1998 - Nitrogen Uptake Regime and Phytoplankton Community Structure in the Atlantic and Indian Sectors of the Southern Ocean

8/11/2019 1998 - Nitrogen Uptake Regime and Phytoplankton Community Structure in the Atlantic and Indian Sectors of the

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.Journal of Marine Systems 17 1998 159177

Nitrogen uptake regime and phytoplankton community structurein the Atlantic and Indian sectors of the Southern Ocean

M. Semeneh a, F. Dehairs a, M. Elskens a, M.E.M. Baumann b, E.E. Kopczynska c,C. Lancelot d, L. Goeyens a,)

a ( ) ( )Analytische Chemie ANCH , Vrije Uniersiteit Brussel VUB , Pleinlaan 2, B-1050 Brussels, Belgiumb ( )Alfred Wegener Institut fur Polar-und Meeresforschung AWI , Postfach 120161, D-27515 Bremerhaen, Germany

cDepartment of Antarctic Biology, Polish Academy of Sciences, Ustrzycka 10, 02141 Warsaw, Poland

d ( ) ( )Groupe de Microbiologie des Milieux Aquatiques GMMA , Uniersite Libre de Bruxelles ULB , Campus de la Plaine, CP 221,Bouleard du Triomphe, 1050 Brussels, Belgium

Received 15 December 1995; accepted 15 October 1996

Abstract

Phytoplankton nitrogen uptake is studied in relation to the biomass and structure of phytoplankton community in the

Atlantic and Indian sectors of the Southern Ocean. Two scenarios of seasonal evolution of uptake regime and phytoplankton

community structure are described. The first scenario includes the Marginal Ice Zone areas of the Weddell Sea and adjacent

areas where a predominantly nitrate based, diatom dominated assemblage, thriving in a stable water column at the beginning

of the season was transformed into a mainly ammonium based, flagellate dominated assemblage, towards the end of the

season. The change in phytoplankton community structure was caused by selective grazing by large grazers and reducedstability of the water column and the shift in uptake regime was due to increased ammonium availability and changes in

.community structure. In the second scenario, in the Coastal and Continental Shelf Zone CCSZ and Open Oceanic Zone .OOZ of the Indian sector, a shift in uptake regime occurred without a big change in phytoplankton community structure.

These areas were sampled late in the growth season and were characterized by prolonged water column stability, less grazing

pressure on large diatoms and high ammonium availability. Diatoms dominated the assemblage and about 80% phytoplank-

ton biomass was on the )10 mm size fraction. Unlike the first scenario, diatoms were largely based on ammonium. Thus,

in areas of persistent water column stability and less selective grazing pressure, a shift in uptake regime can occur without

change in community structure. The dominance of diatoms under regenerated production provides a physiological evidence

for the excess net removal of silicate over nitrate occurring in certain provinces of the Southern Ocean.

Resume

Lassimilation dazote dans les secteurs Atlantique et Indien de lOcean Austral est etudiee en fonction de la biomasse et de la structure de la communaute phytoplanctonique. Deux scenarios decrivant levolution saisonniere du regime dassimila- ` tion et les changements de structure dans la communaute du phytoplancton sont presentes. Au debut de la saison, dans la zone marginale des glaces de la mer de Weddell, les nitrates representent la principale source dazote avec un assemblage

)Corresponding author. Tel.: q32-2-629-3264; Fax: q32-2-629-3274; E-mail: [email protected]

0924-7963r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. .P I I : S 0 9 2 4 - 7 9 6 3 9 8 0 0 0 3 6 - 0


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( )M. Semeneh et al.rJournal of Marine Systems 17 1998 159177 161

the ocean floor bears a direct evidence for theirsignificant contribution in export production Treguer

.et al., 1995 . Their relative importance exhibits large

spatial and temporal variability. In this ecosystem

nitrate concentrations hardly reach limiting levelswith strong preference for ammonium Glibert et al.,

1982; Ronner et al., 1983; Probyn and Painting,1985; Smith and Nelson, 1990; Owens et al., 1991;

.Goeyens et al., 1991a, 1995 . In some parts of the

Southern Ocean the net removal of silicate during

the growth season exceeds that of nitrate and this

uncoupling of Si and N cycles in surface waters isdue to the rapid cycling of N as ammonium Treguer

.and Jacques, 1992; Goeyens et al., 1998 .

Depending on the underlying biological and

physico-chemical factors governing the growth and

maintenance of phytoplankton in the euphotic zone,a shift in nitrogen uptake regime i.e., from new to

.regenerated production can proceed with or withoutbig change in community structure. For example, in

the ScotiaWeddell Confluence marginal ice zone .MIZ a shift in uptake regime from a predominantly

new production to regenerated production was ac-

companied by a change in phytoplankton assemblage

from a rich, diatom dominated, microplankton as-

semblage to a cryptophyte dominated nanoplanktonassemblage Goeyens et al., 1991a,b; Jacques and

.Panouse, 1991 . Whereas the change in community

structure is caused by selective grazing by large

grazers such as krill and reduced water columnstability, the shift in uptake regime is driven by

enhanced ammonium availability and by the change

in community structure itself. In other hydrographiczones such as in the CCSZ of Prydz Bay Kopc-

.zynska et al., 1995 and in the Southern part of the .Weddell Sea Nothig et al., 1991 blooms of di-

atoms, which usually appear early in the season,

were observed late in the growth season. These areas

are characterized by stable water column and en-hanced heterotrophic activity hence high ammonium

.availability . In these areas, unlike the MIZ, diatoms

were dominant under predominance of regenerated .production Semeneh, 1992 .

Here we discuss the biomass and structure of the

phytoplankton community and uptake regime from

several expeditions in the Atlantic and Indian sectors

of the Southern Ocean. For the first time we docu-

ment the nitrogen nutrition of phytoplankton in a

shallow and highly productive coastal and continen-

tal shelf zone of the Prydz Bay area. Our main

objectives are to show that a shift from new to

regenerated production can proceed with or without

a shift in phytoplankton community structure. In

particular, we describe the dominance of diatoms in

a system where regenerated production is predomi-

nant.

2. Materials and methods

2.1. Sampling

Samples were collected during five cruises in the

Atlantic and Indian sectors of the Southern Ocean .Fig. 1 . Table 1 shows the regions and sampling

periods of all cruises. The sampling periods and sitescover different functional zones and different stagesof the growth season. The ANTARKTIS IXr2 ANT

. .IXr2 and ANTARKTIS Xr7 ANT Xr7 cruises

in the Weddell Sea were done on the same transect

except differences in sampling period and directionof cruise track. Moreover, the Larsen Shelf the

.south-western part of the Weddell Sea was investi-

gated during the ANT Xr7 cruise. For each cruise

physico-chemical parameters such as temperature,

salinity, nitrate, ammonium, phosphate and silicate

as well as biological parameters such as Chl a, PON .and particulate organic carbon POC concentrations

were measured according to the standard protocols

and the details are in their respective cruise reportsGoeyens et al., 1991c; Van Bennekom and Veth,

.1991; Fiala, 1994 . B. Tilbrook and S. Wright pro-

vided us the physico-chemical data for the Marine .Science Voyage 6 cruise MSV 6 .

2.2. Phytoplankton counting

Samples for phytoplankton count were taken from .the surface layer upper 20 m and fixed with ahexamine buffered formalin solution final concen-

.tration ;0.4% . A subsample of either 10 or 50 ml

was settled for 24 h and counted under inverted

microscope according to the Utermohl method .Utermohl, 1958 . Cell volume was calculated usingcell dimensions and appropriate cell geometry. Cell


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( )M. Semeneh et al.rJournal of Marine Systems 17 1998 159177162

.Fig. 1. Map showing the locations of the study areas in the Seasonal Ice Zone, SIZ indicated by the solid line , of the Southern Ocean.

. .1sEPOS LEG 2 cruise ScotiaWeddell Confluence zone ; 2 and 3sANTARKTIS IXr2 and Xr7 cruises Weddell Sea ; 4sMarine . . .Science Voyage 6 cruise Prydz Bay ; 5sANTARES 2 cruise 628E meridian transect . The Polar Front PF is indicated by dashed line.

.Adapted from Treguer and Van Bennekom 1991 .

. carbon C, pg was estimated from cell volume V,3.mm using the conversion factor of Eppley et al.

. .1970 : logCs0.76= logV y0.352 for diatoms .and logCs0.94= logV y0.6 for non-diatoms. No

attempt was made to distinguish between autotrophic

and heterotrophic dinoflagellate species except few

known heterotrophic species. With this method only

cells )2 mm were counted. It is worth mentioning

that this method can overestimate cell carbon due to

errors associated with determining cell volumes.

Nevertheless, it provides an invaluable information

on the composition and structure of the phytoplank-

ton community. Data on phytoplankton biomass dur-

ing the EPOS LEG 2 were kindly provided by S.

Becquevort.

2.3. Nitrogen uptake rates

Uptake rates of nitrate and ammonium by phyto- .plankton in the surface layer 020 m were mea-

sured using the 15

N tracer technique. Surface water

sample in 2.7 l sized polycarbonate bottle was en-15 15 15 .riched either with NO or NH 99% N and3 4

incubated for 24 h under natural light in a Plexiglas

on-deck incubator in which surface sea water tem-

perature was maintained by a continuous flow ofsurface sea water. Addition of labelled nitrogen i.e.,

15 15 .NO or NH increased the ambient concentra-3 4tion by about 10%. At the end of the incubation

period the water sample was filtered on a precom- .busted 4508C Whatman GFrF glass-fibre filter,


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Table 1

Locations of the study areas and their sampling periods in the Atlantic and Indian sectors of the Southern Ocean

Sector Cruise Region Position Sampling period Research vessel

Atlantic EPOS LEG 2 ScotiaWeddell Confluence 57861850 S 20.11.8807.01.89 Polarstern .NorthSouth transect 47498W

.ANTARKTIS IXr2 ANT IXr2 Weddell Sea 6382671805 S 14.11.9030.12.90 Polarstern .West East transect 128153837 W

.ANTARKTIS Xr7 ANT Xr7 Weddell Sea 6383271812 S 03.12.9222.01.93 Polarstern .East West transect 0882461811 W . .Indian Marine Science Voyage 6 MSV Prydz Bay Grid 6583769837 S 03.01.9120.03.91 Aurora Australis

6784978811 E

ANTARES 2 Indian Sector 4983566841 S 26.01.9423.03.94 Marion Dufresne .NorthSouth transect 628E

dried and sealed in a petri-dish for later 15

N analysis.

In order to measure the 15

N abundance in particulate

material all PON has to be converted to N gas. A2discharge tube containing a subsample of particulate

material together with CuO rods and CaO bricks was y5 .subjected to a high vacuum pressure -10 mbar

with occasional heating to remove atmospheric and

adsorbed gases. Then the samples were combusted at

7508C f o r 8 h . 15

N abundance was measured by

emission spectrometry.

Uptake rates of nitrate and ammonium were cal- .culated according to Dugdale and Wilkerson 1986 .

The ammonium uptake rates are not corrected for

isotope dilution. For each nutrient two uptake param-

eters were calculated: absolute and specific uptake

y1

.rates. Absolute uptake rates r , nM day repre-Nsent the amount of nitrogen taken up during the

incubation period whereas specific uptake rates n ,Ny1 . .h represent the normalised by PON uptake rates

or the turnover rates of cell nitrogen. f-ratio was .calculated according to Eppley and Peterson 1979 .f-ratio)0.5 indicates predominance of new nitrate

.based production. Other nitrogen sources such as

urea, nitrite and amino acids were not considered.

2.4. Statistical analysis

Multivariate techniques such as principal compo- .nent analysis PCA were used to identify stations

with similar physico-chemical and biological charac-

teristics. Only stations in the seasonally ice covered . .zone SIZ were considered Fig. 1 . The parameters

include: temperature, salinity, nitrate, ammoniumavailability i.e., the percentage of ammonium in the

.total dissolved inorganic nitrogen, DIN , phosphate,

silicate, Chl a and POC concentrations. Prior to the

analysis, all parameters were standardised to over-

come differences in measurement units. Then PCA

was applied using a CANOCOe 3.10 statistical .package Ter Braak, 1990 .

3. Results

3.1. Physico-chemical characteristics

3.1.1. Weddell Sea

Table 2 shows the summaries of the physico-

chemical and biological characteristics of the surfacewater during different cruises. During ANT IXr2

. cruise 1990 near freezing surface temperature ;. .y1.738C , high salinity ;34.36 , high nutrient

.concentration NO s29.4 mM and low phyto-3 y1 .plankton biomass Chl as0.1 mg l indicated a

near winter water situation. Most parameters showed

very small variability indicating a high degree of

spatial homogeneity.

Due to large spatial variability in most parameters

during the ANT Xr7 cruise, principal component

analysis was used to identify stations with similar

physico-chemical and biological characteristics.

Three clusters of stations representing three different

stages were identified: Central Weddell Sea, Eastern .Weddell Sea and Larsen Shelf Table 2 . Separate

PCA analysis on species biomass data gave similar .groups of stations data not shown . The first group

.Central Weddell Sea consisted of stations in the


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Table 2 .Summaries mean and standard deviation of the physico-chemical and biological characteristics of the surface water at various re

.n Temperature Salinity NO PO Si OH NH3 4 4 . . . . . 8C mM mM mM %

.Weddell Sea ANT IXr2 9 y1.73"0.12 34.36"0.03 29.4"1.1 2.0"0.1 72.4"7.3 0.5 .Central Weddell Sea ANT Xr7 16 y1.26"0.42 33.95"0.22 27.6"1.4 1.9"0.1 71.7"2.6 1.0 .Eastern Weddell Sea ANT Xr7 4 y1.36"0.44 34.23"0.02 24.5"3.2 1.7"0.2 58.3"1 0.8

.Larsen Shelf ANT Xr7 5 y0.67"0.61 33.75"0.16 15.9"3.6 1.1"0.2 60.6"13 0.8

ScotiaWeddell, SWC A 8 y1.41"0.27 33.84"0.27 27.9"2.9 1.7"0.2 72.5"6.0 0.7

ScotiaWeddell, SWC B 6 y0.29"0.68 33.59"0.17 22.3"3.4 1.4"0.3 62.5"10.2 4.7 .Prydz Bay, CCSZ MSV 6 4 y0.15"0.47 33.49"0.51 12.9"4.5 0.9"0.3 31.7"11.6 0.9

.Prydz Bay, OOZ MSV 6 5 0.98"1.12 33.81"0.09 26.8"0.9 1.8"0.1 42.2"7.4 1.0

ANTARES 2, OOZ 4 0.40"0.54 33.92"0.13 28.0"0.4 1.8"0 42.6"6.7 1.5

NDsno data.

nsNumber of stations.

NH Av.sammonium availability4 .SWCsScotiaWeddell Confluence zone SWC Asearly season and SWC Bs late season .

CCSZsCoastal and continental shelf zone.

OOZsOpen oceanic zone.


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central Weddell Sea with physico-chemical charac-

teristics very similar to ANT IXr2 but with slightly .lower nitrate concentrations ; 27.6 mM and

slightly higher phytoplankton biomass Chl as0.4y1 .mg l . This group and the ANT IXr2 stations

indicate the early stage of the growth season. The .second group of stations Eastern Weddell Sea were

located in the extreme eastern part of the transect .Table 2; Fig. 1 . These are shallow stations charac-

.terized by low surface temperature ;y1.368C , .high salinity ;34.23 and moderate levels of

.nutrients NO s24.5 mM; Fig. 2 . Phytoplankton3bloom was apparent with surface Chl a concentra-

tions in some stations reaching up to 6 mg ly1. A .very sharp and shallow pycnocline 1030 m indi-

cated a stable water column. Despite shallow pycno- y1 .cline, high Chl a concentrations )1 mg l ex-

tended down to 75 to 100 m depth. The third group

includes the Larsen Shelf, a short transect on the .western side of the Weddell Sea Table 2 . Like the

Eastern Weddell Sea, the Larsen Shelf transect cov-

ers relatively shallow waters showing pronounced

ice melting and maintaining a shallow upper mixed .layer 2050 m; Fig. 2 . High nutrient depletion

.e.g., surface NO concentrations as low as 12 mM ,3 y1 .phytoplankton bloom Chl as3.8 mg l and sub-

.surface ammonium maximum 5075 m character- .ize this area Table 2 . Fig. 2 depicts the vertical

profiles of temperature, nitrate, ammonium and Chl

a concentrations in the Eastern Weddell Sea and the

Larsen Shelf. The most remarkable difference is in

terms of ammonium availability. Despite similar val- .ues in the surface layer Table 1 , ammonium avail-

ability in the upper mixed layer was significantly

higher in the Larsen Shelf. Very high subsurface

ammonium concentrations can only develop in case

there has been high phytoplankton biomass and ex-

tensive heterotrophic activity. By and large, the

growth season was more advanced in the LarsenShelf than in the Eastern Weddell Sea.

. . . y1 . Fig. 2. Vertical profiles of temperature 8C , nitrate mM , ammonium mM and chlorophyll a concentrations mg l at Station 11 70849. . . .S, 8824 W in the Eastern Weddell Sea ( and at Station 76 698S, 57889 W in the Larsen Shelf I during the ANTARKTIS Xr7

cruise.


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( )3.1.2. ScotiaWeddell Confluence SWC .Sampling during the EPOS LEG 2 cruise 1989

was done in the ScotiaWeddell Confluence area, .located north of the Weddell Sea proper Fig. 1 .

Two major groups were identified in the marginal .ice zone of this area Table 2 . The early stage of the

MIZ, SWC A, was characterized by low surface .temperature ;y1.418C , more saline surface water

.;33.83 , high nitrate and silicate concentrations . .NO s27.9 mM; Si OH s72.5mM , low ammo-3 4

.nium availability ;0.7% and high phytoplankton y1 .biomass Chl a ;1.2 mg l ; T able 2 . On the

contrary, relatively high surface temperature ;. .y0.228C , less saline surface water ;33.53 ,

decreased nitrate and silicate concentrations NO s3 . .22.3 mM; Si OH s62.5 mM , high ammonium4

.availability ;4.7% and relatively low phytoplank- y1 .ton biomass Chl as1.0 mg l in SWC B indi-

cate a more advanced stage of the MIZ.

3.1.3. Prydz Bay

Surface water circulation in the Prydz Bay areaas observed during Marine Science Voyage 6, Table

.1 and Fig. 1 is characterized by a cyclonic gyre and

by a continuous supply of fresh melt water from two

ice shelves: the Amery Ice Shelf in the south and

West Ice Shelf on the south-eastern part of the Bay .Jacques and Fukuchi, 1994 . The Antarctic diver-

.gence AD in this area is located around 668S which

coincides with the shelf break. PCA analysis of the

physico-chemical data separated stations in theCoastal and Continental Shelf Zone CCSZ, south of

.AD or shelf break from the Open Oceanic Zone .OOZ .

At the moment of sampling the CCSZ was free of

ice and the water column was very stable. Severedepletion of nutrients e.g., the lowest NO and3

.Si OH concentrations were 2.8 and 11.0 mM, re-4

. spectively and huge build up of ammonium 23

. . . . .Fig. 3. Profiles of temperature 8C , salinity , nitrate mM and ammonium concentrations mM at Station 38 658S, 758E in the Open . . .Ocean Zone OOZ, I and at Station 29 698S, 74830 E in the Coastal and Continental Shelf Zone CCSZ, ( of the Prydz Bay

area during the Marine Science Voyage 6 cruise.


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.mM at the subsurface layer suggest extended high .productivity during the growth season Fig. 3 . Al-

though Chl a values are not available, POC and .phytoplankton carbon biomass PPC data indicate

.dense phytoplankton bloom Tables 2 and 3 .

The OOZ, which was far from ice shelves for a

continuous fresh water supply, was subjected tostrong wind stress and hence deep mixed layer Fig.

.3; Semeneh, 1992 . Nitrate and silicate concentra-tions were relatively high NO s26.8 mM and3

. .Si OH s42.2 mM; Table 2 . The vertical profiles4of temperature, salinity, nitrate and ammonium in

two typical stations representing the CCSZ and OOZ

parts of the Prydz Bay are shown in Fig. 3.

3.1.4. Meridian transect along 628EDuring ANTARES 2, JanuaryMarch 1994,

. Table 1 and Fig. 1 , the area was free of ice hence

. OOZ . Nutrient concentrations were high e.g., NO3 . .s28.0 mM and Si OH s42.6 mM; Table 2 . The4

Antarctic divergence was located between 64 and

658S. South of the divergence surface temperature .was low 08C and the vertical profile in the upper

200 m was rather uniform. Between the divergenceand the northern limit of the ice cover i. e. between

.648 and 588S surface water temperature increased .slightly ;0.51.58C and the vertical profile was

characterized by a pronounced subsurface minimum .E. Charriaud, personal communication . A very

sharp vertical temperature gradient in this area indi-cated stable water column conditions. Despite this

stable water column, the area was very oligotrophic y1 .Chl as0.2 mg l ; Table 2; Fiala et al., 1998 .

3.2. Biomass, species composition and size structure

of phytoplankton community

Table 3 shows the summaries of phytoplankton 2carbon biomass of all cruises. A slope of 0.20 r s

.0.71, p-0.05 was obtained when phytoplankton .carbon PPC of all cruises was plotted against par-

.ticulate organic carbon POC , indicating that phyto-plankton represented about 20% of POC. However,

when phytoplankton biomass was very low e.g.,. .ANT IXr2 , PPC was not related to POC p)0.05 .

3.2.1. Weddell Sea .Early in the growth season ANT IXr2 flagel-

.lates non-Phaeocystis sp dominated the phyto-

.plankton assemblage Table 3 . The autotrophic

community structure was characterized by greater

importance of small cells. About 66% of the total

biomass was represented by the -20 mm size frac-

tion, although the -10 mm fraction in Table 3

contributed only 40.6%.During ANT Xr7 the Central Weddell Sea ex-

.cept few stations was relatively poor in phytoplank-

ton biomass and flagellates dominated the assem- .blage Table 3 . The community structure was domi-

.nated by the -10 mm fraction ;61% . On the

other hand, in the Eastern Weddell Sea and the

Larsen Shelf, where dense phytoplankton blooms

were observed, diatoms were dominant with a rela- . tive contribution of 46% 1867% and 40% 16

.56% , respectively. Moreover, a greater proportion

of the total biomass was on the )10 mm fraction: .4474% means59% in the Eastern Weddell Sea

.and 3087% means54% in the Larsen Shelf.Phaeocystis sp. was a very important component of

these blooms with a mean relative contribution of

25%. Thus, the apparently high relative abundance

of the -10 mm size fraction in bloom stations of

the Weddell Sea was entirely due to Phaeocystis sp. .Table 3 .

In terms of species composition, the Central Wed-

dell Sea was characterized by abundance of small

diatoms and autotrophic flagellates. Cryptophytes and

other unidentified flagellates formed the bulk of the

biomass. Phaeocystis sp. contributed only 7.4%.Pennate diatoms contributed about 18% to the total

biomass. These include small pennates such as

Nitzschia cylindrus, chain forming species of thegroup Pseudonitschia Nitzschia prolongatoides and

.Nitzschia lineola , and Tropidoneis sp. Centric di-

atoms of the genus Coscinodiscus, Thalassiosira,

Corethron, Asteromphalus and Actinocyclus were

also important. On average, centric diatoms repre-

sented about 13% of the total biomass.

The phytoplankton assemblages in the Eastern

Weddell Sea and the Larsen Shelf were characterizedby high abundances of diatoms and Phaeocystis sp.

Unlike the Central Weddell Sea, Phaeocystis sp. in

these areas was very important with an average

contribution of 26.5% in the Eastern Weddell Sea

and 23% in the Larsen Shelf. Differences in diatom

species composition were observed between the

Eastern Weddell Sea and Larsen Shelf. In the East-


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Table3

.

.

TotalphytoplanktonbiomassPPC,absoluteandrelativecontributionsofdifferenttaxonomicgroupsandsize-fractions-

10mmand)

10mm

equivalentsphericaldiameter,ESD

y1

.

.

n

PhytoplanktonbiomassmgCl

Relativecontribution

%

PPC

Pen.Diat.

Cent.Diat.

Diat.

Dino.

Fla.

-

10mm

)

10mm

Diat.

Dino.

Fla.

-

10mm

)

10mm

.

WeddellSeaANTIXr2

6

19.3"

10.2

2.9"

2.7

1.9"

1.9

4

.8"

4.4

6.2"

5.2

8.3"

3.0

6.7"

2.1

12.6"

8.9

22.5"

10.4

28.7"

16.6

48.8"

19.2

40.6"

15.3

59.4"

15.3

.

CentralWeddellSeaANTXr7

16

15.7"

10.5

3.6"

3.5

1.9"

1.7

5

.5"

4.9

6.3"

5.6

4.0"

9.8

8.9"

6.1

7.0"

5.6

33.0"

12.2

35.9"

19.0

31.5"

19.2

59.1"

18.1

40.9"

17.8

.

EasternWeddellSeaANTXr7

4

53.0"

43.9

8.3"

4.3

11.0"

9.0

19

.4"

12.7

13.9"

15.0

19.7"

19.1

21.2"

16.8

31.7"

27.8

43.2"

22.0

22.1"

12.6

34.7"

12.6

40.9"

10.65

9.1"

10.6

.

LarsenShelfANTXr7

5

95.0"

50.0

24.3"

14.3

15.1"

20.8

39

.5"

28.8

29.0"

27.9

26.5"

14.5

38.2"

23.6

56.5"

54.2

39.6"

15.5

30.1"

12.0

30.3"

15.1

45.8"

22.9

54.2"

22.9

.

ScotiaWeddellSWCA

EPOS

6

31.4"

26.1

ND

ND

18

.4"

24.0

1.8"

0.7

11.2"

7.4

ND

ND

48.7"

22.2

6.0"

2.3

45.3"

22.7

ND

ND

.

ScotiaWeddell,SWCB

EPOS

6

26.9"

27.6

ND

ND

1

.8"

2.25

0.5"

0.43

24.9"

28.74ND

ND

19.7"

31.2

3.0"

5.2

77.3"

31.6

ND

ND

.

PrydzBay,CCSZ

MSV6

4

110.4"

37.0

64.0"

32.1

26.9"

5.3

90

.9"

28.4

6.7"

5.1

12.8"

17.2

25.9"

17.2

84.5"

21.0

83.6"

13.2

6.4"

4.3

10.0"

10.4

21.7"

7.3

78.3"

7.3

.

PrydzBay,OOZ

MSV6

5

87.9"

51.8

28.0"

43.6

48.5"

54.1

76

.5"

51.0

6.9"

12.7

4.4"

6.8

7.9"

8.0

80.0"

50.0

85.6"

14.9

10.3"

14.3

4.1"

5.2

12.2"

10.8

87.8"

10.8

ANTARES2,OOZ

4

16.5"

9.5

2.7"

1.5

2.5"

1.8

5

.1"

1.9

7.3"

9.4

4.1"

1.1

3.2"

2.9

13.3"

10.8

34.4"

12.4

32.4"

30.8

33.2"

24.3

28.4"

37.2

71.6"

37.2

.

n

s

Numberofsamplesstations.

Pen.

Diat.s

Pennatediatoms.

Cent.Diat.s

Centricdiatoms.

Diat.s

Diatoms.

Dino.s

Dinoflagellates.

Fla.s

Flagellates.

NDs

Nodata.


11/19


ern Weddell Sea centric diatoms such as Thalas-

siosira sp., Coscinodiscus sp. and Corethron crio-

philum were very important. The genus Thalas-

siosira alone represented about 14% of the total

biomass. On the contrary, in the Larsen Shelf pen-

nate diatoms were dominant with a relative contribu-

tion of 32%. The main pennate diatoms in the Larsen

Shelf include species such as N. cylindrus, Thalas-

siothrix sp., Fragilariopsis sp. and chain forming

Pseudonitzschia species such as N. prolongatoides

and N. lineola. Centric diatoms contributed only 9%,

compared to the 21% relative contribution in the

Eastern Weddell Sea.

3.2.2. ScotiaWeddell Confluence .The two MIZ groups i.e., SWC A and SWC B ,

despite similar biomass, showed differences in .species composition Table 3 . Diatoms dominated

the assemblage during the early stage of the MIZ . SWC A . However, late in the growth season SWC. .B flagellates non-Phaeocystis dominated over di-

atoms. In some stations bloom of cryptophytes were

observed late in the growth season. A greater propor-

tion of the total biomass during early season was due

to the )20 mm size fraction but as the season

progressed the dominance shifted to the -20 mm

size fraction. This seasonal shift in community struc-

ture was caused by deepening of the mixed layer andselective grazing of diatoms by krill Jacques and

.Panouse, 1991 .

3.2.3. Prydz Bay

This area was characterized by a stable water

column as well as by a rich and diverse phytoplank- .ton assemblage Table 3 . Both POC and PPC data

indicate dense phytoplankton bloom Tables 2 and 3;.Kopczynska et al., 1995 . Like the Eastern Weddell

Sea and the Larsen Shelf, the CCSZ is a relatively

shallow area with depth ranging from a few hundred .meters to the shelf break ;1000 m . However, we

observed important differences in species composi-

tion between the CCSZ and the bloom stations of the .Weddell Sea Eastern Weddell Sea and Larsen Shelf .

While the bloom in the Eastern Weddell Sea and the

Larsen Shelf was due to diatoms and Phaeocystis

sp., the bloom in the CCSZ was entirely due to .diatoms; about 83.6% 67.799.7% of the total

phytoplankton biomass was due to diatoms. The

phytoplankton community in the CCSZ was domi-

nated by the )10 mm size fraction 67.7 84.3%,.means78.3% . Pennate diatoms were particularly

very abundant, representing 70% of the diatom

biomass and 58% of the total phytoplankton biomass.

In particular, N. curta was the most dominant pen-

nate species which, on average, contributed 28.2%.

At one station this species virtually formed a

monospecific bloom. This species together with N.

antarctica and N. subcurata contributed over 45%

to the total biomass. Centric diatoms, compared to

the pennate species, were less dominant with a mean

relative contribution of 24.4%. Important centric

species include: C. criophilum, Rhizosolenia hebe-

tata, Thalassiosira sp., Dactyliosolen sp. and Bid-

dulphia sp. The relative contribution of Phaeocystis .sp. in the CCSZ was very small 5.4% compared to

a mean contribution of about 25% in the Eastern

Weddell Sea and the Larsen Shelf.

As in the CCSZ, the assemblage in the OOZ wasalso dominated by diatoms 62.699.2%, means

. 85.6% and the )10mm size fraction 71.5 98.8%,.means87.8%; Table 3 . Unlike the CCSZ, the dom-

inant diatoms were centric species with a mean

relative contribution of 55%. These include Thalas-

siosira, R. hebetata, C. criophilum and Chaetoceros

dichaeta. Pennate diatoms such as N. curta, N.

lecontei and Fragilariopsis sp. were also important.

Dinoflagellates such as Prorocentrum sp. were very

abundant in some stations. Therefore, in Prydz Bay

the phytoplankton assemblage was dominated bydiatoms, mainly by pennate species in the CCSZ and

by centric species in the OOZ.

3.2.4. Meridian transect along 628E

During ANTARES 2, although the water column

was stable, phytoplankton biomass was very low y1 .Chl as0.2 mg l ; Table 2; Fiala et al., 1998 .

.Diatoms dominated the assemblage Table 3 and .much of the phytoplankton biomass ;72% was on

the )10 mm size fraction. As in the OOZ of Prydz

Bay, the dominant diatoms were centric specieswhich represented about 31% of the PPC. The main

species include C. criophilum, Proboscia sp., Rhi-

zosolenia sp and Cha. dichaeta. Among pennatediatoms, species of the genus Nitzschia N. kergue-

. lensis and N. cylindrus , Thalassiothrix T. antarc-.tica , Tropidoneis sp. and Pseudonitzscha sp. were

very important. Cryptophytes were very abundant


12/19


Table 4

Specific and absolute nitrogen uptake rates and f-ratios in the surface waters of the Southern Ocean

y3 y1 y1 . .n Specific uptake rate =10 h Absolute uptake rate nM day f-ratio

n n n r r rNO N H N NO NH N3 4 3 4

.Weddell Sea ANT IXr2 9 3.5"2.6 1.3"0.5 4.8"2.8 22.8"16.9 9.1"5.6 31.9"19.3 0.69"0.1 .Central Weddell Sea ANT Xr7 14 3.3"3.5 1.3"0.6 4.6"3.3 109.0"131.5 40.3"29.4 149.3"132.5 0.62"0.2 .Eastern Weddell Sea ANT Xr7 5 8.7"4.5 0.9"0.3 9.6"4.5 919.6"757.8 84.7"39.0 1004.3"784.8 0.89"0.1

.Larsen Shelf ANT Xr7 4 5.8"3.8 0.9"0.4 6.7"3.5 678.4"286.1 119.6"67.2 798.0"222.4 0.83"0.1 .ScotiaWeddell, SWC A EPOS 8 5.4"2.1 2.6"1.6 8.0"3.4 194.3"155.1 78.4"34.4 272.7"180.7 0.66"0.1 .ScotiaWeddell, SWC B EPOS 6 2.7"3.0 2.0"0.7 4.7"3.3 122.8"127.8 98.8"42.9 221.7"151.2 0.44"0.2

.Prydz Bay, CCSZ MSV 6 4 0.5"0.4 0.7"0.3 1.2"0.7 61.4"37.1 85.8"47.7 147.2"71.9 0.42"0.1 .Prydz Bay, OOZ MSV 6 5 1.0"0.9 0.4"0.2 1.3"1.0 27.6"20.5 11.3"7.1 38.9"24.7 0.68"0.1

ANTARES 2, OOZ 4 1.0"0.6 1.9"0.6 3.0"1.2 22.9"14.8 39.2"16.2 62.0"31.1 0.34"0.1

.nsNumber of samples stations .

n sn qn .N NO NH3 4 r sr qr .N NO NH3 4nNO 3f-ratios .

nN

.14% . Silicoflagellates contributed about 8%.

Phaeocystis sp. biomass was very low.

3.3. Nitrogen uptake regime

The results of the nitrogen uptake rates are sum-

marised in Table 4. Specific nitrate uptake rates .n of all five cruises in the SIZ ranged fromNO 30.0002 to 0.06 hy1 and specific ammonium uptake

. y1rate n from 0.0002 to 0.006 h . A wide rangeNH 4

of variations in n compared to n indicatesNO NH3 4greater spatial and temporal variability in nitrate

uptake rate than ammonium.

3.3.1. Weddell Sea

High specific nitrate uptake rates in this regionwere associated with bloom conditions i.e., Eastern

.Weddell Sea and Larsen Shelf; Table 4 . Ammonium

uptake rates, on the other hand, were rather similarboth during bloom and non-bloom conditions Table

. .4 . Absolute nitrate uptake rates r for bloomNO 3stations were one order of magnitude higher than for

non-bloom stations. High r can be due to eitherNhigh n , high biomass or both. Fast dividing cellsNexhibit high n . Very high n in the EasternNO NO3 3Weddell Sea, compared to the Larsen Shelf, indi-

cates more active phytoplankton. .Early in the season ANT IXr2 both diatoms

.and flagellates depended on nitrate Table 4 . f-ratios

were )0.5, indicating the predominance of new

production. Although flagellates dominated the as- .semblage Table 3 , r was equally related toNO 3

2 .both diatoms slopes0.0048 and r s0.70 and 2 .flagellates slopes0.0047 and r s0.76 . Among

diatoms the biomass of pennate species correlated 2 .strongly with nitrate uptake rate r s0.76 . During

ANT Xr7, as a whole, only diatom biomass showedstrong correlation with nitrate uptake rate Figs. 4

.and 5 . Despite large variability in biomass and

species composition during ANT Xr7 Tables 2 and.3 , the production regime was rather similar with

y1 .Fig. 4. Relationship between diatom biomass mg C l and y1 .specific nitrate uptake rate n , h during spring time in theNO 3

.Weddell Sea ANTARKTIS Xr7 cruise .


13/19


y1 .Fig. 5. Relationship between diatom biomass mg C l and y1 .absolute nitrate uptake rate r , mM day during spring timeNO 3

.in the Weddell Sea ANTARKTIS Xr7 cruise .

overwhelming importance of new production Table.4 .

3.3.2. ScotiaWeddell Confluence

The uptake regime in this area varied depending

on dominance of diatoms or flagellates. At the begin-

ning of the season a diatom dominated assemblage

thriving in stabilised surface water was characterized

by high specific nitrate uptake rates and predomi- .nance of new production SWC A; Tables 3 and 4 .

.With the progress of the season SWC B , the phyto-

plankton composition shifted to dominance by flagel-

.lates mainly cryptophytes . This was accompanied .by an increase in ammonium availability Table 1 , a

y1 .Fig. 6. f-ratio vs. total diatom biomass mg C l in Prydz Bay .area during summer ( s OOZ and I sCCSZ .

y1 .Fig. 7. f-ratio vs. pennate diatom biomass mg C l in the Prydz

Bay area.

decrease in nitrate uptake rate and preponderance of .regenerated production Table 4 .

3.3.3. Prydz Bay

In the CCSZ phytoplankton biomass was still veryhigh but growth rate was low, i.e., low n Tables 3N

.and 4 . High biomass under low growth rate imply

little or no loss rate. Stable water column, high .subsurface ammonium concentrations Fig. 3 and

.dominance of diatoms, mainly by pennates Table 3 ,

were the salient features of this area. Specific nitrate y1 .uptake rates were very low n s0.0005 h , anNO 3

order of magnitude less than the bloom stations in

.the Weddell Sea Table 4 . The production regimewas characterized by predominance of regenerated

y1 .Fig. 8. Relationship between centric diatom biomass mg C l

and f-ratio in the Prydz Bay area.


14/19


y1 .Fig. 9. Specific ammonium uptake rate n , h vs. pennateNH 4 y1 .diatom biomass mg C l in Prydz Bay area.

.production f-ratio-0.5, Table 4 . In the OOZ, .however, diatoms mainly centric species were dom-

inant and the community was largely based on new

.production f-ratio)0.5 . In Prydz Bay, as a whole,f-ratio correlated negatively with diatom biomass .Fig. 6 . However, pennate and centric diatoms

showed different relationships with the f-ratio.

Whereas the biomass of pennate diatoms correlated

negatively with f-ratio, the opposite was true for .centric diatoms Figs. 7 and 8 . Moreover, the

biomass of pennate diatoms was positively correlated .to specific ammonium uptake rate Fig. 9 .

3.3.4. Meridian transect along 628E

During ANTARES 2, the OOZ was characterized

by a stable water column, a diatom dominated as-

semblage and predominance of regenerated produc- .tion Tables 3 and 4 . Unlike in the CCSZ, phyto-

y1 .Fig. 10. Relationship between total diatom biomass mg C l y1 .and specific nitrate uptake rate n , h during the ANTARESNO 3

2 cruise in summer period.

y1 .Fig. 11. f-ratio vs. total diatom biomass mg C l during the

ANTARES 2 cruise.

.plankton biomass was very low Table 3 . As in the

CCSZ, diatom biomass correlated negatively with

.n and f-ratio Figs. 10 and 11 .NO 3

4. Discussion

Two lines of seasonal evolution are apparent from

the physico-chemical characteristics of the environ-

ment as well as from the relationship between nitro-

gen uptake regime and phytoplankton biomass, com-

position and structure. In the first scenario, repre-

sented by the MIZ areas of the Weddell Sea and

ScotiaWeddell Confluence, a shift in uptake regime .new to regenerated production during the growth

season was accompanied by a change in phytoplank-ton community structure i.e., from a diatom domi-

nated microplankton assemblage to a flagellate dom-.inated nanoplankton assemblage . In the second sce-

nario, represented by the CCSZ and OOZ of Prydz

Bay and the OOZ of ANTARES 2, a shift in the

uptake regime occurred without change in commu-

nity structure. In discussing the seasonal evolution,

the factors that control the growth and maintenance

of phytoplankton biomass, community structure and

uptake regime are considered.

The first scenario is typical for the MIZ areas of

the Weddell Sea and ScotiaWeddell Confluence.

High ice cover, high nutrient levels, low ammonium

availability characterized the early stage of MIZ .ANT IXr2 and Central Weddell Sea, Table 2 .

.Autotrophic flagellates non-Phaeocystis dominated


15/19


16/19


can be seen in Table 4, uptake rates were low at thebeginning of the season ANT IXr2 and Central

.Weddell Sea , increased as the season progressed . SWC A , reached its maximum Eastern Weddell

. .Sea , decreased slightly Larsen Shelf and became .very low at the end of the season SWC B . This

trend in specific nitrate uptake rate was mirror- .imaged by the f-ratio Table 4 . Thus, in the MIZ, a

diatom-dominated, predominantly nitrate-based, mi-

croplankton assemblage thriving in a stable water at

the beginning of the season was transformed into a

flagellate dominated, ammonium-based, nanoplank-

ton assemblage towards the end of the season.

In the second scenario, applying to the CCSZ and

OOZ of Prydz Bay and the OOZ of the ANTARES 2

cruise, only a shift in uptake regime was observed.

The main features of these areas include stable water

column, high phytoplankton biomass, dominance of

diatoms, greater importance of the larger size frac- .tion, lown , high ammonium availability CCSZNO 3and predominance of regenerated production Tables

.24 . A bloom can develop only when the rate of

biomass increase exceeds the loss rate, i.e., grazing .rate and sedimentation rate Sakshaug et al., 1991 .

Once the bloom has developed it can be sustained

for a longer period under low growth rate if the loss

rate is small. This appears to be the case in the

CCSZ of the Prydz Bay. Stable water column due to

continuous freshwater supply from the ice shelves,

high biomass, low n and high ammonium avail-NO 3ability indicate prolonged phytoplankton bloom. High

ammonium availability can develop without the con-

tribution of migratory herbivores such as krill through

prolonged heterotrophic activity by the micro-

heterotrophs. This is corroborated by high hetero-

trophic activity of dinoflagellates, ciliates andnanoflagellates in this area Archer et al., 1995;

.Kopczynska et al., 1995 . Microheterotrophs can

consume as much as 48% of the daily production .Becquevort et al., 1992 and this results in high

ammonium availability. These heterotrophs preferen- .tially graze on small cells -20 mm and their

grazing pressure increases with the progress of the .season Archer et al., 1995; Froneman et al., 1995 .

This keep the biomass of small cells low thereby

reducing their competition for ammonium with the

large cells.

The dominance of diatoms late in the growth

season implies little or no selective grazing pressure

by meso- and macroheterotrophs and persistence of

stable water column. This also suggests that diatoms

were dominant throughout the season. In a similarenvironment, the Ross Sea ice edge, diatoms mainly

.pennate diatoms dominated the assemblage from thebeginning to the end of the season El-Sayed et al.,

1983; Smith and Nelson, 1985; Nelson and Smith,. .1986 . Although El-Sayed et al. 1983 reported high

abundance of Phaeocystis sp. in the Ross Sea, we

have no evidence to suggest that this species was

abundant at the beginning of the season.

Despite bloom conditions in the CCSZ, phyto-

plankton production was largely based on ammo- .nium Tables 1 and 4; Semeneh, 1992 . In particular,

large diatoms, which often are considered to live

mainly on nitrate and export organic material to the

deep sea, were predominantly based on ammonium

.or regenerated production Fig. 6 . On the contrary,in MIZ a mixed diatom-Phaeocystis sp. bloom was

based on nitrate, new production Tables 3 and 4;.Figs. 4 and 5 . In Prydz Bay, despite preponderance

of regenerated production, pennate and centric di-

atoms exhibited differences in nitrogen nutrition.

Centric diatoms were more abundant in the OOZ and .were mainly based on nitrate Tables 2 and 4 . Their

.biomass correlated positively with f-ratio Fig. 8 .

On the other hand, pennate diatoms tended to be

abundant in shallow areas with high ammonium

.availability e.g., in the CCSZ and Larsen Shelf and .showed negative correlation with f-ratio Fig. 7 . In

these areas pennate diatoms constituted a major frac-

tion of the total biomass. Moreover, these diatomsoften dominate ice assemblages Garrison and Buck,

.1985 . Elevated ammonium concentrations can occur .in the ice 1.52.4 mM; Fritsen et al., 1994 . When

encountered with high ammonium concentrations as.in the CCSZ , these diatoms can increase their up- .take capacity Smith and Nelson, 1990; Fig. 9 . This

enables them to change their nitrogen source depend-

ing on the availability of ammonium. Thus, pennatediatoms are opportunistic species with a flexible

physiology that enables them to live successfully

both in the ice and water column. Such flexible

nitrogen nutrition is particularly important in systems

such as the Southern Ocean where nitrate utilisationcan be limited by iron availability Martin et al.,

.1990 . Greater dependence on ammonium offers


17/19


more advantage for slowly dividing large diatom .cells in the CCSZ low n ; Table 4 . This isNO 3

because large cells have high iron requirement and,

in particular, this requirement is higher when theyassimilate nitrate Raven, 1988; Doucette and Harri-

.son, 1991 . The shift in nitrogen nutrition from

nitrate to ammonium reduces the overall community

demand for iron, thus, enables part of the community

to subsist on nitrate. Although the relative contribu- .tion of nitrate in the CCSZ was low f-ratios0.42 ,

its absolute uptake rate was high due high phyto-

plankton biomass. This explains the observed verylow ambient nitrate concentrations in the CCSZ Ta-

.ble 2 . In the MIZ, however, large diatoms which

formed the bulk of the bloom were selectively re-

moved from the euphotic zone through grazing and

water column destabilization and the remaining com-

munity had low biomass, mainly composed of

nanoplanktonic flagellates, was sustained by ammo-nium.

The community structure, unlike in the MIZ, did

not change during the season. The )10 mm size

fraction in Prydz Bay represented 28.9 to 98.8% .means80.6% of total phytoplankton biomass. The

)10 mm size fraction was dominant concurrent

with the predominance of regenerated production .Tables 3 and 4 . The situation in the CCSZ is the

extension of the first stages of the MIZ in the

Weddell Sea and adjacent areas. Therefore, under

persistent physical stability and absence of selectivegrazing pressure, a diatom dominated microplankton

assemblage persisted to late season and shifted its .uptake regime from nitrate to ammonium in re-

sponse to increased ammonium availability. The

dominance of diatoms under regenerated production

provides a further physiological evidence for the

excess net removal of silicate over nitrate occurringin certain provinces of the Southern Ocean Treguer

.and Jacques, 1992; Goeyens et al., 1998 .

From the export production point of view, export

can occur in both systems. The first system proceeds

with abundance of diatoms which are either grazed

or directly exported to deep water due to destabiliza-

tion of the water column. The proportion of organic

material channelled through a higher trophic level or

directly exported to bottom water and sediments

depends on the degree of grazing pressure, degree of

destabilization and rate of remineralization. More-

over, it also depends on the time at which grazing

starts as well as the time at which stable water

column lasts. Early termination of phytoplankton

bloom by heavy grazing pressure or water column

destabilization will lead to small absolute export

production.

Traditionally, nitrate uptake represents new pro- .duction hence export production and is mediated

by a diatom dominated microplankton assemblage .Eppley and Peterson, 1979 . In the CCSZ, although

diatoms were dominant, the assemblage was pre-

dominantly based on ammonium or regenerated pro-

duction. The predominance of regenerated produc-

tion in the CCSZ, according to Eppley and Peterson .1979 , would suggest little export production but the

dominance of large diatom cells implies export pro-

duction. This contradiction underlines the importance

of phytoplankton community analysis in interpreta-

tion of field nitrogen uptake results. The significancethe CCSZ for export production can be resolved by

using other measures of export production such as

sediment trap or accumulation of barite at .mesopelagic layer Dehairs et al., 1992 .

Acknowledgements

We are grateful to the captains and crew members

of the R.V. Polarstern, Marion Dufresne and Au-

rora Australis. We thank M. Leermakers for herassistance. This is AWI publication 1458.

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