chlorophyll a biomass of netplankton in surface waters in...

581

Journal of Oceanography, Vol. 57, pp. 581 to 592, 2001

Keywords:⋅ Netplankton,⋅ chlorophyll a,⋅ surface water,⋅ Southern Ocean,⋅ marginal ice zone.

* Corresponding author. E-mail: han@mbrij .co.jp [email protected]

* Present address: Marine Biological Research Institute of Japan Co.,Ltd., 4-3-16 Yutaka-cho, Shinagawa-ku, Tokyo 142-0042, Japan.

Copyright © The Oceanographic Society of Japan.

Chlorophyll a Biomass of Netplankton in SurfaceWaters in the Pacific Sector of the Southern Oceanin Austral Summer

DONG-HOON HAN1* and MASAYUKI MAC TAKAHASHI1,2

1Department of Systems Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan2International Arctic Research Center, Frontier Research System for Global Change, Seavans North 7F, 1-2-1 Shibaura, Minato-ku, Tokyo 105-7103, Japan

(Received 15 March 2000; in revised form 2 April 2001; accepted 9 April 2001)

Chlorophyll a of total and particles retained on 30 µm mesh plankton net were bothdetermined in surface waters along two cruise tracks ranging from the Subtropicalwater to the marginal ice zone in the Pacific sector of the Southern Ocean in australsummer. Total surface chlorophyll a in the study area was mostly less than 1µg chl a l–1, and showed distributions with no obvious trend associated with differentwaters masses of the Antarctic and the Subantarctic, although total chlorophyll aconcentrations changed greatly within each water mass. Particularly low concentra-tions of chlorophyll a were detected in the marginal ice zone. Chlorophyll a containedin 30 µm netplankton made up 5~60% of total chlorophyll a: large near the marginalice zone and becoming small with travel towards the north. High percentage sharesof netplankton chlorophyll a were confirmed even in low total chlorophyll a concen-trations in summer in the Southern Ocean. A positive relation was observed betweenthe percentage of 30 µm netplankton and the “average total chlorophyll a”, althoughthere was great scatter.

Water conditions in the Southern Ocean are unique,with the development of circular, homogeneous watermasses in the east-west direction, such as the subtropi-cal, the Subantarctic and the Antarctic waters (describedfrom north to south). Fronts are located between eachwater mass, such as the subtropical convergence, theSubantarctic polar front, and the Antarctic front (fromnorth to south) (Mackintosh, 1946; Gordon, 1972). Eachwater mass and front changes its location and area, bothtemporally and spatially (Gordon, 1972; Tchernia, 1980).The Antarctic water is covered with sea ice extending fromthe Antarctic Continent in winter and the open water areaexpands in summer due to melting ice. High biologicalactivity has been reported in the marginal ice zone (Bunt,1963; Ackley et al., 1979; Clarke and Ackley, 1984).

Ship observations of chlorophyll concentrations inthe Southern Ocean were started in about 1942 (cf.Burkholder and Burkholder, 1967), and have become ac-tive since 1960 (Fukuchi, 1980), including satellite ob-servations after the late 1970s (Sullivan et al., 1993).

Surface chlorophyll measurements conducted by theJapanese Antarctic Research Expedition (JARE), alongcruise tracks of FUJI are available for the permanently

1. IntroductionIt has been pointed out that the individual size of

organisms is important for food chain dynamics in ma-rine ecosystem dominated by planktonic organisms(Ryther, 1969; Sheldon et al., 1972; Harrison, 1986).During plankton sampling in the field, we often noticedthat it is easy to clog up net with 10 or 20 µm mesh butnot with 30 µm mesh.

This suggests that particular microplankton some-times predominate in given water. Han and Takahashi(2000) reported the percentage shares of chlorophyll aretained on a 30 µm mesh varied from 0.2% to 11.9% oftotal chlorophyll a in a concentration range of 0.2 to 4.3µg chl a l–1 in the surface waters of the northern northPacific Ocean.

582 D.-H. Han and M. M. Takahashi

open ocean zone (POOZ) through 1965~1976. These ob-servations are mainly concentrated in the Indian sector,and fortunately complement previous observations ofother sectors of the Antarctic Ocean. A total of 631 val-ues acquired south of the polar front clearly confirm theoligotrophic nature of this ocean in terms ofphytoplankton biomass: about half of the stations showedchlorophyll a concentrations of <0.25 µg chl a l–1, withonly are surpassing 1 µg chl a l–1 (Fukuchi, 1980). Thesame range has been reported by Ichimura and Fukushima(1963) (0.1~0.6 µg chl a l–1) and by Yamaguchi andShibata (1982) (0.1~0.4 µg chl a l–1). For the POOZ, theaverage reaches a higher value (0.46 µg chl a l–1) in mid-dle-late December. Chlorophyll a in surface water of theorder of 1 µg chl a l–1 appeared to be the maximum forthe POOZ in December (Kanda and Fukuchi, 1979; LeJehan and Treguer, 1983).

Savidge et al. (1996) evaluated areal changes of sur-face chlorophyll a in two periods, from October to De-cember and from January to March in summer, using thedata obtained from 1977 to 1995 in the Southern Ocean.As a result, they also found that chlorophyll a concentra-tions in the surface water were mostly low, e.g., less than1 µg chl a l–1 in the entire Southern Ocean, except for themarginal ice zone, Weddell-Scotia Confluence andWeddell Sea, where surface chlorophyll a seldom roseover 1 µg chl a l–1. This type of evaluation has not beendone in other seasons than summer because of an insuffi-ciency of available data. Winter data, in particular, areextremely poor. According to the data obtained in winterin particular areas, surface chlorophyll a concentrationswere always low, e.g., less than 0.1 µg chl a l–1 off theShowa Base (Satoh et al . , 1986) and 0.02 to 0.3µg chl a l–1 around the Antarctic Peninsula area (<0.05µg chl a l–1 on average) (Kottmeier and Sullivan, 1987;Smith and Dierssen, 1996).

Significant positive linear relations were observedin chlorophyll a concentrations between the surface wa-ter and both the subsurface chlorophyll a maximum andthe integrated chlorophyll a from 0 m to 50 m in the en-tire Southern Ocean (cf. Heywood and Whitaker, 1984;El-Sayed, 1987; Knox, 1994). A similar linear relationwas also reported in chlorophyll a concentrations betweenthe surface and the concentration integrated between 0 mand 50 m (Mitchell and Holm-Hansen, 1991) and between0 m and 100 m (Smith and Dierssen, 1996) around theAntarctic Peninsula. These results indicate that chloro-phyll a concentrations in the water column can be esti-mated from the surface chlorophyll a concentrations inthe Southern Ocean.

It has long been believed that large size planktoncomposed of diatoms predominates in the Southern Ocean(Hart, 1942; Hasle, 1969). Size fractionation studies ofchlorophyll samples having been popular since 1970s in

the Southern Ocean, revealing that percentage shares ofchlorophyll a in particles retained on 8 µm pore filter in-creased in the south of the Antarctic convergence (Sheldonet al., 1972; Sasaki, 1984; Kosaki et al., 1985), and thatthe percentage shares of diatoms particularly increasedthere (Sheldon et al., 1972; Suskin, 1985; Priddle, 1990).According to the size fractionation studies of surface chlo-rophyll a for each representative sector of the SouthernOcean carried out in summer by Jacques (1989), the mostabundant fraction was 3 to 20 µm, occupying about 55%,the fraction <3 µm being negligibly small. It has also beenmentioned that picoplankton of <2 µm were seldom domi-nant, and microplankton of >20 µm were abundant at atime with high chlorophyll a concentrations whenphytoplankton bloom occurred.

Based upon past reports, it has become clear that theSouthern Ocean is characterized by higher percentage ofnano- and micro-phytoplankton compared to the water invarious other areas, even with low chlorophyll concen-trations of <1 µg chl a l–1.

In the present study, we focused on largenetplanktonic algae retained on a 30 µm mesh net. Weevaluated the biomass contribution of the largenetplankton algae according to their percentage shares inthe phytoplankton community as well as their concentra-tions. Observations were made during austral summer(December~January) ranging from the subtropical waterto the marginal ice zone in the Pacific sector of the South-ern Ocean.

2. Materials and MethodsSampling was carried out from 13 December 1994

to 4 January 1995 in the Pacific sector of the SouthernOcean by R/V Hakuho-Maru of the Ocean Research In-stitute, University of Tokyo (KH-94-4 cruise), as shownin Fig. 1. Two lines were occupied: the New Zealand linefrom New Zealand (14 December) to the point 140°E and64°S, and the Australian line from 65°S to 44°S mainly140°E along (from 18 December to 3 January). Observa-tions on the Australian line from 52°S to Tasmania Islandwere off 140°E. Surface water samples were continuouslypumped up from the bottom of the ship (5 m depth) dur-ing the cruise, together with continuous, simultaneousdeterminations of temperature, salinity, and in vivo chlo-rophyll fluorescence. Vertical measurements of variousparameters were only made along the Australian line.

Netplankton samples were collected by a concentra-tor attached to a 30 µm net (conical NITEX net, mouthdiameter 30 cm, total length 60 cm, total filtration area0.29 m2) (Han and Takahashi, 2000). The net was sub-merged and continuously supplied the surface water sam-ples at flow rates varying from 16 to 17 l min–1, and sam-ples were concentrated to 500 to 1000 ml from 2.3 to 13.6m3: actual sampling distance for each sample varied from

Netplankton Chlorophyll a in the Southern Ocean 583

14 km to 240 km along the cruise track, and samples werealso collected at fixed stations. A net having a 30 µm meshsize is believed to capture mainly microplankton or largersizes of plankton having a sphere equivalent volume from20 µm, because the spring form of most microplanktoncells makes trapping easy on a mesh size larger than thesphere diameter of the equivalent cell volume.Unconcentrated water samples were collected at the be-ginning and at the end of each concentration process andwere used to determine total chlorophyll a concentrations.A part of the concentrated sample was used to determinechlorophyll a concentration.

A certain volume of each water sample (200 ml or500 ml for unconcentrated sample, 5 ml for concentratedsample) was vacuum filtered at <100 mmHg through a25 mm glass fiber filter (Whatman GF/F). Materials re-tained on the filter paper were extracted with 6 ml of N,N-dimethylformamide at –20°C in the dark overnight, fol-lowed by fluorometric determination using a fluorometer(Turner Designs, Model 10-005R) (Strickland and

Parsons, 1972; Suzuki and Ishimaru, 1990). Errors en-countered in the chlorophyll a for unconcentrated and con-centrated samples in the present study are 5.1% (n = 4)and 3.4% (n = 3), respectively.

3. Results

3.1 Characteristics of water masses in the study areaAs shown in Fig. 1, field observations were made

within the area covered from 140°E to 165°E and from44°50′ S to 65°06′ S of the marginal ice zone.

According to the locations of Subtropical,Subantarctic and Antarctic fronts in the area reported byGordon (1972) and the criteria of each water mass setdown by Tchernia (1980), the water masses along the NewZealand line were evaluated along cruise track in termsof surface water temperature and salinity (Fig. 2).

Along the Australian line, the subtropical conver-gence (STC) evaluated from vertical temperature profiledown to 800 m was found around 46°S, and theSubantarctic surface water was found to the south of STC(Nakai et al., 1986; KH-94-4 Cruise Report, 1996). Basedupon temperature and salinity profiles down to 500 m(Gordon, 1972; Tchernia, 1980), it has been decided thiswas the Subantarctic polar front (SAPF) around 50°S. Thesouthern Subantarctic surface water to the south of SAPF,the Antarctic polar front (APF) around 53°30′ S, andAntarctic surface water in the south of 54°S extended tothe marginal ice area around 65°06′ S. Antarctic diver-gence (AD) was found in the area between 64°20′ S and64°30′ S (Fig. 3). It can be seen in Fig. 3 that the Antarc-tic deep seawater upwelled to the surface in the AD, andtraveled northwards with decreasing salinity due to pos-sible dilution by low salinity surface water.

3.2 Total chlorophyll a concentrations in the surface wa-ter in the study areaTotal chlorophyll a concentrations in surface water

determined in the present study along two lines (south-bound and northbound) are shown graphically in Fig. 4according to the differences in water masses. The 39 datapoints obtained from 0.09 to 1.23 µg chl a l–1, displayingabout a 14-fold difference. Simple “average total chloro-phyll a” of surface water in the study area was 0.44 ±0.24 µg chl a l–1 with 55.8% of CV (Table 1). Overallchanging patterns were similar between the two lines,although high and low peaks were not the same, oftenbehaving rather in opposition, changing directions. Therewere 10 data points in the marginal ice zone (MIZ), andall of them were low, e.g., less than 0.4 µg chl a l–1. Therewere no obvious differences in total chlorophyll a con-centrations depending upon differences in water mass, norobvious high concentrations of chlorophyll a in MIZ, al-though there were slightly high concentrations of >1

Fig. 1. Cruise track of the study area (Leg 2, KH-94-4). Shortbars along cruise tracks represent the area where chloro-phyll a concentrations of netplankton in the surface waterwere collected between two neighboring bars. Short arrowsindicate cruise direction. STC, SAPF, APF, AD and MIZrepresent subtropical convergence, Subantarctic polar front,Antarctic polar front, Antarctic divergence and marginal icezone, respectively.


Fig. 2. Surface water temperature and salinity determined on board by pumping up the surface water between 44°S and 59°Salong the New Zealand line. STC, SAPF and APF represent subtropical convergence, Subantarctic polar front and Antarcticpolar front, respectively.

Fig. 3. Vertical profiles of temperature and salinity in the water column above 500 m between 48°S and 65°S on the Australianline, mostly along 140°E in the study area. SAPF, APF and AD represent Subantarctic polar front, Antarctic polar front andAntarctic divergence, respectively.


Fig. 4. Geographical distributions of total chlorophyll a concentrations in the surface water in the study area of the Pacific sectorof the Southern Ocean. Open and closed circles represent for the New Zealand (NZ) and the Australian (AUS) lines, respec-tively. STC, SAPF, APF, AD and MIZ represent subtropical convergence, Subantarctic polar front, Antarctic polar front,Antarctic divergence and marginal ice zone, respectively.

Fig. 5. Geographical distributions of netplankton chlorophyll a concentrations (>30 µm) in the surface water in the study area ofthe Pacific sector of the Southern Ocean. Open and closed circles represent for the New Zealand (NZ) and the Australian(AUS) lines, respectively. STC, SAPF, APF, AD and MIZ represent subtropical convergence, Subantarctic polar front, Ant-arctic polar front, Antarctic divergence and marginal ice zone, respectively. Short horizontal bars for each data points indicatesampling distance.


µg chl a l–1 around APF in the New Zealand line. Exceptfor the northern half of the Antarctic and the Subantarcticwaters, the New Zealand line gave higher chlorophyll aconcentrations than the Australian line. Chlorophyll aconcentrations in the surface water varied greatly withineach water mass, rather than due to differences in watermass and frontal structure, and average chlorophyll aconcentrations between water masses were not very dif-ferent.

3.3 Netplankton chlorophyll a in the surface water in thestudy areaChlorophyll a determined in the particles retained

on a 30 µm mesh net along the two lines were plottedaccording to differences in water masses in Fig. 5 whereplots were made at the mid-point in each sampling area.A total of 34 data points obtained from 0.016 to 0.333µg chl a l–1, displaying about a 21-fold variation, whichwas greater than that of the total chlorophyll a mentionedabove. Changing patterns of netplankton chlorophyll awere quite similar to those of the total chlorophyll a shownin Fig. 4.

All 10 data points in MIZ along the Australian linewere low, varying between 0.016 and 0.092 µg chl a l–1.Netplankton chlorophyll a concentrations in the south-ern part of the Antarctic water were mostly constantaround 0.13 µg chl a l–1. On the other hand, those of theNew Zealand line in the southern part of the Antarcticwater varied greatly from 0.09 to 0.33 µg chl a l–1 and asudden drop in the northern part in the Antarctic watercorresponded well with that observed in the total chloro-phyll a. In the northern half of the Antarctic water, bothlines showed similar changing patterns. There were noobvious changes in chlorophyll a concentrations ofnetplankton associating with frontal structures. Further-more, netplankton chlorophyll a concentrations were al-ways higher along the New Zealand line except for a partof the Antarctic water compared to the Australian line.

The average total chlorophyll a concentrations dur-ing netplankton sampling were estimated by averagingtwo total chlorophyll a concentrations at the starting andat the ending periods during the sampling. The averagetotal chlorophyll a concentrations were evaluated to seehow representative they are of the water between the twodata points according to the continuous in vivo chloro-phyll fluorescence readings determined during cruising.In 29 out of total 34 cases for the average total chloro-phyll a, all 1 minute interval measurements of in vivochlorophyll fluorescence showed only less than 20% oftheir standard deviation for each sampling period. Eachaverage in vivo chlorophyll fluorescence for the above29 cases showed a positive linear relation with the “aver-age total chlorophyll a” concentrations, although the re-gression coefficient (r2 = 0.334) was not high. Chloro-

Tab

le 1

. T

otal

and

net

plan

kton

(>

30 µ

m)

chlo

roph

yll

a co

ncen

trat

ion

rang

e, a

vera

ge (

avg.

), s

tand

ard

devi

atio

n (S

D)

and

coef

fi-

cien

t of

vari

atio

n (C

V)

of th

e su

rfac

e w

ater

in th

e S

outh

ern

Oce

an in

aus

tral

sum

mer

. ST

C, S

AP

F, A

PF,

AD

and

MIZ

rep

rese

ntsu

btro

pica

l co

nver

genc

e, S

uban

tarc

tic

pola

r fr

ont,

Ant

arct

ic p

olar

fro

nt, A

ntar

ctic

div

erge

nce

and

mar

gina

l ic

e zo

ne, r

espe

c-ti

vely

.

*Re t

rain

e d o

n >

30 µ

m m

e sh

net.

**M

inim

um a

nd m

axim

um c

onc e

ntra

tion

s.**

*Ave

rage

of

tota

l c h

loro

phyl

l a

c onc

e ntr

a tio

ns a

t th

e be

ginn

ing

a nd

the

e nd

of e

a ch

netp

lank

ton

sam

plin

g pe

riod

.


Fig. 6. Relations between netplankton chlorophyll a concentrations (>30 µm) and the “average total chlorophyll a”. Open andclosed circles represent for the New Zealand (NZ) and the Australian (AUS) lines, respectively. Five data points marked witha small asterisk were obtained from water samples having great variations of in vivo chlorophyll fluorescence readings suchas greater than 20%.

Fig. 7. Geographical distributions of the percentage share of netplankton chlorophyll a concentration (>30 µm) against the“average total chlorophyll a” in the surface water in the study area of the Pacific sector of the Southern Ocean. Open andclosed cycles represent for the New Zealand (NZ) and the Australian (AUS) lines, respectively. STC, SAPF, APF, AD andMIZ represent subtropical convergence, Subantarctic polar front, Antarctic polar front, Antarctic divergence and marginal icezone, respectively. Five data points marked with a small asterisk were obtained from water samples having great variations ofin vivo chlorophyll fluorescence reading such as greater than 20%. Short horizontal bars for each data points indicate sam-pling distance.


phyll a concentrations of >30 µm netplankton chlorophylla were plotted against the “average total chlorophyll a”,as shown in Fig. 6. Even though data points showed agreat scattering, a general trend where the netplanktonchlorophyll a of >30 µm increased with the increase ofthe “average total chlorophyll a” was clearly observed inboth the Australian and New Zealand lines. The higher“average total chlorophyll a” readings tended to agreewith the higher chlorophyll a of the >30 µm netplankton.At the same time, low chlorophyll a concentrations of>30 µm netplankton also occurred, even at high “averagetotal chlorophyll a”.

According to microscopic observations, dominantspecies composing >30 µm netplankton were all colonyforming diatoms: Corethron criophilum colony, 2~4 cellshaving a total cell volume of 1.2 × 105 µm3 with 22 ± 10µm cell length; Dactyliosolen antarcticus colony, 3~5cells having a total cell volume of 3.1 × 105 µm3 with52 ± 16 µm cell length; Fragilariopsis kerguelensiscolony, 10~30 cells having a total cell volume of 0.013 ×105 µm3 with 8 ± 1 µm cell length; and Thalassiothrixantarctica colony, 2~4 cells having a total cell volume of0.32 × 105 µm3 with 7 ± 1 µm cell length.

3.4 Percentage shares of chlorophyll a of >30 µmnetplankton compared with average total chlorophylla concentrationFigure 7 represents the percentage shares of chloro-

phyll a of >30 µm netplankton shown in Fig. 5 plottedagainst the “average total chlorophyll a” according to

different water masses. Ten out of a total of 34 data pointswere obtained in MIZ. The percentage share immediatelyoutside the ice edge was less than 20%, but it increasedrapidly upon leaving the ice edge, reaching nearly 60%at its maximum. The percentage share of the netplanktontended to be higher to the north of AD. Even though thepercentage shares varied greatly, they decreased gradu-ally towards the north from the highest of 50~60% nearAD to less than 20% in the south area of the subtropicalconvergence. A similar decrease in the percentage shareof netplankton towards the north was also clearly observedin the chlorophyll a concentrations of netplankton. Thepercentage shares of netplankton were also large in theNew Zealand line compared to the Australian line as ob-served in chlorophyll a of netplankton (Fig. 7), but theactual difference was not as large as the chlorophyll aconcentrations. There was no general tendency observedbetween the “average total chlorophyll a” concentrationsand the percentage shares of >30 µm netplankton, but thepercentage shares varied greatly from 8 to 60% regard-less of differences in the “average total chlorophyll a”concentrations in almost the entire range (Fig. 8).

4. DiscussionThe present study, carried out in austral summer in

the Pacific sector of the Southern Ocean, revealed onlytwo of a total of 39 locations where surface total chloro-phyll a exceeded 1 µg chl a l–1. The “average total chlo-rophyll a” concentration was 0.46 ± 0.28 µg chl a l–1 (Ta-ble 1), which supported the results of Savidge et al.

Fig. 8. A relation between the percentage shares of netplankton chlorophyll a and the “average total chlorophyll a”. Open andclosed circles represent for the New Zealand and the Australian lines, respectively. Five data points marked with a smallasterisk were obtained from water samples having great variations of in vivo chlorophyll fluorescence readings such as greaterthan 20%.


(1996). No obviously great increase in surface chloro-phyll a concentrations was noticed in any frontal struc-ture between different water masses, as mentioned bySavidge et al. (1996) in association with some frontalstructures. No clear differences were detected in the sur-face chlorophyll a concentrations between the Antarcticand the Subantarctic waters, but surface chlorophyll avaried greatly within each water mass. In the Antarcticwater, from MIZ to AD there was 0.23 ± 0.07µg chl a l–1 (CV, 29.3%) and the average concentrationfrom AD to APF was 0.53 ± 0.25 µg chl a l–1 (CV, 46.8%),where the latter was characterized with high chlorophylla concentrations having a large variation. In theSubantarctic water, total chlorophyll a concentrationswere high with a great variability in the northern partwhere the average concentration was 0.57 ± 0.22µg chl a l–1 (CV, 37.6%), whereas that in the southernpart was 0.26 ± 0.03 µg chl a l–1 (CV, 10.7%).

Surface chlorophyll a concentrations in summer ofthe Southern Ocean were low compared to the average of0.5 to 2 µg chl a l–1 reported in the Bering Sea and thenorthern North Pacific Ocean in the same high latitudesbetween summer and autumn in the northern hemisphere(Fukuchi, 1980; Odate, 1996; Han and Takahashi, 2000).The Southern Ocean is included as a representative areacharacterized by high nutrient and low chlorophyll a con-centrations (HNLC), such as the eastern North and theeastern Equatorial Pacific Ocean (Martin and Fitzwater,1988; Frost, 1991). Compared with the summer surfacechlorophyll a concentrations between the HNLC waters,

the Southern Ocean showed slightly higher values thanthe eastern Equatorial Pacific (0.2 to 0.4 µg chl a l–1:Chavez et al., 1996) and the eastern North Pacific Ocean(0.2 to 0.3 µg chl a l–1: Parsons and Lalli, 1988; Miller etal., 1991).

Surface chlorophyll a concentrations near MIZ werelow in the present study. There are similar observations(Weber and El-Sayed, 1987), but by contrast, there havebeen several reports showing high concentrations of chlo-rophyll a (Kosaki et al., 1985; Weber and El-Sayed, 1987;Savidge et al., 1996). It is known that chlorophyll a con-centration in the water is generally low immediately af-ter sea ice has melted, increasing after a certain period oftime due to ice edge effects (Ackley et al., 1979; Platt etal., 1982; Priddle et al., 1994). Low concentrations ofchlorophyll a at MIZ observed in the present study canbe explained by not enough time being allowed after theice had melted to allow phytoplankton to increase in thewater. Actually, high total chlorophyll a concentrationswere detected in the surface water about 50 km away fromMIZ in the present study. Furthermore it is also possiblethat the strong wind prevailing in the area might reducethe vertical stability, which resulting in a depression ofphytoplankton growth in the water column (Savidge etal., 1996).

The percentage share of chlorophyll a contained in>30 µm netplankton in the surface water was 18.4% inthe area from MIZ to AD, increasing to 35.0% for AD toAPF, followed by an obvious decrease in order such as15.3, 12.1 and 8.2% for APF to SAPF, SAPF to STC and

Fig. 9. Relations between the netplankton chlorophyll a concentrations (>30 µm) and the “average total chlorophyll a” in thesurface water in the Southern Ocean (open circle, New Zealand line; closed circle, Australian line) in the present study and inthe northern North Pacific Ocean (square, Han and Takahashi, 2000). Five data points marked with a small asterisk wereobtained from water samples having great variations of in vivo chlorophyll fluorescence readings such as greater than 20%.


Fig. 10. Relations between the percentage shares of netplankton chlorophyll a and the “average total chlorophyll a”. Open andclosed circles and open squares represent for the New Zealand and the Australian lines in the present study and for thenorthern North Pacific Ocean (Han and Takahashi, 2000), respectively. Five data points marked with a small asterisk wereobtained from water samples having great variations of in vivo chlorophyll fluorescence readings such as greater than 20%.

the north of STC, respectively (Table 1 and Fig. 7). Con-centrations of chlorophyll a in >30 µm netplankton basi-cally showed the same geographical trend as its percent-age shares, being highest at the AD and the lowest at theSTC with the lowest concentrations in MIZ, althoughactual differences were smaller than the percentage shares.Sasaki (1984) also reported similar results in Decemberin the Indian Ocean sector, the percentage shares of chlo-rophyll a for >10 µm microplankton being lowest in thesubtropic (24.1%) and increasing to 64.8 and 68.9% inthe Subantarctic and the Antarctic, respectively. His re-sults were 2 to 4 times larger than the present results,which might be due to the difference in year and to thedifferent mesh size used for collecting samples (10 µmrather than 30 µm). The results reported by Kosaki et al.(1985) also showed increased percentage shares of chlo-rophyll a for >20 µm netplankton towards the south inthe Southern Ocean.

As Fig. 6 shows, a trend where the higher the “aver-age total chlorophyll a” concentrations corresponded tothe higher chlorophyll a concentration of >30 µmnetplankton was also recognized in the northern NorthPacific Ocean (Han and Takahashi, 2000). However, com-paring their results with the present study, the chlorophylla concentrations of >30 µm netplankton in the SouthernOcean were much higher, even at low “average total chlo-rophyll a” concentrations, as shown in Fig. 9. This indi-cates that the percentage shares of >30 µm netplanktonin the “average total chlorophyll a” were high in the

Southern Ocean, as shown in Fig. 10. They were 0~10%in the northern North Pacific Ocean but 5~60% in theSouthern Ocean, although there is a great scattering inthe data distributions regardless of actual differences inchlorophyll a concentrations. This shows that the domi-nance of large planktonic algae was significantly high inthe Southern Ocean compared to the northern North Pa-cific Ocean. According to past reports, the dominance ofdiatoms increased in the Atlantic sector of the circumpolarcurrent (Suskin, 1985) and netplankton also increased thepercentage shares from Africa to the Antarctic Continentin the southwest Indian Ocean sector, with increased thepercentage shares of diatoms towards the south (Fukase,1962; Kopczynska et al., 1986). High percentage sharesof >30 µm netplankton chlorophyll a observed in theSouthern Ocean were also confirmed as being due to thecontribution of large diatoms by microscopic observationsin the present study.

AcknowledgementsThe present study was supported by Grant-in-Aid for

Scientific Research (No. 06454009) from the Ministry ofEducation, Science and Culture, Japan. We extend ourthanks to the captain and crew of the R/V Hakuho-Maruof the Ocean Research Institute, University of Tokyo. Wealso thank Dr. Kouichi Kawaguchi for his help during thestudy. This study was also supported in part by theSasagawa Scientific Research Grant from the Japan Sci-ence Society awarded by D.-H. Han.


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