Dataset: Water column data sampled aboard the R/V Pelican during August and September 2016 and May 2017 in Northern Gulf of Mexico, specifically the Louisiana Shelf region dominated by the discharge of the Mississippi River plume.

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 total dissolved nitrogen (TDN), nitrate+nitrite (NO3 + NO2), nitrite, soluble reactive phosphate (SRP), and ammonium (NH4) colorimetrically using Skalar autoanalyzer (Dzwonkowski et al. 2017), and for dissolved silicic acid (Si(OH)4) using a manual colorimetric method (Krause et al. 2009). Water for Chlorophyll a was filtered through a 0.45 μm-pore 47mm diameter HAWP Millipore filter, immediately frozen, and analyzed on shore (<2 weeks) using an acetone extraction/acidification method (Lomas et al. 2019). 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 abundance was quantified by fixing samples with Bouin's solution; in the laboratory, cells were settled in a chamber and enumerated (Utermöhl 1958).

Diatom rate processes were quantified using a radioisotope (32Si) and fluorescent dye (PDMPO) tracers.  Sample bottles were incubated for 12 or 24 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). Measurement for ambient (Amb) conditions (i.e. no enrichment of Si(OH)4) and enhanced (Enh) conditions (i.e. +20 µM enrichment of Si(OH)4) were made at most stations and depths using 32Si. The gross 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 ambient or enriched Si(OH)4, incubated in the same conditions as the 32Si samples, and processed as in McNair et al. (2015). The net rate of biogenic silica production was calculated from biogenic silica standing stock at the time of sampling and after a 24-hour incubation, under the same conditions as the 32Si and PDMPO, as described in Krause et al. (2010).