Sampling:

Seawater for the incubation was collected from ~15m at 35°55.4’N, 121°32.4’W using acid-cleaned HDPE tubing taped directly to the amsteel blue line. A lead weight coated with fiberglass and epoxy paint was at the bottom of the line. The pump was a teflon Wilden air-operated double-diaphragm pump. The tubing was run into a trace metal clean positive pressure bubble, where it was homogenized in acid-cleaned 50 gallon plastic barrels. From there, 4L cubitainers for the different treatments were rinsed and filled with the homogenized seawater, and then spiked appropriately based on treatment. These cubitainers had been pre-cleaned using the methods of Crawford et al. 2003.

The incubation setup:

We had five different treatments, with three replicates each:
control: no addition
+Fe: 5 nmol/kg added dissolved Fe
+Sc: 5 nmol/kg added dissolved Sc
+Fe and +Sc: 5 nmol/kg each of dissolved Sc and Fe added
filtered +Fe and + Sc: the seawater was filtered with 0.2 micrometer pre-cleaned supor Acropak filter before rinsing and filling the cubitainer, and then spiking with 5 nmol/kg each of dissolved Sc and Fe.

The cubitainers were incubated in an on-deck plexiglass incubator that was surface-seawater chilled and covered with a screening to achieve 30% of the incident irradiance. After 24 hours, the incubation was ended and each treatment was harvested.

Sample analysis:

Dissolved metals: Samples were filtered in a trace metal clean bubble to 0.2 micrometers with a pre-cleaned supor Acropak filter, and acidified to pH 1.8 at sea with optima HCl. Samples were analyzed post-cruise using the methods of Biller and Bruland (2012) with modifications as described in Parker et al. (2016). Briefly this method involves preconcentrating the metals of interest on a PA1 chelating resin at pH 6.0 +/- 0.2 and analyzing the extracts on an Element 2 Extended Range ICP-MS. Standard addition curves are made in low-metal seawater for calibration, and blanks are quantified by loading less than 0.5 mL of low-metal seawater on the columns and extracting as usual. Rhodium is used as an internal standard. Samples were analyzed in the Till lab by PI Till and undergraduate Freiberger.

Nutrients samples were filtered to 0.2 micrometers using the same Acropak filters and frozen at sea. They were analyzed by the Wetland Biogeochemistry Analytical Services at Louisiana State University using OI Analytical Flow Solutions IV segmented flow auto analyzer methologies.

Chlorophyll concentrations were measured by Emily Pierce and YuanYu Lin in the Marchetti lab, with two different filter sizes: 0.7 and 5 micrometers. Chlorophyll a (Chl a) measurements were obtained by gravity filtering seawater through a 5 μm polycarbonate filter followed by vacuum filtration through a GF/F filter using a series filter cascade for size fractionation. Filters were frozen at −80degC until analysis. Extractions were performed ship-board using 90% acetone kept at −20decC for 24 h followed by fluorometric quantification with a Turner Designs 10-AU fluorometer using the acidification method (Parsons et al. 1984).