Beschreibung: Atmospheric iron and underway sea-surface dissolved (〈0.2 μm) iron (DFe) concentrations were investigated along a north-south transect in the eastern Atlantic Ocean (27°N/16°W-19°S/5°E). Fe concentrations in aerosols and dry deposition fluxes of soluble Fe were at least two orders of magnitude higher in the Saharan dust plume than at the equator or at the extreme south of the transect. A weaker source of atmospheric Fe was also observed in the South Atlantic, possibly originating in southern Africa via the north-easterly outflow of the Angolan plume. Estimations of total atmospheric deposition fluxes (dry plus wet) of soluble Fe suggested that wet deposition dominated in the intertropical convergence zone, due to the very high amount of precipitation and to the fact that a substantial part of Fe was delivered in dissolved form. On the other hand, dry deposition dominated in the other regions of the transect (73-97), where rainfall rates were much lower. Underway sea-surface DFe concentrations ranged 0.02-1.1 nM. Such low values (0.02 nM) are reported for the first time in the Atlantic Ocean and may be (co)-limiting for primary production. A significant correlation (Spearman's rho = 0.862, p〈0.01) was observed between mean DFe concentrations and total atmospheric deposition fluxes, confirming the importance of atmospheric deposition on the iron cycle in the Atlantic. Residence time of DFe in the surface waters relative to atmospheric inputs were estimated in the northern part of our study area (17 ± 8 to 28 ± 16 d). These values confirmed the rapid removal of Fe from the surface waters, possibly by colloidal aggregation. © 2003 Elsevier Ltd. All rights reserved.

Beschreibung: A shipboard analytical intercomparison of dissolved (〈0.2 μm) iron in the surface waters of the Atlantic Ocean was undertaken during October 2000. A single underway surface (1-2 m) seawater sampling and filtration protocol was used, in order to minimise differences from possible sample contamination. Over 200 samples (1/h) were collected over 12 days and analysed immediately using four different analytical methods, based on three variants of flow injection with luminol chemiluminescence (FI-CL) and cathodic stripping voltammetry (CSV). Dissolved iron concentrations varied between 0.02 and 1.61 nM during the intercomparison. On average, CSV Electroanalysis 12 (2000) 565 measured 0.08 nM higher iron concentrations than one FI-CL method Anal. Chim. Acta 361 (1998) 189, which measured 0.13 nM higher iron values than the other two Anal. Chem. 65 (1993) 1524; Anal. Chim. Acta 377 (1998) 113, Statistical analyses (paired two-tailed t-test) showed that each analytical method gave significantly different dissolved iron concentrations at the 95% confidence interval. These data however, represent a significant improvement over earlier intercomparison exercises for iron. The data have been evaluated with respect to accuracy and overall inter-laboratory replicate precision, which was generally better than the 95% confidence intervals reported for the NASS Certified Reference Materials. Systematic differences between analytical methods were probably due to the extraction of different physico-chemical forms of iron during preconcentration, either on the micro-column resin (in the FI methods) or with competing ligand equilibration (in the CSV method). Small systematic concentration differences may also have resulted from protocols used for quantification of the analytical blank and instrument calibration. © 2003 Elsevier B.V All rights reserved.

Beschreibung: Blooms of large diatoms dominate the CO2 drawdown and silicon cycle ofthe Southern Ocean in both the past and present. The growth of theseAntarctic diatoms is limited by availability of iron (and light). Here wereport the first assessment of growth rates in relation to ironavailability of two truly oceanic Antarctic diatom species, the large,chain-forming diatom Chaetoceros dichaeta and the small, uni-cellulardiatom C. brevis. In filtered natural, untreated Southern Ocean water, amaximum specific growth rate of 0.62 ± 0.09 d-1 and a Km for growth of 1.12x 10-9 M dissolved iron was calculated for C. dichaeta. This response couldonly be seen during a long-day light period. C. brevis maintained growthrates of 0.39 ± 0.09 d-1 with and without iron addition, even under-shortday light conditions, and could only be forced into iron limitation byadding the siderophore desferri-ferrioxamine B (DFB), an iron immobilisingagent. Using this approach, the low Km value for growth of 0.59 x 10-12 Mdissolved Fe was calculated for this species. The size class dependentgrowth response to iron (and light) confirms the key role of theseparameters in structuring Southern Ocean ecosystems, and thus the CO2dynamics and the silicon cycle.