Deployment: KH04xx-02

Tsuda, A., et al. (2007): Evidence for the grazing hypothesis: Grazing reduces phytoplankton responses of the HNLC ecosystem to iron enrichment in the western subarctic pacific (SEEDS II). J. Oceanogr. 63(6), 983-994.

The first iron addition was carried out from 0:50 GMT on 20 July to 0:00 GMT on 21 July (GMT). Day 1 was defined as 21 July (GMT).

The ship started to inject iron and sulfur hexafluoride (SF6) as an inert tracer of the water mass, executing an 8 km by 8 km grid pattern centered on the buoy with an interval of 400 m. The ship was navigated with a lagrangian coordination system (Tsumune et al., 2005), and buoy position was transmitted to the ship every 10 min to update the navigation frame of reference to account for surface water advection. The amount of iron added to the patch was 332 kg Fe as FeSO4. During the iron fertilization, 4000 L of saturated SF6 solution was also  simultaneously injected. The saturated SF6 solution was made onboard using the method previously detailed in Tsumune et al. (2005). Note that the saturated SF6 concentration in seawater is about 0.2 mM (Ledwell and Watson, 1991). A second iron addition was performed on day 6 without SF6 tracer, when an additional 159 kg of iron was added to the patch, which was traced using the SF6 signal.