Southern Ocean Iron Experiment Research Articles

Mesoscale biogeochemical responses to iron fertilization in the upper layers of the Southern Ocean Iron Experiment areas

During the Southern Ocean Iron Experiment (SOFeX), January–February 2002, two iron fertilization experiments were conducted at the south (66.45°S, 171.8°W) and north (56.23°S, 172°W) patches. The south patch was replete with all macronutrients, whereas the north patch was nearly depleted of silicate. Using a towed water sampling/measurement system, high resolution three‐dimensional observations of temperature, salinity, CO2 partial pressure (pCO2) and the concentrations of total CO2 (TCO2), nitrate, phosphate and silicate in the upper 100 m were obtained 10 and 28 days after initial iron additions in the south and north patches, respectively. TCO2 and nutrient drawdowns observed in the south patch were small due to wind events that limited biological response in the early postfertilization period. At the north patch, drawdown signals were stronger, resulting from the longer time since fertilization. The north patch was dispersed along a frontal zone between two distinct waters, with the denser subducted under the lighter. Subduction limited CO2 uptake from the air, but provided alternative mechanisms for vertical carbon export. Based on the biogeochemical drawdowns and published estimates of the rate of the entrainment of nonfertilized waters into the patch, the mean net community productivity over 28 days was estimated to be 94 ± 12 mmol carbon m−2 day−1, to which the air‐to‐sea flux contributed only 2%. Despite low silicate concentrations and potential limitation of growth of silicifying plankton in the north patch, our estimates of the enhanced carbon uptake are similar to estimates from higher silicate environments.

Production and air–sea flux of halomethanes in the western subarctic Pacific in relation to phytoplankton pigment concentrations during the iron fertilization experiment (SEEDS II)

Abstract Iron could play a key role in controlling phytoplankton biomass and productivity in high-nutrient, low-chlorophyll regions. As a part of the iron fertilization experiment carried out in the western subarctic Pacific from July to August 2004 (Subarctic Pacific iron Experiment for Ecosystem Dynamics Study II—SEEDS II), we analysed the concentrations of trace gases in the seawater for 12 d following iron fertilization. The mean concentrations of chlorophyll a in the mixed layer (5–30 m depth) increased from 0.94 to 2.81 μg L –1 for 8 d in the iron patch. The mean concentrations of methyl bromide (CH 3 Br; 5–30 m depth) increased from 6.4 to 13.4 pmol L –1 for 11 d; the in-patch concentration increased relative to the out-patch concentration. A linear correlation was observed between the concentrations of 19′-hexanoyloxyfucoxanthin, which is a biomarker of several prymnesiophytes, and CH 3 Br in the seawater. After fertilization, the air–sea flux of CH 3 Br inside the patch changed from influx to efflux from the ocean. There was no clear evidence for the increase in saturation anomaly of methyl chloride (CH 3 Cl) due to iron fertilization. Furthermore, CH 3 Cl fluxes did not show a tendency to increase after fertilization of the patch. In contrast to CH 3 Br, no change was observed in the concentrations of bromoform (in-patch day 11 and out-patch day 11: 1.7 and 1.7 pmol L –1 ), dibromomethane (2.1 and 2.2 pmol L –1 ), and dibromochloromethane (1.0 and 1.2 pmol L –1 , respectively). The concentration of isoprene, which is known to have a relationship with chlorophyll a , did not change in this study. The responses of trace gases during SEEDS II differed from the previous findings ( in situ iron enrichment experiment—EisenEx, Southern Ocean iron experiment—SOFeX, and Subarctic Ecosystem Response to Iron Enrichment Study—SERIES). Thus, in order to estimate the concomitant effect of iron fertilization on the climate, it is important to assess the induction of biological activity and the distributions/air–sea fluxes of trace gases by iron addition.

The effects of dilution and mixed layer depth on deliberate ocean iron fertilization: 1-D simulations of the southern ocean iron experiment (SOFeX)

To better understand the role of iron in driving marine ecosystems, the Southern Ocean Iron Experiment (SOFeX) fertilized two surface water patches with iron north and south of the Antarctic Polar Front Zone (APFZ). Using 1-D coupled biological–physical simulations, we examine the biogeochemical dynamics that occurred both inside and outside of the fertilized patches during and shortly after the SOFeX field campaign. We focus, in particular, on three main issues governing the biological response to deliberate iron fertilization: the interaction among phytoplankton, light, macronutrient and iron limitation; dilution and lateral mixing between the fertilized patch and external, unfertilized waters; and the effect of varying mixed layer depth on the light field. At the patch south of the APFZ, sensitivity simulations with no dilution results in the maximum bloom magnitude, whereas dilution with external water extends the development of the north patch bloom by relieving silicon limitation. In model sensitivity studies for both sites, maximum chlorophyll concentration and dissolved inorganic carbon depletion inside the fertilized patches are inversely related to mixed layer depth, similar to the patterns observed across a number of iron fertilization field experiments. Our results suggest that Southern Ocean phytoplankton blooms resulting from natural or deliberate iron fertilization will tend to become iron-light co-limited unless the mixed layer depth is quite shallow.

High biomass, low export regimes in the Southern Ocean

This paper investigates ballasting and remineralization controls of carbon sedimentation in the Twilight Zone (100–1000 m) of the Southern Ocean. Size-fractionated (<1 μm, 1–51 μm, >51 μm) suspended particulate matter was collected by large-volume in-situ filtration from the upper 1000 m in the Subantarctic (55°S, 172°W) and Antarctic (66°S, 172°W) zones of the Southern Ocean during the Southern Ocean Iron Experiment (SOFeX) in January–February 2002. Particles were analyzed for major chemical constituents (POC, P, biogenic Si, CaCO3), and digital and SEM image analyses of particles were used to aid in the interpretation of the chemical profiles. Twilight Zone waters at 66°S in the Antarctic had a steeper decrease in POC with depth than at 55°S in the Subantarctic, with lower POC concentrations in all size fractions at 66°S than at 55°S, despite up to an-order-of magnitude higher POC in surface waters at 66°S. The decay length scale of >51-μm POC was significantly shorter in the upper Twilight Zone at 66°S (δe=26 m) compared to 55°S (δe=81 m). Particles in the carbonate-producing 55°S did not have higher excess densities than particles from the diatom-dominated 66°S, indicating that there was no direct ballast effect that accounted for deeper POC penetration at 55°S. An indirect ballast effect due to differences in particle packaging and porosities cannot be ruled out, however, as aggregate porosities were high (∼97%) and variable. Image analyses point to the importance of particle loss rates from zooplankton grazing and remineralization as determining factors for the difference in Twilight Zone POC concentrations at 55°S and 66°S, with stronger and more focused shallow remineralization at 66°S. At 66°S, an abundance of large (several mm long) fecal pellets from the surface to 150 m, and almost total removal of large aggregates by 200 m, reflected the actions of a single or few zooplankton species capable of grazing diatoms in the euphotic zone, coupled with a more diverse particle-feeding zooplankton community immediately below. Surface waters with high biomass levels and high proportion of biomass in the large-size fraction were associated with low particle loading at depth, with all indications implying conditions of low export. The 66°S region exhibits this “high biomass, low export” (HBLE) condition, with very high >51-μm POC concentrations at the surface (∼2.1 μM POC), but low concentrations below 200 m (<0.07 μM POC). The 66°S region remained HBLE after iron fertilization. Iron addition at 55°S caused a 10 fold increase in >51-μm biomass concentrations in the euphotic zone, bringing surface POC concentrations to levels found at 66°S (∼3.8 μM), and a concurrent decrease in POC concentrations below 200 m. The 55°S region, which began with moderate levels of biomass and stronger particle export, transitioned to being HBLE after iron fertilization. We propose that iron addition to already HBLE waters will not cause mass sedimentation events. The stability of an iron-induced HBLE condition is unknown. Better understanding of biological pump processes in non-HBLE Subantarctic waters is needed.

THE ROLE OF PHYTOPLANKTON SIZE ON PHOTOCHEMICAL RECOVERY DURING THE SOUTHERN OCEAN IRON EXPERIMENT1

Phytoplankton primary productivity in the Southern Ocean is controlled by complex interactions among iron, light, and grazing. This project interfaced with the Southern Ocean iron experiment (SOFeX) that created two iron‐enriched patches north and south of the Polar Front each with distinct silicic acid concentrations. We used pulse amplitude modulated fluorometry and measured the recovery of the maximum quantum yield of photochemistry (Fv/Fm) for three size fractions (whole, &lt;5, &lt;20 μm) and light adapted quantum yield (ΔF/F′m) for single phytoplankton cells. The rates of recovery from iron stress were found to be unrelated to average cell size for both size‐fractioned and single‐celled measurements. The smallest cells appeared to exhibit more severe iron stress at the onset of the experiment than the larger taxa. The largest response detected in regression parameters was that of the pennate diatoms, which took only ∼3.4 days to reach the maximum quantum yield, whereas the centric diatom Asteromphalus sp. reached maximum ΔF/F′m after ∼10.4 days. The north patch measurements showed a different response; the smallest cells never reached maximum ΔF/F′m, whereas the size fraction containing the largest cells did. Single‐celled measurements made nearly 30 days after the initial iron enrichment suggested that diatoms were experiencing either silicic acid or iron limitation, whereas measurements of Phaeocystis sp. did not. These data represent the first study of in situ recovery rates of PSII for groups of diatoms, and may help elucidate the mechanisms of species change in response to environmental perturbation.

Remote sensing observations of ocean physical and biological properties in the region of the Southern Ocean Iron Experiment (SOFeX)

Satellite remote sensing estimates of surface chlorophyll, temperature, wind speed, and sea ice cover are examined in the region of the Southern Ocean Iron Experiment (SOFeX). Our objectives are to place SOFeX into a regional context and highlight regional mesoscale spatial and monthly temporal variability. SOFeX fertilized two patches with iron, one south of the Antarctic Polar Front (PF) and one north of the PF but south of the Subantarctic Front (SAF). Satellite‐observable phytoplankton blooms developed in both patches. The spring sea ice retreat near the south patch site was delayed in the 2001–2002 season, in turn delaying the naturally occurring, modest spring bloom in this region. Ambient surface chlorophyll concentrations for the area surrounding the southern patch during January 2002 are low (mean 0.26 mg/m3) compared with climatological January values (0.42 mg/m3). Regions east and west at similar latitudes exhibited higher mean chlorophyll concentrations (0.79 and 0.74 mg/m3, respectively). These modest phytoplankton blooms were likely stimulated by melting sea ice via changes in the light‐mixing regime and release of iron and were smaller in magnitude than the iron‐induced bloom within the SOFeX southern patch (&gt;3 mg/m3). Iron inputs from melting ice may drive much of the natural spatial and temporal variability within the seasonal ice zone. Mean chlorophyll concentrations surrounding the SOFeX northern patch site were similar to climatological values during the SOFeX season. The northern patch was stretched into a long, thin filament along the southern boundary of the SAF, likely increasing the mixing/dilution rate with surrounding waters.

Nutrient and carbon parameters during the Southern Ocean iron experiment (SOFeX)

During the 2002 austral summer, two in situ mesoscale iron fertilization experiments were conducted in two distinct silicic acid regimes of the Southern Ocean (SOFeX). The iron was added to the low Si(OH) 4 waters of the Subantarctic (<3 μmol kg −1, North Patch) and the high Si(OH) 4 waters poleward of the southern boundary of the Antarctic Circumpolar Current (ACC), (∼63 μmol kg −1, South Patch). The low iron availability in these two regimes of the Southern Ocean limits biological utilization of the ambient nutrients, influences phytoplankton species composition and species-specific nutrient consumption rates. The response of the carbon dioxide system and macronutrients to initial and subsequent iron additions were monitor relative to measurements outside the iron-enriched waters. These iron fertilization experiments demonstrate that iron addition to the High Nutrient Low Chlorophyll (HNLC) regions of the Southern Ocean increases primary productivity and decreases the fugacity of carbon dioxide. Dilution of the iron fertilized sites diminished the observed geochemical signals produced as a result of phytoplankton growth, while the entrainment of silicic acid sustained the increase of diatom biomass in the low Si(OH) 4 waters of the Subantarctic. Empirical observation of the elemental composition of the iron fertilized ecosystems and the observed nutrient-depletion ratios upon the addition of iron were indicative of changes in the community structure and/or changes in the unique physiological nutrient consumption rates of a species within the community. Ultimately, variations in the nutrient-depletion ratios induced by increased iron concentrations influences biogeochemical cycles in the Southern Ocean and the export of carbon. Molar ratios in the north and south iron fertilization sites, respectively, were determined to be ΔN:ΔP=14.1±0.2 and 14.2±0.4; ΔC:ΔN=9.1±1.7 and 7.2±0.6; ΔC:ΔP=106±4 and 99±4; ΔC:ΔSi=7.7±0.8 and 6.2±0.5; ΔC:ΔO 2=−0.75±0.02 and −0.73±0.02. Combining these results, the following stoichiometry is obtained for the growth of phytoplankton during the SOFeX in the north and south iron enrichment sites, 106CO 2+120H 2O+14HNO 3+H 3PO 4+14SiO 2→ ( CH 2 O ) 92 ( CH 2 ) 14 ( NH 3 ) 14 ( H 3 PO 4 ) ( SiO 2 ) 14 +141O 2 and 99CO 2+113H 2O+14HNO 3+H 3PO 4+16SiO 2→ ( CH 2 O ) 81 ( CH 2 ) 18 ( NH 3 ) 14 ( H 3 PO 4 ) ( SiO 2 ) 16 +136O 2, respectively. Variations in the ratios of carbon to macronutrients utilized by the ecosystem under iron replete conditions are important for the interpretation of the effect of iron fluxes on atmospheric CO 2 concentrations in glacial times, highlighting the integral role of Fe and the Southern Ocean regulation of atmospheric carbon dioxide.

Control of silica production by iron and silicic acid during the Southern Ocean Iron Experiment (SOFeX)

We examined the role of Si limitation in mediating the response of siliceous biomass and silica production to mesoscale Fe fertilization in the high‐silicic acid high‐nitrate waters of the Antarctic and in the low‐silicic acid high‐nitrate waters of the Subantarctic during austral summer. Iron fertilization stimulated biogenic silica production and silicic acid depletion in both regions. Si limitation significantly curtailed the response of silica production to Fe in the Subantarctic, but not in the Antarctic. Additions of silicic acid to Fe‐enriched waters of the Subantarctic more than doubled specific silica production rates beyond the increase caused by Fe alone. This result, combined with the presence of Si limitation both inside and outside of the fertilized patch in the Subantarctic, indicates that silica production in the Subantarctic is regulated by both Fe and Si during austral summer. Fe increased the ability of Subantarctic diatom assemblages to take up low concentrations of Si over an order of magnitude by increasing maximum uptake rates and lowering half‐saturation constants for silicic acid uptake. Our observations indicate that the dramatic gradient in Si availability across the Antarctic Circumpolar Current exerts a strong control on the contribution of diatoms to new production following Fe enrichment in the Southern Ocean during austral summer. With sufficient Fe to allow nitrate depletion, the abundant silicic acid in the Antarctic would allow diatoms to dominate nitrate use; however, low silicic acid concentrations in the Subantarctic would restrict the fraction of available nitrate that would be consumed by diatoms to ca. 5%.

Particle export during the Southern Ocean Iron Experiment (SOFeX)

We studied the effect of iron addition on particle export in the Southern Ocean by measuring changes in the distribution of thorium-234 during a 4-week iron (Fe) enrichment experiment conducted in the high-silicate highnitrate waters just south of the Southern Antarctic Circumpolar Current Front at 172.58W. Decreases in 234 Th activity with time in the fertilized mixed layer (0‐50 m) exceeded those in unfertilized waters, indicating enhanced export of 234 Th on sinking particles after Fe enrichment. The addition of Fe also affected export below the fertilized patch by increasing the efficiency of particle export through the 100-m depth horizon. Extensive temporal and vertical Lagrangian sampling allowed us to make a detailed examination of the 234 Th flux model, which was used to quantify the fluxes of particulate organic carbon (POC) and biogenic silica (bSiO 2). Iron addition increased the flux of both POC and bSiO2 out of the mixed layer by about 300%. The flux at 100 m increased by more than 700% and 600% for POC and bSiO2, respectively. The absolute magnitude of the POC and bSiO2 fluxes were not large relative to natural blooms at these latitudes or to those found in association with the termination of blooms in other ocean regions. Our results support the hypothesis that Fe addition leads directly to significant particle export and sequestration of C in the deep ocean. This is a key link between ocean Fe inputs and past changes in atmospheric CO2 and climate.

Quantification of &lt;Superscript&gt;234&lt;/Superscript&gt;&lt;Superscript&gt; &lt;/Superscript&gt;Th recovery in small volume sea water samples by inductively coupled plasma-mass spectrometry

Development of a small volume (2-4 liter) technique for measuring 234 Th in sea water has been instrumental in bringing to light small-scale structures in upper-ocean particle removal processes previously missed by standard 234 Th measurement techniques. Further development of this method to evaluate removal efficiencies of 234 Th via MnO2 precipitation quantified using ICP-MS are presented in this work. Advantages to this approach are precise knowledge of 234 Th recovery, while maintaining high sample throughput afforded by ICP-MS analyzes. The improved technique includes the acidification of 4-liter sea water samples and the addition of 230 Th as a yield monitor prior to MnO2 precipitation. Subsequent filtration and beta-counting of the high-energy daughter, 234mPa, was followed by a final background count after 6 half-lives (144 d) of decay. Filtered precipitates were dissolved with H2O2, and an internal standard of 229 Th was added. Samples were purified using anion-exchange chromatography to remove high levels of manganese, and recoveries were determined by measured ratios of 230 Th/229 Th by ICP-MS. Application of this procedure for 234 Th derived export in the recent Southern Ocean Iron Experiment showed average recoveries of 91%. Corrections for rare low recoveries (25-80%) noticeably change 234 Th profiles, thus impacting subsequent elemental flux calculations.

Cellular iron contents of plankton during the Southern Ocean Iron Experiment (SOFeX)

Iron (Fe) availability limits phytoplankton biomass and production in large regions of the Southern Ocean and influences community composition and size structure, which may affect C export and other system-level functions. To improve our understanding of Fe partitioning within communities and the response of different components to fertilization, we assessed the cellular Fe contents of individual diatoms, autotrophic flagellates, and heterotrophic flagellates during the recent Southern Ocean Fe Experiment using synchrotron-based X-ray fluorescence (SXRF). Dual 55Fe/ 14C radioisotope incubations were also conducted to estimate Fe:C ratios in size-fractionated plankton. Cellular Fe quotas determined by the two techniques were in close agreement when low amounts of 55Fe (0.2 nM) were added, but 55Fe additions of 2 nM resulted in 2–3-fold higher quotas. SXRF assessments of cellular Fe quotas (normalized to C) were generally in good agreement with prior bulk analyses of natural assemblages, but revealed compositional differences among protistan taxa not previously detected. Mean Fe:C ratios for diatoms, autotrophic flagellates, and heterotrophic flagellates from unfertilized waters were 6.0, 8.7, and 14.1 μmol mol C −1, respectively. Smaller cells had higher Fe:C ratios than larger cells. Fertilization enhanced Fe quotas in all cell types, with mean Fe:C ratios increasing approximately 4-fold (from about 10 to about 40 μmol mol C −1) after two Fe additions. This study provides some of the first measurements of Fe quotas in phytoplankton cells from natural communities and the first measurements of Fe quotas in natural protozoa.

Element stoichiometries of individual plankton cells collected during the Southern Ocean Iron Experiment (SOFeX)

During the Southern Ocean Iron Experiment (SOFeX), we analyzed Si, P, S, Mn, Ni, and Zn in individual diatoms, autotrophic flagellates, and heterotrophic flagellates with synchrotron‐based X‐ray fluorescence (SXRF) and calculated cellular C from measurements of cell size. Element stoichiometries for the different types of protists (normalized to either C, S, or P) were generally in good agreement with prior bulk analyses of natural assemblages but also revealed previously undocumented differences in elemental composition among cell types. Flagellated cells contained 39% more P than diatoms, which in turn contained 79% more Mn, 3‐fold more Ni, and 2.6‐fold more Zn than flagellates. Heterotroph cells contained approximately 40% more P and twice as much Zn as autotrophs. Manganese and Ni stoichiometries were negatively related to cell volume, while larger cells contained more Zn per mole C. Iron fertilization resulted in an approximate doubling of Mn, Ni, and Zn quotas and smaller increases in cellular P, but the timing of the stoichiometric changes varied between the two patches. Silicon contents of diatoms dropped approximately 40% after the first Fe addition but returned to prefertilization levels as cellular P doubled following the second addition, resulting in 40% lower Si : P ratios in the fertilized waters. The mean elemental stoichiometries calculated for all cells analyzed were comparable with previously published extended Redfield ratios for mixed plankton assemblages, but the observed differences between diatoms and flagellates and between autotrophs and heterotrophs indicate that valuable information is lost when all types of co‐occurring plankton are grouped together for analysis.

The heterotrophic bacterial response during the Southern Ocean Iron Experiment (SOFeX)

We studied heterotrophic bacterial dynamics as part of the Southern Ocean Iron Experiment (SOFeX), January‐February 2002. Two phytoplankton blooms were monitored following infusions with iron sulfate (FeSO4). The first bloom was initiated north of the Antarctic Polar Front Zone (APFZ) in silica‐poor waters (North Patch) and was observed sporadically over 42 d, whereas the second was south of the APFZ in silica‐rich waters (South Patch) and was continuously observed for 30 d. In both experiments, iron additions resulted in increased chlorophyll a (Chl a), particulate organic matter (POC + PN), and a drawdown of inorganic nutrients. Heterotrophic bacteria responded by increasing their abundance (110% and 60% increases in bacterial abundance in the North and South Patch, respectively, relative to nonenriched waters). Thymidine (TdR) and leucine (Leu) incorporation rates in the North Patch increased by 400% and 120%, respectively, with more modest increases in the South Patch (80% and 70%, respectively). In the South Patch, bacterial production (BP) was significantly correlated with net particulate primary production (PP) and Chl a. Bacterial abundance was also significantly correlated with Chl a. Net bacterial accumulation rate in the South Patch was 0.02 d−1 over 17 d, excluding physical dilution by mixing with water outside the South Patch. The evidence suggests that bacterial growth during Southern Ocean Iron Experiment (SOFeX) was limited by labile carbon rather than by iron. Despite the close association of BP to PP, BP remained a small fraction of PP (&lt;10%).

The effects of iron fertilization on carbon sequestration in the Southern Ocean.

An unresolved issue in ocean and climate sciences is whether changes to the surface ocean input of the micronutrient iron can alter the flux of carbon to the deep ocean. During the Southern Ocean Iron Experiment, we measured an increase in the flux of particulate carbon from the surface mixed layer, as well as changes in particle cycling below the iron-fertilized patch. The flux of carbon was similar in magnitude to that of natural blooms in the Southern Ocean and thus small relative to global carbon budgets and proposed geoengineering plans to sequester atmospheric carbon dioxide in the deep sea.

Robotic observations of enhanced carbon biomass and export at 55 degrees during SOFeX.

Autonomous floats profiling in high-nitrate low-silicate waters of the Southern Ocean observed carbon biomass variability and carbon exported to depths of 100 m during the 2002 Southern Ocean Iron Experiment (SOFeX) to detect the effects of iron fertilization of surface water there. Control and "in-patch" measurements documented a greater than fourfold enhancement of carbon biomass in the iron-amended waters. Carbon export through 100 m increased two- to sixfold as the patch subducted below a front. The molar ratio of iron added to carbon exported ranged between 10(4) and 10(5). The biomass buildup and export were much higher than expected for iron-amended low-silicate waters.

Bicarbonate uptake by Southern Ocean phytoplankton

Marine phytoplankton have the potential to significantly buffer future increases in atmospheric carbon dioxide levels. However, in order for CO2 fertilization to have an effect on carbon sequestration to the deep ocean, the increase in dissolved CO2 must stimulate primary productivity; that is, marine phototrophs must be CO2 limited [Riebesell et al., 1993]. Estimation of the extent of bicarbonate (HCO3−) uptake in the oceans is therefore required to determine whether the anthropogenic carbon sources will enhance carbon flux to the deep ocean. Using short‐term 14CO2‐disequilibrium experiments during the Southern Ocean Iron Experiment (SOFeX), we show that HCO3− uptake by Southern Ocean phytoplankton is significant. Since the majority of dissolved inorganic carbon (DIC) in the ocean is in the form of bicarbonate, the biological pump may therefore be insensitive to anthropogenic CO2. Approximately half of the DIC uptake observed was attributable to direct HCO3− uptake, the other half being direct CO2 uptake mediated either by passive diffusion or active uptake mechanisms. The increase in growth rates and decrease in CO2 concentration associated with the iron fertilization did not trigger any noticeable changes in the mode of DIC acquisition, indicating that under most environmental conditions the carbon concentrating mechanism (CCM) is constitutive. A low‐CO2 treatment induced an increase in uptake of CO2, which we attributed to increased extracellular carbonic anhydrase activity, at the expense of direct HCO3− transport across the plasmalemma. Isotopic disequilibrium experimental results are consistent with Southern Ocean carbon stable isotope fractionation data from this and other studies. Although iron fertilization has been shown to significantly enhance phytoplankton growth and may potentially increase carbon flux to the deep ocean, an important source of the inorganic carbon taken up by phytoplankton in this study was HCO3−, whose concentration is negligibly affected by the anthropogenic rise in CO2. We conclude that biological productivity in this region of the world's ocean is unlikely to be directly regulated by natural or anthropogenic variations in atmospheric CO2 concentrations because of the presence of a constitutive CCM.