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:

Samples for labile particulates were sampled and analyzed by the Hurst lab. The cubitainers were sampled into 2 L polycarbonate bottles with three rinses before filling. They were filtered immediately through pre-cleaned 0.2 mm pore-size polycarbonate track-etched (PCTE) membrane filters (47 mm dia., Nuclepore, Whatman) mounted in Teflon filter sandwiches (Millipore). Filters were stored in the freezer before digestion with a Berger et al. (2008) leach (acetic acid and hydroxylamine), used to extract the labile particulates. Extracts were analyzed for Fe and Sc on the Thermo Fisher Element 2 extended range ICP-MS at UC Santa Cruz.