Sampling was conducted aboard the R/V Neil Armstrong (cruise AR16) in May 2017. Seawater was collected from Niskin bottles deployed on a rosette with a CTD.

Samples for seawater methane (CH4) and ethylene (C2H4) concentration measurements were collected from the CTD rosette in 250 mL glass serum vials (pre-combusted) crimp-sealed with aluminum collars and with Teflon-lined septa. Samples were typically analyzed the same day of sampling aboard the R/V Armstrong with an Agilent 7980A gas chromatograph equipped with a flame ionization detector (FID) and a gas stripping and cryo-trap concentration method as described previously (Repeta et al. 2016). The FID was calibrated by injecting different sized loops of a gas mixture standard containing 10 ppm of CH4 and C2H4 in pure nitrogen gas (Scott-Marrin, Riverside, CA, USA).

Samples for nutrient analyses were collected in acid-clean HDPE plastic bottles. Samples were shipped frozen to the Center for Microbial Oceanography: Research and Education at the University of Hawaii at Manoa where they were stored frozen until analysis.

Seawater phosphate (PO4) concentrations were determined following the autoanalyzer colorimetric procedure of Foreman et al. (2019). Seawater nitrate+nitrite (NOx) concentrations were determined following the autoanalyzer colorimetric procedure outlined of Foreman et al. (2016). Both PO4 and NOx analyses were performed on a SEAL Analytics autoanalyzer model 3 HR.

Seawater samples with low level N+N (LLN) concentrations were measured using the modified chemiluminescent method based on titanium (III) trichloride reduction of N+N to nitric oxide gas and detection with an Antek model 7090 as described by Foreman et al. (2016).

MAGIC soluble reactive phosphorus (SRP) concentrations (i.e. phosphate) were determined in seawater samples using the MAGnesium Induced Coprecipitation (MAGIC) technique as described by Karl and Tien (1992) with modifications based on Thompson-Bulldis and Karl (1998) and Cavendar-Bares et al. (2001) to increase the concentration factor of phosphate. Specifically, SRP was concentrated from 150 mL sweater samples using 0.75 mL (0.5% v/v) of 1M NaOH to precipitate Mg(OH)₂ and the Mg(OH)₂ pellets were dissolved with 5.25 mL of 0.1M HCl.

Total dissolved P (TDP) concentrations were measured by a photo-oxidation procedure in which controlled exposure to ultraviolet (UV) radiation converts organic P in seawater to phosphate which is then measured by the modified molybdenum blue method adapted for the autoanalyzer technique. The UV photo-oxidation methodology for TDP analysis is described in Foreman et al. (2019). The difference between TDP and PO4 or TDP and SRP is calculated to estimate the concentration of dissolved organic phosphorus (DOP) in seawater.