4.1. High Productivity and Low Salinity in the EWG
 Our results present strong evidence for the existence of substantial bioproductivity at the EWG/ACC boundary. The previous identification of such a region by inverse modeling [Usbeck et al., 2002], high biogenic silica fluxes to the underlying sediment [Geibert et al., 2005], and high whale abundances [Tynan, 1998] strongly suggests that this is a recurrent feature, though variable in location and extent, because the location of the boundary wanders longitudinally between years (indicated by patchiness around the 0.5°C isotherm in Figure 1).
 The salinity minimum in the EWG, which is too pronounced to be fed from any local water mass, has not only been observed in this season, but it has also been found in other studies, e.g., expedition Polarstern ANT XVI/3 [Boye et al., 2001], or the recent ANDREX study (D. C. E. Bakker, personal communication, 2010). We therefore conclude that excess melting ice is also a recurrent phenomenon in the EWG.
 Our results show that the productivity is linked to melting ice at the sea surface. The link between melting sea ice and phytoplankton blooms has been described previously [Savidge et al., 1996; Smith and Nelson, 1985; Sokolov, 2008]. However, it has been demonstrated that stratification due to sea ice alone is not sufficient to generate large phytoplankton blooms [Bathmann et al., 1997]. Consequently, we must not only consider the role of melting sea ice for increased stratification of surface waters, but also for the enhanced supply of limiting micronutrients, especially iron. We will therefore focus our discussion on two specific aspects of the observations. First, we explore mechanisms that might explain why the EWG/ACC boundary receives consistently more freshwater (from sea ice or icebergs) than other regions of the SO, and second, we investigate how this may relieve iron limitation.
4.2. Enhanced Supply of Ice to the EWG Boundary
 Freshwater supply to the EWG/ACC boundary is controlled by sea ice, icebergs and precipitation (mainly onto sea ice). Sea-ice transport in the WG generally follows wind-forcing, which results in a general pattern of eastward transport in the northern WG [Kimura, 2004; Uotila et al., 2000], see Figure 5b. For most parts of the WG, the atmospheric circulation patterns roughly coincide with the ocean circulation, which means that sea ice remains within the same water mass. The EWG/ACC boundary is an exception to this rule. Here, a longitudinal boundary in surface water masses is found at ∼25°E (Figure 1), across which sea ice drifts under wind-forcing (see Figure 5). This situation holds not only in spring, but also persists in winter, then slightly further northeastward, leading to enhanced sea-ice melting rates at the EWG/ACC boundary. With ice drift velocities of 15 cm/s [Kimura, 2004] and an average ice thickness of 48 cm [Worby et al., 2008], approximately 42 L of sea-ice volume cross each meter of this boundary per minute.
Figure 5. (a) Schematic drawing of the proposed mechanism for enhanced sea ice and iceberg melting at the EWG/ACC boundary. Note that the underlying graph of temperature distribution is not based on data from this expedition, which were mainly oriented in the S–N direction, but were taken from World Ocean Circulation Experiment line SO4 (source: electronic atlas of World Ocean Circulation Experiment data, available at http://www.ewoce.org). This transect is mostly W–E directed, following the prevailing wind direction in the Southern Ocean. The latitude ranges from ∼58°S (W) to 54°S (E). Our in situ transect (Figures 3b and 4) is oriented perpendicularly to this section, intersecting at 23°E. (b) Mean ice motion for 10 years in a polar stereographic projection. Sea ice and enclosed icebergs will be forced eastward by wind pressure, where they reach the warmer waters of the ACC, which float on top of WG WW. This causes enhanced melting of ice at the EWG/ACC boundary. Modified after Kimura , with permission.
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 Icebergs might be expected to follow the circulation of the underlying water masses, as they penetrate to considerable water depths, whereas the cross-sectional area exposed to wind is comparatively small. However, a combination of observational data and modeling has shown that even large Antarctic icebergs follow the wind-drifted sea ice in which they are enclosed [Lichey and Hellmer, 2001]. Therefore, a closed sea-ice surface in the winter months also means wind-driven icebergs, which are exposed to higher water temperatures at the EWG/ACC boundary, representing a permanent melting hot spot for wind-drifted ice, as depicted in Figure 5.
 This advective mechanism explains why the eastern rim of the Weddell Gyre consistently receives an excess of freshwater from melting ice, potentially both icebergs and sea ice.
 In order to disentangle the proportions of sea ice versus icebergs, we compare our salinity data to a recent modeling study [Jongma et al., 2009]. Jongma et al. have investigated the potential salinity decrease resulting from melting icebergs in the Southern Ocean. In the region that is affected most by melting icebergs according to their study, Jongma et al. report a salinity decrease of up to 0.3 units, with typical values in the WG of around 0.1. In contrast, we observe a salinity decrease of up to 1.5 units compared to local water masses. We conclude that, from a modeling perspective, icebergs alone would not be sufficient to produce the salinity minimum found in the EWG, and sea ice must be the main actor. There is, however, episodic shipboard evidence of high iceberg densities in the region, which partly agrees with satellite observations that suggest a decrease in iceberg density east of the EWG/ACC boundary [Tournadre et al., 2008], which would be consistent with increased iceberg melting here. Still, we conclude that melting of excess sea ice is the main reason for the low salinity in the EWG.
 This contributes to the persistence of high productivity at this specific location by creating stratified conditions. However, primary production also requires the micronutrient iron, which has repeatedly been found to be limiting in Southern Ocean environments. Therefore, we investigate potential iron sources in the following section.
4.3. Potential Iron Sources
 In particulate plankton samples in the bloom area, we measured Fe/C ratios of 2*10−4 mol mol−1 (Table 1). The observed values for Fe/C ratios are exceptionally high for an open ocean system, which are usually expected to be in the order of 10−5 mol mol−1 or less [Sunda and Huntsman, 1995; Twining et al., 2004]. We rule out sampling artifacts, because the type of 18,000 × g centrifuge used for the collection of particulate samples has previously been shown to be efficient and reliable in collecting trace element samples of marine particulate matter [Schussler and Kremling, 1993]. Inspection of the collected material by Scanning Electron Microscopy (SEM) with energy dispersive analysis of secondary X-rays (EDX) gave no evidence of contaminating terrigenous particles. Approximately 1/20 of the iron was found within the diatom shells, after cleaning from all potential traces of terrigenous matter. As the Fe/C ratio observed here exceeds known values of cellular iron requirements, we consider the possibility that a part of the iron may be present in adsorbed form, or there is a case of “luxury iron uptake and storage” [Sunda and Huntsman, 1995].
 With an organic carbon production of 2000 mmol*m−2 in the productive layer of the bloom, as derived from nutrient depletion, this corresponds to a particulate iron stock of 400 μmol m−2 (Table 1). Irrespective of the form of iron present in the particulate samples, we conclude that our inferred iron stock of 400 μmol m−2 must be supported by an efficient supply mechanism, as rapid water mass exchange and particle export limit residence times in this dynamic region. Particulate iron export might be less than the 25% (= 100 μmol m−2) suggested by 234Th export data (Table 1), if the Fe/C ratio in exported particles is lower than in suspended matter.
 Various pathways of iron to the Southern Ocean have been investigated recently, including airborne iron supply from terrestrial sources by dust [Martínez-Garcia et al., 2009], supply from underlying water masses by deep upwelling and vertical mixing [Meskhidze et al., 2007], detrital material and mixing effects from islands [Blain et al., 2007], sea ice [Lannuzel et al., 2008], melting icebergs [Hegner et al., 2007; Raiswell et al., 2008; Smith et al., 2007] or extraterrestrial dust [Johnson, 2001].
 In order to assess the potential contribution from atmospheric deposition, we use published values of dust or iron in snow. Lannuzel et al.  report concentrations of total dissolvable iron in snow on sea ice of up to 20 nM (= 1.1 ng/g) in the western Weddell Gyre, and similar and lower concentrations are found in Eastern Antarctic sea-ice environments [Lannuzel et al., 2007]. Schodlok et al.  calculate with a concentration of 10 ng/g dust in snow, which translates into 0.3 ng/g total iron assuming 3% iron in dust, a fraction of which will be dissolvable. No atmospheric iron deposition data are available from the immediate neighborhood of our study, but aerosol measurements at Neumayer station (70°39′S, 8°15′W) display strong dust flux maxima in austral summer [Weller et al., 2008]. Summarizing, we assume an iron concentration of 0.5 ng/g in snow, and a deposition of 100 kg snow per m2 of sea ice, after Worby et al. . This results in a contribution of 50 μg m−2 yr−1, or ∼1 μmol m−2 yr−1 onto sea ice, 0.25% of the iron stocks we find in the productive layer. Higher iron fluxes may be expected in the ice-free season, but these can only play a minor role for the ice melt related bloom observed here, and these fluxes could still only account for a small fraction of the iron stocks.
 Deep upwelling is also a source that might be of importance here, as it is prevalent in the WG, especially in its eastern and southern parts, and in the Maud Rise region. Therefore, we will investigate its potential impact based on our data. Typical deep water dissolved iron concentrations in the WG are in the order of 0.2–0.4 μmol/m3 [Boye et al., 2001; Croot et al., 2004] at 18°E and 6°E, respectively. Vertical transports are high, with annual entrainment rates of deep water on the order of 50 m yr−1 [Geibert et al., 2002; Gordon and Huber, 1990]. Calculating with the higher value (0.4 μmol/m3), this means an approximate annual supply from below of ∼20 μmol m−2 yr−1, which corresponds to only 5% of the inferred stock in the bloom.
 We measured 227Acxs (Table 1) to investigate whether the origin of the bloom-forming water mass in the EWG is indeed upwelled WG water, or rather water from the ACC. The uniquely high 227Acxs values known from the WG allow discrimination of WG from ACC waters [Geibert et al., 2002] despite the alteration in temperature and salinity characteristics by melting ice. Three out of seven 227Acxs values at the WG/ACC boundary point to an ACC source (∼0.25 dpm*m−3), whereas the four other values indicate WG waters or mixtures. The highest 227Acxs was in the northernmost part of the WG. Because high productivity spans both ACC and WG surface waters, we can state that deep upwelling in the WG does not seem to be the major iron source that controls this bloom, at least not without sea ice acting as a transporting agent. We can also infer from the 227Acxs pattern that ACC waters can be found floating on underlying WG waters here.
 In order to evaluate the potential importance of sea ice for iron inputs, we take values of sea-ice iron concentrations typical for the western WG from the literature. In early spring, a depth-integrated total iron concentration of 59.4 μmol/m−2 was reported for sea ice [Lannuzel et al., 2008]. Observations of iron release associated with the spring melt of sea ice from the WG at 6°E [Croot et al., 2004] confirm that sea ice is indeed a likely transporting agent for iron in the WG. We concluded in the freshwater budget of our study (section 3.1) that the volume of sea ice delivered to the eastern boundary of the WG is at least 2.7 times higher than the regional average, associated with a higher than average iron supply of >160.4 μmol m−2. Cumulative advection of sea ice to the EWG boundary, followed by melting when encountering warmer ACC waters, therefore accounts directly for 40% of the calculated particulate iron inventory of 400 μmol m−2. Considering that the sea-ice enrichment factor of 2.7 can be an underestimate because the freshwater lens spreads horizontally, and adding the uncertainty of highly variable iron concentrations in sea ice, and then taking into consideration internal recycling, the proposed mechanism can sustain the stock of 400 μmol m−2 Fe observed here, and create Fe levels that may be temporarily sufficient to alleviate iron limitation.
 Elevated iron levels in sea ice may be explained by sorption or uptake of dissolved iron, as depicted by Lannuzel et al. . However, sea ice can theoretically not contain more iron than delivered by upwelling and atmospheric fluxes together in winter. Therefore, we suggest a mechanism that delivers additional iron into sea ice during winter, when large icebergs continue melting due to their penetration into deeper water layers (up to >300 m). This means that they are exposed to warmer waters even during winter, when sea ce is present and growing. Continuous melting of icebergs in winter will lead to rising fresher and potentially iron-enriched waters from below, in the immediate vicinity of icebergs. This water would spread under the sea ice as a thin lens of fresher water, where it can refreeze due to its comparatively low salinity, and it can undergo processes of sorption and biological uptake. This hypothesis is consistent with maxima of iron concentrations in the lowermost parts of sea ice prior to the onset of spring melting [Lannuzel et al., 2008], and the generally high iron concentration observed in sea ice that is not fully supported by atmospheric deposition and upwelling.