Sources of Fe-binding organic ligands in surface waters of the western Antarctic Peninsula

Organic ligands are a key factor determining the availability of dissolved iron (DFe) in the high nutrient low chlorophyll (HNLC) areas of the Southern Ocean. In this study, organic speciation of Fe is investigated along a natural gradient of the western Antarctic Peninsula, from an ice covered shelf to the open ocean. An electrochemical approach, competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE15 AdCSV) was applied. Our results indicated that organic ligands in surface water on the shelf are associated with ice-algal exudates, possibly combined with melting of sea-ice. Organic ligands in deeper shelf water are supplied via resuspension of slope or shelf sediments. Further offshore, organic ligands are most likely related to the development of phytoplankton blooms in open ocean waters. On the shelf, total ligand concentrations ([Lt]) were between 1.2 nM eq. Fe and 6.4 nM eq. Fe. The organic ligands offshore ranged between 1.0 and 3.0 nM eq. Fe. 20 The southern boundary of the Antarctic Circumpolar Current (SB ACC) separated the organic ligands on the shelf from bloom-associated ligands offshore. Overall, organic ligand concentrations always exceeded DFe concentration (excess ligand concentration, [L ́] = 0.8 5.0 nM eq. Fe). The [L ́] made up to 80% of [Lt], suggesting that any additional Fe input can be stabilized in the dissolved form via organic complexation. The denser modified Circumpolar Deep Water (mCDW) on the shelf showed the highest complexation capacity of Fe 25 (αFe ́L ; the product of [L ́] and conditional binding strength of ligands, KFe'L ). Since Fe is also supplied by shelf sediments and glacial discharge, the high complexation capacity over the shelf can keep Fe dissolved and available for local primary productivity later in the season, upon sea ice melting. 1 https://doi.org/10.5194/bg-2020-357 Preprint. Discussion started: 18 November 2020 c © Author(s) 2020. CC BY 4.0 License.

between 1.2 nM eq. Fe and 6.4 nM eq. Fe. The organic ligands offshore ranged between 1.0 and 3.0 nM eq. Fe. 20 The southern boundary of the Antarctic Circumpolar Current (SB ACC) separated the organic ligands on the shelf from bloom-associated ligands offshore. Overall, organic ligand concentrations always exceeded DFe concentration (excess ligand concentration, [L΄] = 0.8 -5.0 nM eq. Fe). The [L΄] made up to 80% of [Lt], suggesting that any additional Fe input can be stabilized in the dissolved form via organic complexation. The denser modified Circumpolar Deep Water (mCDW) on the shelf showed the highest complexation capacity of Fe 25 (αFe΄L ; the product of [L΄] and conditional binding strength of ligands, K Fe'L cond ). Since Fe is also supplied by shelf sediments and glacial discharge, the high complexation capacity over the shelf can keep Fe dissolved and available for local primary productivity later in the season, upon sea ice melting.

Introduction
The Southern Ocean is a High Nutrient Low Chlorophyll (HNLC; e.g. Sunda et al., 1989) region where the 30 phytoplankton biomass is relatively low despite high ambient macronutrient concentrations, i.e. nitrogen (N), phosphorus (P) and silicon (Si) (e.g. Martin et al., 1991;Schoffman et al., 2016). The generally limited availability of light and the micronutrient iron (Fe) prevents phytoplankton from depleting P and N in the vast majority of HNLC areas (de Baar et al., 2005;de Baar, 1990;Martin et al., 1991;Viljoen et al., 2018). Indeed, Fe regulates the dynamics of primary production as it is involved in various cellular processes (Schoffman et al., 2016;Sunda, 35 1989). In the HNLC Southern Ocean, the availability of Fe has a direct impact on the early spring phytoplankton bloom, and thus on primary productivity (Moore et al., 2013). The Fe limitation in the Southern Ocean could thus have a direct effect on the amount of atmospheric CO2 sequestration (Le Quéré et al., 2016;Arrigo et al., 2008;Raven and Falkowski, 1999) to the deep ocean via the biological pump (De La Rocha, 2006;Lam et al., 2011).
Accordingly, the availability of Fe in the Southern Ocean is not only important for sustaining the food web, but 40 also has a substantial impact on global climate (Hanley et al., 2019 and references therein).
The low solubility of Fe in seawater, coupled with low atmospheric and terrestrial input of Fe, result in the scarcity of dissolved-Fe (DFe) in the Southern Ocean. In oxygenated seawater, Fe is mainly present in its oxidized form, Fe(III), predominantly as Fe(III)oxy-hydroxide species. These species tend to undergo further hydrolysis (Liu and Millero, 2002) and are thereby removed from the water column by scavenging or precipitation processes. Organic 45 Fe-binding ligands greatly elevate Fe solubility in seawater (Kuma et al., 1996) by stabilizing Fe in Fe-ligand complexes, and thus allowing Fe to remain longer in the water column. Moreover, Fe bound to organic ligands appears to be bioavailable to marine phytoplankton (Maldonado et al., 2005;Rijkenberg et al., 2008;Hassler et al., 2020). As such, organic ligands are a key component of Fe chemistry and bioavailability, notably in HNLC regions, as illustrated by Lauderdale et al. (2020). These authors showed, with an idealized biogeochemical model 50 of the ocean, that the interaction between microbial ligand production and binding of Fe by these ligands functions as a positive feedback to maintain the DFe standing stock in the oceans.
Various ligand groups exist and are classified based on their origin. Fe binding ligand properties can be measured by the competition against well-characterized artificial ligands with known stability constants. Analysis is done using an electrochemical technique, competitive ligand exchange (CLE) -adsorptive cathodic stripping 55 voltammetry (AdCSV). The application of AdCSV gives the total concentration ([Lt]) and conditional binding strength (K Fe'L cond ) of the dissolved organic ligands but does not provide information on the identity of ligands.
Although the sources and identities of Fe-binding ligands are still largely unknown, these ligands have a biological origin, being either actively produced or passively generated through microbial activity.
Laboratory studies have documented the active production of Fe-binging ligands under Fe-limited conditions 60 (Boiteau et al., 2013;Boiteau et al., 2016;Butler, 2005). Several types of siderophores, low-molecular-weight organic compounds which have strong affinity to Fe, are produced by mixed marine bacteria communities under Fe stress (Butler, 2005), suggesting that high ligand concentrations are related to a mechanism of Fe acquisition in an Fe-limited environment. These compounds have also been extracted (Boiteau et al., 2016;Macrellis et al., 2001; or identified (Mawji et al., 2008;Velasquez et al., 2016) in field samples. However, 65 they generally occur at picomolar levels (Boiteau et al., 2019) and are a small contributor to the total ligand pool.
Other ligand types, such as polysaccharide compounds, are passively generated in situ from microbial excretion and grazing (Sato et al., 2007;Laglera et al., 2019b). The polysaccharides, such as exo-polymeric substances (EPS), are excreted abundantly by a large number of microbial cells, especially in surface water covered by with sea-ice Lannuzel et al., 2015). Although EPS are relatively labile macromolecules, they can 70 be present in up to micromolar concentrations in seawater, showing the potential to outcompete stronger binding siderophores (Hassler et al., 2017). In addition, humic substances (HS) or HS-like substances from various origins constitute another type of ligand (Krachler et al., 2015;Laglera et al., 2019a;Whitby et al., 2020). Typically, HS are derived from remineralization and degradation of organic matter (Burkhardt et al., 2014). Terrestrial input of organic matter can supply HS to estuarine and coastal areas, whereas sediment resuspension and upwelling often 75 supply HS-like substances to the continental shelf (Gerringa et al., 2008;Buck et al., 2017). HS-like substances have also shown to be part of Fe binding ligands in biologically refractory deep ocean dissolved organic matter (rDOM) with low Fe-bioavailability. However, photodegradation of rDOM was shown to increase the Fe bioavailability making Fe bound to such substances an important source in HNLC areas where upwelling plays a role (Hassler et al., 2020;Lauderdale et al., 2020;Whitby et al., 2020;Laglera et al., 2019a). 80 The Bellingshausen Sea along the western Antarctic Peninsula (WAP) region has a distinct natural DFe gradient (Sherrell et al., 2018;Moffat and Meredith, 2018). The hydrography is strongly influenced by the dynamics of shelf-ocean water exchange. The shoreward intrusion of Circumpolar Deep Water (CDW) provides macronutrients to the shelf region, whereas offshore-flowing waters supply the micronutrients Fe and Mn to the open ocean from local sources (De Jong et al., 2015;Sherrell et al., 2018), such as glacial meltwater, sediments 85 and upwelling. The shelf sea of the WAP is a biologically-rich marine ecosystem in the Southern Ocean. The abundance, community composition and trophic structure of marine primary producers are directly impacted by the changing ice conditions and longer periods of open water due to climate change (Turner et al., 2013). https://doi.org/10.5194/bg-2020-357 Preprint. Discussion started: 18 November 2020 c Author(s) 2020. CC BY 4.0 License.
Moreover, rapid increases in anthropogenic CO2 has enhanced the air-sea CO2 fluxes, decreasing the bulk seawater pH, resulting in ocean acidification (Mikaloff Fletcher et al., 2006), which alters the physicochemical properties 90 of seawater and impacts the organic complexation of Fe (Ye et al., 2020). As the WAP has undergone significant warming (Turner et al., 2020), the changes in ice conditions will influence the supply of Fe and organic ligands, shaping the net primary production in this region. Understanding the sources and distribution of organic ligands provides important information on DFe availability, which is a fundamental step towards understanding the impact of warming of the Antarctic region on primary productivity in the Southern Ocean. 95 In this study, surface waters were sampled in a region of mixing between shelf-influenced waters and HNLC waters in the Bellingshausen Sea along the WAP. In order to probe sources and distributions of Fe-binding ligands along a natural gradient of Fe, the CLE-AdCSV technique was used to quantify the total concentrations and conditional stability constants of Fe-binding ligands. Hydrographic parameters were measured using a conventional conductivity-temperature-depth (CTD) rosette equipped with a fluorometer (WET Labs ECOAFL/FL) and an oxygen sensor (SBE-43). Seawater samples for DFe and Fe-binding ligands in this study were obtained using GO-FLO bottles attached to a Kevlar® wire. 110 Seawater samples were filtered over 0.2 µm filters (Satroban 300, Sartorius®) into pre-cleaned sample bottles inside a trace metal clean van. Sample bottles were pre-cleaned following a three-step cleaning protocol for trace element sample bottles (Middag et al., 2009).

Analysis of DFe and nutrients
The DFe analysis is described in detail by Seyitmuhammedov (2020). In short the DFe analysis was conducted 125 using high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) using a Thermo Fisher Element XR instrument at NIOZ, the Netherlands and with additional inter-calibration using an Amtek Nu Attom instrument at University of Otago, New Zealand, after preconcentration using an automated seaFAST system (SC-4 DX seaFAST pico; ESI). The quantification was done via standard additions. Accuracy and reproducibility were monitored by regular measurements of the reference materials SAFe D1 and GEOTRACES South Pacific (GSP) 130 seawater, and an in-house reference seawater sample, North Atlantic Deep Water (NADW). Results for DFe https://doi.org/10.5194/bg-2020-357 Preprint. Discussion started: 18 November 2020 c Author(s) 2020. CC BY 4.0 License.
analyses of reference materials were within the range of 0.722 ± 0.008 nM (n = 3) for SAFe D1 2013 and 0.155 ± 0.045 nM (n = 13) for GSP 2019 consensus values. The average overall method blank (seaFAST and ICP-MS) concentration, determined by measuring acidified ultrapure water as a sample, was 0.05 ± 0.02 nM (n = 21).
Macronutrients (N and Si) were analyzed simultaneously with a discrete autoanalyzer TRAACS 800 (Technicon) 135 in the shore-based laboratory at NIOZ.

Analysis of Fe-binding ligands
Samples were thawed in the dark and vigorously shaken prior to further treatment. Electrochemical analysis CLE-AdCSV with salicylaldoxime (SA) as a competing added ligand (Abualhaija and van den Berg, 2014) was used.
In short, the voltammetric system consisted of a BioAnalytical System (BASi) controlled growth mercury 140 electrode connected to an Epsilon 2 analyzer (BASi). The voltammetric system was controlled using ECDsoft interface software. The electrodes in the voltammetric stand included a standard Hg drop working electrode, a platinum wire counter electrode, and a double-junction Ag/AgCl reference electrode (3M KCl).
For the titration, 10 mL sample aliquots were added to 12 pre-conditioned Teflon (Fluorinated Ethylene Propylene (FEP), Savillex) vials and buffered to seawater a pH of 8.2 with 0.1 M ammonium-borate buffer. The sample 145 aliquots were titrated with Fe from 0 to 10 nM (with a 0.5 nM interval from 0 to 3 nM; and with a 2 nM interval from 4 to 10 nM Fe) and vials without Fe addition were prepared twice. Then, the competing ligand, SA, was added at a final concentration of 5 µM. The mixture was left to equilibrate for at least 8 hours or typically overnight (Abualhaija and van den Berg, 2014). Before analysis, the Teflon vials for titration were pre-conditioned at least three times with seawater containing SA and the intended Fe addition. For each titration point, duplicate scans 150 were done in the same Teflon vial as voltammetric cell.

Calculation of Fe speciation
Ligand parameters, [Lt] and K Fe'L cond , were obtained by fitting the data from the CLE-CSV titration into a non-linear Langmuir model. One-and a two-ligand models were applied, assuming one ligand and two ligand groups existed, respectively. The R software package was used for data fitting (Gerringa et al., 2014

Hydrography
Water masses were identified by plotting the Conservative Temperature (Ɵ) versus the Absolute Salinity (SA) (Tomczak and Godfrey, 2003) as generated by the freeware ODV (Schlitzer, 2018) from CTD data (Figure 2a).
The water mass description follows the definitions of Klinck et al. (2004) and Smith et al. (1999). A detailed 170 description of hydrographic features of the WAP is described elsewhere (Moffat and Meredith, 2018;Klinck et al., 2004;Smith et al., 1999) and briefly summarized here.
Two distinct horizontal currents exist in the study area, the Coastal Current (CC) and the Antarctic Circumpolar Current (ACC) (Figure 1). In the vicinity of the WAP, the ACC is a large strong eastward flowing current bordering the outer continental shelf. The CC is a strong but narrow southwesterly flowing current that is forced 175 by freshwater discharge and wind over the shelf (Grotov et al., 1998;Moffat and Meredith, 2018)  During the austral spring sampling period, Winter Water (WW; Ɵ < -1.8 o C and SA ~34.1) still existed in the upper 100 m at stations in the shelf sea (Figures 2b and 2c). In spring, Antarctic Surface Water (AASW) forms that has a higher temperature ( -1.5 -1 o C) and a lower salinity (33.0 > SA< 33.7) than WW (Tomczak and Godfrey, 2003;Orsi et al., 1995). During sampling, AASW was present at stations outside the shelf region (St. 84,90 and 96;190 Figures 2b and 2c). The position of the SB marks the southern terminus of water with CDW properties. From the shelf break toward the open ocean, upper Circumpolar Deep Water (uCDW) existed at 300 m depth, characterized by Ɵ max = 2 o C and a maximum of SA ~34.7. Near the WAP, the Antarctic Slope Front is missing (Klinck et al., 2004;Moffat and Meredith, 2018); hence, there is no barrier in the outer shelf region (Klinck et al., 2004). As a consequence, the shelf region is directly affected by the presence of the SB, resulting in subsurface intrusion of 195 uCDW onto the continental shelf. This water mass is modified into cooler and less saline water, referred to as modified CDW (mCDW) (Hofmann and Klinck, 1998) (offshore) flowing surface water that maintains the continuation of offshore-onshore water mass exchange (Klinck et al., 2004;Moffat and Meredith, 2018), although it did not seem to occur along our transect.  and L2 (1.10±4.32 and 1.34±3.74 nM eq. Fe, at 40 and 300 m, respectively), which implies that the one-ligand 225 model fit the data better. Therefore, results of data fitting with the two-ligand model will not be presented.

Discussion 240
The ligand concentrations measured during our study (1.07 -6.43 nM eq. Fe; Figure 3a) are consistent with the broad range of Fe-binding ligand concentrations measured in DFe speciation studies in the Southern Ocean (Boye et al., 2001;Lin and Twining, 2012;Nolting et al., 1998;Thuróczy et al., 2011). Previously reported [Lt] in the Southern Ocean varies from 0.5 -1.84 nM eq. Fe in the Atlantic sector (Thuróczy et al., 2011), 2.2 to 12.3 nM eq. Fe in the Pacific sector (Nolting et al., 1998), and 0.44 -1.61 nM eq. Fe in the Indian sector (Gerringa et al., 245 2008). The [Lt] in Antarctic polynyas ranges between 0.3 -1.6 nM eq. Fe (Gerringa et al., 2019;Thuróczy et al., 2012), whereas in regions with sea ice-coverage, [Lt] in underlying water is relatively high, with values of 4.9 -9.6 nM eq. Fe and up to 72.1 nM eq. Fe within the sea ice Genovese et al., 2018).

Fe-binding ligands along the transect from the shelf to the open ocean
The high sea ice cover on the continental shelf obstructs the light penetration into the water column, inhibiting the 250 development of an early spring bloom. Therefore bloom generated ligands are less likely to be found. However, microbial excretion from sea-ice algae and bacteria within and just beneath the sea-ice release EPS, which can form Fe-binding ligands Norman et al., 2015;Genovese et al., 2018;Hassler et al., 2017).
The planktonic community in spring is dominated by diatoms and haptophytes (Phaeocystis antarctica) (Joy-Warren et al., 2019; Arrigo et al., 2017). According to Lannuzel et al. (2015), the omnipresence of tube dwelling 255 diatoms (Berkelaya sp.) attached via EPS to the bottom of the sea-ice was responsible for relatively high [Lt] in under-ice seawater, indicating that EPS could elevate seawater [Lt] in areas of sea-ice cover. In addition, a laboratory study has shown that cultured P. antarctica appears to excrete EPS in relatively high concentrations , with similar binding strength (log K Fe'L cond 11.5 -12) to those measured in this study (log water column. Indeed, ligand input in the proximity of sediments was previously observed in upwelling regions over the continental shelf or in coastal areas (Gerringa et al., 2008;Buck et al., 2017). Subsequent upwelling processes may transport the ligands to the upper water column, including rDOM. Moreover, intrusion of uCDW 270 also provides heat (Smith et al., 1999), which may cause glacial and sea ice melt. The melting of sea ice (i.e. first year pack ice) supplies ligands to surrounding seawater (Genovese et al., 2018), whereas glacial ice is not expected  (Orsi et al., 1995;Klinck et al., 2004). Here the ACC interacts with the continental slope (Orsi et al., 1995), 280 propagating ocean eddies that subsequently cause cross-shelf water intrusion (Moffat and Meredith, 2018). The subsurface intrusion of uCDW and its associated turbulence may cause vertical water mass mixing at the proximate location of the SB. The little ice-cover at the shelf break compared to the inner shelf allows more light penetration, triggering a bloom, as indicated by fluorescence maxima observed at St. 96 (Figure 5b). The bloom and its related microbial activities could release Fe-binding ligands. However, given the consistently low and 285 constant distribution of [Lt] at the shelf break, it seems that mixing determines the distribution and net concentrations of ligands (Figure 3a). This is confirmed by the relatively constant distribution of DFe and macronutrients (i.e. nitrate; Figure 5c) at the same station, indicating that prominent mixing at the shelf break indeed is the major factor governing the distribution of ligands, DFe and nutrients.
Further oceanward from the shelf break, [Lt] was >1 nM eq. of Fe (St. 84 and 90; Figure 3a), probably related to 290 the spring bloom at this location. Satellite-based data (Arrigo et al., 2017), showed that open water formed one month earlier offshore than near-shore (St. 70 and 72), implying that the melting of sea ice offshore (St. 84 and 90) occurred preceding and during our occupation. The melting of sea ice released nutrients and micronutrients such as Fe (Lannuzel et al., 2016;Sherrell et al., 2018), which together with the availability of light stimulated the spring bloom. Such a bloom in turn is a source of Fe-binding ligands in the upper water column (Gledhill and 295 Buck, 2012;Boye et al., 2001;Croot et al., 2004;Gerringa et al., 2019). Arrigo et al. (2017) reported that a bloom in its early stages was observed underneath variable sea ice cover seaward from the shelf break. Siderophores are expected to be produced upon Fe depletion by marine microbes as a strategy to acquire Fe 300 (Butler, 2005;Buck et al., 2010;Mawji et al., 2008;. Similarly, as detailed above for the shelf stations, the exudation of EPS from diatoms and haptophytes could be an important addition to the organic ligand pool. Moreover, polysaccharide ligands will be released by microbial cells during the bloom as well as via grazing (Sato et al., 2007;Laglera et al., 2019b) and viral lysis (Poorvin et al., 2011;Slagter et al., 2016).
Additionally, the ratios of labile particulate Fe to labile particulate Mn (Seyitmuhammedov, 2020) indicate that 305 Fe has a biogenic origin in the offshore waters. Therefore, we suggest that the origin of [Lt] offshore was, next to the melting of sea ice, the result of in situ production of organic ligands during the bloom and passive generation from microbial processes associated with the bloom.  (Figures 6a and 6b). The value of Si* serves as a proxy for Fe limitation, where Fe stress leads to preferential drawdown of Si compared to N by diatoms in surface water (Takeda, 1998). A negative Si* indicates Fe limiting conditions, assuming that Si and N are required in a 1:1 ratio by diatoms (Brzezinski et al., 2002). Typically, organic ligands excreted under 315 Fe-limited conditions have strong affinity for Fe (Maldonado et al., 2005;Mawji et al., 2008), i.e. a high log K Fe'L cond (>12). However, a relatively low log K Fe'L cond is observed in AASW relative to deeper uCDW and mCDW ( Figure   4a). This indicates that in offshore AASW where Fe limitation is expected, the contribution of siderophores is modest. Indeed, recent studies showed that only <10% of Fe is complexed by siderophores (Boiteau et al., 2019;Bundy et al., 2018), suggesting that the binding strength of the overall ligand pool is not always a good indicator 320 of the presence of particular ligand group if multiple ligand sources are present. Moreover, in the presence of light, organic ligands can undergo photo-degradation (Hassler et al., 2020), and thus the chemical structure can be altered into a slightly weaker ligand type (Barbeau et al., 2001;Powell and Wilson-Finelli, 2003). Mopper et al. (2015) suggested that the absorption of solar radiation by chromophoric dissolved organic matter as part of the ligand pool which commonly produced by sea ice algae (Norman et al., 2011), leads to the photochemical 325 transformation of these compounds. These photo-oxidative processes can thus also explain the shift in log K Fe'L cond in the AASW to lower values compared to deeper uCDW and mCDW.
The presence of the SACCF and SB fronts affects bloom conditions. As shown by Arrigo et al. (2017), high chlorophyll-a concentrations were observed in surface waters in between the SB and SACCF, which suggest the distribution of phytoplankton biomass is affected by physical processes in the area. The SB also appears to mark 330 the boundary between offshore organic ligands that result from a combination of the earlier sea ice melt and in situ production and/or generation associated with offshore blooms, and organic ligands on the shelf that result from a combination of ice-algae exudation, sea-ice melt, and sediment resuspension. In the region near the SB at the shelf break, water mass mixing due to the baroclinically unstable water column seems to have caused consistent distributions of [Lt] (Figure 3a). Further offshore, ligands are most likely associated with the bloom, 335 but the distribution of ligands is also affected by enhanced vertical mixing and intensified currents proximal to the area of the fronts. Solar radiation enhances stratification and drives the formation of the spring bloom whereas deep mixing can both hinder as well as stimulate bloom formation based on the balance between availability of light and nutrients (Arrigo et al., 2017). It thus seems likely that the balance between mixing and stratification results in variable [Lt] in the area around the SACCF (St. 84 and 90; Figure 3a). 340 https://doi.org/10.5194/bg-2020-357 Preprint. Discussion started: 18 November 2020 c Author(s) 2020. CC BY 4.0 License.

Implications for primary productivity
In general, we found [L΄] >0.75 nM eq. Fe (Figure 5a), which accounts for approximately 80% of [Lt]. This implies that at least 80% of total ligands measured are available to bind Fe, although the total complexation capacity of ligands is also determined by its log K Fe'L cond . The highest complexation capacity log αFe΄L was found in mCDW on the shelf (Figure 4b), and concurred with the highest concentrations of DFe in mCDW (Figure 3b and Figures  345   4b). The high complexation capacity of ligands on the shelf increases the potential of organic ligands to stabilize additional Fe input to the shelf waters Gerringa et al., 2019;Thuróczy et al., 2012) and lengthen the residence time of DFe . A longer residence time has a positive feedback on the development of local primary productivity upon sea ice melting (Arrigo et al., 2017), supplying DFe to phytoplankton on the shelf. Moreover, based on the results of oxygen isotope ( 18 O/ 16 O, conventionally reported 350 into delta-notation as δ 18 O) analysis (Seyitmuhammedov, 2020), meltwater associated with runoff and glacial discharge is present in the upper 200 m of the shelf, and probably is a source of particulate and dissolved Fe that will increase under continued climate change. However, whether Fe in particulate form will partition into the dissolved pool via ligand driven dissolution of Fe, also depends on the fraction of labile particulate Fe. In addition, local primary productivity not only relies on the DFe input from, for example, meltwater and glacial debris 355 Lannuzel et al., 2016), but probably also on the input of Co and Mn (Saito et al., 2010;Wu et al., 2019;Middag et al., 2013) as these elements showed co-limitation in the Southern Ocean (Middag et al., 2013;Saito and Goepfert, 2008).
Besides affecting the shelf conditions, ice melt also produces buoyant northward-flowing surface water, which may facilitate DFe transport from the shelf to the open ocean, supplying DFe for primary production offshore, but 360 this effect was not noticeable in the transport data for this specific transect. However, the conditions along the WAP are not homogenous and elevated Fe (dissolved and total-dissolvable; (Seyitmuhammedov, 2020) concentrations northeast of our transect were observed in the upper 100 m, suggesting that some of the observed ligands might have been transported southwesterly with the CC. This high DFe stabilized by organic ligands will probably be transported further to the southwest where a coastal polynya is commonly observed in Marguerite 365 Bay (Arrigo et al., 2015). Such transport would supply DFe to the highly productive Marguerite Bay polynya and fuel a phytoplankton blooms in these ice-free waters but could also be partly transported offshore in the region southwest of our transect. However, the relative amount of DFe bound to organic ligands can vary, and is also strongly influenced by the continued change in environmental conditions due to global warming (Slagter et al., 2017;Ye et al., 2020), making it likely such a transport of DFe to the southwest or offshore will change as well. 370 https://doi.org/10.5194/bg-2020-357 Preprint. Discussion started: 18 November 2020 c Author(s) 2020. CC BY 4.0 License.
Global warming has caused glaciers to retreat and induced significant loss of sea-ice, particularly in the Antarctic Peninsula area (Henley et al., 2019;Stammerjohn et al., 2012;Turner et al., 2020). The sea ice extent over the southern Bellingshausen Sea, has decreased in recent decades, creating open water and lengthening the ice-free season (Turner et al., 2013). This results in increased solar irradiance and enhanced stratification (Henley et al., 2019), which can lead to an alteration of the phytoplankton community structure. As previously reported, variable 375 light conditions favor the growth of Phaeocystis antarctica over diatoms (Joy-Warren et al., 2019;. In contrast, smaller-cell diatoms are better adapted to increased sea surface temperature (Schofield et al., 2017). Changes in planktonic community composition affect net primary production and overall carbon drawdown, which lead to further alteration of the food web and carbon cycling (Schofield et al., 2017;Joy-Warren et al., 2019;Arrigo et al., 1999;Alderkamp et al., 2012). These and other ongoing changes in the food web will 380 also affect production of dissolved organic carbon (DOC) and thus ligands as they form a fraction of the DOC pool (Gledhill and Buck, 2012;Whitby et al., 2020). Generally, one expects that increased DOC production would lead to more ligands, but the binding strength depends on which molecules are formed (Gledhill and Buck, 2012;Hassler et al., 2017). Additionally, intensified light exposure alters log K Fe'L cond by photo-oxidative processes, possibly reducing the complexation capacity and binding strength for Fe (Barbeau et al., 2001;Powell and Wilson-385 Finelli, 2003;Mopper et al., 2015) as well as the bioavailability (Hassler et al., 2020). Furthermore, complexation capacity is affected by pH, implying that ongoing ocean acidification also influences the speciation of Fe (Ye et al., 2020). Overall, the continued sea-ice melt and glacial retreat can be expected to increase the supply of Fe (Lannuzel et al., 2016), other micronutrients (Co, Mn, etc.), and Fe-binding ligands (Lin and Twining, 2012), but the consequences for their complexation capacity and overall bio-availability of Fe remain elusive. If DFe 390 becomes progressively more available in the Southern Ocean, phytoplankton growth could increase until another process becomes limiting, such as the availability of another micronutrient or macronutrient. Many uncertainties remain, but the changing environmental conditions of the WAP due to climate change will affect marine biogeochemical cycles and influence productivity beyond the Southern Ocean as the Southern Ocean is an important hub in ocean circulation and its waters eventually supply nutrients to other regions (e.g. Middag et al. 395 (2020).

Summary
Our results indicate that organic Fe-binding ligands in surface water on the continental shelf of the WAP are associated with ice-algal exudates and addition of ligands from melting sea ice. In the water column close to the https://doi.org/10.5194/bg-2020-357 Preprint. Discussion started: 18 November 2020 c Author(s) 2020. CC BY 4.0 License. continental slope and shelf sediments, resuspension of sediment followed by upwelling processes appears to be 400 another source of ligands. From the continental shelf-break oceanward, sources of Fe-binding ligands are likely related to offshore phytoplankton blooms, either actively produced during the bloom, or passively generated by microbial processes associated with the bloom. The distribution of ligands is affected by the two major fronts in the region, the SACCF and SB. The SB along the shelf break not only marks the boundary between the shelf and open ocean, but also marks the border between organic ligands associated with the bloom offshore and organic 405 ligands on the shelf originating from sea-ice and sediment related sources, such as ice-algae exudation, sea-ice melt, and sediment resuspension. Overall, excess ligands comprised up to 80% of the total ligand concentrations, implying the potential to solubilize additional Fe input. The ligands in denser mCDW on the shelf have a higher complexation capacity for Fe, and are thus capable of increasing the residence time of Fe as DFe and fuel local primary production later in the season upon ice melt.

Acknowledgements 415
The authors would like to thank the captain and his crew of the R/VIB N. B. Palmer, as well as Anne-Carlijn Alderkamp and all other participants, for their efforts and support. Our colleagues in the nutrients lab at NIOZ are acknowledged for analyzing nutrients. IA was financed by Indonesia Endowment Fund for Education (LPDP), and KS received a scholarship from the University of Otago. KRA was funded by a grant from the National Science Foundation Office of Polar Programs (ANT-1063592). The IAEA is grateful to the Government of the 420 Principality of Monaco for the support provided to its Environment Laboratories.