Water column samples were collected in July 2023 aboard the R/V Endeavor using a trace metal clean rosette (SeaBird) outfitted with 12-L Niskin-X samplers (Ocean Test Equipment), a trace metal clean pump system, or a custom surface pump “towfish” system (Mellett and Buck, 2020). Seawater samples for macronutrient analysis were filtered through 0.2 µm Pall Acropak Super membrane filter capsules, collected in acid-cleaned and triple-rinsed 15-mL Falcon tubes, stored in ziplock bags and frozen (-20 ºC) until analyzed at Oregon State University following recommended best practices (Becker et al., 2020).
Samples from seawater incubation experiments were filtered through consecutive acid-cleaned 5 µm and 0.4 µm polycarbonate track-etched (PCTE, Whatman) filters in a dual stage Teflon filter rig (Savillex) on a custom acrylic tower. Filtrate was collected in acid-cleaned and triple-rinsed 15-mL Falcon tubes and stored in ziplock bags and frozen (-20 ºC) until analyzed at Oregon State University following recommended practices (Becker et al., 2020).
Samples were analyzed for phosphate, nitrate+nitrite, silicate, and nitrite on a QuAAtro39 AutoAnalyzer (SEAL Analytical) according to standard colorimetric methods (Strickland and Parsons, 1972). All reagents were prepared in dedicated labware with high purity Milli-Q (>18 MΩ cm) water. Working standards were prepared fresh daily in an artificial seawater (ASW; 35 g/L sodium chloride, 0.5 g/L sodium bicarbonate) matrix using calibrated volumetric pipettes.
Nine-point standard curves made in artificial seawater were analyzed at the beginning of each run following a series of reagent blanks. Quality control checks were conducted every twelfth sample using a combination of ASW blanks and standards. The highest standard from the calibration curves was analyzed approximately every twenty samples to check/correct for drift during the runs. Subsamples of reference material for nutrients in seawater (Kanso) were measured in each run (see accompanying Table 1 in supplemental document).
Detection limits for each parameter were determined from three times the standard deviation of replicate analyses of the lowest standards for each parameter. Limits of detection (LOD) were 0.01 µM for phosphate, 0.04 µM for nitrate+nitrite, 0.04 µM for silicate, and 0.01 µM for nitrite. Values below detection are reported as “0” µM with accompanying QC Flag 6. Limits of quantification (LOQ) for each parameter were determined as ten times the standard deviation of replicate analyses of the lowest standards. These are provided in the supplemental document, Table 2, as an alternative estimate of the analytical limitations for these parameters.
Sample analyses for macronutrients were performed by senior researcher Dr. Salvatore Caprara in the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University.
