Seawater samples were collected using an epoxy coated CTD-rosette mounted with Go-Flo samplers and a Sea-Bird Electronics CTD (SBE9plus). Go-Flo bottles were transferred to a trace metal clean van for subsampling into polypropylene tubes (nutrients), polypropylene bottle (biogenic silica and particulate carbon and nitrogen) or TM acid-cleaned polycarbonate incubation bottles (Si-32 & C-14 incubation experiments).

Nutrient samples were filtered through 0.2 micrometer (μm) polycarbonate filters and frozen at -20° Celsius (C). Samples for biogenic silica concentrations were size fractionated by serial filtration through 5 μm and 0.6 μm polycarbonate filters Filters were stored frozen at -20°C. Particulate organic carbon and nitrogen were measured on samples from experiments examining the effect of added Fe and Si on carbon fixation. These samples were filtered through precombusted GFF filters placed in glass scintillation vials and frozen at -20°C.

Samples for silicic acid uptake profiles were spiked with the radioisotope Si-32. Nutrient limitation assays were performed on pairs of samples where the rate of silicic acid uptake (Si-32) or carbon fixation (C-14 in paired light/dark bottles) were determined in unaltered controlled samples and in samples augmented with either silicic acid (20 μM), nitrate (20 μM) or iron chloride (1 nanomolar (nM)). All samples were incubated on deck in simulated in situ incubators cooled with flowing surface seawater for 24 hours. Profiles sampled six depths from near surface to the 1% light level. Nutrient limitation assays were performed at the 40% and 10% light levels.

Particles from incubated samples were size fractionated by serial filtration through 5 μm and 0.6 μm 25-millimeter (mm) polycarbonate filters. For C-14 incubations, total radioactivity in each sample was determined by sampling 100 μl of sample seawater prior to filtration. Filters from Si-32 incubations were placed on plastic planchettes and dried before covering with mylar film and stored for analysis ashore using low-level beta counters (Riso Inc). Filters from C-14 incubations were acidified in glass scintillation vials, scintillation cocktail (Ultima Gold XR) was added followed by liquid scintillation counting. Total radioactivity samples received 100 μL of b-phenethylamine and 5 mL of scintillation cocktail prior to analysis at sea using a Tri-Carb 3110TR scintillation counter.

Biogenic silica concentrations were determined by NaOH digestion followed by colorimetric analysis of the resulting dissolved Si. Particulate organic carbon and nitrogen samples were analyzed by Dumas combustion. Nutrient concentrations were determined by flow injection using a Lachat Instruments QuikChem 8500 Series 2 anayzer.

For more information, see the Protocol documents attached as Supplemental Files and https://msi.ucsb.edu/facilities-services/analytical-lab/services.

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