Nutrient and diatom dynamics during late winter and spring in the Oyashio region of the western subarctic Pacific Ocean

We investigated the nutrient and diatom dynamics during late winter and spring (9-March to 1-May 2007) in the Oyashio region as part of the OECOS-WEST research cruises. Macronutrients, iron, chlorophyll a (Chl-a) and biogenic silica (BSi) concentrations in the upper mixed layer varied remarkably ranges were 1.88-18.8 mu mol L-1 for NO3+NO2, 0.64-1.85 mu mol L-1 for PO4, 3.14-35.7 mu mol L-1 for Si(OH)4, 0.14-0.54 nmol L-1 for D-Fe, 0.64-24.6 nmol L-1 for T-Fe, 0.30-17.4 mu g L-1 for Chl-a, and 0.34-14.1 mu mol L-1 for BSi. Mixed-layer depth (MLD) varied from 8-190 m during the cruises. The growth rate of in situ phytoplankton communities, dominated by centric diatoms, varied in shipboard culture experiments from 0.55 d(-1) for iron-replete to 0.14 d(-1) for iron-limited conditions. A relationship between BSi and Chl-a concentrations indicates that the in situ diatom community in the warmer water system ( > 4 degrees C) was heavily silicified, probably due to iron-limitation. The in situ macronutrient and dissolved iron concentrations below the MLD and estimated macronutrient concentrations during winter were negatively correlated to temperature (1-6 degrees C), that is to the relative proportion of warm modified Kuroshio Water mixed into the colder Oyashio water system. The rate of decrease in Si(OH)(4) per degrees C increase was greater than the rates for NO3+NO2 and PO4 for both in situ and estimated winter values. These results suggest that the spring bloom in the cold-water system with high macronutrients and iron concentrations would progress rapidly and intensely, and then be terminated by nitrogenous nutrient depletion. However, the diatom bloom in warmer waters with lower macronutrients and iron concentrations would be terminated by Si- and/or iron-limitation of heavily-silicified diatoms. In the OECOS study, variation of macronutrients and iron due to the surface intrusions of several water masses and modification from different chemical conditions during winter were the most important factors regulating the progression, magnitude and probably fate of the spring phytoplankton bloom in the Oyashio region. Published by Elsevier Ltd.