Winter iron supply processes fueling spring phytoplankton growth in a sub-polar marginal sea, the Sea of Okhotsk: Importance of sea ice and the East Sakhalin Current

Abstract

To evaluate the winter iron (Fe) supply processes fueling spring phytoplankton growth in the southern Sea of Okhotsk, the distributions of the Fe concentrations in seawater were investigated using hydrographic observations in late November (in the absence of sea ice) and mid-February (in the presence of sea ice). The results obtained from the hydrographic observations indicated a temporal increase in the Fe concentration in the surface mixed layer from November to February, which coincided with a decrease in seawater salinity. Changes in the alkalinity and oxygen isotopic composition of seawater indicated that the mixed layer in February was affected by sea ice meltwater as well as the less saline East Sakhalin Current (ESC) water, with a significant Fe supply originating from these waters. The magnitude of the Fe supply from these waters is greater than the upward Fe supply from deep water. The sea ice meltwater enriches the Fe in the near-surface water of the mixed layer, whereas the ESC water enriches the Fe at the depth of the mixed layer by the entrainment of sedimentary Fe associated with deep mixing due to Ekman transport. Additionally, the Fe nutritional status of the phytoplankton in the surface water of the ice-covered area was investigated by a shipboard incubation experiment. The results showed that bio-available Fe existed in the surface water and that large (>10 μm) phytoplankton were not stressed by low Fe availability. The Fe supply processes related to sea ice melting and the ESC water inflow play important roles in sustaining the suitable Fe nutritional status of phytoplankton in the southern Sea of Okhotsk in early spring.