2.1 Sediment trap and sensors.
The sediment trap mooring was deployed during the SOCLIM cruise (doi/10.17600/16003300) on October 13th 2016, at a station located on the central Kerguelen plateau (50°38’344 °S, 71°59’854 °E) (Figure 1A) in the core of a productive region which is naturally iron fertilized (Blain et al. 2007). The bottom depth is 527 m. Two consecutive diatom blooms occur annually (Blain et al. 2020) with peaks in chlorophyll concentrations within the mixed layer in November and in December (Figure 1B).
We used a Technicap PPS3 sediment trap (0.125 m2collecting area, 4.75 aspect ratio) located at 292 m below the surface. The cups were prepared using trace metal clean protocols in a clean room. The cups were washed with warm solution alkalin detergent (Extran) for 24 hours, rinsed 3 times with distilled water, then soaked in 2M HCl (analytical grade) for one week, rinsed 3 times with MQ water, soaked in 0.2 M HCl (ultrapure) for 1 week and finally rinsed 3 times with MQ water. The cups were then stored in plastic bags until used. Following the protocol of the preparation of the trace metal clean AQUIL medium (Price et al. 1989), the hypersaline formalin solution buffered at pH=8 with sodium tetraborate was passed through a Chelex resin to remove trace metal contamination. Trace metal concentrations of this solution can be found in Table S2. Just prior to the deployment, the 12 cups (250mL) were filled with the 5% preservative solution and mounted on the sediment trap carrousel. The collection time for each cup was 11 days (Table 1). A current meter (Aquadopp) and an inclinometer were attached to the sediment trap to record measurements of current speeds and inclination of the trap at a frequency of 1 h-1.
After recovering the sediment traps on April 3rd 2017, 1 mL of the supernatant of the cups was immediately replaced by fresh hypersaline formalin buffered (pH=8) solution before storage at room temperature until further processing. Four months later, at the home laboratory, samples were first transferred to a Petri dish and examined under a stereomicroscope (Leica MZ8, x10 to x50 magnification) to remove swimmers (i.e. organisms for which the structure was well preserved and that actively entered the cup). Then the samples were split into eight aliquots using a Jencons peristaltic splitter (Rembauville et al. 2015a).