See Hardisty et al., 2020 for a detailed description. A brief description is provided below.

Seawater was collected from Martha's Vineyard Sound at the Woods Hole Oceanographic Institution's (WHOI) Environmental Systems Laboratory (ESL). ESL intakes seawater from polypropylene pipes located approximately 200 meters (m) offshore from WHOI's Quissett Campus where water depths reach ~5 m. The pipes are located at approximately 0.5 m from the bottom and contain approximately 2.5 cm mesh.

We performed four separate incubation treatments. A large t0 batch was made for each experimental treatment, but each individual time point was allocated into a separate incubation vial, such that incubations were performed individually for each time point. Each treatment included independently incubated duplicates for each time point (10 mL). A t0 sub-sample was collected immediately following the addition of the Iodine-129 spike and sub-samples were harvested periodically over the course of 67 days. In order to prevent de-oxygenation and subsequent IO3− reduction, the seawater was kept aerated through the use of loose caps, which minimized any contamination through airborne particles but maintained equilibrium between the headspace and ambient atmosphere. The incubations were maintained on a shaker table at 180 rpm throughout the duration of the experiment.

The treatments from seawater collected from the ESL coastal station included: A.) unamended seawater; B.) seawater passed through passive sand-bed filtration (~0.5 mm) C.) a "kill treatment" which consisted of seawater passed through a 0.22-micrometer (um) filter and subsequently irradiated with UV light at 254 nm.

Incubations were conducted under both ambient laboratory light near an uncovered window and dark conditions, all at room temperature. For each experiment, the 129I spike was gravimetrically added, allowing for quantification of initial seawater I− concentrations.

Upon harvesting each time point, samples were immediately filtered (0.22 um syringe-filter) and stored at 4°C to maintain speciation.

The oxidized iodine, or IOX (IO3− + dissolved organic iodine), and I− were separated according to the chromatographic method described in detail in Hardisty et al., 2020. For the unamended seawater and "kill treatment", the time points were put through the chromatographic separation <7 days after collection.