# Role of plankton communities in sea-air variations in pCO2 in the SW Atlantic Ocean

Irene R. Schloss, Gustavo A. Ferreyra, Martha E. Ferrario, Gastón O.

Almandoz, Raúl Codina, Alejandro A. Bianchi, Carlos F. Balestrini, Héctor

A. Ochoa, Diana Ruiz-Pino, Alain Poisson

► To cite this version:

Irene R. Schloss, Gustavo A. Ferreyra, Martha E. Ferrario, Gastón O. Almandoz, Raúl Codina, et al. Role of plankton communities in sea-air variations in pCO2 in the SW Atlantic Ocean. Marine Ecology Progress Series, 2007, 332, pp.93-106. 10.3354/meps332093. hal-01245615

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# Role of plankton communities in sea-air variations in pCO₂ in the SW Atlantic Ocean

Irene R. Schloss<fnref n="1" />,<fnref n="2" />*, Gustavo A. Ferreyra<fnref n="1" />, Martha E. Ferrario<fnref n="2" />,<fnref n="3" />,
Gastón O. Almandoz<fnref n="2" />,<fnref n="3" />, Raúl Codina<fnref n="3" />, Alejandro A. Bianchi<fnref n="4" />,<fnref n="5" />, Carlos F. Balestrini<fnref n="5" />,
Héctor A. Ochoa<fnref n="1" />, D. Ruiz Pino<fnref n="6" />, Alain Poisson<fnref n="6" />

**ABSTRACT:** The influence of the plankton community structure on carbon dynamics was studied in the surface waters of the Argentinean continental shelf (SW Atlantic Ocean) in summer and fall 2002, 2003 and 2004. The horizontal changes in plankton community respiration (R), net community production (NCP) and gross primary production (GPP) were (1) compared with the difference in the partial pressure of CO₂ (pCO₂) between the sea surface and the atmosphere (ΔpCO₂), (2) compared with oxygen saturation and (3) related to the microscopic phytoplankton assemblages. This area, which has recently been shown to be a CO₂ sink, had an average surface oxygen saturation of 108.1 %, indicating that net photosynthesis could have played a dominant role in the CO₂ dynamics. At most stations, the production:respiration (GPP:R) ratio was greater than 1, indicating that planktonic communities were autotrophic; the average GPP:R ratio for the whole study was 2.99. Phytoplankton biomass (chlorophyll a) and NCP showed an inverse relationship with ΔpCO₂ and a direct relationship with %O₂ saturation when phytoplankton assemblages were dominated by diatoms (30 % of the stations). This was not the case when small (≤5 µm) flagellates were the most abundant organisms. Although NCP was mostly positive for both groups of stations (i.e. diatom-dominated or small flagellate-dominated), other physical and biological processes are thought to modify the CO₂ dynamics when small flagellates are the prevailing phytoplankton group.

**KEY WORDS:** Gross primary production · Net primary production · Respiration · Phytoplankton composition · pCO₂ · Oxygen saturation · SW Atlantic · Continental shelf

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## INTRODUCTION

Oceans play a key role in modulating CO₂ dynamics, storing nearly 48% of the anthropogenic CO₂ emitted into the atmosphere (Sabine et al. 2004, Takahashi 2004). Although some latitudinal differences have been observed in natural CO₂ concentrations (Taylor & Orr 2000), the seasonal and geographical variations in

the partial pressure of CO₂ (pCO₂) in surface water are much greater than those of atmospheric pCO₂ (Takahashi et al. 2002). The flux of CO₂ between the ocean and the atmosphere, which strongly depends on the difference between seawater and atmospheric pCO₂ (ΔpCO₂), will therefore ultimately be a function of seawater pCO₂: the lower the seawater pCO₂ with respect to atmospheric pCO₂, the more negative the ΔpCO₂.

<footnote n="1">¹Instituto Antártico Argentino, Cerrito 1248 1010 Ciudad de Buenos Aires, Argentina</footnote>

<footnote n="2">²CONICET, Consejo Nacional de Investigaciones Científicas y Tecnicas, Av. Rivadavia 1917 1033, Buenos Aires, Argentina</footnote>

<footnote n="3">³Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina</footnote>

<footnote n="4">⁴Departamento Ciencias de la Atmósfera y los Océanos, FCEN, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón 2, 1428 Ciudad de Buenos Aires, Argentina</footnote>

<footnote n="5">⁵Departamento Oceanografía, Servicio de Hidrografía Naval, Avenida Montes de Oca 2124, 1212 Ciudad de Buenos Aires, Argentina</footnote>

<footnote n="6">⁶LOCEAN, Laboratoire D'Oceanographie et du Climat: Experimentation et Analyse Numerique, UPMC, Universite Pierre et Marie Curie, Case 100, 4 Place Jussieu, 75252 Paris Cedex 05, France</footnote>
Seawater pCO₂ depends on physical factors and bio-
geochemical processes (Takahashi et al. 1993). The
main factors controlling seawater pCO₂ are tempera-
ture, affecting CO₂ solubility and acid constants;
winds, influencing the gas transfer velocity; carbonate
dissolution in deep water, eventually neutralizing the
effect of CO₂ on ocean acidity; calcification, producing
a net pCO₂ increase; and organic carbon production
(photosynthesis) and consumption (respiration), de-
creasing and increasing seawater pCO₂, respectively.
Organic carbon production and export to the deep
ocean are partly mediated by biological activity (the
so-called 'biological pump', Eppley & Peterson 1979).
Measuring seawater pCO₂ and dissolved O₂ simultane-
ously can help to identify the biological processes con-
trolling pCO₂ (Bender et al. 2000, Carrillo et al. 2004).
For instance, a pCO₂ undersaturation together with an
O₂ supersaturation relative to atmospheric concentra-
tions of both gases has been shown to be related to
active photosynthesis (Carrillo et al. 2004). The oppo-
site could indicate net respiration by the plankton com-
munity.

The biological effects on seawater pCO₂ vary
according to the characteristics of the plankton com-
munity. Marine autotrophic production provides the
organic material to support the producers' own meta-
bolic requirements as well as those of planktonic con-
sumers and decomposers. Ocean areas where
autotrophic production (P) exceeds respiration (R) (P:R
> 1) contribute to biological CO₂ drawdown, while het-
erotrophic areas (P:R < 1) contribute to CO₂ produc-
tion. Moreover, this ratio is expected to vary season-
ally, geographically, with water column depth and with
the dominant trophic pathway. In herbivorous food
webs (Legendre & Rassoulzadegan 1995), often
observed in nitrate-rich areas and where production
and grazing are uncoupled, high carbon export out of
the surface layer and P:R > 1 are expected. In contrast,
P:R is expected to be less than 1 in microbial loops,
which are characterized by high recycling rates and
ammonium-based production as well as by a domi-
nance of autotrophic and heterotrophic bacteria
grazed by zooflagellates. Consequently, herbivorous
food webs should significantly contribute to a CO₂ sink
while systems dominated by microbial loops would
add CO₂ to the atmosphere (Legendre & Rassoulzade-
gan 1996).

Duarte & Agustí (1998) calculated that 80% of the
ocean's surface, comprising different biogeochemical
regimes and different time scales (from daily to
annual), is heterotrophic. It has been suggested that
autotrophic coastal and shelf areas produce the
organic matter needed to fuel the excess respiration in
the open ocean (Bauer & Druffel 1998, del Giorgio &
Duarte 2002). Takahashi et al. (2002) indicated that the

southern Atlantic Ocean, between 40° and 60°S, is
expected to be a CO₂ sink. In fact, Bianchi et al. (2005)
used ΔpCO₂ data collected between 2000 and 2004 to
show that during summer and fall the continental shelf
of southern South America was an intense CO₂ sink.
The transition between source and sink regions closely
followed the location of tidal fronts, suggesting a link
between the vertical stratification of the water column
and the regional CO₂ balance. These fronts typically
have one nearshore homogenized side and one deep
well-stratified side. Phytoplankton is influenced by the
frontal environment. Thus, on the nearshore side, bot-
tom friction contributes to the intense mixing of the
water column, which limits the light availability for
phytoplankton photosynthesis. On the other hand, the
stratification in offshore waters allows phytoplankton
to grow and draw down CO₂.

The SW Atlantic shelf has been classified as a
'Class 1 large marine ecosystem' (primary productivity
>300 g C m⁻² yr⁻¹, Bisbal 1995). There is very little local
data available on primary production, community res-
piration, community composition and distribution in
this extensive area (Lange 1985, Lutz & Carreto 1991,
Carreto et al. 1995). In addition, high zooplankton
abundances have been described (Sabatini et al. 2004).
This region is also a very rich fishing ground (Castello
& Castello 2003) as well as an area with a high abun-
dance of upper trophic level organisms (Campagna et
al. 2000).

The aim of the present study was to determine
whether there is a relationship between the plankton
community composition and (1) its activity (production
and respiration), (2) the distribution of ΔpCO₂ levels
and (3) the distribution of O₂ saturation levels in the
surface waters of the SW Atlantic Ocean. For this, a
series of *in vitro* O₂ production and respiration rate
measurements were performed for different plankton
assemblages in the studied area during 3 consecutive
summers and falls (from 2002 to 2004).

MATERIALS AND METHODS

**Study area.** The SW Atlantic coast constitutes the largest continental shelf in the Southern Hemisphere (Bisbal 1995); its main characteristics have been described by Acha et al. (2004), Bianchi et al. (2005) and references therein. Briefly, the Argentinean shelf covers a 10⁶ km² area, extending from approximately 41°S to Cape Horn (near 55°S). We set the southern limit of this study at 57°S, as in Acha et al. (2004). Since temperature shows strong seasonal variability, salinity is frequently used to classify the water masses (see the detailed description in Bianchi et al. 2005). Surface N-NE circulation over the southern Patagon-
ian shelf is mainly dominated by local westerly and south-westerly winds. The propagation of strong tidal currents, freshwater inputs from several Patagonian rivers (Glorioso & Flather 1995, Piola & Rivas 1997, Glorioso 2000) and the low salinity water outflowing from the Magellan Strait (Piola & Rivas 1997) influence the circulation on the Patagonian continental shelf.

**Sampling.** The SW Atlantic Ocean was studied on 9 occasions from January 2002 to April 2004 on board the Argentinean icebreaker 'Almirante Irizar' (Fig. 1, Table 1). An integrated sampling system developed at the Laboratoire de Biogéochimie et Chimie Marines (Poisson et al. 1993) allowed the continuous acquisition (sampling interval ~10 s), averaging (to intervals of 10 min) and recording of meteorological, atmospheric and seawater data. An automated SAB600AGMA CR2M weather station provided information on photosynthetically active radiation (PAR). The partial pressure of atmospheric CO₂ (pCO₂) was determined on air obtained from an intake placed on

<table><caption>Table 1. Dates of the transects on which production and respiration measurements were performed during the 2002 to 2004 cruises on board 'Almirante Irizar' in SW Atlantic Ocean. The only outer shelf sampling during fall was in 2004</caption><thead><tr><th>Location</th><th>Date</th></tr></thead><tbody><tr><td><strong>2002</strong></td><td></td></tr><tr><td>Outer shelf</td><td>Jan 31-Feb 2</td></tr><tr><td>Coastal</td><td>Mar 23-26</td></tr><tr><td>Outer shelf-Coastal</td><td>Apr 10-13</td></tr><tr><td>Coastal</td><td>May 3-5</td></tr><tr><td><strong>2003</strong></td><td></td></tr><tr><td>Outer shelf-Coastal</td><td>Feb 7-10</td></tr><tr><td>Coastal</td><td>May 15-18</td></tr><tr><td><strong>2004</strong></td><td></td></tr><tr><td>Outer shelf</td><td>Feb 27-March 1</td></tr><tr><td>Outer shelf</td><td>March 13-16</td></tr><tr><td>Coastal</td><td>April 14-17</td></tr></tbody></table>

the bow of the ship by means of a flow-through equilibrator and an infrared (IR) analyzer (Siemens, Type Ultramat 5F). The analyzer was calibrated every 6 h with 3 gas standards containing 270.0, 361.0 and 489.9 ppm mole fraction of CO₂. pCO₂ was corrected for warming effects using temperature data obtained from high-accuracy sensors placed in the equilibrator and in the seawater intake. In addition, several variables were measured on surface seawater. Seawater was obtained from a pumping system placed at 9 m depth on the ship's exterior. A 550 µm stainless steel filter was used to eliminate large zooplankton. Microscopic *in situ* observations of the water samples showed that the pumping system did not break delicate planktonic organisms. Seawater pCO₂ was measured on the IR analyzer described above. Data were corrected for atmospheric pressure variations, drift and moisture effects as in Bianchi et al. (2005). The standard deviation of water pCO₂ was about 1 µatm (Metzl et al. 1995). The differences between ocean and atmosphere pCO₂ (ΔpCO₂) were later computed. Other variables that were measured continuously on the seawater from the same system included temperature (Sea-Bird SBE 38 sensor), salinity (Sea-Bird SBE 37) and *in vivo* fluorescence (Turner Designs Model 10 fluorometer).

Discrete samples were collected every 3 h from the water pumping system (N = 274). They were used for several analyses:

**Dissolved oxygen.** Dissolved oxygen concentrations were measured on duplicate 125 ml samples following the Winkler method and using an automatic potentiometer Mettler DL21 titrator. The average coefficient of variation for replicate samples was 0.4. These data were further compared to oxygen saturations in terms of percentage. Dissolved oxygen saturations were cal-

<figure><img src="image_2.png" /><figcaption>Fig. 1. Tracks of 2002 to 2004 cruises on board icebreaker 'Almirante Irizar', showing stations where NCP and R were measured; (□) offshore stations, (■) coastal stations</figcaption></figure>
culated as in García & Gordon (1992) using the solubil-
ity coefficients of Benson & Krause (1984) and the tem-
perature and salinity data obtained from the continu-
ous sampling system.

**Chlorophyll a.** For chlorophyll a (chl a) determina-
tion, 2 to 4 l of seawater were filtered through What-
man GF/F filters and frozen at -20°C until analysis.
This was done at the Instituto Antártico Argentino
within 1 mo of the end of the cruise. Chl a samples
extracted with 90% acetone were left for 24 h, read
for absorbance in a Beckman DU 650 spectropho-
tometer, and corrected for phaeopigments. The final
concentrations, which have an average precision of
±0.05 mg m⁻³, were calculated as in Strickland &
Parsons (1972).

**Microscopic analysis.** Quantitative plankton sam-
ples were taken directly from the pumping system,
fixed with acid Lugol's solution and kept in cool and
dark conditions. Living samples were examined on
board. Cell counting was done at the phycology labo-
ratory of La Plata University using an inverted light
microscope (Iroscope SI-PH) following the traditional
Utermöhl method (Lund et al. 1958). Further taxo-
nomic observations were done on qualitative Lugol's-
fixed samples collected with a 20 µm mesh net. Speci-
mens were examined at 400× magnification and
photographed using a Wild M20 microscope equipped
with a camera; they were also examined with a scan-
ning electron microscope (JEOL JSMT 100 and JEOL
JSM6360 LV at the SEM service, Museo de La Plata).
Although many samples were examined alive, flagel-
lates generally lose their flagella when fixatives are
added and could thus be confused with other round-
shaped organisms. Therefore, in the present work,
small flagellates and small (≤5 µm) round-shaped
organisms with or without flagella were counted as a
single group of 'small flagellates.' Abundances were
converted into phytoplankton carbon according to
published conversions and data on species biovolume
measured under the microscope (Montagnes et al.
1994, Hillebrand et al. 1999, Menden-Deuer & Lessard
2000).

**Production—respiration experiments.** We col-
lected 46 samples to study phytoplankton production
and community respiration (Fig. 1). In 2002 and 2003,
samples were incubated in the ship's wet laboratory:
samples from stations with low fluorescence signals
were incubated for 12 h; when signals were higher in
relative terms, incubations were for 6 h to enable the
detection of oxygen changes and to avoid the possible
nutrient limitation that could result from longer incu-
bations. In 2004, incubations were done on the deck
and lasted for 24 h; nutrient data analyzed after the
experiments indicated that there was no nutrient limi-
tation for the incubation periods used. For the deter-

mination of net community production (NCP), gross
primary production (GPP) and community respiration
(R), water from the pumping system was first trans-
ferred to a 5 l container in order to reduce replicate
variability. We then carefully filled 250 ml borosilicate
BOD bottles, allowing approximately 3 bottle volumes
of seawater to overflow before closing them. Borosili-
cate bottles do not transmit UV light, which affects
phytoplankton and micrograzers; therefore, we could
compare data obtained in 2004 (when incubations
were done with natural light) with data from 2002 and
2003, when incubations were done with artificial
light. Seawater from the continuous sampling system
circulated through the incubator to keep the system at
*in situ* temperatures. Temperature varied latitudinally
from 19°C in the northern part of the Argentinean
continental shelf during summer to 3.5°C in the
southern latitudes during fall. Light penetration at the
sampling depth (9 m) was measured with a Biospheri-
cal PUV radiometer deployed from a launch, about
100 m away from the icebreaker. The depth of the
euphotic zone in the area is always >10 m (Angelescu
& Prenszki 1987, in Bisbal 1995) and gets deeper at
the limit of the continental shelf. Irradiance in the
incubator was measured with a LI-COR quantum-
radiometer/photometer Model LI-250 with a micros-
pheric sensor. The irradiance inside the incubator
simulated PAR at the sampling depth. In 2002 and
2003, irradiance varied between 40 and 100 µmol
photons m⁻² s⁻¹. In 2004 (24 h on-deck incubations),
bottles were wrapped with neutral filters to reach
~50% of the sea-surface irradiance, corresponding to
the irradiance measured at 9 m (sampling depth). The
average daily values of surface PAR during 2004
ranged between 45 and 257 µmol photons m⁻² s⁻¹,
depending on the time of year and cloud conditions.
Two to 3 replicates were used to determine the initial
oxygen concentration, and there were 3 replicates for
each light and each dark incubation. Dissolved oxy-
gen concentrations were measured as described
above. The coefficient of variation for replicate sam-
ples averaged 2.1%. R was calculated from the differ-
ence between the dark incubations and the initial
oxygen concentrations while NCP was calculated
from the difference between the initial and light-incu-
bated oxygen concentrations. For 2002 and 2003, both
R and NCP were converted to daily estimates by mul-
tiplying hourly estimates by 24. The day length for
each sampling date and latitude was considered for
NCP calculations following Agusti et al. (2004) as
NCP d⁻¹ = DL × GPP [h⁻¹] – 24 × R [h⁻¹], where DL =
length of the day. Gross primary production (GPP)
was computed as R + NCP.

**Data treatment and analysis.** To remove the temper-
ature effect, the observed seawater pCO₂ values were
normalized to seasonal temperature averages as in
Takahashi et al. (2002). Average sea-surface tempera-
tures for the summer and fall of 2002 to 2004 were cal-
culated after dividing the area into grids of 4° latitude
× 5° longitude. In this study, TcΔpCO₂ refers to the dif-
ference between seawater pCO₂ and the atmospheric
pCO₂ after the effect of temperature was removed
from seawater pCO₂. A 1-way ANOVA was used to
determine significant differences among years. In
addition, the whole data set was classified according to
the season (summer: January to March; fall: April to
May) and the sampling area (coastal: depth <60 m;
outer shelf waters: depth >60 m). It should be noted
that the outer shelf waters were sampled in fall only in
2004 (see Table 1 legend). Stations were further
classified according to the dominant phytoplankton
group into diatom-, small flagellate- or dinoflagellate-
dominated stations. After converting cell numbers to
carbon (see above), a group was considered dominant
when its carbon content represented more than 50% of
the total phytoplankton carbon.

Data were log transformed to satisfy the assumptions
of the analysis. Since some production values were
negative, a constant value was added to shift its mini-
mum log value to 0, i.e. log (data + constant) = 0. Aver-
ages and standard errors of means (SE) were calcu-
lated. A major axis Model II regression analysis was
carried out to describe the relation between either
TcΔpCO₂ or %O₂ and biomass (as chl *a*) or NCP. Confi-
dence intervals for the regressions were calculated
according to Ricker (1973).

RESULTS

ΔpCO₂ and oxygen saturation

The observed range of atmospheric pCO₂ was
10.6 µatm, while the sea pCO₂ was about 268 µatm
(ranging between -139 and 129 µatm). Average areal
ΔpCO₂ in the SW Atlantic Ocean over all years and
seasons was -23 µatm, leading to a net CO₂ sink dur-
ing the summer-fall period (Fig. 2A). The average sea-
surface temperatures were not significantly different
between seasons (11.8°C, SE = 3.4 and 10.7°C, SE =
3.9 for summer and fall, respectively; p = 0.21). The
TcΔpCO₂ distribution (Fig. 2B) showed a similar pat-
tern before and after removing the temperature effect
on seawater pCO₂. The average areal TcΔpCO₂
remained negative (-21 µatm). The distribution of %O₂
saturation followed a similar regional pattern (Fig. 2C).
The regional average was 108.1% (range = 86 to
116%), and the stations with the highest oxygen super-
saturation and the lowest TcΔpCO₂ (-110 µatm) corre-
sponded to offshore waters.

Chlorophyll *a* and phytoplankton groups

Fig. 2D shows discrete-sampled spectrophotometric chl *a* data obtained in the area from 2002 to 2004. To avoid distortion, chl *a* values >15 mg chl *a* m⁻³ are indicated on the map but were not used to construct the figure. Average surface chl *a* for summer and fall were 1.45 (SE = 2.10) and 1.21 (SE = 1.08) mg m⁻³, respectively. Maximum chl *a* concentrations were consistently found on the offshore side of the tidal fronts off Valdés Peninsula (42°S), San Jorge Gulf (close to 46°S), its southern tip (Blanco Cape) and Grande Bay (close to 52°S) (Fig. 2D). The highest value measured on the Patagonian shelf during the whole study period was 19.2 mg chl *a* m³, which was found in a wide area off Grande Bay on 9 February 2003. Almost half of this high biomass was from phytoplankton >10 µm (data not shown) and was associated with oxygen supersaturation (105%) and low temperature-corrected seawater pCO₂ (-118.5 µatm). Values around 4 mg chl *a* m⁻³ were found during March 2002 and values up to 5 mg chl *a* m⁻³ were measured during the 2004 survey in some coastal locations at 45.7°S, this time corresponding to the <5 µm phytoplankton fraction (data not shown). In general, all 2004 samples were dominated by small flagellates. The high chl *a* concentration found in Grande Bay in 2003 was associated with a massive bloom of the centric diatom *Chaetoceros debilis*, with more than 2 × 10⁶ cells l⁻¹, accompanied by a high number (>3 × 10⁶ cells l⁻¹) of 6 to 10 µm flagellates. In front of Blanco Cape (around 48°S, Fig. 1), the high phytoplankton biomass (>3 × 10⁶ cells l⁻¹) was due to the presence of small (<5 µm) flagellates. This had been previously observed in the 2002 transsects, although with smaller numbers both in summer (>1 × 10⁶ cells l⁻¹) and in fall (>0.7 × 10⁶ cells l⁻¹). For the San Jorge Gulf area, once again small phytoplankton (<5 µm) organisms dominated the phytoplankton assemblages in summer and fall 2002 as well as in summer 2003. Dinoflagellates dominated the plankton assemblages at 1 station only. A detailed description of the phytoplankton species composition will be presented elsewhere (Almandoz et al. 2007).

At stations where diatoms represented more than
50% of the phytoplankton carbon, a significant linear
correlation was observed between chl *a* and both
TcΔpCO₂ and %O₂ saturation (Fig. 3A,B), although this
correlation was weaker in the latter case (r² = 0.62, p <
0.05 and r² = 0.31, p < 0.05, respectively). In contrast,
no significant relation was found between chl *a* and
TcΔpCO₂ or %O₂ saturation at the stations dominated
by small flagellates (r² = 0.02, p = 0.53 for chl *a* vs.
TcΔpCO₂ and r² = 0.08, p = 0.15 for chl *a* vs. %O₂
saturation; Fig. 3C,D).
<figure><img src="image_3.png" /><figcaption>Fig. 2. (A) Average ΔpCO₂ (μatm) distribution for summer and fall 2002, 2003 and 2004; negative values indicate pCO₂ sink. (B) Average TcΔpCO₂ (μatm) distribution for summer and fall 2002, 2003 and 2004 (i.e. after effect of temperature was removed from seawater pCO₂ values; see 'Materials and methods'). (C) Surface %O₂ saturation for same years. (D) Surface distribution of discrete chl a samples, averaged from 2002 to 2004. In (A), (C) and (D), white dashed line indicates approximate locations of tidal fronts (after Bianchi et al. 2005)</figcaption></figure>

## Production and respiration

One-way ANOVAs showed that there were no significant differences in respiration rate, R (p = 0.29, F = 1.29) or in NCP (p = 0.23, F = 1.59) among years. Therefore, data from the 3 yr were pooled for all the following analyses. No significant differences were found in R when comparing summer and fall (p = 0.17, F = 1.99).

R was correlated neither to temperature (p = 0.28 for log-transformed R data) nor to chl a concentration (p = 0.18 for log-transformed data of both series). The average daily respiration rate for all stations was 4.38 (SE = 0.60) mmol O₂ m⁻³ d⁻¹ (Table 2). The highest R values were obtained for summer 2003 and fall 2002 (Table 2). High R values were mostly found at stations located close to the coast (Fig. 4A). In summer 2002, values of
<figure><img src="image_4.png" /><figcaption>Fig. 3. (A,B) Relationship between chl a content and (A) TcΔpCO₂ and (B) %O₂ saturation for diatom-dominated stations. (C,D) Relationships between chl a content and (C) TcΔpCO₂ and (D) %O₂ saturation for small flagellate-dominated stations. Continuous lines: Major Axis Model II regressions; dashed lines: 95 % confidence intervals</figcaption></figure>

<table><caption>Table 2. Mean (SE) minimum (Min.) and maximum (Max.) (N = 46) chlorophyll a, net community production (NCP), dark community respiration (R), and gross primary production (GPP) for the years studied. Data separated according to season (summer or fall); average yearly data are also presented (summer + fall). All values are in mmol O₂ m⁻³ d⁻¹. -: no data</caption><thead><tr><th rowspan="2">Season</th><th rowspan="2">N</th><th colspan="2">Chl a</th><th colspan="2">NCP</th><th colspan="2">R</th><th colspan="2">GPP</th></tr><tr><th>Mean (SE)</th><th>Min.</th><th>Mean (SE)</th><th>Min.</th><th>Mean (SE)</th><th>Min.</th><th>Mean (SE)</th><th>Min.</th></tr></thead><tbody><tr><td colspan="10"><strong>2002</strong></td></tr><tr><td>Summer</td><td>5</td><td>0.48 (0.14)</td><td></td><td>3.24 (1.85)</td><td></td><td>4.22 (1.56)</td><td></td><td>7.47 (2.67)</td><td></td></tr><tr><td></td><td></td><td>0.04</td><td>0.89</td><td>-3.48</td><td>10.02</td><td>0.86</td><td>11.33</td><td>-1.43</td><td>13.2</td></tr><tr><td>Fall</td><td>7</td><td>0.52 (0.15)</td><td></td><td>1.14 (1.73)</td><td></td><td>6.61 (1.28)</td><td></td><td>7.75</td><td>(2.16)</td></tr><tr><td></td><td></td><td>0.18</td><td>0.98</td><td>-3.03</td><td>5.88</td><td>2.90</td><td>9.68</td><td>1.96</td><td>12.52</td></tr><tr><td>Summer + fall</td><td>12</td><td>0.50 (0.10)</td><td></td><td>2.29 (1.26)</td><td></td><td>5.31 (1.05)</td><td></td><td>7.60</td><td>(1.50)</td></tr><tr><td colspan="10"><strong>2003</strong></td></tr><tr><td>Summer</td><td>10</td><td>3.44 (1.45)</td><td></td><td>18.8 (6.07)</td><td></td><td>7.72 (1.50)</td><td></td><td>26.52 (7.26)</td><td></td></tr><tr><td></td><td></td><td>0.21</td><td>15.90</td><td>-0.47</td><td>62.38</td><td>1.17</td><td>13.72</td><td>0.70</td><td>75.94</td></tr><tr><td>Fall</td><td>17</td><td>1.16 (0.34)</td><td></td><td>2.28 (0.86)</td><td></td><td>2.32 (0.69)</td><td></td><td>4.60 (0.94)</td><td></td></tr><tr><td></td><td></td><td>0.11</td><td>4.81</td><td>-2.20</td><td>10.87</td><td>0.02</td><td>7.64</td><td>1.04</td><td>12.39</td></tr><tr><td>Summer + fall</td><td>27</td><td>2.15 (0.69)</td><td></td><td>9.04 (3.02)</td><td></td><td>4.53 (0.92)</td><td></td><td>13.57 (3.75)</td><td></td></tr><tr><td colspan="10"><strong>2004</strong></td></tr><tr><td>Summer</td><td>6</td><td>1.75 (0.47)</td><td></td><td>4.59 (1.15)</td><td></td><td>2.29 (0.55)</td><td></td><td>6.88 (1.55)</td><td></td></tr><tr><td></td><td></td><td>0.43</td><td>3.28</td><td>0.3</td><td>6.88</td><td>1.05</td><td>4.59</td><td>1.34</td><td>11.39</td></tr><tr><td>Fall</td><td>1</td><td>2.85</td><td></td><td>4.36</td><td></td><td>3.28</td><td></td><td>7.64</td><td></td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Summer + fall</td><td>7</td><td>2.85</td><td></td><td>4.56 (0.97)</td><td></td><td>2.43 (0.49)</td><td></td><td>6.99 (1.31)</td><td></td></tr><tr><td colspan="10"><strong>Total</strong></td></tr><tr><td>Summer</td><td>21</td><td>2.17 (0.71)</td><td></td><td>9.87 (3.19)</td><td></td><td>4.96 (0.95)</td><td></td><td>15.68 (3.89)</td><td></td></tr><tr><td>Fall</td><td>25</td><td>1.08 (0.26)</td><td></td><td>1.91 (0.65)</td><td></td><td>3.19 (0.65)</td><td></td><td>5.10 (0.82)</td><td></td></tr><tr><td>Summer + fall</td><td>46</td><td>1.67 (0.40)</td><td></td><td>6.40 (1.75)</td><td></td><td>4.38 (0.60)</td><td></td><td>10.77 (2.15)</td><td></td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Fig. 4. (A) Dark community respiration (R), (B) net community production (NCP) and (C) production:respiration ratio (GPP:R) measured experimentally in 2002, 2003 and 2004</figcaption></figure>

R >5 mmol O₂ m⁻³ d⁻¹ coincided with relatively low (around 10 mmol O₂ m⁻³ d⁻¹) NCP values. In contrast, R values >5 mmol O₂ m⁻³ d⁻¹ in summer 2003 were accompanied by the highest average NCP values (Fig. 4A,B).

NCP was higher than R at 61% of the stations. Significant differences were found for NCP when comparing summer and fall data (p < 0.05, F = 6.18). The highest NCP value (62.38 mmol O₂ m⁻³ d⁻¹; Table 2) was measured at 51.7° S, 67.4° W (Fig. 4B) on 9 February 2003 and corresponded to the *Chaetoceros debilis* bloom described in the foregoing subsection. High NCP values were mostly located offshore, on the stratified side of the tidal fronts. Moreover, NCP was significantly correlated with biomass (chl a) concentration (r² = 0.61, p < 0.001) (Fig. 5A). NCP represented 59.4% of the GPP for all the data, with 62.9% during summer and 37.5% during autumn. The correlation between NCP and GPP was higher (r² = 0.63, p < 0.001) (Fig. 5B) than that between GPP and R (r² = 0.53, p < 0.001) (Fig. 5C). The distribution of the GPP:R ratio showed values <1 for 4 outer shelf stations, with 1 of them located north and the other 3 south of the Argentinean shelf, near Drake Passage (Fig. 4C). GPP:R ratios averaged 2.46 for the whole data set with no significant differences between seasons (average GPP:R summer = 3.16, fall = 2.67). The GPP:R ratio was >1 in 91% of the cases, indicating that gross primary production exceeded community respiration in most incubations (Table 2) regardless of phytoplankton biomass concentrations or stratification conditions.

## Community metabolism, ΔpCO₂ and %O₂ saturation

The relationship between NCP and the TcΔpCO₂ dynamics shows a different pattern for the 2 studied seasons. In summer, most of the NCP data were positive and accompanied by negative TcΔpCO₂ data (Fig. 6A). Oxygen saturation was frequently >100%, although all data were very close to the equilibrium (Fig. 6B). During fall (mainly coastal stations), TcΔpCO₂ was generally >0, indicating that even though NCP was positive, the area would not be acting as a CO₂ sink; in addition, waters were not saturated in oxygen for these same NCP positive values (Fig. 6C, D). Comparing production and respiration between diatom- and small flagellate-dominated stations, only R and GPP were significantly different among groups, although this was not the case for the GPP:R ratio (Table 3). The 1-way ANOVA did not show differences for NCP or for chl a concentration (Table 3). However, as shown for chl a (Fig. 3), the relationships between NCP and both TcΔpCO₂ and %O₂ saturation were consistently different for both the diatom-dominated (Fig. 7A,B) and the small flagellate-dominated (Fig. 7C,D) communities. A significant linear correlation was observed between NCP and both TcΔpCO₂ and %O₂ saturation (r² = 0.49, p < 0.01 and r² = 0.52, p < 0.01, respectively) (Fig. 7A,B). In contrast, no significant relation was found between NCP and TcΔpCO₂ or %O₂ saturation at the stations dominated by small flagellates (r² = 0.13, p = 0.06 for NCP vs. TcΔpCO₂ and r² = 0.00, p = 0.92 for NCP vs. %O₂ saturation) (Fig. 7C, D). This could indicate that
<figure><img src="image_6.png" /><figcaption>Fig. 5. Model II regressions between (A) chl *a* and net community production (NCP), (B) NCP and gross community production (GPP), and (C) community respiration (R) and GPP. Dashed lines: 95% confidence intervals. Since some production values were negative, a constant value was added to shift its minimum log value to 0 (see last subsection of 'Materials and methods')</figcaption></figure>

the influence of phytoplankton on seawater pCO₂ differs among groups, regardless of total biomass.

DISCUSSION

Bianchi et al. (2005) showed that the SW Atlantic Ocean acts as a CO₂ sink during summer and fall. In that study, the simultaneous presence of the lowest seawater pCO₂ and the highest chl *a* concentrations on the stratified side of tidal fronts led to the conclusion that

the combined physical-biological processes control pCO₂ dynamics in surface waters over almost all the Argentinean shelf. In the present study, TcΔpCO₂ data are complemented by information on %O₂ saturation and phytoplankton production and respiration in relation to the composition and distribution of phytoplankton assemblages for the first time in this area. After removing the effect of temperature on surface water pCO₂, average areal TcΔpCO₂ was still negative and had values similar to ΔpCO₂. This indicates that other factors affect seawater pCO₂. However, thermal stratification and water column structure are essential to phytoplankton photosynthesis, and high phytoplankton concentration is only possible under the appropriate physical environment. Therefore, the temperature effect on water column stratification and biological production are intimately related and probably act jointly to produce the negative ΔpCO₂ that we observed.

The production estimates in this study are representative of the surface mixed layer of the water column, which averages 25 m during summer and fall (Rivas & Piola 2002). The few previous estimates of primary production in the area indicate that overall production on the Patagonian shelf south of 41°S and west of the Malvinas (Falkland) Islands was about 150 to 250 mg C m⁻² d⁻¹. In the slope region from 23°S to the Malvinas (Falkland) Islands, it varied between 250 and 500 mg C m⁻² d⁻¹ (Bisbal 1995). In order to allow a proper comparison, we converted our oxygen-based production data set into carbon using a photosynthetic quotient (PQ = Δ[O₂]/Δ[CO₂]) of 1.4 (based on nitrate assimilation) (Dickson & Orchard 2001). NCP data (in mmol O₂ m⁻³ d⁻¹) obtained in the present 3 yr study were converted into carbon by multiplying the number of mmol oxygen by 12 to obtain mg of carbon. The estimated average net primary production in the area, without considering the exceptional 2003 bloom in Grande Bay, was 34 ± 7.4 mg C m⁻³ d⁻¹. When that bloom was included, production reached 45.4 ± 9.6 mg C m⁻³ d⁻¹. A rough estimation indicates that in either case, our primary production values are much higher than earlier (1972) estimates of the FAO (Food and Agriculture Organization of the United Nations; see Bisbal 1995). Even assuming that the values reported in Bisbal (1995) result from integrating production only in the upper 25 m, the estimated production would fall between 10 and 25 mg C m⁻³ d⁻¹. However, our data are in agreement with other production estimates for the area based on benthic carbon content (Seiter et al. 2004) and remote sensing (Antoine et al. 1996). They are also similar to values corresponding to other continental shelf areas (de Haas et al. 2002, Verity et al. 2002).

Tidal amplitude on the Patagonian shelf is one of the highest in the World Ocean (Kantha et al. 1995). Tidal and shelf-break fronts show elevated levels of algal
<figure><img src="image_7.png" /><figcaption>Fig. 6. Relationships between TcΔpCO₂ and %O₂ saturation, and microplanktonic net community production (NCP) for (A,B) summer and (C,D) fall. Since some NCP values were negative, a constant value was added to shift its minimum log value to 0</figcaption></figure>

biomass due to either *in situ* growth or aggregation at a frontal convergence (Longhurst 1995), as stated for the frontal areas in the SW Atlantic (Acha et al. 2004). Close to the coastline, river discharges of suspended sediments may limit light availability. In addition to intense mixing, this contributes to explaining the low productivity and low phytoplankton biomass values consistently found near the continent. In this region, low GPP:R ratios and %O₂ saturations were measured both in summer and in fall (Figs. 2C & 4C). However, coastal stations still showed GPP:R >1. This suggests that plankton production does not compensate for

other processes that convert this area into a CO₂ source (Fig. 2A). Then, seawater pCO₂ in coastal zones could have several origins other than phytoplankton respiration. River input not only limits irradiance for photosynthesis, but also adds inorganic and organic carbon to the coastal Patagonian ecosystem (Depetris 1996). In addition, both benthic respiration (Middelburg et al. 2005) and the remineralization of dissolved organic matter (Broecker & Peng 1982) contribute to seawater pCO₂ in coastal oceans. Conversely, high primary production and negative ΔpCO₂ were generally found in the stratified zones of the frontal regions. High phyto-

plankton biomass and production (as found off Grande Bay) could be explained by stratification leading to adequate light conditions and to an intense nutrient supply, especially from waters that originated in the subantarctic region. Preliminary results on nutrients in the area (where no nutrient limitation was evident) support this hypothesis (Paparazzo et al. 2004).

Phytoplankton concentration along the Argentinean coast has well-defined maxima in spring and fall (Acha et al. 2004). Since the present study did not cover the start of the phytoplankton growth season, it is possible that the low ΔpCO₂ values measured during summer were the result of spring production, later accompanied by episodic

<table><caption>Table 3. Average (SE) (N = 45) and significance of 1-way ANOVAs for different variables measured in relation to the dominant phytoplankton group in the studied samples. One station, where phytoplankton carbon was dominated by dinoflagellates, was not included in the analysis. ΔpCO₂ (μatm) is the difference between seawater and atmospheric pCO₂ without correcting for sea-surface temperature (SST). ns = not significant</caption><thead><tr><th rowspan="2"></th><th colspan="2">Stations dominated by</th><th rowspan="2">p</th></tr><tr><th>Diatoms<br>(31% of stations)</th><th>Small flagellates<br>(67% of stations)</th></tr></thead><tbody><tr><td>SST (°C)</td><td>10.36 (1.16)</td><td>12.16 (0.67)</td><td>ns</td></tr><tr><td>%O<sub>2</sub> saturation</td><td>103.89 (1.92)</td><td>101.16 (1.11)</td><td>ns</td></tr><tr><td>TcΔpCO<sub>2</sub> (μatm)</td><td>21.67 (23.89)</td><td>8.08 (14.11)</td><td>ns</td></tr><tr><td>ΔpCO<sub>2</sub> (μatm)</td><td>-1.85 (13.9)</td><td>-3.81 (8.33)</td><td>ns</td></tr><tr><td>R (mmol O<sub>2</sub> m<sup>-3</sup> d<sup>-1</sup>)</td><td>6.77 (1.23)</td><td>3.22 (0.55)</td><td>p < 0.01</td></tr><tr><td>NCP (mmol O<sub>2</sub> m<sup>-3</sup> d<sup>-1</sup>)</td><td>9.51 (5.02)</td><td>4.95 (0.94)</td><td>ns</td></tr><tr><td>GPP (mmol O<sub>2</sub> m<sup>-3</sup> d<sup>-1</sup>)</td><td>17.17 (5.75)</td><td>7.93 (1.32)</td><td>p < 0.05</td></tr><tr><td>GPP:R</td><td>2.46 (0.67)</td><td>3.03 (0.37)</td><td>ns</td></tr><tr><td>Chl a (mg m<sup>-3</sup>)</td><td>2.38 (1.20)</td><td>1.33 (0.20)</td><td>ns</td></tr></tbody></table>
<figure><img src="image_8.png" /><figcaption>Fig. 7. (A,B) Relationships between microplanktonic net community production (NCP) and (A) TcΔpCO₂ and (B) %O₂ saturation for diatom-dominated stations. (C,D) Relationships between NCP and (C) TcΔpCO₂ and (D) %O₂ saturation for small flagellate-dominated stations. Continuous lines: Model II regression; dashed lines: 95% confidence intervals. Since some NCP values were negative, a constant value was added to shift its minimum log value to 0</figcaption></figure>

high production events during the summer season. To
complete the understanding of the SW Atlantic, spring
and winter studies are necessary. Local discrepancies
that can be seen between the distribution patterns of
both TcΔpCO₂ and %O₂ saturation (Fig. 2A,B,C) can
be explained by the lower solubility of O₂ compared
to CO₂, since O₂ approaches atmospheric equilibrium
concentrations faster than CO₂ (Carrillo et al. 2004). In
fact, an intensive latitudinal sampling like the present
one has an incertitude related to the temporal evolu-
tion of the water masses. On occasion, low seawater
pCO₂ is the result of a previous bloom that was not
retained in the surface layer (probably through sedi-
mentation) but that left a residual CO₂ deficiency in
surface waters. Such a time lag between primary pro-
duction and CO₂ exchange has been described for
other areas (Borges & Frankignoule 2001, Anadón &
Estrada 2002). A similar situation was found when
comparing NCP distribution and %O₂ saturation. In
some areas, as in the open waters between 46 and
49°S, high %O₂ saturations were not accompanied by
high NCP values. Since incubations were fewer in
number than the discrete oxygen sampling, some
highly productive areas may have been missed.

In the Argentinean shelf region, plankton communi-
ties appeared to be autotrophic. Moreover, the correla-
tion between NCP and GPP was much higher (r² = 0.7)
(Fig. 5B) than between GPP and R (Fig. 5C), in agree-
ment with what has been found in other ocean areas
(i.e. the Southern Ocean; Agustí et al. 2004). Although

GPP might be underestimated because of enhanced
respiration under illuminated conditions (Dickson &
Orchardo 2001) and due to the grazing activity of small
(<500 µm) zooplankton and protozoans, NCP oxygen
incubation results represent the maximal potential
productivity in the absence of physical disturbances.
Following the criteria of Duarte & Agustí (1998) and
Agustí et al. (2004), the parameters of a power equa-
tion (R = a GPPb) relating GPP and R were calculated in
order to estimate the GPP value required to balance
the seasonal community average R (i.e. GPP:R = 1) and
render the communities autotrophic. This resulted in a
GPP of 8.95 mmol O₂ m⁻³ d⁻¹ for summer + fall,
12.65 mmol O₂ m⁻³ d⁻¹ for summer and 4.67 mmol O₂
m⁻³ d⁻¹ for fall. These estimated GPP values were then
compared to the average GPP values obtained experi-
mentally (Table 2). Both the estimated values for sum-
mer + fall as well as for summer are well below the sea-
sonal measured GPP averages. However, the
estimated fall GPP value needed to render GPP:R = 1
was higher than the average value measured in 2003
and higher than some of the fall values found during
the 3 yr studied (Table 2). Furthermore, according to
the linear relationship between chl a and NCP (Fig.
5A) and between NCP and GPP (Fig. 5B), the phyto-
plankton biomass levels required to reach these GPP
values were 0.9 mg chl a m⁻³ for summer + fall, 0.5 mg
chl a m⁻³ for summer and 1.67 mg chl a m⁻³ for fall.
Again, this last value is higher than the seasonal mean
we found in the area (see 'Results'). Less negative
TcΔpCO₂ values and lower %O₂ saturations were found during fall (Fig. 6C,D) than during summer (Fig. 6A,B). Increased vertical mixing during fall is probably responsible for the uncoupling between NCP and gas exchanges. Excess fall heterotrophy has to be compensated with organic matter coming from an external source. Since fall sampling has been mainly conducted in the coastal area, terrigenous (allochtonous) inputs (Duarte & Agustí 1998) as well as dissolved organic carbon produced locally (Bauer & Druffel 1998) could account for the excess respiration. In outer shelf waters, respiration could be due to the activity of ciliates, which can be abundant in the area (Thompson 2004), or other microzooplankton. Bacterial respiration, which has not been considered separately in the present study, probably plays a central role in NCP and R uncoupling, as it does in other continental shelf areas (Griffith & Pomeroy 1995).

Communities with NCP >0 should significantly contribute to the CO₂ sink while those with NCP <0 should be a source of this gas from the ocean to the atmosphere (Ducklow & McCallister 2004). Although NCP is similar in both diatom- and small flagellate-dominated stations on average, a linear relationship between TcΔpCO₂ and %O₂ saturation with phytoplankton biomass and NCP is only evident when the phytoplankton community is dominated by diatoms (Figs. 3 & 7, Table 3). Such a relationship has been described before for different environments (Anadón et al. 2002, Brown et al. 2002, Tremblay et al. 2002, Sarthou et al. 2005). Diatoms dominate the phytoplankton assemblages in nutrient-rich and frontal areas (Sarthou et al. 2005). The dominance of diatoms during the few high production events favors the establishment of a herbivorous food web (Legendre & Rassoulzadegan 1995) and can lead the area to act as a CO₂ sink. As a consequence, the non-oxidized particulate matter could be exported towards the sea bottom, therefore contributing to the carbon sink. The excess production could be buried in the sediments and/or exported to fuel respiration in the open-ocean interior (Bauer & Druffel 1998).

Diatoms dominated the plankton assemblages in 31% of the studied stations. In terms of total biomass, these stations represented 47% of the chl *a* in the area and 49% in terms of phytoplankton carbon. This means that most stations were dominated by small flagellates and that more than half of the phytoplankton biomass and carbon on the Patagonian shelf was due to this group, for which no relationship was found between their abundance or between measured NCP and either TcΔpCO₂ or %O₂ saturation. Moreover, the whole area still behaves as a CO₂ sink: 64% of the stations dominated by small flagellates showed negative TcΔpCO₂ values and 78% of them had GPP:R ratios >1.

It is thus evident that small flagellates certainly play a significant role in the dynamics of seawater pCO₂ and %O₂ saturation. This type of community, which leads to microbial food webs and microbial loops, is in general much less studied than communities dominated by diatoms. The recycling of nutrients and carbon is enhanced in these communities, and therefore no linear or simple power relation is found regarding TcΔpCO₂ or %O₂ saturation. It must be noted that although microscopic examination only allowed us to detect small flagellates and identify diatom taxa, production and respiration experiments consider the whole microplanktonic community. Furthermore, in a climate change scenario (i.e. increases in atmospheric CO₂, sea-surface temperature and stratification, and a decrease in nutrient supply to surface waters), decreases in chlorophyll, primary production and export from the euphotic zone are expected, as well as a shift in phytoplankton taxa, from diatoms to relatively smaller phytoplankton cells (Legendre & Rivkin 2002). In the present study, small flagellates were not further characterized. In addition, we did not consider the effect of grazers >500 µm. To study the role of biology on CO₂ dynamics, it is essential to consider the complete composition of plankton assemblages. Diatoms are easier to identify than organisms making up the small flagellate group. The identification and comprehension of the physiology and ecology of the small flagellates, as well as those of picoplankton and bacteria, which will be subject of further studies, are the 'black box' that must be opened to understand the relationship between plankton composition and CO₂.

## CONCLUSIONS

Over most of the Argentinean continental shelf, the GPP:R ratio was >1, suggesting that phytoplankton production greatly influences the negative average ΔpCO₂ values found in the area. The present study confirms the importance of continental shelves and slopes, which comprise <20% of the World Ocean area (Bauer & Druffel 1998), in the global carbon cycle. Excess production in this area is a source of biological carbon fueling the vast heterotrophy found in the open ocean (del Giorgio et al. 1997). Our results show that there is a linear relationship of biomass and production with TcΔCO₂ and %O₂ concentrations at stations dominated by diatoms, while no such relationships were found when small flagellates dominated the phytoplankton assemblages. This was the case in 67% of the stations studied. As shown for other processes in the ocean (i.e. export flux), food web structure and phytoplankton composition affects the CO₂ dynamics. The lack of a linear relation between chl *a* and
TcpΔCO₂ in these situations indicates that community composition (i.e. more than just biomass) is an important aspect to be considered in future works as well as in modeling efforts. Changes in environmental conditions leading to the dominance of either group could then have consequences on the biogeochemical cycles and especially on CO₂ dynamics. Knowledge of system controls and feedbacks on the primary production in coastal areas will contribute to our understanding of the system's responses to the anthropogenic-induced climatic changes that are under way.

*Acknowledgements.* This study was conducted within the framework of a cooperative research program (ARGAU, Programme de coopération avec la ARGentine pour l'étude de l'océan Atlantique AUstral) between the Laboratoire de Biogéochimie et Chimie Marines at the Université Pierre et Marie Curie in Paris (France), the Instituto Antártico Argentino and the Servicio de Hidrografía Naval (Argentina) from 2002 to 2004. We thank L. Cantoni, A. Ulrich, B. Schaueri, C. Brunet and J. Guigand, and technical personnel from the Servicio de Hidrografía Naval for assistance during fieldwork, as well crew members of the 'Almirante Irizar'. Additional thanks are extended to Professors M. Gosselin, D. W. R. Wallace, S. Roy, F. Momo, J. P.nutan, Z. P. Mei and S. Rodriguez. This project benefited from the PEI-2001, CONICET, and PICTO 6524/1108/03-ANPCYT 01-11563 grants to I.R.S. Financial support also came from the French INSU (Institut National des Sciences de l'Univers) Program PROOF/Flamenco (Flux Air-Mer en CO₂ to D.R.P. for the ARGAU cruises.

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