Water column sampling:
The full water column of 21 stations near the West Antarctic Peninsula was sampled between 11 September 2016 and 10 October 2016 aboard the R/V IB Nathaniel B. Palmer. Depth profile samples were collected using 12-liter (L) Niskin bottles (OceanTestEquipment, Inc) mounted on a trace metal clean rosette sampling system (SBE32, Seabird; TMC CTD).

From the trace metal clean rosette, samples for humic-like substances were filtered through pre-cleaned 0.2-micrometer (µm) polyethersulfone membrane filters (Acropak 200, Pall Corporation). Samples were collected in acid-cleaned, narrow-mouth 500-milliliter (mL) fluorinated high-density polyethylene bottles (FLPE; Nalgene) and stored at -20 degrees Celsius (ºC) until shore-based humic-like substance and ligand analyses at Oregon State University.

Sample analyses – humic-like substances:
Electroactive, Fe-binding humic-like substances (HS) were measured using adsorptive cathodic stripping voltammetry based on the method first developed by Laglera et al. (2007) and updated by Sukekava et al. (2018). All samples were analyzed using a polarographic 797 VA Computrace (Metrohm) stand equipped with a hanging mercury drop electrode, Ag/AgCl reference electrode, platinum rod auxiliary electrode, and an acid-cleaned Teflon analytical cell. Briefly, frozen samples were set out to thaw overnight at room temperature in the dark. The following day, 10 mL sub-samples were aliquoted into each of 2 paired acid-cleaned, pre-conditioned 50 mL polypropylene vials (MetalFree, Labcon), followed by the addition of 50 microliters (μL) of 1.5 molar (M) boric acid buffer to maintain a solution pH of 8.2. Iron standard was added to one of the paired vials (vial B, final concentration 60 nanomolar (nM)) to saturate all the humic-like substance binding groups in the solution, while vial A was left under ambient Fe conditions. Vial A represented the amount of Fe bound to HS in-situ (Fe-HS) and vial B represented the total amount of Fe-binding HS in the sample (HS_T). Both aliquots were left to equilibrate for a minimum of 14 hours. After the equilibration period, vial A was added to the Teflon voltammetric cell with an addition of 500 μL of 0.4M KBrO₃- catalyst. The sample was then purged for 300 seconds with high-purity N₂ gas, and then electrochemical analysis was performed with a 60-120 second deposition period with stirring at -0.1 volts (V) followed by linear sweep voltammetry (-0.1 to -1V, scan rate 50 millivolts per second (mV s-1)). Vial B was analyzed immediately following vial A under the same conditions, then three successive standard additions of Fe-saturated Suwannee River Fulvic Acid (SRFA) standard were added. Default standard additions were 0.1, 0.2, and 0.3 milligrams (mg) SRFA per liter, and up to 0.6 mg SRFA per liter for samples with high HS concentrations. A 70 second N₂ purge was included after each addition to ensure oxygen removal. All measurements were measured in triplicate, and the Teflon analytical cell was rinsed thoroughly with Milli-Q water between samples to minimize carryover. Due to high HS concentrations, 13 samples required dilutions with Milli-Q water prior to analysis. Dilutions were 1:10 (n = 2), 1:5 (n = 10), and 5:1 (n = 1) volume sample : volume Milli-Q.

Peak heights were extracted from scans using ECDSOFT software (Omanović, 2025; Omanović and Branica, 1998), then the peak heights from vial A and vial B were converted into Fe-HS and HS_T concentrations, respectively in micrograms (μg) SRFA eq per liter using the slope of curve generated by the three standard additions. Units were then converted to nM Fe eq using the measured binding capacity of SRFA in seawater 14.6 ± 0.4 nmol Fe per mg SRFA. Median variability between replicate scans was 0.02 nM Fe eq (average 0.03) for Fe-HS and 0.02 nM Fe eq (average 0.06) for HS_T. The limit of detection (LOD) was calculated as three times the standard deviation of triplicate scans of a UV seawater blank at 120 s (0.03 ± 0.01 nM Fe eq) and was estimated to be 0.03 nM Fe eq. Only one sample had a concentration at or below the LOD (Stn 16–33m, Fe-HS = 0.03 nM Fe eq).