Seawater for six shipboard incubation experiments was collected using a surface towfish (Mellett and Buck 2020) at ~2 meters (m) depth on the R/V Roger Revelle between 15 August 2018 and 29 August 2018. Water was prefiltered through a 150-µm mesh to remove large grazers. For incubations (Inc) 1-2 and 4-5, water was homogenized into 20-liter (L) carboys, and three carboys were amended for each treatment indicated in Table 1 (see Supplemental Files), yielding triplicates for each treatment and timepoint sampled. Each carboy was sampled once at the indicated timepoints. Control carboys sampled at the beginning and end of each incubation were unique to the timepoint (i.e., no carboy was resampled over time). For Incs 3 and 6, water was first homogenized into two 20-L carboys, from which 4-L bottles were filled and amended with the treatments indicated in Table 1. Three 4-L bottles were amended for each treatment, and each 4-L bottle was sampled once at the indicated timepoints, yielding triplicates.
Incubations 1 and 4 were carried out for 24 hours, Inc 2 and 3 for 6 days, and Inc 5 and 6 for 8 days (Table 1). The nutrient amendments for each experiment were selected to induce nutrient stress. For Inc 2 and Inc 5, we refer to the +20 μM nitrate +1.25 μM phosphate +20 μM silicic acid treatment as "AllButFe" and the +20 μM nitrate +1.25 μM phosphate +5 nM ⁵⁷FeCl₃ treatment as "AllButSi".
Dissolved (<0.4 μm) trace metal samples were collected in acid-cleaned 125-milliliter (mL) low-density polyethylene (LDPE, Nalgene) bottles, and acidified to 0.024 M hydrochloric acid (HCl, Fisher Optima). Dissolved trace metal (Fe, Cu, Mn, Co, Ni, Cu, Zn, Cd, Pb) concentrations were determined by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) at the University of South Florida (USF) on an Element XR (Hollister et al. 2020; Burns et al. 2023).
Leachable particulate trace metals were collected on 0.4-μm and 5-μm polycarbonate track etch (PCTE) filters, transferred into 1.5-mL high-density polyethylene (HDPE) snap-cap vials, and frozen at -20 degrees Celsius until processed. Filters were leached in a heated 25% acetic acid leach with a reducing step, "Berger Leach", at National High Magnetic Field Laboratory (NHMFL) to extract the leachable particulate trace metals (Berger et al. 2008). Leachable particulate elemental (P, V, Fe, Cu, Mn, Co, Ni, Cu, Zn, Cd, Pb) concentrations were determined by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) on an Element 2 with the assistance of Dr. Peter Morton of Florida State University (FSU).
Sample analyses for dissolved trace metals and leachable particulate trace metals were performed by Shannon Burns (USF). Detection limits and quality control results are presented in Table 2 (see Supplemental Files).
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