Southern Ocean Iron RElease Experiment Research Articles

Effects of changing continuous iron input rates on a Southern Ocean algal assemblage

The upwelling of nutrients and iron (Fe) sustains biological production in much of the Southern Ocean. Using a shipboard natural community continuous culture system (Ecostat), we supplied a single added Fe concentration at two dilution rates chosen to examine the effects of variations in realistic growth and loss rates on an Fe-limited algal community in the Antarctic Zone south of Australia. A parallel growout experiment provided “no-dilution” +Fe and −Fe controls. In the continuous flow experiment, phytoplankton biomass was lower and more constant throughout the incubation and major nutrients were never depleted. Nanophytoplankton abundance remained similar in both growout treatments, and therefore, growth of this group did not appear to be Fe-limited. The addition of Fe in a continuous fashion resulted in a community co-dominated by both small diatoms and nanophytoplankton. Increases in dilution rate favored diatom species that were smaller and faster-growing, as well as non-silicified algal groups. Particulate carbon (PC) to particulate nitrogen (PN) ratios increased above the Redfield ratio when Fe was added in a continuous fashion, while biogenic silica (BSi) to PC and PN ratios decreased 2–3 fold in the continuous flow experiment compared to the initial conditions and the parallel growout control experiment. Photosynthetic efficiency increased in the continuous flow treatments above the control but remained significantly lower than in the 1.4 nM Fe addition. The results of our shipboard continuous flow experiments are compared and contrasted with those of the mesoscale Southern Ocean Iron RElease Experiment (SOIREE) carried out at the same site. Our results suggest that increases in natural dilution rates (i.e. vertical turbulent diffusion) in polar Antarctic waters could shift the algal community towards smaller, faster-growing algal species, thus having a major effect on nutrient cycling and carbon export in the Southern Ocean.

Matching carbon pools and fluxes for the Southern Ocean Iron Release Experiment (SOIREE)

The Lagrangian Southern Ocean Iron Release Experiment (SOIREE) allowed study of a gradually evolving iron-mediated phytoplankton bloom in water labelled with the inert tracer sulfur hexafluoride, SF 6. This article describes a pelagic carbon budget for the mixed layer in SOIREE and assesses the extent to which closure of the budget is achieved. Net community production (NCP) converted 837 mmol m −2 of inorganic carbon to organic carbon in 12.0 d after the first iron addition. A large fraction (41%) of NCP remained as particulate organic carbon in the mixed layer of the iron-enriched patch, while 23% was lost by horizontal dispersion and 0–29% was exported. The closure of the carbon budget is hampered by the lack of measurements of dissolved organic carbon (DOC), by a major uncertainty in carbon export, and by use of empirical conversion factors in estimates of carbon biomass and metabolic rates. Lagrangian carbon-budget studies may be improved by direct measurement of all major carbon parameters and conversion factors. Carbon cycling in the SOIREE bloom resembled that in ‘natural’ algal blooms in the open Southern Ocean in some respects, but not in all. Daily NCP in the SOIREE bloom (70 mmol m −2 d −1) was higher than in natural blooms, partly because other studies did not account for horizontal dispersion, were for longer periods or included less productive areas. The build-up of POC stock and carbon export as a fraction of NCP in SOIREE were in the lower range of observations elsewhere.

Development of a robust marine ecosystem model to predict the role of iron in biogeochemical cycles: A comparison of results for iron-replete and iron-limited areas, and the SOIREE iron-enrichment experiment

A new mixed layer multi-nutrient ecosystem model, incorporating diatoms, non-diatoms and zooplankton, is described that models the role of iron in marine biogeochemical cycles. The internal cell biochemistry of the phytoplankton is modelled using the mechanistic model of Flynn [2001. A mechanistic model for describing dynamic multi-nutrient, light, temperature interactions in phytoplankton. Journal of Plankton Research 23, 977–997] in which the internal cell concentrations of chlorophyll, nitrogen, silica, and iron are all dynamic variables that respond to external nutrient concentrations and light levels. Iron stress in phytoplankton feeds back into chlorophyll synthesis and changes in photosynthetic unit (PSU) size, thereby reducing their growth rate. Because diatom silicon metabolism is inextricably linked with cell division, diatom population density (cell m −3) is modelled as well as C biomass. An optimisation technique was used to fit the model to three time-series datasets at Biotrans (47°N, 20°W) and Kerfix (50°40′S, 68°25′E) and the observations for the Southern Ocean Iron-Release Experiment (SOIREE) iron-enrichment experiment (61°S, 140°E). The model gives realistic simulations of the annual cycles of nutrients, phytoplankton, and primary production at Biotrans and Kerfix and can also accurately simulate an iron fertilisation experiment. Specifically, the model predicts the high values of diatom Si:N and Si:C ratios observed in areas where iron is a limiting factor on algal growth. In addition, the model results at Kerfix confirm previous suggestions that underwater light levels have a more limiting effect on phytoplankton growth than iron supply. The model is also used to calculate C budgets and C and Si export from the mixed layer. The implications of these results for developing biogeochemical models incorporating the role of iron are discussed.

Role of algal aggregation in vertical carbon export during SOIREE and in other low biomass environments

Additions of iron to surface regions of the ocean have induced an increase in phytoplankton biomass, but do not necessarily trigger increases in carbon export from surface waters. Using new size‐characterization of settling particles from the Southern Ocean Iron Release Experiment (SOIREE) and an extremely simple, mechanistic aggregation model, we show that removal of phytoplankton from surface waters via sedimentation was controlled by the formation of larger, faster sinking particles via coagulation. We demonstrate that in low biomass regions, where concentrations do not reach the critical concentration needed for massive sedimentation, carbon export is nevertheless still contingent upon the detailed mechanics of particle aggregation. There is therefore a direct dependence of export flux on the extent of aggregation during blooms, verification critical to the success of hydrodynamic models in predicting export.

Iron and mixing affect biological carbon uptake in SOIREE and EisenEx, two Southern Ocean iron fertilisation experiments

This study explores the changes in the surface water fugacity of carbon dioxide ( fCO 2) and biological carbon uptake in two Southern Ocean iron fertilisation experiments with different hydrographic regimes. The Southern Ocean Iron Release Experiment (SOIREE) experiment was carried out south of the Antarctic Polar Front (APF) at 61°S, 141°E in February 1999 in a stable hydrographic setting. The EisenEx experiment was conducted in a cyclonic eddy north of the APF at 48°S, 21°E in November 2000 and was characterised by a rapid succession of low to storm-force wind speeds and dynamic hydrographic conditions. The iron additions promoted algal blooms in both studies. They alleviated algal iron limitation during the 13-day SOIREE experiment and probably during the first 12 days of EisenEx. The fCO 2 in surface water decreased at a constant rate of 3.8 μatm day −1 from 4 to 5 days onwards in SOIREE. The fCO 2 reduction was 35 μatm after 13 days. The evolution of surface water fCO 2 in the iron-enriched waters (or ‘patch’) displayed a saw tooth pattern in EisenEx, in response to algal carbon uptake in calm conditions and deep mixing and horizontal dispersion during storms. The maximum fCO 2 reduction was 18–20 μatm after 12 and 21 days with lower values in between. The iron-enriched waters in EisenEx absorbed four times more atmospheric CO 2 than in SOIREE between 5 and 12 days, as a result of stronger winds. The total biological uptake of inorganic carbon across the patch was 1389 ton C (±10%) in SOIREE and 1433 ton C (±27%) in EisenEx after 12 days (1 ton=10 6 g). This similarity probably reflects the comparable size of the iron additions, as well as algal growth at a similar near-maximum growth rate in these regions. The findings imply that the different mixing regimes had less effect on the overall biological carbon uptake across the iron-enriched waters than suggested by the evolution of fCO 2 in surface water.

Simulated biogeochemical responses to iron enrichments in three high nutrient, low chlorophyll (HNLC) regions

To fill temporal gaps in iron-enrichment experimental data and gain further understanding of marine ecosystem responses to iron enrichments, we apply a fifteen-compartment ecosystem model to three iron-enrichment sites, namely SEEDS (the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study; 48.5°N, 165°E) in the western North Pacific, SOIREE (the Southern Ocean Iron RElease Experiment; 61°S, 140°E) in the Southern Ocean, and IronExII (the second mesoscale iron enrichment experiment; 3.5°S, 104°W) in the Equatorial Pacific. The ecological effects of iron in the model are represented by changing two photosynthetic parameters during the iron-enrichment period. The model results successfully reproduce the observed biogeochemical responses inside and outside the iron patch at each site, such as rapid increases in plankton biomass and biological productivity, and decreases in surface nutrients and pCO 2, inside the patch. However, the modeled timing and magnitude of changes differ among the sites because of differences in both physical environments and plankton species. After the iron enrichment, the diatom productivity is strongly controlled by light at SOIREE and by silicate at IronExII and SEEDS. Light limitation due to self-shading by the phytoplankton is significant during the bloom at all sites. Sensitivity analysis of the model results to duration of the iron enrichment reveals that long-term multiple infusions over more than a week would not be effective at SEEDS because of strong silicate limitation on diatom growth. Sensitivity of the model to water temperature shows that export production is higher at lower temperatures, because of slower recycling of particulate organic carbon. Therefore, the e-ratio (the ratio of export production to primary production) is inversely correlated with temperature, and the relationship can be described with a linear function. Through this study, we conclude that ecosystem modeling is a powerful tool to help design future iron-enrichment experiments and observational plans.

Relationship of nitrogen isotope fractionation to phytoplankton size and iron availability during the Southern Ocean Iron RElease Experiment (SOIREE)

The 15N composition of sediments has been used as a proxy for nitrate utilization in surface waters to assess the role of Southern Ocean export production in glacial/interglacial changes in atmospheric CO2 concentration. Interpretation has relied on a temporally constant isotope effect (ε) associated with uptake and assimilation of nitrate by phytoplankton. To investigate the reliability of this approach, we examined the relationships between the 15N compositions of dissolved nitrate, bulk and size‐fractionated (200, 70, 20, 5, 1 µm) suspended particulate organic nitrogen (PON), and sinking particles obtained from sediment traps during the Southern Ocean iron release experiment (SOIREE). We found variations in phytoplankton nitrogen isotopic compositions with both cell size and iron availability. δ15NPON increased by >2‰ with increasing size, both within and outside the ironenriched patch. In comparison to unfertilized waters, δ15NPON within the iron‐fertilized patch was a further 3–4‰ higher in those size fractions dominated by large diatoms (20–70, 70–200 µm). We speculate that this iron response might result from (1) variation in ε of nitrate utilization or (2) an iron‐stimulated shift from ammonium‐based to nitrate‐based production. Comparing the δ15N of the large diatom—dominated size fractions to the δ15N of nitrate suggests relatively low ε values of 4–5‰, in contrast to estimated values of 7–10‰ from seasonal nitrate depletion and export production. This suggests that higher glacial δ15N in Southern Ocean sediments could, in part, reflect increases in iron availability, dominant cell size, and possibly growth rates, and these effects must be considered in any quantitative scaling of δ15N variations, including those of diatom‐bound δ15N, to the extent of nitrate utilization.

The role of iron in the biogeochemistry of the Southern Ocean and equatorial Pacific: a comparison of in situ iron enrichments

Abstract A better understanding of the relationship between iron supply and the biogeochemical functioning of high nitrate low chlorophyll (HNLC) regions may be obtained by comparing and contrasting observations from oceanic provinces. The open polar Southern Ocean and the eastern equatorial Pacific are HNLC regimes, but have different oceanographic properties. Until recently, there have been insufficient datasets on the role of iron (mainly from deckboard iron enrichments) to enable a comprehensive comparison of these regions. However, the recent Southern Ocean Iron RElease Experiment (SOIREE), the first in situ iron enrichment in polar waters, provides a detailed suite of time-series measurements to compare with those from the equatorial Pacific IronEx II study. As expected, a comparison of these polar and tropical studies yielded differences in the timing of iron-mediated responses that are mainly due to the temperature-dependence of biological rates. However, trends from both studies are similar with respect to the magnitude of iron-mediated changes in bulk signals (such as macronutrient uptake), algal physiological responses, and shifts in algal community structure. There are also parallels between these studies in the response of components of the pelagic ecosystem such as heterotrophic bacteria. Such convergence suggests that it is possible to incorporate considerable detail into future generic models investigating the role of the biota in the biogeochemical cycling of iron. There are also significant differences, such as the degree of herbivory, and the fate of the accumulated algal carbon during these two iron-stimulated phytoplankton blooms. Such departures offer a means to understand better important regional differences in the biogeochemical cycling of iron in HNLC waters, and to investigate the possible effects of physical artefacts—caused by mixing with surrounding HNLC waters at the boundaries of these labelled patches—during such mesoscale perturbation experiments.

Iron uptake and physiological response of phytoplankton during a mesoscale Southern Ocean iron enrichment

Iron supply is thought to regulate primary production in high nitrate, low chlorophyll (HNLC) regions of the sea in both the past and the present. A critical aspect of this relationship is acquisition of iron (Fe) by phytoplankton, which occurs through a complex series of extracellular reactions that are influenced by Fe chemistry and speciation. During the first in situ mesoscale Fe‐enrichment experiment in the Southern Ocean (Southern Ocean iron release experiment [SOIREE]), we monitored the uptake of Fe by three size classes of plankton and their ensuing physiological response to the Fe enrichment. Rates of Fe uptake from both inorganic Fe (Fe′) and organic Fe complexes (FeL) were initially fast, indicative of Fe‐limitation. After Fe enrichment phytoplankton down‐regulated Fe uptake and optimized physiological performance, but by day 12 they had greatly increased their capacity to acquire Fe from FeL. The increase in Fe uptake from FeL coincided with a sixfold decrease in Fe′ that followed the production of Fe‐binding organic ligands. Phytoplankton were able to use organically bound Fe at rates sufficient to maintain net growth for more than 42 d. Adaptation to such shifts in Fe chemistry may contribute to bloom longevity in these polar HNLC waters.

Retention of dissolved iron and FeII in an iron induced Southern Ocean phytoplankton bloom

During the 13 day Southern Ocean Iron RE‐lease Experiment (SOIREE), dissolved iron concentrations decreased rapidly following each of three iron‐enrichments, but remained high (>1 nM, up to 80% as FeII) after the fourth and final enrichment on day 8. The former trend was mainly due to dilution (spreading of iron‐fertilized waters) and particle scavenging. The latter may only be explained by a joint production‐maintenance mechanism; photoreduction is the only candidate process able to produce sufficiently high FeII, but as such levels persisted overnight (8 hr dark period) —ten times the half—life for this species—a maintenance mechanism (complexation of FeII) is required, and is supported by evidence of increased ligand concentrations on day 12. The source of these ligands and their affinity for FeII is not known. This retention of iron probably permitted the longevity of this bloom raising fundamental questions about iron cycling in HNLC (High Nitrate Low Chlorophyll) Polar waters.

Particle transformations and export flux during an in situ iron‐stimulated algal bloom in the Southern Ocean

During the first Southern Ocean Iron RElease Experiment (SOIREE), a suite of biogeochemical measurements (water column 234Th and δ13Corg inventories, particle fluxes from sediment traps, phytoplankton sinking rates) were undertaken to test the hypothesis that the vertical export of particulate organic carbon (POC) is enhanced due to iron‐induced increases in phytoplankton production. During the 13‐days that the SOIREE bloom was monitored, export fluxes within the iron‐fertilised patch were not substantially different to those in waters outside the bloom. On days 11–13, iron enrichment may have caused particle transformations that could lead to elevated future export via particle aggregation and/or diatom chain formation. The unknown time‐lag between increased production and export, the longevity of the SOIREE bloom, and the absence of nutrient limitation over days 1–13, however, prohibit prediction of any iron‐induced export. This conclusion highlights the difficulties of fully testing the “Iron Hypothesis” and for evaluating the implications for global climate change.

Phytoplankton processes. Part 2: Rates of primary production and factors controlling algal growth during the Southern Ocean Iron RElease Experiment (SOIREE)

Abstract The Southern Ocean Iron RElease Experiment (SOIREE) fertilised 200 km2 of polar waters (mixed-layer depth, 65 m) south of Australia in February 1999. During the 13-d SOIREE, iron enrichment stimulated primary production, algal growth and biogenic silica production rates. A floristic shift from small to large cells resulted in a greater contribution to community production by diatoms, which contributed >60% to community production towards the end of SOIREE. Column-integrated rates of primary production increased by more than 10-fold, and community production rates (normalised to chlorophyll) had doubled by day 4, but then declined after day 6 to initial rates. Iron enrichment increased the growth rates of the algal community from 0.02 to 0.15 d−1 (based on changes in in-situ chlorophyll concentrations), from 0.02 to 0.2 d−1 (based on algal carbon), and more than doubled initial rates (based on 14C) to 0.2 d−1 during SOIREE. However, these iron-elevated rates were less than the maximum rates (0.69 d−1) predicted for waters at 2.5°C. Community biogenic silica production rates increased by 6-fold, i.e. to a lesser extent than primary production rates, resulting in a 2-fold reduction in the silicic-acid : carbon uptake ratio. Shipboard perturbation experiments indicated that iron, rather than both iron and light, limited algal growth in these waters. However, a significant increase in light attenuation in situ due to algal self-shading, suggested that light limitation of algal growth might become significant after day 13 at this locale. Limitation of algal growth by both iron (and silicic acid and/or zinc) within iron-fertilised waters after day 8 of SOIREE was not evident from the results of a shipboard experiment. A simple mathematical model of particulate organic carbon (POC) potential export during SOIREE predicted large increases in export, due to both elevated production rates and a floristic shift to large cells. However, no significant increase in POC export was observed during SOIREE.

The Southern Ocean Iron RElease Experiment (SOIREE)—introduction and summary

Abstract This volume is dedicated to the Southern Ocean Iron RElease Experiment (SOIREE), the first in situ iron fertilisation experiment performed in the polar waters of the Southern Ocean. SOIREE was an interdisciplinary study involving participants from six countries, and took place in February 1999 south of the Polar Front in the Australasian-Pacific sector of the Southern Ocean. Approximately 3800 kg of acidified FeSO4.7H2O and 165 g of the tracer sulphur hexafluoride (SF6) were added to a 65-m deep surface mixed layer over an area of ∼50 km2. Initially, mean dissolved iron concentrations were ∼2.7 nM, but decreased to ambient levels within days, requiring subsequent additions of 1550–1750 kg of acidified FeSO4.7H2O on days 3, 5 and 7 of the experiment. During the 13-day site occupation there were iron-mediated increases in phytoplankton growth rates, with marked increases in chlorophyll a (up to 2 μg l−1) and production rates (up to 1.3 g C m−2 d−1). These resulted in subsequent changes in the pelagic ecosystem structure, and in the cycling of carbon, silica and sulphur, such as a 10% drawdown of surface CO2. The SOIREE bloom persisted for >40 days following our departure from the site, as observed via SeaWiFS remotely sensed observations of Ocean Colour. Papers in this volume report in detail on aspects of this study, from the oceanographic setting of the experiment to a modelling simulation of the SOIREE bloom. A CD-ROM accompanies this volume and contains the main SOIREE datasets and ancillary information including the pre-experiment ‘desktop’ database study for site-selection, and satellite images of the SOIREE bloom.

The impact of in situ Fe fertilisation on the microbial food web in the Southern Ocean

During the Southern Ocean iron release experiment (SOIREE) in February 1999, the composition and dynamics of the microbial food web were studied. SOIREE was a mesoscale experiment with four infusions of Fe into the patch to elevate Fe concentrations inside the patch. During the 13 d experiment, samples were collected from the mixed layer inside and outside the patch for the enumeration of bacteria, picophytoplankton, phyto and heterotrophic nano#agellates, ciliates, and for estimation of bacterial production and microzooplankton grazing. Inside the patch, bacterial numbers remained constant throughout SOIREE although bacterial production increased three-fold. A strong relationship between the increase in bacterial and primary production suggested that dissolved organic carbon and nitrogen, rather than Fe, potentially limited bacterial growth. The picophytoplankton population, was composed solely of eukaryotic cells and increased three-fold over the rst 7 d of the experiment before decreasing to initial concentrations of approximately 4000 cells ml. In contrast to the bacterial and picophytoplankton populations, the nanophyto#agellate population increased six-fold in numbers and 23-fold in biomass. This resulted in a three-fold increase in carbon #ow through the microbial food web inside the patch by the end of the experiment. The increased carbon #ow resulted in a small increase in total microzooplankton biomass. Ciliate abundances tripled and biomass, doubled; however, the ciliate population only contributed 3}10% of the microzooplankton biomass, which was dominated by the heterotrophic nano#agellate population. The heterotrophic nano#agellate numbers decreased three-fold by the end of the experiment; however, there was no signicant change in biomass throughout the experiment. The changes in the dynamics and structure of the microbial food web during the SOIREE experiment suggest that microzooplankton grazing controlled the bacterial and possibly the picophytoplankton populations. In contrast, the nanophyto#agellates were initially controlled by the Fe concentration, with microzooplankton having an impact on the population towards the end of the experiment. The addition of Fe to a small patch of the Southern Ocean had a considerable impact on the microbial components of the food web, even though the overall importance of the microbial pathways decreased as a result of Fe addition. 2001 Published by Elsevier Science Ltd.

Iron-mediated changes in phytoplankton photosynthetic competence during SOIREE

Abstract Active fluorescence (fast repetition rate fluorometry, FRRF) was used to follow the photosynthetic response of the phytoplankton community during the 13-day Southern Ocean Iron RElease Experiment (SOIREE). This in situ iron enrichment was conducted in the polar waters of the Australasian–Pacific sector of the Southern Ocean in February 1999. Iron fertilisation of these high nitrate low chlorophyll (HNLC) waters resulted in an increase in the photosynthetic competence (Fv/Fm) of the resident cells from around 0.20 to greater than 0.60 (i.e. close to the theoretical maximum) by 10/11 days after the first enrichment. Although a significant iron-mediated response in Fv/Fm was detected as early as 24 h after the initial fertilisation, the increase in Fv/Fm to double ambient levels took 6 days. This response was five-fold slower than observed in iron enrichments (in situ and in vitro) in the HNLC waters of the subarctic and equatorial Pacific. Although little is known about the relationship between water temperature and Fv/Fm, it is likely that low water temperatures — and possibly the deep mixed layer — were responsible for this slow response time. During SOIREE, the photosynthetic competence of the resident phytoplankton in iron-enriched waters increased at dissolved iron levels above 0.2 nM, suggesting that iron limitation was alleviated at this concentration. Increases in Fv/Fm of cells within four algal size classes suggested that all taxa displayed a photosynthetic response to iron enrichment. Other physiological proxies of algal iron stress (such as flavodoxin levels in diatoms) exhibited different temporal trends to iron-enrichment than Fv/Fm during the time-course of SOIREE. The relationship between Fv/Fm, algal growth rate and such proxies in Southern Ocean waters is discussed.

Mesozooplankton community composition, feeding, and export production during SOIREE

The community composition and feeding rates of mesozooplankton (>200 μm length) were determined using plankton hauls, bottle incubations and gut pigment determinations during Southern Ocean Iron RElease Experiment (SOIREE) in the Southern Ocean in February 1999. Upper-ocean (0–65 m) mesozooplankton biomass (4.2 and 3.2 g m−2, inside and outside the iron-fertilised patch, respectively) was dominated by large copepodites (>1.5 mm). Salps and euphausiids were absent and very rare, respectively. Incubations using large copepods showed no significant difference in clearance rates of nano- (2–20 μm) and net- (>20 μm) plankton. Mean clearance rates inside and outside the iron-fertilised patch also did not differ and were very low (ca. 50 ml mg DW−1 d−1). Mean ingestion rate, however, was significantly greater in the patch due to higher algal and heterotrophic nanoflagellate (HNAN) biomass there. Gut pigment analysis showed that most ingestion by large, medium and small copepods occurred at night, and that specific ingestion was greatest in small copepods. Daily integrated ingestion rates determined by the incubation and gut pigment methods were similar for comparable large copepods. Phytoplankton and HNAN ingestion met only 14% of the estimated daily respiratory carbon requirements of the large copepods inside the patch, and 4% outside. Little ciliate or detrital carbon was available in the system, which could have further supplemented the food supply. A number of other studies have found a similar disparity between ingestion and nutritional requirements in copepods. Reasons for this include the possibility that fine-scale aggregations of copepods and their food have not been adequately sampled, or that measured metabolic rates have been systematically overestimated. Ingestion of phytoplankton by the total copepod community was low, with <1% of standing stock removed per day (inside and outside the patch) and 4% and 8% of primary production removed (inside and outside the patch, respectively). Estimated egestion accounted for only 4% of the total particulate carbon flux found in sediment traps, within the patch. Little of the POC observed in traps was copepod faecal material. This supported the finding that feeding rates were low, and combined with the finding of an apparently weak diel vertical migration, suggested that the copepod assemblage contributed little passive and active export flux during SOIREE.

Phytoplankton processes. Part 1: Community structure during the Southern Ocean Iron RElease Experiment (SOIREE)

The first in situ iron-fertilisation in polar Southern Ocean waters, SOIREE (Southern Ocean Iron RElease Experiment) was a mesoscale, 13-day experiment that resulted in a phytoplankton bloom (chlorophyll-a increased six-fold; algal carbon, three-fold). Just prior to the iron enrichment, community chlorophyll-a at the study site was dominated by picoplankton, with an ‘innoculum’ of large diatoms. Iron enrichment resulted in a floristic shift, with increased diatom abundances, and to a lesser extent elevated nanoplankton stocks. Picoplankton stocks initially increased in response to iron supply, but were then grazed down to initial levels by microzooplankton. The chain-forming pennate diatom Fragilariopsis kerguelensis was the dominant bloom species, although <20 μm haptophytes also increased in abundance. Other algal responses to iron enrichment included elevated nanoplankton cell size, increased chain length of Fragilariopsis kerguelensis, and higher cellular chlorophyll-a levels within the pico-, nano- and microplankton. During the SOIREE bloom, chlorophyll-a levels of up to 3.0 mg m−3 were recorded at depth, and averaged 1.8 mg m−3 in the 65-m mixed layer. The floristic shift from a picoeukaryote-dominated community to one dominated by large diatoms was similar to that observed prior to and during algal blooms in the vicinity of open-ocean fronts in the Southern Ocean.

Southern Ocean iron enrichment promotes inorganic carbon drawdown

Abstract The effect of iron supply on phytoplankton growth and the marine carbon cycle was tested in situ at 61°S 141°E in the Southern Ocean Iron Release Experiment (SOIREE). On 9 February 1999 iron and the tracer sulphur hexafluoride (SF6) were added to the mixed layer with additional iron infusions after 3, 5 and 7 days. A small decrease of the fugacity of carbon dioxide (fCO2) and dissolved inorganic carbon (DIC) by iron-induced algal growth was observed 4–5 days after the first infusion. From then onwards fCO2 and DIC steadily decreased, and the iron-enriched waters became a sink for atmospheric CO2. The region with surface-water fCO2 drawdown closely matched the shape of the patch, as indicated by SF6. Surface-water fCO2 and DIC drawdown were relatively constant across the patch, whereas SF6 decreased from the patch centre outwards. This pointed to uniform algal carbon uptake, not limited by iron, in the patch. After 13 days surface-water fCO2 and DIC in the patch centre had decreased by 32–38 μatm and 15–18 μmol kg−1, respectively. Surface-water fCO2 outside the patch had increased by 8 μatm, partly as a result of surface-water warming. The iron-induced fCO2 change exceeded seasonal fCO2 variability in this region by a factor of two. From the surface-water fCO2 distribution we estimate a net DIC drawdown of 1353 t of carbon (±14%) (1 t=106 g) across the patch after 12 days, assuming uniform drawdown in the upper 50 m. Correction for vertical diffusion and air–sea exchange results in a gross DIC drawdown of 1408 t of carbon. The decrease of fCO2 and DIC, integrated over the mixed layer, was remarkably similar in size after 13 days of SOIREE as changes observed after 6–9 days during IronEx II, if we consider the 4–5 days lag in algal carbon uptake at the Southern Ocean site. SOIREE has demonstrated in situ the occurrence of algal iron limitation and of iron-induced carbon uptake in these Southern Ocean waters. The subsequent fate of the fixed inorganic carbon can only be speculated upon.

Circulation and seasonal evolution of polar waters south of Australia: implications for iron fertilization of the Southern Ocean

Abstract The Southern Ocean Iron Release Experiment (SOIREE) was carried out in late summer (February 1999) south of Australia (61°S, 140°E). This region of the southern Antarctic Zone (AZ-S), between the southern branch of the Polar Front (PF) and the southern front of the Antarctic Circumpolar Current (SAACF), is characterized by weak currents and is remote from the influence of sea-ice or coastal waters. The SOIREE site exhibits high nutrient concentrations year-round (phosphate, nitrate and silicate remain above 10 μM), low chlorophyll accumulations (

The fate of added iron during a mesoscale fertilisation experiment in the Southern Ocean

The first Southern Ocean Iron RElease Experiment (SOIREE) was performed during February 1999 in Antarctic waters south of Australia (61°S, 140°E), in order to verify whether iron supply controls the magnitude of phytoplankton production in this high nutrient low chlorophyll (HNLC) region. This paper describes iron distributions in the upper ocean during our 13-day site occupation, and presents a pelagic iron budget to account for the observed losses of dissolved and total iron from waters of the fertilised patch. Iron concentrations were measured underway during daily transects through the patch and in vertical profiles of the 65-m mixed layer. High internal consistency was noted between data obtained using contrasting sampling and analytical techniques. A pre-infusion survey confirmed the extremely low ambient dissolved (0.1 nM) and total (0.4 nM) iron concentrations. The initial enrichment elevated the dissolved iron concentration to 2.7 nM. Thereafter, dissolved iron was rapidly depleted inside the patch to 0.2-0.3 nM, necessitating three re-infusions. A distinct biological response was observed in iron-fertilised waters, relative to outside the patch, unequivocally confirming that iron limits phytoplankton growth rates and biomass at this site in summer. Our budget describing the fate of the added iron demonstrates that horizontal dispersion of fertilised waters (resulting in a quadrupling of the areal extent of the patch) and abiotic particle scavenging accounted for most of the decreases in iron concentrations inside the patch (31-58 and 12-49 of added iron, respectively). The magnitude of these loss processes altered towards the end of SOIREE, and on days 12-13 dissolved (1.1 nM) and total (2.3 nM) iron concentrations remained elevated compared to surrounding waters. At this time, the biogenic iron pool (0.1 nM) accounted for only 1-2 of the total added iron. Large pennate diatoms (> 20 μm) and autotrophic flagellates (2-20 μm) were the dominant algal groups in the patch, taking up the added iron and representing 13 and 39 of the biogenic iron pool, respectively. Iron regeneration by grazers was tightly coupled to uptake by phytoplankton and bacteria, indicating that biological Fe cycling within the bloom was self-sustaining. A concurrent increase in the concentration of iron-binding ligands on days 11-12 probably retained dissolved iron within the mixed layer. Ocean colour satellite images in late March suggest that the bloom was still actively growing 42 days after the onset of SOIREE, and hence by inference that sufficient iron was maintained in the patch for this period to meet algal requirements. This raises fundamental questions regarding the biogeochemical cycling of iron in the Southern Ocean and, in particular, how bioavailable iron was retained in surface waters and/or within the biota to sustain algal growth. © 2001 Elsevier Science Ltd.