POM sample collection:
General equipment preparation in the lab before shipment included an HCl bath (1.0 M HCl overnight) and milli-Q rinse for the 8 L carboys, tubing, and filter holders. Additionally, the carboys were autoclaved. The 25 mm, 0.7 μm GF/F filters and aluminum foil used to wrap the filters were combusted (500 °C for 5 hours) in aluminum foil packets to remove any traces of carbon present. The forceps that came in contact with the samples were wiped with 70% ethanol before and between uses. Seawater for the POM samples was collected from the onboard flow through the underway system. Before sampling, the carboys used were rinsed twice with the pre-filtered underway seawater.

POM samples smaller than < 30µmwere collected hourly and at noon samples were collected in triplicate. 30 μm nylon mesh pre-filter was attached to the underway outlet for all standard samples to separate and remove POM samples larger than > 30µm (e.g., large plankton and particulates). After filtration, all POP triplicates were rinsed with approximately 2-5 mL of a 0.17 M Na2SO4 solution to remove dissolved phosphorus.

POM samples larger than > 30µm were collected on nitex mesh for ~1 hour. The flow rate was measured by measuring volume in an 8L carboy over a duration of several minutes. Mesh was rinsed into a container with filter seawater and then split onto duplicate GF/F samples for POC/PON.

All POM samples were folded in half with the top sides toward each other, sealed inside pieces of precombusted aluminum foil, and stored in a -20 °C freezer until analysis.

POP assay:

The POP data were obtained using an ash/hydrolysis method and comparing the samples to a set of standard phosphorus concentrations (Lomas et al., 2010, Biogeosciences, 7(2), 695-710, doi:10.5194/bg-7-695-2010). The sample filters were unfolded and placed face up into acid-bathed and combusted scintillation vials. Along with each set of samples, 10 different volumes (ranging 0 - 0.5 mL) of 0.1 M KH2PO4 solution were added to scintillation vials. 2 mL of a 0.017 M MgSO4 drying solution was added to each scintillation vial and then all vials were placed into an 80-90 °C oven overnight to dry. After drying, the vials were heated at 500 °C for 2 hours, then left to cool before adding 5 mL of 0.2 M HCl to each vial and being returned to the 80-90 °C oven for 30 minutes after being brought up to temperature. The solutions were then transferred to 15 mL glass centrifuge tubes (prepared in the same way as the scintillation vials). The sample vials were rinsed with 5 mL of milli-Q each, which was also added to the centrifuge tubes. 1 mL of a mixed reagent containing 2:5:1:2 parts Ammonium Molybdate ((NH4)6Mo7O24), 5.0 N H2SO4, Potassium Antimonyl Tartrate (C8H4K2O12Sb2), and Ascorbic Acid (C6H8O6) respectively was added to each centrifuge tube in 30-second intervals. Each of the sample tubes was centrifuged to isolate any glass fibers that could interfere with the absorbance reading. Lastly, after allowing the mixed reagent to react for exactly 30 min, the standards and samples were analyzed in 30-second intervals in a spectrophotometer at an 885 nm wavelength using a blank of ~0.1 M HCl solution and rinsing the cuvette with the blank solution between measurements.

POC/PON assay:
POC and PON measurements were analyzed both in-lab at UCI. The preparation for all samples was the same; the samples were each removed from their foil packets and placed into acid-bathed and combusted scintillation vials and dried in a 55 °C oven overnight. The scintillation vials were then placed in a desiccator containing a beaker of 12 M HCl overnight before being dried at 55 °C at least overnight once more. Samples sent to UCSB were then capped and shipped to the lab. Samples analyzed at UCI were packed into tin packets alongside Atropine (C17H23NO3) standards (ranging from 0.2-1.5 mg) and measured using a Flash EA elemental analyzer.

Associated GO-SHIP I07N underway and bottle datasets can be found on the CLIVAR and Carbon Hydrographic Data Office Section I07N homepage: https://cchdo.ucsd.edu/cruise/33RO20180423.