Hydrocasts were conducted at identified stations. A SeaBird CTD and rosette system, owned and maintained by Louisiana Universities Marine Consortium (LUMCON), operating institution for the R/V Pelican, was used for sampling. Calibration information can be found associated with the CTD data. Unless otherwise stated, samples used for rate measurements were collected based on the percent irradiance relative to that just below the surface.

Water was sampled from Niskin bottles and pooled into 10 L acid-cleaned carboys. For inorganic nutrients, water was filtered using 0.6 μm pore size polycarbonate membrane and immediately frozen until analysis. Filtered water was analyzed for dissolved silicic acid (Si(OH)4) using a manual colorimetric method (Krause et al. 2009). For biogenic silica analysis, seawater was filtered through a 1.2 μm-pore polycarbonate filter (47 mm diameter) and frozen immediately. On shore, filters were dried and analyzed using a sodium-carbonate time course digestion, to correct for lithogenic silica interference, in polymethylpentene tubes (Pickering et al. in review). In 2016, NaOH digestions were also done for biogenic silica, followed by an HF digestion to quantify lithogenic silica as in Krause et al. (2009).

Diatom rate processes were quantified using a radioisotope (32Si) and fluorescent dye (PDMPO) tracers. Samples bottles were incubated for <12 hours in acrylic incubators cooled with continually flowing surface water under a series of neutral density screens to simulate light levels at the depth of collection (i.e. see above). Kinetic experiments were set up by filling eight 250-mL bottles and then adding increasing enrichments of Si(OH)4 (ambient to +20 μM). The rate of biogenic silica production was measured using the radioisotope tracer 32Si with high specific activity (>40 kBq µg Si-1) as described in Krause et al. (2011). For PDMPO uptake, dye was added to samples at enriched in Si(OH)4 as described above, incubated in the same conditions as the 32Si samples, and processed as in McNair et al. (2015).