Sample Collection:
Trace metal clean techniques were employed to collect seawater samples from the BATS site (31°40′N, 64°10′W) in 2019 on 12 March, 14 May, 19 August, and 19 November aboard the R/V Endeavor (March) and R/V Atlantic Explorer (May-November). A trace metal clean carousel (SeaBird Electronics) with a conductivity-temperature-depth sensor (SBE 19 plus, SeaBird Electronics) was fitted with custom-modified 5-liter (L) Teflon-lined external-closure Niskin-X samplers (General Oceanics) and deployed on an Amsteel non-metallic line for the collection of hydrographic data and water column samples from twelve depths (Sedwick et al. 2023). In August, an additional surface (0.3 meter (m)) sample was collected ~500 m upwind of the ship by deploying a Niskin-X sampler from an inflatable dinghy. All seawater samples collected for dissolved nickel analyses were filtered (0.2-micrometer (µm)) (AcroPak, Pall) into acid-cleaned low-density polyethylene (LDPE; Nalgene) bottles, acidified to pH 1.7 (0.024 molar (M) Q-HCl, Fisher Optima) by addition of 6 M ultrapure hydrochloric acid (Fisher Optima), and stored double bagged in buckets at room temperature. For soluble (<0.02 µm) nickel analyses, 0.2 µm-filtered seawater was passed through a Milli-Q (≥18.2 MΩ cm) and sample-rinsed 0.02 µm Anotop syringe filter (Sigma-Aldrich) using a peristaltic pump and collected in acid-cleaned LDPE bottles (Ussher et al. 2010); soluble nickel samples were also acidified to pH 1.7 and stored at room temperature. Seawater samples collected for dissolved nickel speciation analyses were 0.2 µm-filtered into acid-cleaned and Milli-Q conditioned fluorinated high-density polyethylene (FPE; Nalgene) bottles and stored at -20 degrees Celsius (°C) until analysis.

Total Dissolved and Soluble Ni Concentrations:
Concentrations of total dissolved and soluble nickel (Ni) were determined in the acidified seawater filtrates using inductively-coupled plasma mass spectrometry (ICP-MS, Thermo Fisher Scientific ElementXR), with in-line separation-preconcentration (Elemental Scientific SeaFAST SP3) modified after Lagerström et al. (2013). Calibration standards were prepared in low-analyte concentration filtered seawater for which initial concentrations were determined using the method of standard additions. Calibration standards were introduced using the same in-line separation-preconcentration procedure as the seawater filtrate samples, with yttrium was used as an internal standard for all samples except where indicated. Analytical blank concentrations were assessed by applying the in-line separation-preconcentration procedure including all reagents and loading air in place of the seawater filtrate sample ("air blank").

Labile Dissolved Ni:
A stock solution of dimethylglyoxime (DMG; Acros Organics, 99+%) was made in methanol (Optima, Fisher) to a concentration of 0.1 M (Saito and Moffett 2001). A working solution of DMG was prepared by diluting the 0.1 M stock with methanol to a concentration of 0.01 M. A 1.5 M borate-ammonium buffer (Ellwood and Van den Berg 2000) was prepared by dissolving 18.55 grams (g) boric acid (Aldrich, 99.999% trace metals basis) in 200 milliliters (mL) 0.4 N ammonium hydroxide (Optima, Fisher). The borate-ammonium buffer was purified of trace metal contaminants by passing through two consecutive chromatography columns containing cleaned and preconditioned Chelex® 100 resin (100-200 mesh, Bio-Rad Laboratories) at a speed of 2 milliliters per minute (mL min-1) (Mellett and Buck 2020). Ni and copper (Cu) standards at various concentrations were prepared from 1000-fold dilutions of atomic absorption standards in 0.024 M HCl (Optima, Fisher).

Labile dissolved Ni (DNi) concentrations were measured in seawater samples using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV) with DMG as the added competitive ligand (van den Berg and Nimmo 1987, Saito et al. 2004, Boiteau et al. 2016). Seawater samples were thawed in the refrigerator overnight and then allowed to reach room temperature before 10 mL aliquots were dispensed into acid-cleaned Teflon vials (Savillex) and amended with 7.5 millimolar (mM) borate-ammonium buffer (pH=8.2, NBS scale, Ellwood and Van den Berg 2000) and 200 micromolar (µM) DMG (Boiteau et al. 2016). Samples were then allowed to equilibrate for at least 12 hours prior to analysis (Boiteau et al. 2016) and analyzed within 24 hours of the DMG addition (van den Berg and Nimmo 1987). Previous work indicated that some natural ligands preferentially bind Cu over Ni (Boiteau et al. 2016). To investigate the specificity of Ni-binding ligands in this study, a duplicate of each seawater sample was equilibrated with Cu prior to the DMG addition. For these Cu replicates, 10 mL seawater sample aliquots were amended with 7.5 mM borate-ammonium buffer and 10 nanomolar (nM) Cu and allowed to equilibrate for 12 hours. After the Cu equilibration period, DMG was added to a concentration of 200 µM, and samples were allowed to equilibrate for an additional 12 hours before analysis.

A BioAnalytical Systems (BASi) controlled growth hanging mercury drop electrode (CGME) was interfaced with an Epsilon 2 (BASi) analyzer for measurement of labile DNi concentrations. At the time of analysis, samples were purged for 5 minutes with high-purity nitrogen gas (N2) to minimize oxygen interference. For each run, a 30-second deposition time at -0.7 volts (V) was followed by a 10-second quiet time, and a linear sweep from -0.7 V to -1.4 V with a scan rate of 10 volts per second (V s-1) was performed (Dupont et al. 2010). A drop size of ten was used with a stirrer speed of 600 rpm. Samples were measured for ambient labile DNi at least three times before Ni standard additions. For each standard addition, 0.5 nM or 1 nM Ni was dispensed into the electrochemical cell, which was subsequently purged for another 30 seconds before the sample was measured again in at least triplicate. Ni additions continued until the resulting peak heights were double those produced by the natural sample.