Temporal variability and bioavailability of iron and other nutrients during the spring phytoplankton bloom in the Oyashio region

Iron [dissolved Fe (D-Fe) and total dissolvable Fe (T-Fe)] and nutrient concentrations in the surface water of the Oyashio region (northwestern North Pacific) were measured before and during spring phytoplankton bloom (March to May in 2007). During the pre-bloom period (middle of March), we observed vertically uniform concentrations of iron (0.3–0.5 nM [D-Fe] and 3–5 nM [T-Fe]), macronutrients (10–15 μM NO3+NO2, 1.0–1.5 μM PO4 and 20–30 μM Si(OH)4) and chlorophyll a (Chl-a, 0.3–0.4 μg l−1) throughout the upper 125 to 150 m due to vertical mixing during winter. Water temperature and salinity before the bloom were also vertically uniform with >5 °C and S>33.5, the conditions of a modified Kuroshio warm-water (MKW) ring, higher than those of the Coastal Oyashio Water (COW). The cold COW (<2 °C, S<33.2) intruded a few times into the surface during the bloom period with high iron (0.4–0.6 nM [D-Fe] and 10–25 nM [T-Fe]) and Chl-a (10–23 μg l−1) concentrations. The high Chl-a observed in the COW is due to the higher original concentrations of iron and macronutrients in the COW than in the MKW. An incubation experiment conducted during a COW intrusion in early April resulted in robust phytoplankton growth and complete exhaustion of nutrients, indicating an adequate supply of bioavailable iron. This result is also consistent with robust in situ phytoplankton growth and Chl-a production due to the presence of high iron levels in the COW. The most important mechanisms transporting iron to the surface water, which would regulate the primary production during spring bloom in the Oyashio region, are the surface intrusions of iron- and nutrient-rich COW derived from vertical and lateral mixing processes and vertical mixing in MKW during winter and spring.