Natural Iron Fertilization Research Articles

Ocean iron fertilization: Why further research is needed

Despite large uncertainties in the fertilization efficiency, natural iron fertilization studies and some of the purposeful iron enrichment studies have demonstrated that Southern Ocean iron fertilization can lead to a significant export of carbon from the sea surface to the ocean interior. From an economic perspective the potential of ocean iron fertilization (OIF) is far from negligible in relation to other abatement options. Comparing the range of cost estimates to the range of estimates for forestation projects they are in the same order of magnitude, but OIF could provide more carbon credits even if high discount rates are used to account for potential leakage and non-permanence. However, the uncertainty about undesired adverse effects of purposeful iron fertilization on marine ecosystems and biogeochemistry has led to attempts to ban commercial and, to some extent, scientific experiments aimed at a better understanding of the processes involved, effectively precluding further consideration of this mitigation option. As regards the perspective of public international law, the pertinent agreements dealing with the protection of the marine environment indicate that OIF is to be considered as lawful if and to the extent to which it represents legitimate scientific research. In this respect, the precautionary principle can be used to balance the risks arising out of scientific OIF activities for the marine environment with the potential advantages relevant to the objectives of the climate change regime. As scientific OIF experiments involve only comparatively small negative impacts within a limited marine area, further scientific research must be permitted to explore the carbon sequestration potential of OIF in order to either reject this concept or integrate it into the flexible mechanisms contained in the Kyoto Protocol.

An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean)

Abstract. Total dissolvable iron (TDFe) was measured in the water column above and in the surrounding of the Kerguelen Plateau (Indian sector of the Southern Ocean) during the KErguelen Ocean Plateau compared Study (KEOPS) cruise. TDFe concentrations ranged from 0.90 to 65.6 nmol L−1 above the plateau and from 0.34 to 2.23 nmol L−1 offshore of the plateau. Station C1 located south of the plateau, near Heard Island, exhibited very high values (329–770 nmol L−1). Apparent particulate iron (Feapp), calculated as the difference between the TDFe and the dissolved iron measured on board (DFe) represented 95±5% of the TDFe above the plateau, suggesting that particles and refractory colloids largely dominated the iron pool. This paper presents a budget of DFe and Feapp above the plateau. Lateral advection of water that had been in contact with the continental shelf of Heard Island seems to be the predominant source of Feapp and DFe above the plateau, with a supply of 9.7±3.6×106 and 8.3±11.6×103 mol d−1, respectively. The residence times of 1.7 and 48 days estimated for Feapp and DFe respectively, indicate a rapid turnover in the surface water. A comparison between Feapp and total particulate iron (TPFe) suggests that the total dissolved fraction is mainly constituted of small refractory colloids. This fraction does not seem to be a potential source of iron to the phytoplankton in our study. Finally, when taking into account the lateral supply of dissolved iron, the seasonal carbon sequestration efficiency was estimated at 154 000 mol C (mol Fe)−1, which is 4-fold lower than the previously estimated value in this area but still 18-fold higher than the one estimated during the other study of a natural iron fertilisation experiment, CROZEX.

Theory and observations of Ekman flux in the chlorophyll distribution downstream of South Georgia

A large phytoplankton bloom occurs downstream of South Georgia, an island on the northern edge of the Scotia Sea, Atlantic sector of the Southern Ocean. This is due to natural iron fertilisation being advected downstream in the Antarctic Circumpolar Current. Mapping of SeaWiFS chl‐a in dynamic height/longitude space reveals the trajectory of surface water under the dual influences of geostrophic flow and Ekman flux. A theoretical estimate of the trajectory shows good agreement with observations. The trajectory is dependent on the wind stress:mixed layer depth ratio, but independent of the latitudinal separation of dynamic height contours. The chl‐a distribution suggests that iron reaches 20°W below the mixed layer, away from the effects of Ekman flux, but beyond the iron is confined to the surface layer. Chl‐a reduces monotonically east of South Georgia with the signal detectable to 15°E, over 3500 km downstream, indicating the basin‐scale biogeochemical influence of the iron supply.

Origin of iron and aluminium in large particles (> 53 µm) in the Crozet region, Southern Ocean

Natural iron fertilization processes are occurring around the Crozet Islands (46°26′S–52°18′E), thus relieving the water masses from the normally encountered High Nutrients Low Chlorophyll (HNLC) conditions of the Southern Ocean. During austral summers 2004/2005 and 2005/2006, iron and aluminium concentrations were investigated in large particles (> 53 µm) collected from just below the mixed layer at stations under the influence of island inputs, and also in adjacent HNLC waters. These large particles are anticipated to sink out of the mixed layer, and to reflect the net effects of input and cycling of these elements in the overlying mixed layer. Labile and refractory fractions were determined by a two-stage leaching technique. Data showed that water masses downstream of the islands were enriched in total iron and aluminium (0.25–2.68 nmol L − 1 and 0.34–3.28 nmol L − 1 respectively), relative to the southern HNLC control sites (0.15–0.29 nmol L − 1 for Fe and 0.12–0.29 nmol L − 1 for Al), with only a small fraction (typically < 1%) being acid leachable in both environments. Particulate iron predominantly derived from the island system represents a significant fraction of the total water column iron inventory and may complement dissolved Fe inputs that help support the high summer productivity around the Crozet islands.

Southern Ocean deep-water carbon export enhanced by natural iron fertilization

The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon. Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified. Here we report data from the CROZEX experiment in the Southern Ocean, which was conducted to test the hypothesis that the observed north-south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean. The CROZEX sequestration efficiency (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600 mol mol(-1) was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron, but 77 times smaller than that of another bloom initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom(6). The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.

Algal biomarkers in surface waters around the Crozet plateau

The region around the Crozet plateau (Southern ocean) exhibits enhanced annual primary productivity, occurring during a distinct spring bloom, due to natural iron fertilisation of the macro nutrient-replete polar frontal zone. We describe differences in algal biomarker abundances and distributions, with particular emphasis on alkenones and sterols, around the Crozet plateau during an algal bloom (December 2005 to January 2006). In the centre of the bloom, diatom-derived sterols dominate, whereas alkenones are proportionally more abundant at lower productivity times or sites. This is consistent with biomarker analyses for other settings and suggests that high productivity and high nutrient, low chlorophyll (HNLC) regimes can be distinguished on the basis of algal community structure and the associated lipid biomarker assemblages.

234Th-based export fluxes during a natural iron fertilization experiment in the Southern Ocean (KEOPS)

Abstract Five iron-fertilization experiments in the Southern Ocean have clearly demonstrated that adding iron increases primary production, but the implications for carbon export to the ocean interior have been less clear. This reflects both observational limitations of short-term experiments and their uncertain relevance to quantifying ecosystem level processes that are likely to be structured differently under conditions of punctual versus persistent stimulation. To avoid these biases, KEOPS (KErguelen Ocean and Plateau compared Study) investigated the naturally iron-fertilized Kerguelen Plateau region in the Indian Sector of the Southern Ocean that exhibits an annual phytoplankton bloom. Here, we report particulate organic carbon (POC) and nitrogen export from this system based on the 234 Th approach. Results indicate that the export fluxes were variable both on and off the Kerguelen Plateau (9.0–38.4 mmol C m −2 d −1 and 1.6–4.8 mmol N m −2 d −1 ) and were in the range of values reported for natural Southern Ocean ecosystems. Export fluxes were compared at two reference stations, one above and one outside the Plateau. The station above the plateau was characterized by higher iron supply and export fluxes compared to the station outside the plateau. The difference in the export flux between these two reference stations defines the export excess induced by iron fertilization. It was 10.8±4.9 mmol C m −2 d −1 and 0.9±0.7 mmol N m −2 d −1 at 100 m, and 14.2±7.7 mmol C m −2 d −1 and 2.0±1.3 mmol N m −2 d −1 at 200 m. This POC export excess was similar to those found during other studies of artificial (SOFeX) and natural (CROZEX) iron fertilization in the Southern Ocean. The examination of the export efficiency (defined as the ratio of export to primary production) revealed significant variability over the plateau related to the temporal decoupling of production and export during the demise of the bloom. On average, the export efficiency was lower over the plateau than in surrounding waters, suggesting that increased iron supply may increase total export but lower export efficiency. Our findings are very important for evaluating present and past carbon cycling in the Southern and global oceans and for assessing predictive scenarios of carbon cycling and budget.

Distribution of dissolved iron during the natural iron-fertilization experiment KEOPS (Kerguelen Plateau, Southern Ocean)

Abstract The Southern Ocean is the largest high-nutrient low-chlorophyll (HNLC) region of the world ocean. It is now well recognized that the low biological productivity in this region is mainly due to the limitation of phytoplankton growth by iron. However, in the core of the Southern Ocean, a massive bloom occurs annually above the Kerguelen Plateau. In the context of KEOPS (Kerguelen Plateau and Ocean compared Study) we investigated the three-dimensional distribution of dissolved iron (DFe) concentrations southeast of the island. We show that in surface waters, DFe was low and not significantly different above and outside the plateau (0.090±0.034 and 0.073±0.014 nM, respectively). By contrast, below 150 m major differences in vertical profiles of DFe were observed. The deep waters above the plateau were clearly enriched, with DFe concentrations up to 0.6 nM near the seafloor. The regeneration from sinking biogenic material and the input from the sediments are very likely the major processes delivering DFe to the deep water. The deep iron-rich reservoir existing above the plateau was responsible for the natural iron fertilization of the surface water by two different mechanisms: (i) winter mixing provided 5 times more DFe to the surface waters above the plateau than outside and (ii) enhanced vertical gradients of DFe and elevated vertical diffusivity resulted in an 8-fold higher diapycnal diffusive flux. The natural iron fertilization presented major differences from artificial iron-fertilization experiments carried out in the Southern Ocean. The amount of DFe required to stimulate the biological activity is much less and the mode of addition is clearly different. The quantification of the excess of DFe supplied in the naturally fertilized area leads to the conclusion that the natural iron fertilization is 10–150 times more efficient to export carbon below 200 m than in artificial fertilization experiments. The bloom above the Kerguelen Plateau was contingent to iron fertilization but it was also sustained by continuous input of major nutrients. Thus, the natural fertilization described here mimics quite well glacial period scenarios where iron fertilization of the Southern Ocean might have occurred from below.

Radium isotopes to investigate the water mass pathways on the Kerguelen Plateau (Southern Ocean)

Abstract Natural iron fertilization promotes the phytoplankton bloom that takes place on the Kerguelen Plateau in the Indian sector of the Southern Ocean. We measured 228Ra (T1/2=5.75 yr) and 226Ra (T1/2=1602 yr) in waters above the Kerguelen Plateau in order to provide information on the water-mass pathways, which in turn could help elucidate the mechanisms controlling iron fertilization in that area. 228Ra activities are extremely low in this region, being in most cases Using the 228Ra profile obtained at station A3, we could estimate the vertical eddy diffusivity (Kz) on the Kerguelen Plateau. Our Kz estimate (1.5×10−4±1.3×10−4 m2/s) compares well with that reported by Park et al. [2008a. Internal tides and vertical mixing over the Kerguelen Plateau. Deep-Sea Research II, doi:10.1016/j.dsr2.2007.12.027] based on CTD/LADCP data. By combining this Kz estimate with the gradient of dissolved Fe observed at A3 [Blain, S., Sarthou, G., Laan, P., 2008. Distribution of dissolved iron during the natural iron fertilization experiment KEOPS (Kerguelen Plateau, Southern Ocean). Deep-Sea Research II, doi:10.1016/j.dsr2.2007.12.028], we calculate a vertical Fe flux of 1.0–14.3 nmol/m2 d.

Probing natural iron fertilization near the Kerguelen (Southern Ocean) using natural phytoplankton assemblages and diatom cultures

Abstract Natural phytoplankton assemblages collected in surface waters above the Kerguelen Plateau or in the open-ocean and single-species cultures of Southern Ocean diatoms were used to address the existence and effects of natural iron fertilization near the Kerguelen Islands (Southern Ocean). The phytoplankton was transferred during so-called translocation experiments into water collected at the surface over the Plateau, open-ocean surface water or water collected close to the sediment of the Plateau. These watertypes differed in iron (iron-rich deep water and iron-poor surface water) and silicic acid concentration (silicic acid-rich Plateau deep and open-ocean surface water, silicic acid-poor Plateau surface water). As a general trend in the natural phytoplankton assemblages, cell numbers, chlorophyll autofluorescence, photosynthetic efficiency of photosystem II, chlorophyll a and phytoplankton carbon concentrations increased especially after translocation into Plateau deep water. This response was most pronounced in terms of increase in carbon assimilation in the larger-sized phytoplankton (>8 μm in cell diameter), mainly diatoms. Effects of translocation on bacteria and viruses followed those of the phytoplankton. Experiments with single-species cultures of large diatoms ( Fragilariopsis kerguelensis , Thalassiosira sp., Chaetoceros dichaeta ), which have high iron requirements, confirmed the observations made for the natural phytoplankton assemblages. Assuming a continuous flux of deep water to the surface over the Kerguelen Plateau, the translocation experiments provide evidence that this water contains the growth-stimulating factor, most likely iron, responsible for the formation of a phytoplankton bloom as is observed over the Kerguelen Plateau.

Rapid bacterial mineralization of organic carbon produced during a phytoplankton bloom induced by natural iron fertilization in the Southern Ocean

The response of heterotrophic bacteria (Bacteria and Archaea) to the spring phytoplankton bloom that occurs annually above the Kerguelen Plateau (Southern Ocean) due to natural iron fertilization was investigated during the KErguelen Ocean and Plateau compared Study (KEOPS) cruise in January–February 2005. In surface waters (upper 100 m) in the core of the phytoplankton bloom, heterotrophic bacteria were, on an average, 3-fold more abundant and revealed rates of production ([3H] leucine incorporation) and respiration (<0.8 μm size fraction) that exceeded those in surrounding high-nutrient low-chlorophyll (HNLC) waters by factors of 6 and 5, respectively. These differences in bacterial metabolic activities were attributable to high-nucleic-acid-containing cells that dominated (≈80% of total cell abundance) the heterotrophic bacterial community associated with the phytoplankton bloom. Bacterial growth efficiencies varied between 14% and 20% inside the bloom and were <10% in HNLC waters. Results from bottle-incubation experiments performed at the bloom station indicated that iron had no direct but an indirect effect on heterotrophic bacterial activity, due to the stimulation by phytoplankton-derived dissolved organic matter. Within the Kerguelen bloom, bacterial carbon demand accounted for roughly 45% of gross community production. These results indicate that heterotrophic bacteria processed a significant portion of primary production, with most of it being rapidly respired.

Microbial food web structure in a naturally iron-fertilized area in the Southern Ocean (Kerguelen Plateau)

Abstract The objective of this study in the framework of the KErguelen Ocean and Plateau compared Study, 2005–2007 (KEOPS) project was to examine the microbial food web structure within a phytoplankton bloom induced by natural iron fertilization. Integrated bacterial production (BP, 0–100 m) varied 12-fold over the study area (23.5–304 mg C m−2 d−1), while bacterial abundance (0–100 m) varied only by a factor of 2.8. Highest bacterial abundances and rates of BP were observed in the center of the diatom-dominated bloom, and substantial decreases in BP towards the later bloom stage were detectable. The abundance of bacterial predators (heterotrophic nanoflagellates, HNF) showed a significant coupling with BP in the high-nutrient low-chlorophyll (HNLC) area only. In the core of the bloom, BP consumed by HNF was 27%, 29%, 52% and 34% during the four consecutive visits that extended over 4 weeks and was much higher (80–95%) in HNLC waters. The relative contribution of the small-sized (

The natural iron fertilization experiment KEOPS (KErguelen Ocean and Plateau compared Study): An overview

When the first scientific voyages went to the Southern Ocean, high levels of living biomass were observed in the vicinity of islands. Hart (1942) was the first to mention that the abundant biomass might be due to the release of oligoelements like iron from the island. This hypothesis, however, remained unverified for more than half a century because of the experimental difficulties in properly manipulating water with subnanomolar iron concentrations. When John Martin formulated the iron hypothesis (Martin, 1990) and provided the first supporting evidence of it, he also suggested using the chlorophyll plume observed westward of the Galapagos Island in the Equatorial Pacific as a natural iron fertilization experiment.

Fe-binding dissolved organic ligands near the Kerguelen Archipelago in the Southern Ocean (Indian sector)

Abstract During the Ke rguelen O cean and P lateau compared S tudy (KEOPS; January–February 2005) cruise, the area southeast of the Kerguelen Archipelago in the Indian sector of the Southern Ocean was investigated to identify the mechanisms of natural iron fertilization of the Kerguelen Plateau. In this study, the organic speciation of Fe is described. Samples were determined immediately on board using competing ligand-adsorptive cathodic stripping voltammetry (CL-AdCSV). The dissolved organic ligands were always in excess of the dissolved Fe concentration, increasing the residence time in the water column and the potential availability for phytoplankton. The concentration of the dissolved organic ligands ranged from 0.44 to 1.61 n Eq of M Fe (=complexation site for Fe), with an average concentration of 0.91 n Eq of M Fe (S.D.=0.28, n =113) and a mean logarithm of conditional stability constant (log K′) of 21.7 (S.D.=0.28, n =113). A second weaker dissolved organic ligand group was detected in 32% of the samples, with Fe-binding characteristics at the edge of the detection window of the applied method. The occurrence of the highest concentrations of dissolved organic ligands in the wind-mixed surface layer and near the sediment at the bottom of the water column indicated that both phytoplankton and the sediment act as sources. Both sources are in concert with the general conclusions from the KEOPS research on the sources of Fe, where Fe was regenerated, organic Fe-binding ligands were formed in the upper layers, and both Fe and ligands were supplied by the sediment.

Seasonality and scale of the Kerguelen plateau phytoplankton bloom: A remote sensing and modeling analysis of the influence of natural iron fertilization in the Southern Ocean

The phytoplankton bloom that develops over the Kerguelen plateau following natural input of iron is analysed on a regional and seasonal scale. The relation between chlorophyll, bathymetry, and surface advection fields is not as obvious as it first appears from large-scale annual mean field. The high chlorophyll biomass does not always correspond with the shallowest water, and there are portions of the plateau, which persistently exhibit low chlorophyll. Despite this complex dynamic, a one-dimensional model calibrated for HNLC (high-nutrient low-chlorophyll) region is able to capture the observed increase in chlorophyll by increasing the deep iron concentration. The elemental budget shows similarity in terms of carbon, nitrogen, and silicon but differences in terms of iron with the budget calculated during the mission. This discrepancy either has its origin in the structure of the iron cycling in the model or in the temporal scarcity of data that could only be collected during the summer months. In the model, flexibility of the Fe/C ratio associated with high Fe export and input fluxes prevents high carbon sequestration efficiency. This first insight with remote sensing data and the model allows the validation of some of the key mechanisms of natural iron fertilization and exposes problems that will need to be solved to have a complete biogeochemical diagnostic of this natural iron fertilization.

Mesopelagic organic carbon remineralization in the Kerguelen Plateau region tracked by biogenic particulate Ba

Abstract We report on the distribution of excess, non-lithogenic particulate barium (Ba), a proxy for twilight-zone remineralization of organic matter (OM), in the Kerguelen Plateau area during the late austral summer of 2005. This study was part of a broader investigation focusing on natural iron fertilization controlling the characteristic and recurrent phytoplankton blooms in the region. The Plateau area contrasted strongly with the off-shelf HNLC area. Plateau stations had quite high excess of Ba xs contents (up to 5000 pM) in surface waters (upper 150 m) that coincided with enhanced phytoplankton biomass. However, it had a lower mesopelagic (125–450 m) excess of biogenic Ba contents when compared to the off-shelf stations. Particulate excess Ba in the twilight zone (125–450 m depth layer) proved to be significantly set by the vertical distribution of bacterial activity, with higher particulate Ba contents in situations where significant bacterial activity extended deeper in the water column. These observations are in agreement with the role of excess biogenic particulate Ba as a proxy of twilight-zone OM remineralization. Using a transfer function established during earlier work, we calculated organic carbon remineralization rates and weighed these against other carbon fluxes (primary production, bacterial carbon demand), including carbon export from the 125-m horizon ( 234 Th method). Off-shelf HNLC stations had a larger fraction of the organic carbon production exported and subsequently mineralized in the mesopelagic waters (125–450 m). Plateau stations had a smaller fraction of produced carbon exported, but this fraction appeared less prone to remineralization in the twilight zone and therefore had potential to reach the shallow (∼500 m) seafloor or to be exported off-shelf. Differences in trophic structure and in composition of the diatom community would explain the variability in twilight-zone remineralization processes and in transfer efficiency through the mesopelagic zone in the Kerguelen Plateau area.

Effect of natural iron fertilisation on the distribution of DMS and DMSP in the Indian sector of the Southern Ocean

Understanding the processes controlling the distribution and fate of dimethylsulphide (DMS) in marine surface waters is critical to quantifying the flux of DMS to the atmosphere. All fertilisation experiments in the Southern Ocean trying to mimic a short-term (several weeks) atmospheric, dust-related, iron supply so far have shown that DMS concentrations increase when iron is added artificially in small patches to iron-limited regions. During the Kerguelen Ocean and Plateau compared Study (KEOPS), we have investigated the impact on surface concentrations of DMS and its precursor, particulate dimethylsulphoniopropionate (DMSPp), of iron supplied by mixing from below. We found that the massive summer phytoplanktonic bloom sustained by naturally enhanced vertical mixing of deep iron-rich seawater above the Kerguelen Plateau is not a substantially larger source of DMS than the surrounding low-primary productivity waters. The sustained enrichment of iron over the Kerguelen Plateau does not translate into an increase in DMS concentration because: (1) the nano-phytoplankton, which is the main producer of DMSPp in the area, is not favoured by the continuous supply of iron in surface waters, (2) the transfer efficiency of DMSP to DMS is probably low due to the higher levels of bacterial production measured over the plateau, which, likely, also stimulate the bacterial removal of DMS, and (3) nitrates and fresh chromophoric dissolved organic matter, supplied in surface waters from enhanced mixing of deep water over the plateau, increase the photodegradation rates of DMS. Thus, artificial short-term iron supplies from above and naturally sustained ones show fundamental differences in their impacts on the distribution and net production of DMS.

Internal tides and vertical mixing over the Kerguelen Plateau

Within the context of the natural iron-fertilization study KEOPS, time series measurements of CTD and LADCP profiles at a site (50.6°S, 72°E; 528 m) coinciding with an annual phytoplankton bloom over the Kerguelen Plateau were made during the January–February 2005 KEOPS cruise. An important activity of highly nonlinear semidiurnal internal tides having peak-to-peak isopycnal displacements of up to 80 m is identified. These internal tides appear to be a principal agent for promoting elevated vertical mixing indispensable for upward transfer of iron within the seasonal thermocline. We estimate local vertical eddy diffusivities of the order of 4×10?4 m2 s?1 using a Thorpe scale analysis. Although this estimate is higher by an order of magnitude than the canonical value O (0.1×10?4 m2 s?1) in the open ocean away from boundaries, it is consistent with nonlinear internal wave/wave interaction theories, as verified by independent diffusivity estimates using the vertical wavenumber spectral methods for shear and strain. It is also suggested that the general ocean circulation may play an important role in preconditioning the bloom in that the relatively sluggish circulation over the shallow plateau (compared to the much more dynamic neighbouring deep ocean) may foster the bloom's observed annual recurrence over the plateau.

Major differences of bacterial diversity and activity inside and outside of a natural iron‐fertilized phytoplankton bloom in the Southern Ocean

One of the first comparisons of a natural iron fertilized bloom with a high-nutrient low-chlorophyll (HNLC) site was undertaken during the Kerguelen ocean and plateau compared study (KEOPS) cruise. To understand better the bacteria-phytoplankton relationship in the context of natural iron fertilization, bacterial diversity and activity was investigated in the bloom and in the adjacent HNLC region by 16S rDNA clone libraries and by single strand conformation polymorphism (SSCP) analysis. Both libraries were dominated by Alphaproteobacteria, Gammaproteobacteria and the Cytophaga-Flavobacteria-Bacteroides group. Cluster analysis at 99% sequence similarity yielded several microdiverse clusters and revealed striking differences between the two libraries. In the bloom, the dominant operational taxonomic units (OTUs) were the Roseobacter NAC11-7 cluster, SAR92 and a Cytophaga-Flavobacteria-Bacteroides cluster related to the agg58 group, whereas in the HNLC region, SAR11, Roseobacter RCA and Polaribacter dominated. SSCP analysis of 16S rDNA and 16S rRNA revealed contrasting dynamics of three different Roseobacter OTUs. Roseobacter NAC11-7 and NAC11-6 had higher relative abundances and activities in the bloom compared with the HNLC site and NAC11-6 was only detected at the decline of the bloom concomitant with a shift in phytoplankton composi tion. In contrast, Roseobacter RCA was relatively abundant and active both inside and outside of the bloom. These results suggest that the different OTUs within the Roseobacter group represent functional groups that each play an important role in the cycling of carbon.

Dissolved iron in the vicinity of the Crozet Islands, Southern Ocean

Abstract The annual phytoplankton bloom occurring north of the Crozet Plateau provides a rare opportunity to examine the hypothesis that natural iron fertilization can alleviate high-nutrient low-chlorophyll (HNLC) conditions normally associated with the Southern Ocean. Therefore, during CROZet natural iron bloom and EXport experiment (CROZEX), a large multidisciplinary study performed between November 2004 and January 2005, measurements of total dissolved iron (DFe⩽0.2 μm) were made on seawater from around the islands and atmospheric iron deposition estimated from rain and aerosol samples. DFe concentrations were determined by flow injection analysis with N,N-dimethyl-p-phenylenediamine dihydrochloride (DPD) catalytic spectrophotometric detection. DFe concentrations varied between 0.086 and 2.48 nM, with low values in surface waters. Enrichment of dissolved iron (>1 nM) at close proximity to the islands suggests that the plateau and the associated sediments are a source of iron. Waters farther north also appear to be affected by this input of coastal and shelf origin, although dissolved iron concentrations decrease as a function of distance to the north of the plateau with a gradient of ∼0.07 nM km−1 at the time of sampling. Using lateral and vertical diffusion coefficients derived from Ra isotope profiles and also estimates of atmospheric inputs, it was then possible to estimate a DFe concentration of ∼0.55 nM to the north of the islands prior to the bloom event, which is sufficient to initiate the bloom, the lateral island source being the largest component. A similar situation is observed for other Sub-Antarctic Islands such as Kerguelen, South Georgia, that supply dissolved iron to their surrounding waters, thus enhancing chlorophyll concentrations.