Search

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.

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.

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 (

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.