Availability of particulate Fe to phytoplankton in the Sea of Okhotsk

In a shipboard incubation study, we investigated the availability of particulate iron (Fe) to a Fe-starved phytoplankton community through the addition of suspended particulate matter (SPM; > 1 μm) collected from the nepheloid layer in the coastal region of the Sea of Okhotsk. Surface seawater incubations were also conducted at three stations around the Bussolʹ Strait where the SPM that possibly originated from the coastal nepheloid layer could emerge to the surface mixed layer due to strong vertical mixing around the Kuril Islands. In the SPM-added experiment, the growth rate of phytoplankton was significantly enhanced by the addition of SPM compared to the unamended control. This result clearly indicates that Fe in the SPM collected from the nepheloid layer was available to marine phytoplankton. In addition, phytoplankton particularly coastal diatoms in the nepheloid layer were viable and showed healthy growth. In the surface seawater incubation experiments, phytoplankton growth and nutrient drawdown in unamended control conditions in two of the three stations may be supported by Fe from the particulate fraction (> 0.22 μm), as estimated from stoichiometric calculation. We suggest that the bioavailable particulate Fe in SPM of the coastal region supports biological production and nutrient drawdown even after the depletion of dissolved Fe around the Kuril Islands, where strong vertical mixing occurs.