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 underway system. The underway intake was located off the bow of the ship at a depth of approximately 5.3 m from the sea surface and the length of the pipe from the intake location to the output lab was 11.8 m. A 30 μm nylon mesh pre-filter was attached to the underway outlet for all standard samples to remove large plankton and particulates. Before sampling, the carboys used were rinsed twice with the pre-filtered underway seawater. Triplicate sampling occurred roughly at four-hour intervals with a shift forward of one hour each day. For example, if samples were taken at 01:00, 05:00, and 09:00 on one day, samples would be taken at 02:00, 06:00, and 10:00 the following day to avoid any diurnal bias. The water was vacuum filtered through combusted GF/F filters and the volume of seawater filtered was determined on a per station basis, ranging from 3-8 L. The volume was raised if the previous stations’ filtration times exhibited a decreasing trend and lowered if filtration time exhibited an increasing trend in an attempt to normalize the amount of collected material. The large sampling volume and initial rinsing steps limit the effect of a time delay from underway inlet to sampling station. After filtration, all POP triplicates were rinsed with approximately 2-5 mL of a 0.17 M Na2SO4 solution to remove dissolved phosphorus. All POM samples were folded in half with the top sides toward each other, sealed inside pieces of combusted 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 0.2-1.5 mg) and measured using a Flash EA elemental analyzer.

Associated GO-SHIP A13.5 underway and bottle datasets can be found on the CLIVAR and Carbon Hydrographic Data Office Section A13.5 homepage: https://cchdo.ucsd.edu/cruise/33RO20200321