4.1 Effect of Fertilization on Downward Particle Flux
 Neither the 234Th nor the sediment trap data indicate major fertilization-induced export, despite the clear increase in NCP. Moreover, the UVP showed no increase in particles >100 µm upon fertilization. In contrast, evidence is mounting that iron fertilization of Si-replete waters, leading to diatom blooms, can induce severalfold higher export than during LOHAFEX and enhance flux to deep waters (EIFEX [Smetacek et al., 2012], CROZEX [Salter et al., 2007; Morris and Sanders, 2012], SEEDS II [Aramaki et al., 2009], SERIES [Boyd et al., 2005], SOFeX [Buesseler et al., 2004], KEOPS [Blain et al., 2007], and IronEx-II [Bidigare et al., 1999]).
 The LOHAFEX data thus suggest that iron fertilization of Si-limited Southern Ocean waters, which does not stimulate diatom blooms, enhances neither shallow export nor deep POC flux. This is consistent with the view that diatoms are major contributors to new production [Dugdale and Wilkerson, 1998, 2001], given the importance that sinking may have in diatom ecology [Smetacek, 1985; Salter et al., 2012]. It has hence been questioned whether Southern Ocean iron fertilization would work at all to enhance carbon sequestration if it does not do so under Si limitation, because Si is already fully utilized in the Southern Ocean [Trull et al., 2001b]. However, iron fertilization can lower the Si:C ratio of exported material and, thus, can sequester more carbon for the same amount of Si [Smetacek et al., 2012; see also Salter et al., 2012]. Thus, we do not believe that the LOHAFEX results imply that iron fertilization cannot enhance Southern Ocean carbon sequestration.
 However, we cannot readily disentangle the effects on downward POC flux of the lack of diatoms on the one hand and the very high grazing pressure and particle reprocessing by zooplankton on the other. Thus, LOHAFEX provides no conclusive proof that downward POC flux in low-Si sub-Antarctic waters will never be enhanced by iron fertilization, especially since significant export and deep POC flux do occur in low-Si regions [Cardinal et al., 2005; Henson et al., 2012; Honjo et al., 2008; Planchon et al., 2013; Trull et al., 2001a]. Organic carbon did accumulate in the mixed layer (section 4.2), leaving open the possibility that enhanced export occurred after the end of the experiment, although the heavy grazing and particle reprocessing by zooplankton would probably have strongly attenuated any future export event.
 Nevertheless, our results agree with those of SAZ-SENSE, which reported lower export and greater mesopelagic remineralization in naturally iron-replete than in iron-limited low-Si sub-Antarctic waters [Bowie et al., 2011; Ebersbach et al., 2011; Jacquet et al., 2011]. Only a modest response, mostly by nondiatom phytoplankton <20 µm, was found upon iron fertilization of sub-Antarctic low-Si waters during SAGE, suggesting that export was probably not greatly enhanced [Harvey et al., 2010; Peloquin et al., 2010]. In contrast, POC export at the low-Si sub-Antarctic SOFeX North site was enhanced by iron fertilization, with NO3− depletion similar to LOHAFEX [Bishop et al., 2004; Coale et al., 2004]. However, Si(OH)4 in SOFeX North was above limiting concentrations and, apparently, replenished in the elongated patch by admixture of surrounding water. Weakly silicified diatoms contributed 44% to total phytoplankton POC and aggregated eventually [Coale et al., 2004]. Thus, while SOFeX North is nominally considered a “low-Si” experiment, diatoms were not initially Si limited and did bloom, in strong contrast to LOHAFEX.
4.2 Comparison Between NCP, 234Th, and Sediment Traps
 Comparing these three methods is fraught with complications, since export may lag production, the methods integrate over different time scales and depths, and each suffers from biases and uncertainties [Lampitt et al., 2008b; Le Moigne et al., 2013; Morris et al., 2007; Savoye et al., 2008]. However, the long duration and Lagrangian nature of LOHAFEX mitigate some of these problems, and while significant uncertainties are associated with each of our estimates, we do not believe that any of the methods is grossly biased. Figure 8 summarizes our main conclusions.
Figure 8. Overview of carbon fluxes and particle profiles during LOHAFEX. The right side summarizes the carbon fluxes: NCP averaged 21 mmol m−2 d−1 in the mixed layer, of which ≤13 mmol m−2 d−1 accumulated in the mixed layer, leaving at least 8 mmol m−2 d−1 for export below the mixed layer. The dotted line indicates that mixed layer export is not very well constrained, and thus, the degree of flux attenuation between the mixed layer and 100 m is uncertain. 234Th-derived export exceed the flux caught in sediment traps, indicating further attenuation from 100 to 200 m. The left side of the figure summarizes the UVP data, with abundance of different particle types indicated on a relative axis. The UVP data collectively indicate that particle transformation was most intense between the base of the mixed layer and 150 m, most likely owing to zooplankton activity; flux attenuation was most likely intense throughout this range.
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 NCP was 21 mmol POC m−2 d−1, exceeding the 100 m export flux by ~15 mmol m−2 d−1, implying organic carbon accumulation in the mixed layer and/or flux attenuation between the mixed layer depth (MLD) and 100 m. Direct measurements do suggest accumulation in the mixed layer of ≤6 µmol L−1 of total organic carbon in the patch (S. W. A. Naqvi et al., in preparation, 2013), accounting for ≤13 mmol m−2 d−1 of the NCP. This would allow for export out of the mixed layer of at least 8 mmol POC m−2 d−1, of which around 6 mmol m−2 d−1 sank below 100 m (as diagnosed from 234Th). This implies that POC flux was attenuated by around 2 mmol m−2 d−1 between the mixed layer and 100 m. Thus, a little more than half of the in-patch NCP appears to have accumulated in the mixed layer, while the remainder was exported below the mixed layer as sinking POC flux.
 The POC flux diagnosed from 234Th exceeded trap fluxes threefold to sixfold. Since the flux of 234Th itself was just 2–3 times lower in the traps than that diagnosed from the profiles, the discrepancy cannot be attributed purely to biased trap collection. The 234Th and trap data thus indicate a strong reduction in particle flux from 100 to 200–450 m.
 Between the base of the mixed layer and the sediment traps at 200–450 m, POC flux was probably attenuated about eightfold, or about sixfold between 100 and 200–450 m. These estimates must be treated with caution, since the export estimates at each depth carry significant uncertainty. However, such intense attenuation contrasts with the higher transfer efficiencies of flux to depth that have been reported upon collapse of diatom blooms [Buesseler and Boyd, 2009; Martin et al., 2011; Smetacek et al., 2012]. Interestingly, subsurface 234Th excesses indicative of remineralization [Maiti et al., 2010; Savoye et al., 2004] were not consistently found, although excesses are often confined to narrow depth horizons. They might hence have been missed by our 50 m vertical resolution in the mesopelagic.
 The UVP data are also consistent with strong flux attenuation: particle stocks declined with depth below the MLD, and there was a shift from intact fecal pellets to unrecognizable detritus. This shift was most pronounced at the depth of highest copepod abundance, implying coprorhexy [Lampitt et al., 1990] and, generally, particle reprocessing by zooplankton. The high abundance of Oithona spp. during LOHAFEX also suggests substantial flux reprocessing: Oithona spp. are reported to be coprophagous and, hence, likely to attenuate POC flux [González and Smetacek, 1994]. However, intact fecal material contributed ~45% to the sediment trap catches, underscoring the importance of unreprocessed fecal pellets in downward POC flux.
 This contrasts with the enhanced mesopelagic particle stocks seen during the Kerguelen Ocean and Plateau Compared Study (KEOPS) [Jouandet et al., 2011]. Overall, the UVP revealed that the most intense particle transformations took place between the base of the mixed layer and around 150 m (Figures 7 and S8), and flux attenuation probably took place throughout this depth range.
 Mesopelagic communities of high- and low-Si regions may actually respond differently to iron fertilization: mesopelagic remineralization as estimated from excess barium was a relatively low proportion of export flux in the high-Si iron fertilized areas of EIFEX and KEOPS [Jacquet et al., 2008a, 2008b]. In contrast, at the iron-replete low-Si sub-Antarctic site in SAZ-SENSE a greater proportion of export flux was remineralized than at either of the iron-limited sites [Jacquet et al., 2011]. Moreover, export from SOFeX North was initially reduced owing to a response by mesopelagic grazers, though an export event did occur later [Bishop et al., 2004; Lam and Bishop, 2007]. We observed no drastic changes over time, but the upper mesopelagic community appeared to attenuate particle flux heavily.