1997 through January
1998was6.2Gt C or 71.4gCm-2. Theseasonal
netproduction
impliedby the atmospheric02 data would then representanf
ratio (the ratio of new to total production)in the range of
0.38-0.57. Typicalf ratiosfor thesemainlySubantarcticwaters
range from 0.3 to 0.5 [Banse,1996]. Seasonalnet production
can exceedshort-termestimatesof new productionfor a number of reasons[seeBenderet al., 1996]. Seasonalnet production can have highf ratios relative to annual new production
becausesomeof the organicC formedbut not respiredduring
spring/summer
monthsis respiredlater in the season.In addition, somefixed organicmatter sinksand is respiredbelowthe
surfacemixedlayer (but still in the euphoticzone) and thus
mightnot influencespring/summer
02 outgassing.
Thus,taking
these factorsinto account,primary productionin SO midlatitude waters is sufficient
to account for the observed seasonal
nina, pp. 40-53, Nauka, Moscow, 1993.
Belkin, I. M., and A. L. Gordon, Southern Ocean fronts from the
Greenwichmeridianto Tasmania,J. Geophys.Res.,101, 3675-3696,
1996.
Bender,M., T. Ellis, P. Tans, R. Francey,and D. Lowe, Variability in
the O2fN2 ratio of SouthernHemisphere air, 1991-1994: Implications for the carbon cycle, Global Biogeochem.Cycles,10, 9-21,
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Coale, K. H., et al., A massivephytoplanktonbloom inducedby an
ecosystem-scale
iron fertilization experiment in the equatorial Pacific Ocean, Nature, 383, 495-501, 1996.
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ice minimum, coastalpolynyasand bottom water formation in the
Weddell Sea,AntarcticSea Ice: PhysicalProcesses,
Interactions,and
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293-315, AGU, Washington,D.C., 1998.
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Satellite ocean color studiesof Antarctic ice edgesin summer and
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de Baar, H. J. W., J. T. M. de Jong,D.C. E. Bakker, B. M. L6scher,
C. Veth, U. Bathmann,and V. Smetacek,Importance of iron for
plankton blooms and carbon dioxide drawdown in the Southern
cyclein atmospheric02 documentedpreviously[Benderet al.,
1996;Keelinget al., 1998].
SeaWiFS data are providingthe most comprehensivelook
yet at the distributionof chlorophyllin the SO. Severalother
satelliteoceancolorsensorsare scheduledto go into orbit over
the next decade. Thus SeaWiFS is laying the baseline for a
long-termcontinuoustime seriesof globalsurfacechlorophyll
Ocean, Nature, 373, 412-415, 1995.
measurements.Combining satellite data with ocean circula- Duce, R. A., and N. W. Tindale, Atmospherictransportof iron and its
depositionin the ocean,Limnol. Oceanogr.,36, 1715-1726, 1991.
tion, climate,andbiogeochemical
modelswill providevaluable
Eppley,
R. W., and B. J. Peterson,Particulateorganicmatter flux and
tools to improve our understandingof the ocean ecosystems
planktonicnew productionin the deep ocean,Nature,282, 677-680,
and the globalcarboncycle.
1979.
Flierl, G. R., and C. S. Davis, Biologicaleffectsof Gulf Stream meandering,J. Mar. Res.,51,529-560, 1993.
Acknowledgments. The authorswould like to thank Dave Nelson, Fukuchi,M., Phytoplanktonchlorophyllstocksin the AntarcticOcean,
Jim Richman,Ted Strub, and Ricardo Letelier for helpful comments
J. Oceanogr.Soc.Jpn., 36, 73-84, 1980.
and suggestions.
The authorswould also like to thank the SeaWiFS Gordon, A. L., E. Molinelli, and T. Baker, Southern Ocean Atlas,
project(code970.2) and the DistributedActiveArchiveCenter (code
Amerind, New Delhi, India, 1986.
902) at the GoddardSpaceFlight Center, Greenbelt,MD 20771,for Gordon, H. R., D. K. Clark, J. W. Brown, O. B. Brown, R. H. Evans,
the productionand distributionof the oceancolor data, respectively.
and W. W. Broenkow,Phytoplanktonpigmentconcentrations
in the
These activitiesare sponsoredby NASA's Earth ScienceEnterprise
Middle Atlantic Bight: Comparison of ship determinationsand
Program. We would also like to thank the EOS Distributed Active
CZCS estimates,Appl. Opt., 22, 20-36, 1983.
ArchiveCenter(DAAC) at NationalSnowandIce Data Center(Boul- Gordon, R. M., K. H. Coale, and K. S. Johnson, Iron distributions in
der, CO) for the seaice data, the NationalCenterfor Environmental
the equatorial Pacific:Implicationsfor new production,Limnol.
Predictionfor the interpolatedSST data, and the SeaWiFSprojectfor
Oceanogr.,42, 419-431, 1997.
providingthe surfaceradiationdata.Thiswork wasfundedby a NASA
Keeling, R., R. Najjar, M. Bender, and P. Tans, What atmospheric
Earth SystemScienceFellowship(J.K.M.) and by NASA/EOS grant
oxygenmeasurementscan tell us about the global carbon cycle,
NAGW-4596 (M.R.A.).
Global Biogeochem.Cycles,7, 37-67, 1993.
Keeling, R. F., B. B. Stephens,R. G. Najjar, S.C. Doney, D. Archer,
and M. Heimann, Seasonalvariationsin the atmosphericO2/N2
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(ReceivedAugust16, 1999;revisedJuly 19, 2000;
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