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 NO 3+NO 2, 1.0–1.5 μM PO 4 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 (