Sampling and analytical procedures:

For samples in this study, Whatman glass-fiber filters (GF/F) with a nominal 0.7 µm pore size were used to filter samples collected on the P18-2016 and GOA2013 cruises. Polycarbonate filters with a nominal 0.4 µm pore size were used to filter samples collected on the CoFeMUG and WebbPacific2007 cruises. Polyesthersulfone (PES) filters with a nominal 0.2 µm pore size were used to filter samples collected from SCALE, SWINGS, P06-2017, ETSP2010, ETSP2011 and GOM2019 cruises. All samples were stored in high density polyethylene bottles at -20 ˚C until measuring total dissolved phosphorus (TDP) concentration.

DOP (Dissolved Organic Phosphorus) concentrations in this study were calculated as the difference between TDP concentrations ([TDP]) and SRP concentrations ([SRP]) of samples ([DOP] = [TDP] - [SRP]). For DOP concentrations reported here, TDP concentrations were measured by the ash/hydrolysis method based on Monaghan and Ruttenberg, 1999. Briefly, for samples collected >1 year prior to analysis, the sample pH was reduced to 1 by adding ~150 µL 6 M ACS-grade HCl to the sample bottle and placing the bottle in a reciprocal shaker overnight. Then, 6 ml of the acidified sample was added to an acid washed, 500 ˚C combusted glass vial, and 0.6 ml of 4.3 M NaCl/0.3 M MgSO4 solution was added to the acidified sample. Subsequently, vials were put into a drying oven at 70 ˚C until appearing dry (often 4-5 days). Then, each vial was covered with aluminum foil and transferred to a muffle oven to bake at 130 ˚C for 3 hours and then at 500 ˚C for 4.5 hours. Afterwards, 1.8 mL 0.75 M ACS-grade HCl was added to each vial and capped tightly with Teflon-lined caps, then heated at 80 ˚C for 20 min to hydrolyze the polyphosphate left after ashing. After heating, 4.2 mL ultra-pure water (18.2 MΩ_ cm) was added to each vial and heated at 80 ˚C for 10 min to dissolved all remaining solids. I assume quantitative conversion of DOP to SRP and the resulting SRP concentration was measured by using the colorimetric phosphomolybdate-blue method (Hansen and Koroleff, 1999). The final DOP concentration was calculated by the difference between SRP concentration and TDP concentration ([DOP] = [TDP] - [SRP]).

TDP concentrations collected from the published literature were measured by three methods: ash/hydrolysis, UV oxidation, and wet oxidation. The ash/hydrolysis method was also used on samples from the Pulse-26, STN F, KT00A, B, KT01Eq, EN391 and SJ0609 cruises. Wet oxidation is a chemical oxidation where a persulfate reagent is added to seawater and then heated to 120 ˚C for 30 minutes to convert DOP to SRP (Hansen and Koroleff, 1999). Wet oxidation methods were employed to determine TDP concentrations on cruises including BIOSOPE, OUTPACE, KH-11-10, KH-12-3, KH-13-7, KH-14-3, KH-17-4, SATL2004, KM0415, Line P and Latitude II cruises, and at the BATS site. UV oxidation is a photochemical oxidation method using UV radiation to covert DOP to SRP (Armstrong et al., 1966). UV oxidation methods were used to measure the TDP concentrations on cruises including AMT 10, AMT 12, AMT 14, AMT 15, AMT 16, D279, 36N, Line P, GB 93, MAB 94, MAB 96, Cavender-Barres cruises, and at Station ALOHA