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Highlights •A solid knowledgebase and governance process is required to develop policies for sustainable growth of offshore aquaculture in the North Sea. •Environmental goals largely define maximum sustainable production levels. For seaweed culture this is limited to the scale of several hundred km2. •Definition of realistic ambitions is a process rather than a fixed value and greatly benefits from a ‘learning by doing’ approach.

Cultivation of Saccharina latissima, a brown macroalga, is fast developing in Europe and North America for the sustainable production of food and biorefinery materials and important ecosystem services. Prior studies have demonstrated large spatial variability in the yield and chemical composition of the cultivated S. latissima, even within a small coastal bay. Using a validated hydrodynamic-biogeochemical-kelpmodel, this study examined main drivers of the spatial variability in S. latissima growth dynamics in 40 hypothetical farms throughout a Dutch coastal bay, the Eastern Scheldt. Results indicate that temperatureplays a primary role in driving the spatial variability. For example, S. latissima yield in the deeper and better flushed western part is more than double that in the eastern part, mainly due to its 2–3°C warmer seawater in winter. It is also found that S. latissima benefits from co-cultivation with shellfish, since nutrients excreted by shellfish replenish its nitrogen reserve, which fuels a relatively high growth rate in the nitrogen-depleted late spring. The model assessment offers insight into optimal potential locations of S. latissima farms in the Eastern Scheldt. Applicability of our modelling approach to other coastal ecosystems and possible further improvements for assisting in seaweed farming practice are discussed.

Kelp cultivation receives increasing interest for its high-value products and ecological services, especially in Europe and North America. Before industrial kelp farming in marine ecosystems continue to scale up, evaluation of the site-wide production relative to ecological carrying capacity (CC) of the identified system is essential. For this purpose, a mechanistic kelp model was developed and applied for hypothetical numerical experiments of expanding the farming area in a Dutch coastal bay (the Eastern Scheldt), where cultivation of Saccharina latissima (sugar kelp) is emerging. The kelp model was implemented within a three-dimensional hydrodynamic–biogeochemical model to account for the environmental interactions. The model captured the seasonal growth dynamics of S. latissima, as well as its carbon and nitrogen contents measured at the Eastern Scheldt pilot sites. The model results suggest that expanding the kelp farming area to ∼1–30% of the bay (representing ∼3.4–75 kt harvest dry weight in the 350-km2 bay) had the potential to weaken the spring bloom, and thereby affected the coexisting shellfish culture in the bay. Competition between S. latissima and phytoplankton mostly occurred in late spring for nutrients (dissolved inorganic nitrogen). The ecological CC should be weighed according to these negative impacts. However, the production CC was not reached even when farming ∼30% of the Eastern Scheldt, i.e. harvesting totally 75 kt dry mass, given that the simulated overall S. latissima production kept increasing with the farming activity. Our modelling approach can be applied to other systems for S. latissima cultivation and assist in assessing CC and environmental impacts.

The microalgae Rhodomonas sp. is known as an excellent feed source for live feed organisms such as copepods. The main benefits of feeding Rhodomonas to live feed animals are attributed to the relative high polyunsaturated fatty acid (PUFA) level, the combination of containing both docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), and the ratio between these fatty acids (FA). It has been shown that microalgae are able to accumulate valuable metabolites, such as lipids, under adverse conditions. The easiest and most inexpensive method to induce stress to microalgae is through nitrogen (N) starvation. In this study, the effect of N-starvation on biomass concentration, cell volume, and cellular composition, such as fatty acid concentration and composition, and phycoerythrin (PE) concentration of Rhodomonas sp. during a period of 8 days, was investigated. The research was divided into two stages. In the first (growth) stage, Rhodomonas sp. was cultivated in small 400 ml photobioreactors (Algaemist-S) under optimal conditions in turbidostat mode, which reached a biomass concentration of 1.5 gDW L–1 and dilution rate of 1.3 d–1. Samples were taken every 24 h for cell density and volume and productivity measurements in order to ensure a healthy and stable culture. In the next stage (N-starvation), the biomass was washed and transferred in a reactor filled with N-depleted medium. During N-starvation, samples were taken for biomass concentration, cell volume, PE and FA composition. The results of this study demonstrate that the lipid content increased significantly from 9% (t = 0 h) to 30% (t = 120 h) of the dry weight. After 120 h of N-starvation, the total FA content of Rhodomonas sp. remained stable for the remainder of the experiment (next 72 h). The highest increase of the FA concentration was represented by C16:0, C:18:1, C18:2, and C18:3, with highest concentrations after 120 h of starvation. The maximum EPA and DHA concentrations were observed after 48 h of starvation, while the maximum DHA to EPA ratio was detected at the end of the starvation.

Climate change is affecting the hydrodynamics of the world’s oceans. How these changes will influence the productivity, distribution and abundance of phytoplankton communities is an urgent research question. Here we provide a unique high-resolution mesoscale description of the phytoplankton community composition in relation to vertical mixing conditions and other key physicochemical parameters along a meridional section of the Northeast Atlantic Ocean. Phytoplankton, assessed by a combination of flow cytometry and pigment fingerprinting (HPLC-CHEMTAX), and physicochemical data were collected from the top 250 m water column during the spring of 2011 and summer of 2009. Multivariate analysis identified water column stratification (based on 100 m depth-integrated Brunt-Vaisala frequency N2) as one of the key drivers for the distribution and separation of different phytoplankton taxa and size classes. Our results demonstrate that increased stratification (i) broadened the geographic range of Prochlorococcus as oligotrophic areas expanded northward, (ii) increased the contribution of picoeukaryotic phytoplankton to total autotrophic organic carbon (< 20 µm), and (iii) decreased the abundances of diatoms and cryptophytes. We discuss the implications of our findings for the classification of phytoplankton functional types in biogeochemical and ecological ocean models. As phytoplankton taxonomic composition and size affects productivity, biogeochemical cycling, ocean carbon storage and marine food web dynamics, the results provide essential information for models aimed at predicting future states of the ocean.

Relationships between sea surface temperature (SST, > 10 m) and vertical density stratification, nutrient concentrations, and phytoplankton biomass, composition, and chlorophyll a (Chl a) specific absorption were assessed in spring and summer from latitudes 29 to 63° N in the northeast Atlantic Ocean. The goal of this study was to identify relationships between phytoplankton and abiotic factors in an existing SST and stratification gradient. Furthermore, a bio-optical model was used to estimate productivity for five phytoplankton groups. Nutrient concentration (integrated from 0 to 125 m) was inversely correlated with SST in spring and summer. SST was also inversely correlated with near-surface (0–50 m) Chl a and productivity for stratified stations. Near-surface Chl a and productivity showed exponential relationships with SST. Chl a specific absorption and excess light experiments indicated photoacclimation to lower irradiance in spring as compared to summer. In addition, Chl a specific absorption suggested that phytoplankton size decreased in summer. The contribution of cyanobacteria to water column productivity of stratified stations correlated positively with SST and inversely with nutrient concentration. This suggests that a rise in SST (over a 13–23 °C range) stimulates productivity by cyanobacteria at the expense of haptophytes, which showed an inverse relationship to SST. At higher latitudes, where rising SST may prolong the stratified season, haptophyte productivity may expand at the expense of diatom productivity. Depth-integrated Chl a (0–410 m) was greatest in the spring at higher latitudes, where stratification in the upper 200 m was weakest. This suggests that stronger stratification does not necessarily result in higher phytoplankton biomass standing stock in this region.

The accuracy of remote sensing algorithms for phytoplankton biomass and physiology is difficult to test under natural conditions due to rapid changes in physical and biological forcings and the practical inability to manipulate nutrient conditions and phytoplankton composition in the sea. Therefore, an indoor mesocosm was designed to examine the optical properties of phytoplankton under controlled and manipulated conditions of irradiance, temperature, turbulence, and nutrient availability. Equipped with hyperspectral radiometers and bottom irradiance meters, it is shown that under semi-natural environmental conditions biogeochemically relevant species as Emiliania huxleyi and Phaeocystis globosa can be grown with good precision (+/- 20%) between duplicate mesocosms and between duplicate sensors (< 5% deviation). The accuracy of chlorophyll estimates by absorption, using an Integrating Cavity Absorption Meter, and fluorescence using water-leaving radiance was 74% to 80%, respectively, as it was negatively influenced by changes in phytoplankton physiology. Biomass detection was limitedto 1 to 2 mu g chlorophyll/L with an apparent linearity to 50 mu g chlorophyll/L. Estimates of the quantum efficiency of fluorescence (phi approximate to 0.01) were comparable to real-world estimates derived from satellite observations. It is concluded that the mesocosms adequately simulate natural conditions with sufficient accuracy and precision and that they offer an important tool in validating assumptions and hypotheses underlying remote sensing algorithms and models.

In the Eastern North Atlantic Ocean iron (Fe) speciation was investigated in three size fractions: the dissolvable from unfiltered samples, the dissolved fraction (o 0.2 mm) and the fraction smaller than 1000 kDa (o 1000 kDa). Fe concentrations were measured by flow injection analysis and the organic Fe complexation by voltammetry. In the research area the water column consisted of North Atlantic Central Water (NACW), below which Mediterranean Overflow Water (MOW) was found with the core between 800 and 1000 m depth. Below 2000 m depth the North Atlantic Deep Water (NADW) proper was recognised. Dissolved Fe and Fe in the o 1000 kDa fraction showed a nutrient like profile, depleted at the surface, increasing until 500–1000 m depth below which the concentration remained constant. Fe in unfiltered samples clearly showed the MOW with high concentrations (4 nM) compared to the overlying NACW and the underlying NADW, with 0.9 nM and 2 nM Fe, respectively. By using excess ligand (Excess L) concentrations as parameter we show a potential to bind Fe. The surface mixed layer had the highest excess ligand concentrations in all size fractions due to phytoplankton uptake and possible ligand production. The ratio of Excess L over Fe proved to be a complementary tool in revealing the relative saturation state of the ligands with Fe. In the whole water column, the organic ligands in the larger colloidal fraction (between 0.2 mm and 1000 kDa) were saturated with Fe, whereas those in the smallest fraction (o1000 kDa) were not saturated with Fe, confirming that this fraction was the most reactive one and regulates dissolution and colloid aggregation and scavenging processes. This regulation was remarkably stable with depth since the alpha factor (product of Excess L and K 0

International audience; Vertical distributions of iron and iron binding ligands were determined in 2 size classes (dissolved < 0.2 μm, soluble < 200 kDa, e.g., ~ 0.03 μm) in the Southern Ocean. Colloidal iron and complexing capacity (> 200 kDa-< 0.2 μm) were inferred as the difference between the dissolved and soluble fractions. Dissolved iron and ligands exist primarily in the soluble size range in the surface waters, although iron-complexing colloids still represent a significant portion of the dissolved pool and this fraction increases markedly with depth. This work presents evidence for the colloidal nature of a significant portion (37-51% on average) of the 'dissolved' organic Fe pool in these oceanic waters. From the data it was not possible to discern whether iron colloids exist as discrete organic complexes and/or inorganic amorphous colloids. Iron-complexing colloids are the most saturated with iron at the thermodynamic equilibrium, whereas soluble organic ligands occur in larger excess compared to soluble iron. It suggests that the exchangeable fraction for iron uptake through dissociation of Fe complexes likely occurs in the soluble fraction, and that soluble ligands have the potential to buffer iron inputs to surface waters whereas iron colloids may aggregate and settle. Expectations based on Fe diffusion rates, distributions and the stability of the soluble iron complexes and iron colloids also suggest that the weaker soluble Fe complexes may be more bio-available, while the strongest colloids may be a major route for iron removal from oceanic waters. Investigations of the size classes of the dissolved organic iron thus can significantly increase our understanding of the oceanic iron cycle.

The major response to ocean iron fertilization is by large diatoms, which at Fe-replete ambient seawater show an optimum C:Fe elemental ratio of similar to 23 000 and a higher ratio of similar to 160 000 or more under Fe-limited conditions. The efficiency of CO2 drawdown during the several weeks of artificial fertilization experiments with concomitant observations is in the range of 100

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.

Short term (2 days) laboratory experiments were performed to study the change in irradiance induced production of Fe(II) in seawater in the presence of two open oceanic Southern Ocean diatom species, Thalassiosira sp. and Chaetoceros brevis. Three irradiance conditions were applied: 1) UVB+UVA+VIS, 2) UVA+ VIS, and 3) VIS, and Fe concentrations of 0 and 5 nM Fe were added to natural Southern Ocean seawater (containing 0.32 nM dissolved Fe and 1.69 equivalents of nM L(-1) Fe dissolved organic ligands, log K'=22.03). The photoproduced concentration of Fe(II) showed no relationship with the concentration of total dissolved Fe or the concentration of strongly chelated iron. During incubations with the diatoms an increase in the Fe(II) concentration during the second day suggested a modification of the Fe speciation. In the presence of Thalassiosira sp. photoreduction of Fe(III) was observed, whereas in the presence of C brevis irradiance independent Fe(III) reduction played an important role in the Fe(II) production. Furthermore, a decrease in the strongly chelated Fe concentration, in concert with a decrease in the conditional stability constant, suggested a modification of the strongly chelated Fe fraction in the experiments with C brevis. The chelated Fe fraction did not change in cultures with Thalassiosira sp. Overall, the presence of diatoms appeared to enhance the reactive Fe pool improving the biological availability of Fe. (c) 2007 Elsevier B.V. All rights reserved.

Shipboard and rernote-sensed Chlorophyll fluorescence were determined in the natural phytoplankton assemblage above the iron-enriched Kerguelen Plateau and the adjacent high-nutrient, low-Chlorophyll open Southern Ocean. The variance between fluorescence yield and photosynthetic efficiency was determined in combination with Chlorophyll a concentrations, irradiance and phytoplankton species distribution. A co-variance between the fluorescence measurements would allow the refinement of remote-sensing primary production algorithms. Distinct differences were found in photosynthetic efficiency and water-leaving fluorescence, with relatively high values for the Kerguelen Plateau and low values in the open ocean, reflecting the differences in Chlorophyll a concentrations. The co-variance of the fluorescence properties suggested that remote-sensed fluorescence measurements could be used to infer differences in the physiological state of the phytoplankton, hence primary production. Fluorescence yield, however, did not show the differences in the research area, most likely due to the low signal and the diurnal variation in water-leaving fluorescence.

International audience; Aerosol (soluble and total) iron and water-column dissolved (DFe, < 0.2 μm) and total dissolvable (TDFe, unfiltered) iron concentrations were determined in the Canary Basin and along a transect towards the Strait of Gibraltar, in order to sample across the Saharan dust plume. Cumulative dust deposition fluxes estimated from direct aerosol sampling during our one-month cruise are representative of the estimated deposition fluxes based on near surface water dissolved aluminium concentrations measured on board. Iron inventories in near surface waters combined with flux estimates confirmed the relatively short residence time of DFe in waters influenced by the Saharan dust plume (6–14 months). Enhanced near surface water concentrations of DFe (5.90–6.99 nM) were observed at the Strait of Gibraltar mainly due to inputs from metal-rich rivers. In the Canary Basin and the transect towards Gibraltar, DFe concentrations (0.07–0.76 nM) were typical of concentrations observed in the surface North Atlantic Waters, with the highest concentrations associated with higher atmospheric inputs in the Canary Basin. Depth profiles showed that DFe and TDFe were influenced by atmospheric inputs in this area with an accumulation of aeolian Fe in the surface waters. The sub-surface minimum of both DFe and TDFe suggests that a simple partitioning between dissolved and particulate Fe is not obvious there and that export may occur for both phases. At depths of around 1000–1300 m, both regeneration and Meddies may explain the observed maximum. Our data suggest that, in deep waters, higher particle concentrations likely due to dust storms may increase the scavenging flux and thus decrease DFe concentrations in deep waters.

Dissolved Fe and ligand concentrations and the Fe-binding strength of the organic ligands were measured in samples from the upper water column (150 m) of the oligotrophic waters of the Canary Basin (eastern North Atlantic Ocean). Concentrations of major nutrients, phytoplankton abundance and photosynthetic characteristics were also measured in the same samples.The concentrations of dissolved Fe and dissolved organic ligands were low with mean values of 0.31 +/- 0.18 nM Fe and 1.79 +/- 0.73 nEq of M Fe(n =47), respectively. The conditional binding constant varied between 10(19.8)-10(22.7) (n = 47). The largest variation with depth in the ligand concentrations (between 4.78 and 1.1 nEq of M Fe) was observed in the upper layer, above the Deep Chlorophyll Maximum (DCM located between 80 and 100 in), with high surface values in stations at 18 and 34.At the DCM where Fe was depleted, the ligand concentrations were still relatively high showing the same trend with depth as the amount of phytoplankton cells. Here 62% of the vertical variation in ligand concentrations can be explained by parameters describing phytoplankton cell abundance or biomass and orthosilicic acid concentration, which could reflect diatom growth. Ligand concentrations below the maximum of the DCM (n=4) showed good linear positive relationships with the total phytoplankton biomass as well as with 2 out of 4 distinguished groups of phytoplankton (Synechococcus and pico-eukaryote I).In the maximum of the DCM and below this maximum the phytoplankton origin of the dissolved organic ligands of Fe is very probable. Data suggest a release of ligands by cell lysis and not by an active production. However, the origin in the surface layer is more difficult to explain. Although the amount of phytoplankton cells in the surface layer is reduced, it is still similar to 25% of the cell concentration observed in the DCM. High concentrations of organic ligands could then be a remnant of past blooms or present production under nutrient depleted conditions. Input of Sahara dust can be another source of ligands. (c) 2006 Elsevier B.V. All rights reserved.

Measurements of Fe(II) and H2O2 were carried out in the Atlantic sector of the Southern Ocean during EisenEx, an iron enrichment experiment. Iron was added on three separate occasions, approximately every 8 days, as a ferrous sulfate (FeSO4) solution. Vertical profiles of Fe(II) showed maxima consistent with the plume of the iron infusion. While H2O2 profiles revealed a corresponding minima showing the effect of oxidation of Fe(II) by H2O2, observations showed detectable Fe(II) concentrations existed for up to 8 days after an iron infusion. H2O2 concentrations increased at the depth of the chlorophyll maximum when iron concentrations returned to pre-infusion concentrations ( In this work, Fe(II) and dissolved iron were used as tracers themselves for subsequent iron infusions when no further SF6 was added. EisenEx was subject to periods of weak and strong mixing. Slow mixing after the second infusion allowed significant concentrations of Fe(II) and Fe to exist for several days. During this time, dissolved and total iron in the infusion plume behaved almost conservatively as it was trapped between a relict mixed layer and a new rain-induced mixed layer. Using dissolved iron, a value for the vertical diffusion coefficient Kz = 6.7±0.7 cm2 s−1 was obtained for this 2-day period. During a subsequent surface survey of the iron-enriched patch, elevated levels of Fe(II) were found in surface waters presumably from Fe(II) dissolved in the rainwater that was falling at this time. Model results suggest that the reaction between uncomplexed Fe(III) and O2− was a significant source of Fe(II) during EisenEx and helped to maintain high levels of Fe(II) in the water column. This phenomenon may occur in iron enrichment experiments when two conditions are met: (i) When Fe is added to a system already saturated with regard to organic complexation and (ii) when mixing processes are slow, thereby reducing the dispersion of iron into under-saturated waters.

An in situ iron enrichment experiment was carried out in the Southern Ocean Polar Frontal Zone and fertilized a patch of water within an eddy of the Antarctic Circumpolar Current (EisenEx, Nov. 2000). During the experiment, a physical speciation technique was used for iron analysis in order to understand the changes in iron distribution and size-fractionations, including soluble Fe ( 0.2 μm), throughout the development of the phytoplankton bloom. Prior to the first infusion of iron, dissolved (

International audience; Diatoms are a major component of phytoplankton community. They tend to dominate under natural high-nutrient concentrations, as well as during artificial Fe fertilisation experiments. They are main players in the biogeochemical cycle of carbon (C), as they can account for 40% of the total primary production in the Ocean and dominate export production, as well as in the biogeochemical cycles of the other macro-nutrients, nitrogen (N), phosphorus (P), and silicon (Si). Another important nutrient is Fe, which was shown to have a direct or indirect effect on nearly all the biogeochemical parameters of diatoms. In the present paper, an inventory is made of the growth, physiology and fate of many diatom species, including maximum growth rate, photosynthetic parameters (maximum specific rate of photosynthesis, photosynthetic efficiency and light adaptation parameter), nutrient limitation (half-saturation constant for growth/uptake), cellular elemental ratios, and loss terms (sinking rates, autolysis rates and grazing rates). This is a first step for improvement of the parameterisation of physiologically based phytoplankton growth and global 3D carbon models. This review is a synthesis of a large number of published laboratory experiments using monospecific cultures as well as field data. Our compilation confirms that size is an important factor explaining variations of biogeochemical parameters of diatoms (e.g. maximum growth rate, photosynthesis parameters, half-saturation constants, sinking rate, and grazing). Some variations of elemental ratios can be explained by adaptation of intracellular requirements or storage of Fe, and P, for instance. The important loss processes of diatoms pointed out by this synthesis are (i) sinking, as single cells as well as through aggregation which generally greatly increases sinking rate, (ii) cell autolysis, which can significantly reduce net growth rates, especially under nutrient limitation when gross growth rates are low, and (iii) grazing by both meso- and micro-zooplankton. This review also defines gaps concerning our knowledge on some important points. For example, we need to better know which iron species is available for phytoplankton, as well as the impact of Fe on the variation of the elemental ratios, especially in terms of assimilation and regeneration of C and N. A better quantification of prey selection by microzooplankton and mesozooplankton in natural environments is also needed, including preference for the various phytoplankton and zooplankton species as well as for aggregates and faecal pellets.

Picophytoplankton are a small or major component of the phytoplankton community and present in all oceanic systems, from pole to pole. They dominate in the low chlorophyll biomass areas, such as the (sub)tropical regions, but also contribute considerably (up to 20%) in the high chlorophyll biomass areas. The ecosystems of occurrence contrast significantly in physical and chemical settings. This includes a strongly mixed upper water column replete in nutrients as well as a strongly thermally stratified euphotic zone depleted in nutrients at the surface with a steep inverse light and nutrient gradient. These changes impose a strong impact on the composition of the picophytoplankton community but also on the biochemical and physiological properties of the species present. In particular, the pigmentation and cellular carbon, nitrogen and phosphorus quota and requirement will differ from a stratified compared to a well-mixed water column. As a result no characteristic values for the parameters required for this specific algal group in a global phytoplankton carbon model (the SWAMCO model,Lancelot et al. (2000), Deep-Sea Res. I, 47, 1621) can be given. In the present paper an inventory is made of the biochemical, physiological and photosynthetic parameters of two species of cyanobacteria (Prochlorococcus and Synechococcus) and the pico-size class fraction of the eukaryote phytoplankton component. Other groups of phytoplankton, such as diatoms, Trichodesmium, Phaeocystis and coccolithophorids, will be discussed in separate papers in this issue. This inventory is a mixture of laboratory experiments using well-defined algal populations as well as data derived from field surveys including a mixture of species. Where possible, the relevance of the parameters will be discussed in relation to the nature of the physico-chemical conditions of the area of occurrence.

The remote waters of the Pacific region of the Southern Ocean are the furthest away from any upstream and upwind continental Fe sources. This prime area for expecting Fe limitation of the plankton ecosystem was studied (March-April 1995) along a north-south transect at similar to 89 degrees W. At the end of the austral summer the upper wind-mixed layers were in the order of similar to 100 m deep, thus mixing the algae down into the dimly lit part of the euphotic zone where photosynthesis is severely restricted. The dissolved Fe was found at low concentrations ranging from 0.05 nM near the surface to 0.5 nM in deeper waters. Along the transect (52 degrees S-69 degrees S), the dissolved iron was enhanced in the Polar Front, as well as near the Antarctic continental margin (0.6-1.0 nM). In between, the southern ACC branch was depleted with iron; here the concentrations in surface waters were quite uniform at about 0.21 nM. This is only somewhat lower than the 0.49 nM (October 1992) and 0.31 nM (November 1992) averages in early spring in the southern ACC part of Atlantic 6 degrees W sections [de Baar, H.J.W., de Jong, J.T.M., Bakker, D.C.E.. Loscher, B.M., Veth, C., Bathmann, U., Smetacek, V., 1995. Importance of iron for phytoplankton spring blooms and CO2 drawdown in the Southern Ocean. Nature 373, 412-415; Loscher, B.M., de Jong, J.T.M., de Baar, H.J.W., Veth, C., Dehairs, F., 1997. The distribution of iron in the Antarctic Circumpolar Current. Deep-Sea Research II 44, 143-188.]. First, the lower similar to 0.21 nM in March-April 1995 may partly be due to continuation of the seasonal trend where the phytoplankton growth, albeit modest, was removing Fe from the surface waters. Secondly, the 89 degrees W Pacific stations are further downstream continental or seafloor sources than the Atlantic 6 degrees W section. In the latter case, the ACC water had passed through the Drake Passage and also over the Sandwich Plateau. Indeed for Drake Passage, intermediate Fe concentrations have been reported by others. The generally somewhat lower surface water Fe at the ACC and PF at 89 degrees W is consistent with the distance from sources and the late summer. It also would explain the very low abundance of phytoplankton (Chl a) in the region and the conspicuous absence of plankton blooms. In the subAntarctic waters north of the Polar Front there are no diatoms, let alone diatom blooms, due to low availability of silicate. Thus, it appears the biological productivity is suppressed due to iron deficiency in combination with the severe seasonal effects of wind mixing on the light climate, as well as regional silicate limitation for diatoms. (C) 1999 Elsevier Science B.V. All rights reserved.

Concentrations of dissolved Ni, Cu, Zn, Cd and Pb were measured in water samples collected during a cruise with R.V Pelagia (29-6/14-7-1993) in the northern North Sea and N.E. Atlantic Ocean. At least six depths (0–90 m) were sampled with modified Go-Flo samplers from a rubber zodiac. In the study area, the first 25 m were well mixed and stratification occurred below this depth. The local bloom of Emiliania huxleyi hardly affected the trace metals concentration, except for some removal of Cd as seen from its correlation with nitrate. The mean dissolved concentrations were for Ni (3.66 nM), Cu (1.61 nM), Zn (4.5 nM), Cd (48 pM) and Pb (108 pM). These concentrations are among the lowest reported for the North Sea and are of similar magnitude to those found in the eastern North Atlantic at the same latitude. Zn was the only exception with values 10 times higher compared to those in the Atlantic Ocean, suggesting external inputs, mainly atmospheric and possibly from surrounding land masses. The observed ratio Zn:Cd in the North Sea and estuaries is in between the high ratio 600–900 for continental sources and the low ratio 5–10 for oceanic waters. Latter low ratio is consistent with the 21-fold stronger inorganic complexation of Cd in seawater which, in combination with the preferential biological uptake of Zn, may lead to the observed about hundredfold fractionation of Zn versus Cd in the marine system. Other processes may play a role but would need further investigation. The dissolved Pb values tend to be lower than found before in the North Sea, indicating decreasing inventories due to reduced anthropogenic emissions.

Variations in the speciation of iron in the northern North Sea were investigated in an area covering at least two different water masses and an algal bloom, using a combination of techniques. Catalytic cathodic stripping voltammetry was used to measure the concentrations of reactive iron (FeR) and total iron (FeT) in unfiltered samples, while dissolved iron (FeD) was measured by GFAAS after extraction of filtered sea water. FeR was defined by the amount of iron that complexed with 20 μM 1-nitroso-2-napthol (NN) at pH 6.9. FeT was determined after UV-digestion at pH 2.4. Concentrations of natural organic iron complexing ligands and values for conditional stability constants, were determined in unfiltered samples by titration. Mean concentrations of 1.3 nM for FeR, 10.0 nM for FeT and 1.7 nM for FeD were obtained for the area sampled. FeR concentrations increased towards the south of the area investigated, as a result of the increased influence of continental run off. FeR concentrations were found to be enhanced below the nutricline (below ∼40 m) as a result of the remineralisation of organic material. Enhanced levels of FeT were observed in some surface samples and in samples collected below 30 m at stations in the south of the area studied, thought to be a result of high concentrations of biogenic particulate material and the resuspended sediments respectively. FeD concentrations varied between values similar to those of FeT in samples from the north of the area to values similar to those of FeR in the south. The bloom was thought to have influenced the distribution of both FeR and FeT, but less evidence was observed for any influence on FeR and FeD. The concentration of organic complexing ligands, which could possibly include a contribution from adsorption sites on particulate material, increased slightly in the bloom area and in North Sea waters. Iron was found to be fully (99.9%) complexed by the organic complexing ligands at a pH of 6.9 and largely complexed (82–96%) at pH 8. The ligands were almost saturated with iron suggesting that the ligand concentration could limit the concentration of iron occurring as dissolved species.

Short-term iron enrichment experiments were carried out with samples collected in areas with different phytoplankton activity in the northern North Sea and northeast Atlantic Ocean in the summer of 1993. The research area was dominated by high numbers of pico-phytoplankton, up to 70,000 ml−1. Maximum chlorophyll a concentrations varied from about 1.0 μg l−1 in a high-reflectance zone (caused by loose coccoliths, remnants from a bloom of Emiliania huxleyi) and about 3.5 μg l−1 in a zone in which the phytoplankton were growing, to about 0.5 μg l−1 in the northeast Atlantic Ocean. From the high-reflectance zone to the northeast Atlantic Ocean, nitrate concentrations increased from 0.5 μM to 6.0 μM. Concentrations of reactive iron in surface water showed an opposite trend and decreased from about 2.6 nM in the high-reflectance zone to

Fe speciation was measured with competitive ligand equilibration adsorptive cathodic stripping voltammetry [Gledhill, M., Van den Berg, C.M.G., 1994. Determination of complexation of iron (III) with natural organic complexing ligands in sea water using cathodic stripping voltammetry. Mar. Chem., 47, 41–54.] in the Pacific part of the Southern Ocean between 58° and 68°30′S along the 90°W meridian. The conditional stability constant (K′ with respect to [Fe3+]) was between 1020.6 and 1021.6 when one organic ligand was detected. The ligand concentration ([Lt]) varied between 2.2 and 12.3 equivalents of nM Fe (nEq of Fe). The ligand concentration was at least 6 times, and generally more than 10 times, that of the total dissolvable Fe concentration. At one station a depth profile was sampled where below 200 m depth, two organic ligands were measured with K1′=1021 and K2′=1022.4. Organic complexation of Fe was similar to results found elsewhere [(Gledhill, M., Van den Berg, C.M.G., 1994. Determination of complexation of iron (III) with natural organic complexing ligands in sea water using cathodic stripping voltammetry. Mar. Chem., 47, 41–54.); (Van den Berg, C.M.G., 1995. Evidence for organic complexation in seawater. Mar. Chem., 50, 139–159.); (Rue, E.L., Bruland, K.W., 1995. Complexation of iron (III) by natural organic ligands in the Central North Pacific as determined by a new competitive ligand equilibration/adsorptive cathodic stripping voltammetric method. Mar. Chem., 50, 117–138.); (Rue, E.L., Bruland, K.W., 1997. The role of organic complexation on ambient iron chemistry in the equatorial Pacific Ocean and the response of a mesoscale iron addition experiment. Limnol. Oceanogr., 42, 901–910.)] judging from the overall organic alpha value (K′*[Lt]) 1012.4–1013.9. The lower values of organic alpha were within one order of magnitude of our choice of the inorganic alpha (1011.9, [Millero, F.J., Yao, W., Aicher, J., 1995. The speciation of Fe (II) and Fe (III) in natural waters. Mar. Chem., 50, 21–39.]) in which case the organic and inorganic ligands could compete effectively for Fe. Different values of organic alpha and the occurrence of two organic ligand classes were consistent with differences in hydrography. South of the Polar Front, the least organic complexation occurred (organic alpha=1012.4, organic complexation around 80%), where the highest chlorophyll a concentrations were measured.

International audience; The impact of copepod grazing on Fe regeneration was investigated in a naturally iron fertilised area during KEOPS (Kerguelen Ocean and Plateau compared Study, Jan.-Feb. 2005). 55Fe labelled natural plankton assemblages (< 200 μm) were offered as food to copepod predators sampled in the field (Calanus propinquus, Rhincalanus gigas, Metridia lucens and Oithona frigida). Diatoms (Eucampia antarctica, Corethron inerme and Navicula spp.) constituted the bulk of the protists whereas microzooplankton (i.e. ciliates and dinoflagellates) were in very low abundance. Copepod grazing on phytoplankton ranged from 0.3 to 2.6 µgC ind-1 d-1 and reflected low utilisation of the food stocks (1-10% of total Chlorophyll a d-1) and low daily rations (0.2-3.3 % body C d-1). Copepod grazing resulted in a 1.7-2.3-fold increase in Fe regeneration. Fe speciation determined by extraction onto C18 columns showed that less than 1% of the regenerated Fe was complexed with hydrophobic organic ligands. This suggests that Fe was regenerated as inorganic species and/or bound to freely soluble organic ligands. The biogenic Fe budget established from our study and literature based data indicates that most of the primary production is recycled through the detrital pool, which represents the largest Fe pool (49% of total Fe). Our iron budget further indicates that mesozooplankton and diatoms represent the dominant Fe biomasses above the Kerguelen plateau. The rate of Fe regeneration accounts for half of the Fe demand, strengthening the need for new Fe sources to sustain the massive phytoplankton bloom above the Kerguelen plateau.

Viral dynamics, community structure, and the impact of viruses on phytoplankton mortality in comparison with microzooplankton grazing were determined in the natural iron-fertilized waters southeast of the Kerguelen Islands, Southern Ocean, during the austral summer (January-February 2005). The study area was characterized by a phytoplankton bloom above the Kerguelen Plateau and the high-nutrient low-chlorophyll waters surrounding it. During the Kerguelen Ocean and Plateau compared Study (KEOPS), viral abundance was relatively high (1-19 x 10(7) mL(-1)) as compared to the few other studies in the Southern Ocean, significantly correlating with depth and system productivity. Viral abundance showed a strong positive relationship with the numerically dominant bacterial hosts, which in turn were correlated to phytoplankton biomass. In total, 13 different viral genome sizes were detected, with the lower-sized genomes 34 and 68 kb dominating at all stations. The viral community at the low chlorophyll C-transect grouped apart from the more productive transects A and B. Potential algal viruses were recorded for all stations, but only at very low intensities. Virally induced lysis of the smaller-sized (< 10 mu m) phytoplankton was a minor loss factor as compared to microzooplankton grazing (up to 6% and 45% of total < 30 mu m algal standing stock per day, respectively). Grazing was phytoplankton population-specific, but was in all cases able to keep the standing stock of the small-sized phytoplankton low (net growth rates between -0.2 and 0.2 d(-1)). Microzooplankton regenerated on average 1.1 pM Fed(-1) (present study), which represented approximately 30% of the total regeneration rate and at least 15% of the total biogenic Fe demand as calculated by [Sarthou, G., Vincent, D., Christaki, U., Obernosterer, L, Timmermans, K.R., Brussaard, C.P.D., 2008. The fate of biogenic iron during a phytoplankton bloom induced by natural fertilization: impact of copepod grazing. Deep-Sea Research II]. (C) 2008 Elsevier Ltd. All rights reserved.

International audience; Dissolved Fe, Mn and Al concentrations (dFe, dMn and dAl hereafter) in surface waters and the water column of the Northeast Atlantic and the European continental shelf are reported. Following an episode of enhanced Saharan dust inputs over the Northeast Atlantic Ocean prior and during the cruise in March 1998, surface concentrations were enhanced up to 4 nmol L− 1 dFe, 3 nmol L− 1 dMn and 40 nmol L− 1 dAl and returned to 0.6 nmol L− 1 dFe, 0.5 nmol L− 1 dMn and 10 nmol L− 1 dAl towards the end of the cruise three weeks later. A simple steady state model (MADCOW, [Measures, C.I., Brown, E.T., 1996. Estimating dust input to the Atlantic Ocean using surface water aluminium concentrations. In: Guerzoni. S. and Chester. R. (Eds.), The impact of desert dust across the Mediterranean, Kluwer Academic Publishers, The Netherlands, pp. 301-311.]) was used which relies on surface ocean dAl as a proxy for atmospheric deposition of mineral dust. We estimated dust input at 1.8 g m− 2 yr− 1 (range 1.0-2.9 g m− 2 yr− 1) and fluxes of dFe, dMn and dAl were inferred. Mixed layer steady state residence times for dissolved metals were estimated at 1.3 yr for dFe (range 0.3-2.9 yr) and 1.9 yr for dMn (range 1.0-3.8 yr). The dFe residence time may have been overestimated and it is shown that 0.2-0.4 yr is probably more realistic. Using vertical dFe versus Apparent Oxygen Utilization (AOU) relationships as well as a biogeochemical two end member mixing model, regenerative Fe:C ratios were estimated respectively to be 20 ± 6 and 22 ± 5 μmol Fe:mol C. Combining the atmospheric flux of dFe to the upper water column with the latter Fe:C ratio, a 'new iron' supported primary productivity of only 15% (range 7%-56%) was deduced. This would imply that 85% (range 44-93%) of primary productivity could be supported by regenerated dFe. The open ocean surface data suggest that the continental shelf is probably not a major source of dissolved metals to the surface of the adjacent open ocean. Continental shelf concentrations of dMn, dFe, and to a lesser extent dAl, were well correlated with salinity and express mixing of a fresher continental end member with Atlantic Ocean water flowing onto the shelf. This means probably that diffusive benthic fluxes did not play a major role at the time of the cruise.

Towards more rapid ultraclean sampling of deep ocean waters for trace elements, a novel rectangular frame was constructed of titanium, holding two rows of 12 samplers, as well as various sensors. The frame is deployed to deep ocean waters by an 8000 m length Kevlar wire with internal power and signal cables. Closing of each sampler is by seawater hydraulics via silicone tubings connecting each sampler with a central 24 position Multivalve. Upon recovery the complete frame with 24 samplers is placed inside an ultraclean laboratory van, where water is drawn via filters into bottles. Previously the clean sampling of ocean waters has been very time-consuming by attachment of individual ultraclean bottle samplers one by one to a metal-free (e.g. all-Kevlar) hydrowire. The novel Titan system is 3-4 times faster and permits routine collection of deep ocean sections while economizing required shiptime. In a test of the new system in November 2005 in the Canary Basin excellent low dissolved Fe concentrations (similar to 0.1to similar to 0.4nM) are consistent with values obtained of individual samplers on a simple wire, and previous values in a pilot study of 2002 in the same basin, as well as published dissolved Fe values elsewhere in the northeast Atlantic Ocean. (C) 2007 Elsevier B.V. All rights reserved.

The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial-interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization--an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below--as invoked in some palaeoclimatic and future climate change scenarios--may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

This study shows that ultraviolet B (UV-B: 280-315 nm) and UV-A (315-400 nm) have no significant influence on the photodegradation of organic Fe(III)-binding ligands in estuarine waters from Marsdiep and Scheldt (The Netherlands). High salinity estuarine seawater from the Marsdiep and Scheldt contains concentrations of organic Fe(Ill)-binding ligands as high as 24.4 equivalent of nM Fe (eq nM Fe) and similar to 4.6 eq nM Fe, respectively, with conditional stability constants (K) of 10(21.0) and 10(20.1), respectively. Investigation of the relation between the organically bound iron fraction and the photoproduction of Fe(II) in the estuarine Marsdiep and Scheldt water shows that the concentration of Fe(II) produced was very low (

The distribution of dissolved iron and its chemical speciation (organic complexation and redox speciation) were studied in the northeastern Atlantic Ocean along 23°W between 37 and 42°N at depths between 0 and 2000 m, and in the upper-water column (upper 200 m) at two stations further east at 45°N10°W and 40°N17°W in the early spring of 1998. The iron speciation data are here combined with phytoplankton data to suggest cyanobacteria as a possible source for the iron binding ligands. The organic Fe-binding ligand concentrations were greater than that of dissolved iron by a factor of 1.5–5, thus maintaining iron in solution at levels well above it solubility. The water column distribution of the organic ligand indicates in-situ production of organic ligands by the plankton (consisting mainly of the cyanobacteria Synechococcus sp.) in the euphotic layer and a remineralisation from sinking biogenic particles in deeper waters. Fe(II) concentrations varied from below the detection limit ( Synechococcus growth.

An iron enrichment experiment, EisenEx, was performed in the Atlantic sector of the Southern Ocean during the Antarctic spring of 2000. Deck incubations of open ocean water were performed to investigate the influence of ultraviolet B (UVB: 280-315 nm) and ultraviolet A (UVA: 315-400 nm) on the speciation of iron in seawater, using an addition of the radioisotopes Fe-59(III) (1.25 nM) or Fe-55(III) (0.5 nM). Seawater was sampled inside and outside the iron-enriched region. The radioisotopic Fe(II) concentration was monitored during daylight under three different light conditions: the full solar spectrum (total), total minus UVB, and total minus UVB+UVA. A distinct diel cycle was observed with a clear distinction between the three different light regimes. A clear linear relationship was found for the concentration of radioisotopic Fe(II) versus irradiance. UVB produced most of the Fe(II) followed by UVA and visible light (VIS: 400-700 nm), respectively. UVB produced 4.89 and 0.69 pM m(2) W-1 radioisotopic Fe(II) followed by UVA with 0.33 and 0.10 pM M-2 W-1 radioisotopic Fe(II) and VIS with 0.04 and 0.03 pM m(2) W-1 radioisotopic Fe(II). (C) 2004 Elsevier B.V All rights reserved.

[1] Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/ Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate ( 10 - 30 µm), medium ( 30 - 60 µm), and large (> 60 µm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of "dissolved'' Fe ( filtrate < 0.2 µm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the "dissolved'' pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe similar to 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

Experiments were conducted with three species of marine pico-phytoplankton: Synechococcus sp. (CCMP 839), Pelagomonas calceolata (CCMP 1756) and Prasinomonas capsulatus (CCMP 1617) in order to collect physiological parameters for pico-phytoplankton to be utilised in Ocean Biogeochemical Climate Models. The main parameters to follow the effects of ammonium, phosphate, iron and light limitation were cell growth rates (μ), half saturation constants for growth (K m ), N, P and Fe quota (per cell or per mol C), and photochemical quantum efficiency (F v /F m ). The nitrate and phosphate limitation experiments demonstrated that the small phytoplankton species could grow at low N and P concentrations. K m values were in the micro-molar (NH 4 + ) and sub-micro-molar (PO 4 3− ) range. N and P quota were in the femto-molar range per cell and varied from nutrient-deplete to nutrient-replete conditions. F v /F m values were only adversely affected at the lowest N and P concentrations in these experiments. In the Fe limitation experiments, it was shown that all three species were adversely affected only at extremely low Fe concentrations. Iron chelating agents had to be added to force the species in Fe limitation till ultimately growth stopped. K m values with respect to dissolved Fe were in the femto-molar range. Fe quota were in the low zepto-molar (10 −21 M) range per cell, and varied considerably from Fe limiting to Fe replete growth conditions. F v /F m values diminished only at the lowest iron concentrations. In the light limitation experiments, growth rates and photochemical quantum efficiencies were adversely affected only at irradiance levels below 10 μmol photons m −2 s −1 . These results indicate that the pico-phytoplankton species will hardly ever be completely stopped in their growth by NH 4 + , PO 4 3− , Fe or light (separately) under natural conditions.

[ 1] A spectral weighting function describing the wavelength dependency of the photoproduction of Fe( II) in Antarctic seawater was established. The strong wavelength-dependent photoproduction of Fe( II) from amorphous ferric hydroxides can be described as an exponential function: epsilon(lambda) = 3.57 . 10(3) . e (-0.02)((lambda-300)). Solar spectra recorded during the 2000 Antarctic ozone depletion season were used to demonstrate that daily and seasonal variability of the ultraviolet A ( UVA: 315-400 nm) and the visible part of the light spectrum ( VIS: 400-700 nm) dominates Fe( II) production rates in surface waters ( respectively > 60% and about 30%) and in the water column. Although ultraviolet B ( UVB: 280-315 nm) is the most effective wavelength region for Fe( II) photoproduction, the impact of UVB was small due to the relatively low flux of UVB into the ocean surface waters. However, the impact of UVB did indeed increase significantly from 3.54 to 6.15 % during the austral ozone minimum.

International audience; Concentrations of dissolved Al (0.2 Am filtered) have been determined in the Canary basin and on a transect towards the Strait of Gibraltar, in order to allow sampling across the Saharan dust plume. The highest surface water concentrations of up to 25 nM were observed in the northwest part of the studied region, indicating substantial atmospheric input from the Sahara desert. This observation was supported by relatively pronounced Al surface maxima of the shallow casts performed in this region. The positive covariance between dissolved Al and dissolved Fe concentrations on the transect from the Canary Islands towards the Strait of Gibraltar suggests a common atmospheric source of the two metals. Outside the Canary basin, the Al concentrations rapidly dropped to values below 10 nM, coherent with lower aeolian inputs. A good correlation has been found between dissolved Al surface seawater concentrations and dissolved orthosilicic acid (Si(OH) 4), which can most likely be attributed to conservative mixing of warm, more saline Al-and Si-rich water masses with colder waters of lower salinity which are more depleted in both elements. In spite of the oligotrophic waters that were met most of the cruise, a strong minimum in dissolved Al was observed at the chlorophyll a maximum at a few stations, suggesting rapid Al scavenging or biological uptake in this region. D

Hydrogen peroxide (H2O2) was measured in marine surface waters of the eastern Atlantic Ocean between 25degreesN and 25degreesS. H2O2 concentrations decreased from 80 nM in the north to 20 nM in the south, in agreement with earlier observations. A diel cycle of H2O2 production as a function of sunlight in surface waters was followed twice whilst the ship steamed southward. Around 23degreesN a distinct diel cycle could be measured which correlated well with irradiance conditions.The wavelength dependency of H2O2 formation was studied near the equator. For 16 hours, water samples were incubated with wavelength hands of the solar spectrum, i.e. visible (VIS: 400-700 nm), VIS and ultraviolet A radiation (UVAR: 320-400 nm) and VIS, UVAR and ultraviolet B radiation (UVBR: 280-320 rim). A significant relationship was found between wavelength band and the production of H2O2. In addition, a clear positive relationship between intensity and production was found. UVAR was 6.5 times more efficient than VIS in producing 1 nM of H2O2, whereas UVBR was 228 times more efficient than VIS. When these data were weighted with respect to the energy of the solar spectrum at zenith hour, 28% of the H2O2 was formed by VIS, 23% was formed by UVAR and 48% was formed by UVBR. Considering the strong attenuation of UVBR in marine waters as compared with UVAR and VIS radiation, the role of UVAR deeper in the water column is recognised. Furthermore results of this research emphasise the importance of VIS radiation in the formation of H2O2. (C) 2003 Elsevier B.V. All rights reserved.

Four large, open-ocean diatoms from the Southern Ocean (Actinocyclus sp., Thalassiosira sp., Fragilariopsis kerguelensis, and Corethron pennatum) were grown in natural (low iron) Southern ocean seawater with increasing Fe concentrations. With increasing dissolved iron (Fe diss) concentrations, the growth rates increased three- to sixfold. The species with the smallest cells had the highest growth rates. The half-saturation constants ( Km) for growth were low (0.19‐1.14 nmol L 21 Fediss), and close to the ambient Fediss concentrations of 0.2 nmol L 21 . The range in Km with respect to Fediss also varied with the size of the diatoms: the smallest species had the lowest Km and the largest species had the highest Km .A s Fe diss concentrations decreased, silicate consumption per cell increased, but nitrate consumption per cell decreased. Phosphate consumption per cell varied without clear relation to the dissolved iron concentrations. The differences in nutrient consumption per cell resulted in marked differences in elemental depletion ratios in relation to Fe diss concentrations, with the depletion ratios being most affected by iron limitation in the largest cells. These experimental findings are in agreement with previous laboratory and field studies, showing the relatively high requirements of large diatoms for Fe. The size-dependent response of the diatoms with respect to nutrient depletion is a good illustration of the effects of Fe on silicate, nitrate, and phosphate metabolism. While the understanding of the temporal and spatial patterns in the environmental control of phytoplankton in the Southern Ocean has increased over the last years (Boyd 2002), species-specific studies on key species of open Southern Ocean phytoplankton are scarce. An understanding of the physiological response of large Southern Ocean diatoms at the species level to environmental stress factors such as iron and light limitation and the concomitant effects on major nutrient uptake is virtually absent. At the community level, more information is available for Southern Ocean diatoms (Boyd et al. 2000; Blain et al. 2002; Coale et al. 2003), but the relevance of maximum growth rates ( mmax) and half-saturation constants (Km for growth or Ks for nutrient uptake) for assemblages of species or even genera is limited. Strictly speaking, mmax and Km or Ks are only applicable to single species and are unique physiological characteristics of a species, not a community. Even small changes in species composition or changes in dominance of

In analogy to the Fe hypothesis, the Zn hypothesis states that Zn may limit primary production in some regions of the world oceans and therefore influence the global carbon cycle. The proposed mechanism is via carbon limitation due to a lack of the cofactor Zn in carbonic anhydrase. In the current conceptual model for the use of inorganic carbon by E huxleyi, carbonic anhydrase in the chloroplast generates CO2 from HCO3- at the site where CO2 is fixed by ribulose bisphosphate carboxylase oxygenase (Rubisco). The H+ that is required in this reaction comes from calcification. From this it can be expected that carbonic anhydrase affects the use of HCO3- in photosynthesis. First, we grew E. huxleyi under ZN2+ limitation. The K-1/2 for growth of E. huxleyi is 19 +/- 8 pmol L-1 ZN2+ with a minimum requirement of 9 +/- 3 pmol L-1. Additions of both ethylenediaminetetraacetic acid (EDTA) and ZnCl2 show that EDTA is not detrimental to E. huxleyi up to a concentration of 200 mumol L-1. Then we grew E. huxleyi under ZN2+-HCO3- colimitation to test the conceptual model outlined above. The results were partly inconsistent with the model. Contrary to what was expected from the conceptual model, the efficiency of CO2 use decreased when both ZN2+ and HCO3- concentrations were low, even though the experiment was conducted at a constant high concentration of CO2. This shows that ZN2+, and possibly carbonic anhydrase activity, are needed for CO2 fixation also. In accordance with the model, we found that ZN2+ affects the efficiency of HCO3- use by E huxleyi. Since the lowest ZN2+ concentration in the Northeast Pacific is similar to0.4 pmol L-1, Zn limitation of E. huxleyi growth may indeed occur.

Blooms of large diatoms dominate the CO2 drawdown and silicon cycle of the Southern Ocean in both the past and present. The growth of these Antarctic diatoms is limited by availability of iron (and light). Here we report the first assessment of growth rates in relation to iron availability of two truly oceanic Antarctic diatom species, the large, chain-forming diatom Chaetoceros dichaeta and the small, unicellular diatom C. brevis. In filtered natural, untreated Southern Ocean water, a maximum specific growth rate of 0.626 0.09 d 21 and a Km for growth of 1.12 3 10 29 M dissolved iron was calculated for C. dichaeta. This response could only be seen during a long-day light period. C. brevis maintained growth rates of 0.39 6 0.09 d 21 with and without iron addition, even under short-day light conditions, and could only be forced into iron limitation by adding the siderophore desferri-ferrioxamine B (DFB), an iron immobilizing agent. Using this approach, the low Km value for growth of 0.59 3 10 212 M dissolved Fe was calculated for this species. The size-class dependent growth response to iron (and light) confirms the key role of these parameters in structuring Southern Ocean ecosystems and thus the CO2 dynamics and the silicon cycle.

Dissolved Zn concentrations were determined in surface water samples collected on-line along transects in the eastern North Atlantic in spring (March 1998). Two frontal zones could be identified in the research area by a change in salinity, temperature and nutrient concentrations. One zone was identified at 42 degrees N, separating the North Atlantic central water (NACW) and the Atlantic surface water (ASW) from each other, and another one crossing the continental slope at 12 degrees and 8 degrees E, respectively. Variability in Zn concentrations was observed near these zones, not only as a result of a change of water mass, but also due to external Zn sources. Surface Zn concentrations were 0.5-1 nM and 2 nM in the NACW and ASW, respectively, increasing to 4 nM over the continental shelf and finally 5-6 nM in the English Channel. Contributions of Zn derived from shelf sediments appear to be the major source for the enriched surface values in the continental zone.

The solubility of iron in oxic waters is so low that iron can be a limiting nutrient for phytoplankton growth in the open ocean. In order to mimic low iron concentrations in algal cultures, Ethylenediaminetetraacetate (EDTA) is commonly used. The presence of EDTA enables culture experiments to be performed at a low free metal concentration, while the total metal concentrations are high. Using EDTA provides for a more reproducible medium. In this study Fe speciation, as defined by EDTA in culture media, is compared with complexation by natural organic complexes in ocean water where Fe is thought to be limited. To grow oceanic species into iron limitation, a concentration of at least 10−4 M EDTA is necessary. Only then does the calculated [Fe3+] concentrations resemble those found in natural sea water, where the speciation is governed by natural dissolved organic ligands at nanomolar concentrations. Moreover, EDTA influences the redox speciation of iron, and thus frustrates research on the preferred source of Fe-uptake, Fe(III) or Fe(II), by algae. Nowadays, one can measure the extent of natural organic complexation in sea water, as well as the dissolved Fe(II) state, and can use ultra clean techniques in order to prevent contamination. Therefore, it is advisable to work with more natural conditions and not use EDTA to create iron limitation. This is especially important when the biological availability of the different chemical fractions of iron are the subject of research. Typically, many oceanic algae in the smallest size classes can still grow at very low ambient Fe and are not easily cultivated into limitation under ambient sea water conditions. However, the important class of large oceanic algae responsible for the major blooms and the large scale cycling of carbon, silicon and other elements, commonly has a high Fe requirement and can be grown into Fe limitation in ambient seawater.

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.