Deep-Sea Research II 49 (2002) 1691–1706 Net community production in the marginal ice zone and its importance for the variability of the oceanic pCO 2 in the Southern Ocean south of Australia Masao Ishii*, Hisayuki Y. Inoue, Hidekazu Matsueda Geochemical Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan Abstract In the marginal ice zone (MIZ) of the Southern Ocean south of Australia at 1401E, the net community production (NCP) above the remnant temperature-minimum ðT min Þ layer was evaluated in the austral summer of 1994/95 on the basis of the deficit in normalized total inorganic carbon (NTCO 2 ) by reference to the NTCO 2 in the remnant T min layer (2182 mmol kg 1 at S ¼ 34). The NCP integrated from the preceding late winter ranged from 4 to 16 gC m 2 in December 1994 and from 6 to 30 gC m 2 in January 1995, showing a tendency to increase with time and distance from the ice edge. The biological TCO 2 drawdown and temperature-rise after the retreat of sea ice were the dominant factors controlling the spatial and temporal variability of CO 2 partial pressure in surface seawater (pCO 2 sw) in the seasonal ice zone including the MIZ. The water beneath the sea ice is considered to be significantly CO 2 -supersaturated in winter, but the effect of biological CO 2 uptake in summer on average was larger than that of the temperature-rise after the retreat of the sea ice. Hence, the pCO 2 sw showed a tendency to decrease with time and distance from the ice edge in the MIZ. The community uptake ratio of DSi/DC/DN/DP estimated from the deficits in silicic acid, TCO 2 , nitrate+nitrite and phosphate above the T min layer was 39/54/8.7/1 in the MIZ. The community uptake ratios of DSi/DP (=39) and DSi/DN (=4.5) were even higher than the uptake ratios by the diatom-dominated phytoplankton assemblage in the field incubation experiments under iron-deficit conditions (DSi/DP=26, DSi/DN=1.9–2.1; Takeda, Nature 393 (1998) 774). These results suggest that phosphorus and nitrogen, as well as carbon, in biogenic particulate matter have undergone significant preferential remineralization compared with silicate through heterotrophic activities in the surface layer. r 2002 Elsevier Science Ltd. All rights reserved. Re´ sume´ Dans la zone marginale des glaces (MIZ) de l’Oc ! ean Austral (sud de l’Australie, le long du 1401E) la production communautaire nette (NCP) est d! etermin! ee " a partir du d! eficit en carbone inorganique total normalis ! e (NTCO 2 ). La masse d’eau situ! ee sous la glace de mer est sursatur ! ee en CO 2 en hiver mais la teneur en pCO 2 d! ecroit rapidement au cours du printemps et de l’ ! et ! e. Ceci r ! esulte " a la fois d’un d! egazage de l’eau de surface " a temp! erature croissante mais aussi de la consommation biologique dont l’impact est en g! en! eral pr ! epond! erant. NCP, int ! egr ! ee dans la couche de surface au dessus du minimum de temp! erature, varie entre 4-16 gC m 2 en D! ecembre 1994 et 6–30 gC m 2 en Janvier 1995. Elle varie spatialement et temporellements avec une tendance " a augmenter au cours de la saison et au fur et " a mesure que l’on s’ ! ecarte du bord des glaces. Les rapports de consommation communautaire DSi/DC/DN/DP estim! es " a partir du d! eficit en acide silicique, en TCO 2 , en nitrate+nitrite, et en phosphate au dessus du minimum thermique sont *Corresponding author. Tel.: +81-298-53-8727; fax: +81-298-53-8728. E-mail address: [email protected] (M. Ishii). 0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0967-0645(02)00007-3
Transcript
Deep-Sea Research II 49 (2002) 1691–1706
Net community production in the marginal ice zone and itsimportance for the variability of the oceanic pCO2 in the
Southern Ocean south of Australia
Masao Ishii*, Hisayuki Y. Inoue, Hidekazu Matsueda
Geochemical Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan
Abstract
In the marginal ice zone (MIZ) of the Southern Ocean south of Australia at 1401E, the net community production
(NCP) above the remnant temperature-minimum ðTminÞ layer was evaluated in the austral summer of 1994/95 on the
basis of the deficit in normalized total inorganic carbon (NTCO2) by reference to the NTCO2 in the remnant Tmin layer
(2182mmol kg�1 at S ¼ 34). The NCP integrated from the preceding late winter ranged from 4 to 16 gCm�2 in
December 1994 and from 6 to 30 gCm�2 in January 1995, showing a tendency to increase with time and distance from
the ice edge. The biological TCO2 drawdown and temperature-rise after the retreat of sea ice were the dominant factors
controlling the spatial and temporal variability of CO2 partial pressure in surface seawater (pCO2sw) in the seasonal ice
zone including the MIZ. The water beneath the sea ice is considered to be significantly CO2-supersaturated in winter,
but the effect of biological CO2 uptake in summer on average was larger than that of the temperature-rise after the
retreat of the sea ice. Hence, the pCO2sw showed a tendency to decrease with time and distance from the ice edge in the
MIZ. The community uptake ratio of DSi/DC/DN/DP estimated from the deficits in silicic acid, TCO2, nitrate+nitrite
and phosphate above the Tmin layer was 39/54/8.7/1 in the MIZ. The community uptake ratios of DSi/DP (=39) and
DSi/DN (=4.5) were even higher than the uptake ratios by the diatom-dominated phytoplankton assemblage in the field
0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 0 0 7 - 3
de 39/54/8.7/1. Les rapports DSi/DP (=39) et DSi/DN (=4.5) sont !elev!es et vont au del"a des rapports mesur!es quand on
cultive des assemblages de phytoplancton domin!es par des diatom!ees en condition d!eficitaires en fer (DSi/DP=26, DSi/DN=1.9–2.1; Takeda, Nature 393 (1998) 774). Ces r!esultats sugg!erent qu’azote, phosphore et carbone sont
pr!ef!erentiellement recycl!es par l’activit!e des h!et!erotrophes dans la couche de surface, par rapport au silicium.
Mots-cl!es: pression partielle de dioxyde de carbone; carbone inorganique total, production communautaire nette,
rapport des d!eficits en nutriments; Ocean Austral; zone marginale des glaces.
1. Introduction
Distribution of partial pressure of CO2 insurface seawater (pCO2sw) in the Southern Oceanhas been often characterized by a large variabilityin space and time (e.g., Inoue and Sugimura, 1986,1988; Goyet et al., 1991; Metzl et al., 1991;Takahashi et al., 1993; Poisson et al., 1993, 1994;Hashida, 1993; Robertson and Watson, 1995;Bellerby et al., 1995; Bakker et al., 1997; Sabineand Key, 1998; Popp et al., 1999). Data of pCO2swshow both supersaturation and undersaturation ofCO2 in surface seawater, and whether the South-ern Ocean as a whole is a net sink or a net sourceof the atmospheric CO2 has been a matter ofcontroversy. Recent reports based on intensivemeasurements of pCO2sw (Takahashi et al., 1999;Metzl et al., 1999), diagnostic modeling (Louanchiet al., 1999), and synthesis inversion from theatmospheric CO2 data (Rayner et al., 1999) haverevealed that the Southern Ocean, in particular,the Subantarctic Zone, is a net sink of CO2 on anannual basis. The value of pCO2sw is physico-chemically determined as a function of totalinorganic carbon (TCO2), alkalinity (or pH),temperature (T), and salinity (S). Consequently,it is affected by a variety of interlinked processes,including biological activities such as photosynth-esis, respiration and export of organic matter, andphysical factors such as heat flux, freshwater flux,wind stress, and their resultant changes in theocean circulation and sea–ice coverage. However,these processes, and particularly the biologicalactivities, that control the variability in pCO2sw,have not been clearly characterized in the South-ern Ocean, and their responses to climate changeare still uncertain.
It is net community production (NCP) ratherthan primary production that is crucial in con-sidering the effects of biological activities on the
CO2 system in seawater. NCP is defined asprimary production minus respiration by all theautotrophic and heterotrophic organisms presentin the community (Codispoti et al., 1986; Minaset al., 1986). It is inherently equivalent to the netamount of organic carbon produced and, thus, tothe net amount of inorganic carbon biologicallyconsumed. Therefore, the NCP in a euphotic layerintegrated over a certain period of time not onlyconstrains the estimate of export production out ofthe euphotic layer but also determines the role ofbiological activities for the variability of pCO2sw.To quantify the temporal and spatial variability inNCP and to characterize its controlling mechan-isms are crucial in considering the impact ofclimate change on the air–sea CO2 flux, as wellas in understanding the mechanisms controllingcontemporary air–sea CO2 flux in the SouthernOcean.
In this paper, we present the meridional andtemporal variations of pCO2sw, TCO2, tempera-ture and salinity in surface waters of the SouthernOcean south of Australia (1401E) in the australsummer of 1994/95 and demonstrate the impor-tance of biological activities for the variability ofthe oceanic CO2 in the marginal ice zone (MIZ). Inthe following, we quantify the temporal andspatial variation of the NCP in the MIZ fromthe deficit in TCO2 above the remnant tempera-ture-minimum ðTminÞ layer. Finally, the signifi-cance of observing subsurface Tmin layers as aproxy for winter conditions and the implication ofthe deficit ratio of TCO2 and macronutrients in thesurface layer of the MIZ are discussed.
2. Field sampling and methods
We present the results for the oceanic CO2
measurements made aboard the R=V Hakuho
M. Ishii et al. / Deep-Sea Research II 49 (2002) 1691–17061692
Maru during its KH-94-4 cruise in the australsummer of 1994/95 (Kawaguchi, 1996). R=VHakuho Maru departed from Tokyo on November22, 1994, and sailed to the Southern Ocean to thesouth of Australia via the western North Pacific(Ishii et al., 2001) and the western South Pacific(transect 1) (Fig. 1). Measurements along meridio-nal transects in the Southern Ocean were maderepeatedly during southbound and northboundcruises on leg 2 (Lyttelton, New Zealand, toHobart, Australia; December 13, 1994–January4, 1995) (transects 2 and 4) and on leg 3 (Hobart toSydney, Australia; January 9–28, 1995) (transects
5 and 7). Much ship-time was spent in the MIZ tothe south of 641S around 1401E on each leg(transects 3 and 6) with a three-week interval(Fig. 2).
We made underway measurements of the CO2
concentration (mole fraction in dry air) in airequilibrated with seawater (xCO2sw) and inmarine boundary air (xCO2air) along with watertemperature (T) and salinity (S). The underwaymeasurement of TCO2 also was made concurrentlyin all transects except 7. In addition, TCO2 in theupper water columns was analyzed at a total ofeight hydrographic stations in the MIZ (seeFig. 2).
Seawater for these underway measurements wasdrawn continuously at ca. 5m depth by a pumpingsystem on the ship. Semi-continuous measure-ments of xCO2sw and xCO2air were made usingan automated CO2 analyzer (Inoue, 1999). ForxCO2sw analyses, seawater was introduced into ashower-type equilibrator and its water jacket,where it was equilibrated with air in closed circuit.The seawater-equilibrated air was dried with anelectric cooling unit and magnesium perchloratecolumn, and was introduced into a non-dispersiveinfrared (NDIR) gas analyzer (BINOS 4.1, Rose-mount Co.). For xCO2air measurements, a sampleof air pumped continuously from the bow of theship was dried in the same manner and wasintroduced into the NDIR gas analyzer. All NDIRoutput voltages were recorded under the ambientpressure while stopping the air stream temporarily.They were calibrated once every 1.5 h using fourCO2-in-air working standard gases (290, 340, 372,422 ppm) prepared by Nippon Sanso Co. Values ofxCO2sw and xCO2air were corrected to the 1985WMO scale, pCO2sw and pCO2air were calculatedtaking into account ambient pressure and satu-rated water vapor pressure. The temperature risein the equilibrator compared to the sea surface wasgenerally o+0.51C and was corrected for usingthe equation given by Gordon and Jones (1973).The system we used for the underway xCO2sw andxCO2air analyses is the same as that we equippedin an international intercomparison experimentonboard the R=V Meteor in June 1996 in thesubtropical and subarctic North Atlantic (K-.ortzinger et al., 1999, 2000).
120°E 140°E 160°E 180°
70°S
60°S
50°S
40°S
30°S
20°S
10°S
0°
- - - - - -
SAZ
PFZ
POOZ
SIZ
(1)
(2)(5)
(7)
(4)
(3)(6)
Fig. 1. Sites of the underway measurements along the KH-94-4
cruise track (transects 1–7) in the western South Pacific and in
the Southern Ocean. Superposed on transects are the approx-
imate positions of the major oceanic zones: SAZ=Subantarctic
zone, PFZ=polar frontal zone, POOZ=permanently open
ocean zone, SIZ=seasonal ice zone.
M. Ishii et al. / Deep-Sea Research II 49 (2002) 1691–1706 1693
Analysis of TCO2 was made coulometrically(Johnson et al., 1985, 1987) using an automatedCO2 extraction unit (Ishii et al., 1998) and acoulometer (model 5012, UIC Inc., USA). Under-way TCO2 analysis in surface seawater was madetwice every 1.5 h concurrently with xCO2swmeasurements. At hydrographic stations in theMIZ, discrete samples for TCO2 analysis weretaken from Niskin bottles on a CTD/rosettesampler together with sub-samples for salinityand nutrient analyses. They were stored in 250 cm3
ground glass stoppered borosilicate glass bottleslubricated with Apiezon-L grease after poisoningwith 0.1 cm3 of a saturated mercury(II) chloridesolution and were analyzed on board usuallywithin 18 h of the sampler’s arrival on deck. Thecoulometric system was calibrated before and aftereach leg of the cruise with suites of sodiumcarbonate solutions prepared from primary stan-dard grade sodium carbonate anhydrous (99.97%,Asahi glass Co.) dried at 6001C for 1 h and purifiedwater provided from MILLI-Q SP TOC by asimilar method to that of Goyet and Hacker(1992) under a nitrogen stream. Procedures and amethod of calculation were essentially identical tothose described in DOE (1994). For qualitycontrol, we also used the certified referencematerial (CRM) batch #20 provided by Dr. A.G.
Dickson (http://www-mpl.ucsd.edu/people/adick-son/CO2 QC/index.html) and reference materialthat we prepared from a batch of western NorthPacific oligotrophic water by a method similar tothat of Dickson (1991). Analytical precision (71 s)was estimated to be 72.5 mmol kg�1, and differ-ences in our analytical results for the CRM fromits certified value were �0.572.0 mmol kg�1 duringthe cruise.
We calculated total alkalinity (TA) in surfaceseawater from pCO2sw, TCO2, T and S using thesolubility of CO2 in seawater given by Weiss(1974), the dissociation constants of carbonic acidgiven by Roy et al. (1993), and the dissociationconstant of boric acid given by Dickson (1990).Concentrations of phosphate, nitrate, nitrite andsilicic acid in seawater were determined by themethod of Parsons et al. (1984) with the repeat-ability of 0.02, 0.2, 0.01 and 0.4 mM, respectively.
3. Results
3.1. Longitudinal distributions of the oceanic CO2
parameters
Fig. 3 shows the meridional distributions ofpCO2sw, TCO2, TA, temperature and salinity in
-10
-20
-30
-40
-50
-60
-70
Latit
ude
Dec 1
1994 1995Dec 10 Dec 20 Jan 1 Jan 10 Jan 20 Feb 1
(1)
(2)
(3)
(4) (5)
(6)
(7)
ββ
β β βø
1/ 1/ 1/ 1/1/œ œ
Fig. 2. Transects (in parentheses) in the western South Pacific and in the Southern Ocean, and positions of the hydrographic stations in
the marginal ice zone.
M. Ishii et al. / Deep-Sea Research II 49 (2002) 1691–17061694
surface seawaters in the western South Pacific andin the Southern Ocean in December 1994 (trans-ects 1–3). Analytical results of TCO2 and calcu-lated values of TA were normalized to a constantsalinity of S ¼ 34 (NX=X� 34/S, X=TCO2, TA)to correct for the influence of ice-melt andprecipitation/evaporation. These transects coveredthe subtropics in the western South Pacific andfour oceanic zones in the Southern Ocean: Sub-antarctic zone (SAZ) to the south of the sub-tropical convergence, polar frontal zone (PFZ)between the Antarctic Polar front and the Sub-antarctic front, permanently open ocean zone(POOZ), and seasonal ice zone (SIZ) (Tr!eguerand Jacques, 1992). The southern region in the SIZclose to the ice edge to the south of 641S wasrecognized as the MIZ, where significant influencesof the recent ice-melt from receding sea ice wereseen in the surface layer. The colder, less salinesurface water in the MIZ overwhelmed theAntarctic divergence, recognized in this zone asthe upward intrusion of warmer and salty uppercircumpolar deep water (Kawaguchi, 1996; Poppet al., 1999).
A large mesoscale variation in pCO2sw rangingfrom 308 to 384 matm was observed along thesetransects in the Southern Ocean, which presents astriking contrast to its rather smooth meridonalchange in the subtropics. In the MIZ, the pCO2swwas at a similar level to pCO2air on average, but inthe northern part of the SIZ, the pCO2sw wasmostly some tens of micro-atmospheres lower thanpCO2air. NTCO2 recorded its highest value(2175.3 mmol kg�1) near the ice edge at 64.41Sand decreased northward within the MIZ. In thenorthern part of the SIZ and in the southern partof the POOZ between 571S and 641S, the spatialvariation in the surface NTCO2 was large(2151.176.3 mmol kg�1, n ¼ 59), but the meridio-nal gradient was insignificant. It also presents astriking contrast to the small spatial variation inthe NTCO2 in the subtropics between 101S and201S (1887.272.8 mmol kg�1 ðn ¼ 71Þ) and to thesignificant meridional gradient from the northernpart of the POOZ to the SAZ. NTA in the POOZand the SIZ south of 541S, was 2282.574.7 meq kg�1 ðn ¼ 147Þ while NTA in the SAZbetween 481S and 521S was 2259.974.2 meq kg�1
ðn ¼ 31Þ: Since the precision of TCO2 analysis is72.5 mmol kg�1 and that of pCO2sw is expected tobe about 71 matm, the precision of NTA calcu-lated would to be 73 meq kg�1. However, it issubjected to a larger error in the Southern Oceanbecause pCO2sw, TCO2, and salinity showed largespatial variability and the spatial scale that a singledatum represents is different for each parameter.
410
390
370
350
330
310
290
pCO
2/
µat
m
air
sea1
2
3
2250
2200
2150
2100
2050
20001950
19001850
NT
CO
2 /
µm
ol k
g-1
2350
2300
2250
2200
NT
A /
µm
ol k
g-1
0
5
10
15
20
25
30
SS
T /
° C
33
34
35
36
70 60 50 40 30 20 10
Sal
inity
Latitude
! ! ! ! (a)
(b)
(c)
(d)
(e)
Fig. 3. Meridional distribution of (a) partial pressure of CO2 in
surface sea water and in the marine boundary air, (b) total
inorganic carbon normalized at S ¼ 34; (c) total alkalinity
normalized at S ¼ 34; (d) temperature, and (e) salinity in
surface seawater for transects 1–3 of the KH-94-4 cruise.
M. Ishii et al. / Deep-Sea Research II 49 (2002) 1691–1706 1695
Therefore, it seems fair to assume that thevariation in NTA in the zones of the POOZ andthe SIZ (74.7 meq kg�1) is within the uncertaintyof the estimate.
The larger spatial variability in pCO2sw in thePOOZ and in the SIZ is thus mainly ascribed tothe larger variability in surface NTCO2 and alarger buffer factor
b ¼ ðdpCO2sw=pCO2swÞ=ðdTCO2=TCO2Þ ¼ 15;
ð1Þ
as compared with those in the subtropics whereb ¼ 9: Spatial variation in temperature andsalinity ranging from �0.91C to +2.81C and33.46 to 34.01, respectively, to the south of 571S,and the larger sensitivity of pCO2sw to the changein temperature,
ðqpCO2sw=qTÞ=pCO2sw ¼ 0:0531C�1; ð2Þ
than in the subtropics (0.0401C�1) are also factorsthat contribute to the large spatial variation inpCO2sw in the POOZ and in the SIZ.
In addition to the large spatial variations,significant temporal changes in pCO2sw, NTCO2,and temperature also were observed in the MIZand in the northern part of the SIZ within thethree-week interval from December 20–28, 1994(transect 3) to January 14–21, 1995 (transect 6)(Fig. 4). The pack-ice edge retreated from 651080Sin December to 651310S in January. In bothDecember and January, the NTCO2 showed itshighest values at the sites near the ice edge and asignificant meridional gradient within the MIZ.However, from December to January, the NTCO2
on average declined 15 mmol kg�1 in the MIZ and4 mmol kg�1 in the northern SIZ. In contrast, thetemperature values were at their lowest values atthe sites near the ice edge. From December toJanuary, the temperature on average was elevated+2.21C in the MIZ and +1.61C in the northernSIZ. These changes in the NTCO2 and tempera-ture indicate the importance of biological CO2
uptake and the heat storage in these zones after theretreat of sea ice. In the POOZ, a temporaldecrease in the NTCO2 and a temporal increasein temperature also were observed, but theiramplitudes were smaller than those in the MIZand in the northern SIZ.
400
380
360
340
320
300
280
pCO
2 /
µat
m ai r
sea
2200
2150
2100
2050
NT
CO
/
2µ
mol
kg-1
2250
2300
NT
A /
µm
ol k
g-1
0
5
10
15
SS
T /
° C
35.5
35.0
34.5
34.0
33.5
30.065 60 55 50 45
Sal
inity
Latitude
(a)
(b)
(e)
(d)
(c)
Fig. 4. Meridional distribution of (a) partial pressure of CO2 in
surface sea water and in the marine boundary air, (b) total
inorganic carbon normalized at S ¼ 34; (c) total alkalinity
normalized at S ¼ 34; (d) temperature, and (e) salinity in
surface seawater for transects 3 and 4 (�), 5 and 6(+), and
7(B).
M. Ishii et al. / Deep-Sea Research II 49 (2002) 1691–17061696
As a result of changes in the NTCO2 andtemperature, temporal variation pCO2sw fromDecember to January proceeded in oppositedirections between the MIZ and the northernSIZ. In the MIZ, the pCO2sw showed a tendencyto decrease from 359711 matm ðn ¼ 212Þ inDecember to 340713 matm ðn ¼ 93Þ in January.In contrast, the pCO2sw in the northern SIZshowed a tendency to increase from 33379 matmðn ¼ 25Þ in December to 343712 matm ðn ¼ 22Þ inJanuary. Correspondingly, the air–sea CO2 fluxcalculated with in situ wind speed and the formulaof CO2 piston velocity for short-term wind speeddata given by Wanninkhof (1992) showed a trendof strengthening CO2 sink in the MIZ (from0.172.2mmolm�2 d�1 in December to�1.873.2mmolm�2 d�1 in January), but itshowed a weakening trend of CO2 sink in thenorthern SIZ (from �3.572.8mmolm�2 d�1 inDecember to �1.171.7mmolm�2 d�1 in January).
3.2. NTCO2 in the preceding winter mixed layer
In the MIZ, a temperature-minimum ðTminÞlayer was observed in the sub-surface (Fig. 5(a)). Itis the remnant of the mixed layer in the precedinglate winter, and its properties can be used as theproxies for those in the winter mixed layer(Jennings et al., 1984; Minas and Minas, 1992;Ishii et al., 1998; Rubin et al., 1998; Pondavenet al., 2000). At all eight hydrographic stations inthe MIZ at 1401E where we obtained verticalprofiles of TCO2, the Tmin layer was observedbetween depths of 40 and 200m. In those Tmin
layers in which the temperature was close to thefreezing point (Tmino�1.71C), the NTCO2 con-verged to a value of 2182.172.3 mmol kg�1 withinthe uncertainty of analysis (Fig. 6). Since theproperties in the preceding winter mixed layerare thought to have been well preserved in thesecoldest waters, we assume that the NTCO2 in thepreceding winter mixed layer was also2182.172.3 mmol kg�1.
3.3. Net community production in the MIZ
Overlying the cold Tmin layer, the surface layerwith relatively warm and less saline water was
observed (Fig. 5(a) and (b)). The vertical stabilityin the surface layer increased from December toJanuary; sigma-t at a depth of 10m decreasedfrom 27.1070.07 in December to 26.9070.08 inJanuary while sigma-t at 50m, near the base of thestratified surface layer, did not change significantly(27.5170.06 in December and 27.5570.06 inJanuary). In this surface layer, the deficits in theTCO2, NTCO2, phosphate, nitrate+nitrite, andsilicic acid were evident (Fig. 5(d)–(h)), indicatinga significant biological uptake of these compo-nents. We calculated the deficit in the NTCO2 foreach depth (DC(z)) above the Tmin layer byreferring to the value of 2182.1 mmol kg�1 inferredfor the winter mixed layer, and then calculated theTCO2 deficit in summer in the water column aboveTmin layer ðSDTCO2Þ at each hydrographic station,
SDTCO2 ¼Z 0
�z
fDCðzÞSðzÞrðzÞ=34g dz; ð3Þ
DCðzÞ=ðmmol kg�1Þ
¼ NTCO2ðzÞ=ðmmol kg�1Þ � 2182:1; ð4Þ
where SðzÞ and rðzÞ denotes salinity and density ofseawater, respectively, at depth z (Ishii et al., 1998).In general, SDTCO2 is ascribed to the net CO2
transport across the air–sea interface and the netbiological uptake of TCO2 by the community forthe production of organic matter and carbonate. Itis also subject to change due to vertical and lateralmixing. In the MIZ at 1401E, however, the net air–sea CO2 flux was small and the change in NTA wasinsignificant (see previous section). In addition,the entrainment of the subsurface water into thesurface across the Tmin layer is expected to havebeen minimal in summer, because Tmin was close tothe freezing point at most hydrographic stations(see Fig. 5(a)) and vertical stability in the surfacelayer generally increased from December to Jan-uary (see above). Therefore, assuming that hor-izontal mixing was also insignificant, we ascribeSDTCO2 essentially to the net biological uptake ofCO2 by the community for the production oforganic matter:
SDTCO2ENCP ðin the MIZ at 1401EÞ: ð5Þ
Net production of organic matter by the commu-nity, i.e. NCP ranged from 3.6 to 15.5 gCm�2 until
M. Ishii et al. / Deep-Sea Research II 49 (2002) 1691–1706 1697
late December 1994 and reached 5.6–30.1 gCm�2
in mid-January 1995 in the MIZ (Table 1). Inaccordance with the variation in the surfaceNTCO2 (see Fig. 4), the NCP was lower at stations
near the pack ice-edge and was higher in thenorthern region in the MIZ (Fig. 7). Thesetendencies suggest that NCP was higher in theregions where more time had passed after the
Nitrate / µmol kg-1 at S=340 30 60 90Silicic acid / µmol kg-1 at S=34
0 0.5 1 1.5 2
Chl-a / mg m-3
(a) (b) (c)
(d)
(e)
(f)(g) (h)
Fig. 5. Vertical profiles of (a) temperature, (b) salinity, (c) chlorophyll-a, (d) total inorganic carbon, (e) total inorganic carbon
normalized at S ¼ 34; (f) phosphate, (g) nitrate+nitrite, and (h) silicic acid in the top 150m at Stn. 13 (’), Stn. 40 (J), and Stn. 43
(B). See Table 1 for the exact location of the stations.
M. Ishii et al. / Deep-Sea Research II 49 (2002) 1691–17061698
sea-ice retreat. The NCP in the northern region ofthe MIZ at 1401E was comparable to that in theSIZ in the Indian Ocean sector and Australiansector between 301E and 1501E (10–44 gCm�2)
(Ishii et al., 1998), in the Pacific Ocean sector (9–35 gCm�2) (Rubin et al., 1998), and in the north-western Ross Sea (14–50 gCm�2) (Sweeney et al.,2000a), but was lower than in the southwesternRoss Sea (53–130 gCm�2) (Sweeney et al., 2000a).
3.4. Deficit ratio of TCO2 to phosphate, nitrate and
silicic acid
Deficits in phosphate ðDPðzÞÞ; nitrate+nitriteðDNðzÞÞ; and silicic acid ðDSiðzÞÞ above the Tmin
layer also were computed for each hydrographicstation, and they are plotted against DCðzÞ inFig. 8. The deficit ratios of DC=DP; DC=DN andDSi=DC calculated from the slopes in Fig. 8(a)–(c)were 54.372.4, 6.370.2, and 0.7370.02, respec-tively, and the overall ratio of DSi=DC=DN=DPwas 39/54/8.7/1.
The deficit ratio of DC=DN=DP; which shouldbe equivalent to the net uptake ratio by thecommunity observed here, was in good agreementwith those obtained in the MIZ off of L .utzow–Holm Bay (381E), off of Casey Bay (481E), and inPrydz Bay (751E), and in the SIZ between 591Eand 971E, where the composite plots gave the ratioof 58/9.2/1 (Ishii et al., 1998). The deficit ratio of
Table 1
Deficits of TCO2 ðSDTCO2Þ; phosphate ðSDPÞ; nitrate+nitrite ðSDNÞ; and silicic acid ðSDSiÞ in the water column above the Tmin layer
at each station in the MIZ at 1401E
Station Location Date SDTCO2 SDP (mmolm�2) SDN (mmolm�2) SDSi (mmolm�2)
(gCm�2) (mmolm�2)
11 641400S 12/19/94 6.0 500 6.8 66 531
1401010E
13 641220S 12/24/94 9.8 812 10.6 108 522
1401000E
14 651060S 12/26/94 3.6 302 4.0 33 209
1401000E
20 641010S 12/27/94 15.5 1290 20.9 191 796
1391570E
40 641100S 01/12/95 30.1 2500 37.0 371 1860
1401400E
41 651090S 01/17/95 5.6 466 6.7 73 224
1391590E
43 651250S 01/20/95 9.2 762 12.8 122 402
1391530E
45 651300S 01/21/95 9.9 823 16.3 135 514
1401260E
2220
2200
2180
2160
2140
2120
2100-2 -1 0 1 2 3
St.11St.13St.14St.20
St.40St.41St.43St.45
NT
CO
2 /
µm
ol k
g-1
Temperature / °C
Fig. 6. A composite plot of total inorganic carbon normalized
at S ¼ 34 vs. temperature in the water columns above 300m at
a total of eight hydrographic stations in the marginal ice.
M. Ishii et al. / Deep-Sea Research II 49 (2002) 1691–1706 1699
DSi/DN (=4.5) was also in agreement with thatobserved in the SIZ (DSi/DN=4.9) and in thePOOZ (DSi/DN=4.4) of the Indian Ocean sectoralong 621E (Minas and Minas, 1992; Pondavenet al., 2000). It is likely that these deficit ratioswould show little spatial variation over the IndianOcean sector and the Australian sector althoughwith some exceptions in offshore regions (Ishiiet al., 1998).
Although the DC/DN/DP deficit ratio indicatesthat about twice as much phosphate has beenconsumed as that of Redfield (106/16/1) (Redfieldet al., 1963), they are in fair agreement with theratio for particulate organic matter (POM) insurface waters in the Indian Ocean sector (C/N/P=62/10.5/1) (Copin-Mont!egut and Copin-Mont-!egut, 1978). In addition, the increase in dissolvedorganic carbon (DOC) in the surface layer inJanuary at the same stations we observed (Ogawaet al., 1999) corresponds to 10–17% of the deficitin TCO2. These results suggest that TCO2 andmacronutrients assimilated in the surface layerwere transformed mainly into POM.
4. Discussion
The NTCO2 in surface seawater in the MIZsouth of Australia around 1401E exhibited largetemporal and spatial variability that ranged from
2125 to 2175 mmol kg�1 at S ¼ 34: In contrast, theNTCO2 in the subsurface remnant Tmin layer, inwhich Tmino�1.71C, showed little variability(218272.3 mmol kg�1) in the same region. Itshould be noted that the Tmin layer value is inexcellent agreement with the NTCO2 in the surfacewater in the compact pack-ice region and in theTmin layer of Tmino�1.71C in the SIZ of theIndian Ocean sector between 381E and 761Eobserved in February–March 1993(2184.073.7 mmol kg�1) (Ishii et al., 1998). Inaddition, it appears that these values are in goodagreement with the NTCO2 in the surface seawaterin winter and in the remnant Tmin layer in summerin the Weddell Sea (Weiss et al., 1979; Takahashiet al., 1993; Hoppema et al., 1995), beneath sea icein austral spring in the Atlantic Ocean sector at61W (Bakker et al., 1997), in surface seawaters inearly spring in Ross Sea polyna (Bates et al., 1998),and in a significant part of the Pacific Ocean sectorbetween 1101W and 1601W (Rubin et al., 1998).These observations suggest that the NTCO2 in themixed layer in late winter shows little spatialvariation over most of the SIZ in the SouthernOcean, and zonal advection is not problematic incalculating the deficit in TCO2 in the surface layerof this zone in summer.
Furthermore, because of its spatial homogene-ity, a long-term trend of NTCO2 in the wintermixed layer and in the remnant Tmin layer insummer in the SIZ may be detectable. A long-termtrend of pCO2sw and TCO2 in the upper layer ofthe ocean has been documented in the subtropicsin the North and South Pacific and in the NorthAtlantic (e.g., Inoue et al., 1995, 1999; Bates et al.,1996; Win et al., 1998). However, although theincrease in TCO2 has been suggested in the bottomwater of the Weddell Sea (Hoppema et al., 1998), along-term trend of CO2 parameters in the upperlayer has not been determined in the SouthernOcean because of their large summer time tempor-al and mesoscale variability (Inoue et al., 1999).Observations of the CO2 parameters and otheroceanographic properties in Tmin layers in theSouthern Ocean over some decades would help toclarify any trend and its causes with minimalmesoscale variability due to biological activitiesin summer. Consequently, they would help in
0
5
10
15
20
25
30
35
6464.56565.5
NC
P /
gC m
-2 Dec. 26Jan. 20
pack-ice edge
December 1994 January 1995
Latitude
Fig. 7. Variation in the NCP integrated from late winter as a
function of latitude in December 1994 and in January 1995.
NCP was estimated from the deficit in NTCO2 in the surface
layer above temperature-minimum layer.
M. Ishii et al. / Deep-Sea Research II 49 (2002) 1691–17061700
understanding the response of the Southern Oceanto the contemporary atmospheric CO2 increaseand to climate change.
From the condition inferred from the propertiesin the Tmin layer, i.e. NTCO2=218272 mmol kg�1,T=�1.81C, S ¼ 34:35 and NTA=228375 meq kg�1, the value of pCO2sw in the wintermixed layer is calculated to be 390720 matm(DpCO2=+35720 matm). This suggests that thewater beneath the sea ice is supersaturated withCO2 in late winter because of the remineralizationof organic matter and brine-driven convection andthat ice-free regions such as polynya and leads are
sources of CO2 in winter. This is consistent withthe results of the direct pCO2sw measurements inand near ice-covered regions in late winter in theWeddell Sea (Weiss et al., 1992; Takahashi et al.,1993), in the Atlantic Ocean sector at 61W (Bakkeret al., 1997), and in the Pacific Ocean sector(Rubin et al., 1998). Whether surface seawaterimmediately after the retreat of sea ice in earlysummer is a net CO2 source or sink depends on theNCP in and beneath sea ice (Gibson andTrull, 1999). Therefore, it would be related tolight availability, which is controlled by thethickness of sea ice and snow cover, and might
-10
-20
-30
-40
-50
-60
-70
0
10
-1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2
∆C(z
) / µ
mol
kg-1
∆C(z
) / µ
mol
kg-1
∆ P(z) / µmol kg-1
∆ Si(z) / µmol kg-1
∆ C/∆ P = 54.3 +/- 2.4
-70
-60
-50
-40
-30
-20
-10
0
10
-12 -10 -8 -6 -4 -2 0 2
∆N(z) / µmol kg -1
∆ C/∆ N= 6.35 +/ - 0 .15
-10
-20
-30
-40
-50
-60
-70
0
10
-50 -40 -30 -20 -10 0 10
∆ Si/∆ C = 0.73 +/ - 0 .02
(a) (b)
(c)∆C
(z) /
µm
ol k
g-1
Fig. 8. Relationships of (a) deficit in the normalized total inorganic carbon (NTCO2) vs. deficit in the normalized phosphate, (b) deficit
in NTCO2 vs. deficit in the normalized nitrate+nitrite, (c) deficit in NTCO2 vs. deficit in the normalized silicic acid, above the
temperature-minimum ðTminÞ layer in the marginal ice zone at 1401E by reference to the concentrations in the Tmin layer. The overall
deficit ratio was DSi/DC/DN/DP=39/54/8.7/1. Each symbol denotes a hydrographic station as in Fig. 6.
M. Ishii et al. / Deep-Sea Research II 49 (2002) 1691–1706 1701
be subjected to change in response to climatechange together with the extent of sea ice in winterand in summer.
Temporal and spatial variation in pCO2sw in theSIZ in summer is governed by the variation intemperature due to solar heating, by the variationin NTCO2 due to biological activities, and some-what by the variation in salinity due to ice-melt(see Fig. 4). In the MIZ closer to the ice edge,NTCO2 and pCO2sw were higher than those in thenorthern SIZ because the NCP near the ice edgewas still small (4–10 gCm�2). However, the effectof a temporal NTCO2 decline due to the increasein NCP surpassed the effect of a temperature-risethere; hence, the pCO2sw tended to decrease withtime. It is likely that NCP in the MIZ at 1401E thatrecorded 4–16 gCm�2 until late December 1994and reached 6–30 gCm�2 until mid-January 1995(see Fig. 7) continued to increase after January,since the minimum of pCO2sw in the MIZ at 1401Ein February and March was even lower(B270 matm) (Metzl et al., 1999) than our ob-servations in January (B300 matm). In contrast, inthe northern SIZ, where NTCO2 had alreadysignificantly declined due to the larger NCP and,therefore, the pCO2sw was lower than that in theMIZ, the effect of temperature-rise was generallylarger than that of the NTCO2 decline, and,therefore, the pCO2sw showed a tendency toincrease with time.
Deficit ratios of TCO2 and macronutrients in astratified surface layer depend on their ratios foruptake by phytoplankton and the ratios forsubsequent remineralization by the community.Uptake ratios by phytoplankton are thought tochange with species as well as their environmentaland physiological conditions. Arrigo et al. (1999)reported that deficit ratios of DC/DP and DC/DNfor the diatoms-dominated bloom were 94720and 9.271.7 and those for the bloom dominatedby a prymnesiophyte Phaeocystis antarctica were147727 and 7.871.3 in the Ross Sea polynya. Inthe same location, Sweeney et al. (2000b) reportedthat deficit ratios of DC/DP and DC/DN for thediatoms-dominated bloom were 80.572.3 and7.870.2 and those for the bloom dominated byP. antarctica were 13475 and 7.270.1. In theMIZ at 1401E, pigment analysis revealed that
diatoms containing fucoxanthin (40% and 60%)and prymnesiophytes containing 190-hexanoylox-yfucoxanthin (15–35%) were dominant, and theycontributed to about 75% of the total chlorophyll-a in surface waters (Sohrin et al., 2000). However,the deficit ratios of DC/DP and DC/DN were5472.4 and 6.370.2, respectively, which are bothsignificantly lower than those ratios of diatomsbloom and P. antarctica bloom observed in theRoss Sea polynya. Although the difference inphysiological conditions and their impacts on thephytoplankton’s DC/DP and DC/DN uptake ratiosbetween the MIZ at 1401E and the Ross Seapolynya are not known, these differences in thedeficit ratios rather suggest that carbon in theorganic matter might have been preferentiallyremineralized in the upper layer of the MIZ at1401E.
In addition, deficit ratios of DSi/DP and DSi/DNin the MIZ at 1401E were 39 and 4.5, respectively.They were 1.5 times to approximately twice as highas the uptake ratios of DSi/DP=26 and DSi/DN=2.3 in the incubation experiments of largediatoms collected at Stn. 40 (641100S, 1401400E)under iron-deficit conditions (Takeda, 1998).Consequently, the deficit ratio of DSi/DC(=0.7370.02) is more than twice as large asthat inferred from the incubation experimentunder iron-deficient conditions (0.28–0.30). Con-centration of labile iron in the MIZ above the Tmin
layer ranged from 0.15 to 0.27 nM in December1994 and 0.15 to 0.67 nM in January 1995 (Sohrinet al., 2000). These concentrations were compar-able or higher than the initial iron concentrationsin the incubation bottles (0.16–0.20 nM). Thedifference in the availability of iron between thewater column and the incubation bottle, however,does not explain the higher deficit ratios of DSi/DPand DSi/DN in the water column because thehigher abundance of iron results in the reductionof these uptake ratios (Takeda, 1998). Theproduction by non-silicious organisms such asprymnesiophytes also affects the DSi/DN and DSi/DP deficit ratios, but it should reduce these deficitratios. Hence, the growth of prymnesiophytes inthe water column also does not explain the higherdeficit ratios in the water column than in theincubation experiment.
M. Ishii et al. / Deep-Sea Research II 49 (2002) 1691–17061702
It is likely that nitrogen and phosphorus as wellas carbon in biogenic siliceous particulate matterhave undergone significant preferential reminera-lization compared to silicon in the surface layer,and resulted in the higher DSi/DC, DSi/DN andDSi/DP deficit ratios. Phytoplankton growth ratesand microzooplankton grazing rates measured bythe dilution method in parallel with the hydro-graphic observations at Stns. 11, 13 and 43 in theMIZ were �0.02 to 0.66 d�1 and 0.01 and 0.69 d�1,respectively, indicating comparable phytoplanktongrowth rates and microzooplankton grazing rates(Tsuda and Kawaguchi, 1997). Sinking particlescollected by a moored sediment trap deployed at540m from December 25, 1994 through January21, 1995 at 641420S, 1391590E in the MIZ consistedof fecal pellets of krill and copepods (Suzuki andSasaki, submitted). These results also suggestsignificant activities of heterotrophic organismsin the upper layer of the MIZ. Although theavailability of iron appears to be a limiting factorfor the primary production in the stratified surfacelayer in the Southern Ocean south of Australia at1401E (Takeda, 1998; Sohrin et al., 2000; Boydet al., 2000), it is likely that heterotrophic activitiesalso play an important role in controlling the NCPin this region.
5. Conclusion
The activity of heterotrophic organisms as wellas the availability of iron for phytoplankton islikely to be an important factor limiting the NCPand controlling the distributions of TCO2 andpCO2sw in the stratified surface layer in the SIZ ofthe Southern Ocean south of Australia at 1401E.Even so, seasonally integrated NCP above the Tmin
layer reached 30 gCm�2 until January, and it washigh enough for the water beneath the sea ice thatwas supersaturated with CO2 in winter to becomesignificantly under-saturated in summer after theretreat of the sea ice. The majority of CO2
assimilated appears to be transformed into POM.However, lacking data for the standing stock ofsuspended POC, the amounts of assimilatedcarbon that was exported downward below theTmin layer and that was remineralized within the
surface layer until the end of summer areunknown.
Further observations of CO2 parameters andother oceanographic variables in the winter mixedlayer or in the Tmin layers in early summer as wellas those in the surface layer in summer are neededto quantify the temporal and spatial variability ofthe NCP in the SIZ of the Southern Ocean. Theyare useful in understanding the controlling factorsfor the NCP and its role for the air–sea CO2 flux aswell as in constraining the export production outof the surface layer. In addition, their long-termobservations would be helpful in understandingthe changes in the Southern Ocean in response tothe contemporary CO2 increase in the atmosphereand to the climate change.
Acknowledgements
The authors thank Professors Kouichi Kawa-guchi and Makoto Terazaki of the Ocean Re-search Institute of the University of Tokyo whoserved as chief scientists during the KH-94-4 cruiseand the officers and crew of the R=V Hakuho
Maru for their help on board. We also express ourappreciation to Dr. Atsushi Tsuda for providingthe underway data of temperature and salinity andto Dr. Shuichi Watanabe and Mr. HiroshiHasumoto for providing the nutrients and hydro-graphic data from hydrocasts. Data presented inthis paper are available from the authors as well asfrom the Japan Oceanographic Data Center(JODC) and the Carbon Dioxide InformationAnalysis Center (CDIAC).
