Primary productivity and its bio-optical modeling in the Oyashio region, NW Pacific during the spring bloom 2007

Despite large diatom blooms occurring in the Oyashio region of the NW Pacific every spring, our knowledge of factors controlling primary productivity in the region during that season remains incomplete. Therefore, we investigated phytoplankton abundance, size structure, and primary productivity from April to June 2007. Significant changes were observed. Chlorophyll a (Chl a) concentrations in surface waters fluctuated between 0.37 and 17 mg m−3. Micro-sized (>10 μm) phytoplankton dominated the phytoplankton community when Chl a was >1 mg m−3. Depth-integrated daily primary production within the euphotic layer ranged between 328 and 3231 mg C m−2 d−1. Higher values of the water-column light utilization index (Ψ) for phytoplankton photosynthesis were observed in May and June. Although no significant relationships were found between surface primary productivity and macronutrient concentrations or photosynthetically available radiation (PAR), surface primary productivity correlated significantly with Chl a concentration during April, indicating that algal productivity depended on phytoplankton biomass. Furthermore, significant linear relationships were found throughout the observations of phytoplankton absorption coefficient to surface primary productivity and of that coefficient to the optimum algal photosynthetic rate normalized by Chl a level (PBopt) in the water column. Modeling PBopt with the empirical equations of Behrenfeld and Falkowski (1997) or Kameda and Ishizaka (2005) did not accurately reproduce in situ PBopt. These results suggest that the phytoplankton absorption properties could become useful indicators for estimating primary productivity in the Oyashio region during spring.