Hydrographically mediated patterns of photosynthetic pigments in the East/Japan Sea: Low N:P ratios and cyanobacterial dominance

Nutrients and photosynthetic pigments were measured over a north–south transect of the East/Japan Sea (EJS), which includes various oceanographic environments such as the cold and warm-water masses, sub-polar fronts, and eddies in the surface ocean. Sampling was conducted in May 2004, July 2005, and October 2005. The N:P ratios were low (< 4) for high chlorophyll a areas owing to the active Redfieldian consumption of N and P from waters with a low N:P ratio (approximately 13) in the entire EJS. The compositions of photosynthetic pigments were used for identifying phytoplankton groups at a class level using the CHEMTAX program. It is notable that cyanobacteria (40–60%) dominated the phytoplankton community in the frontal zone in 2004 and formed approximately 10–50% of the community over the entire surface layer in 2005. This dominance of cyanobacteria appears to be associated with the seawater conditions of low N:P ratio and optimum temperature. As such, the relative amount of prymnesiophytes increased with a decrease in the N:P ratio in the upper 200 m of the EJS. In contrast, more preferentially in high N:P ratio areas, diatoms (40–80%) dominated the phytoplankton community in May 2004 because of the spring bloom, and pelagophytes (20–60%) dominated the community in July and October 2005. Therefore, the EJS seems to provide an ideal environment for studying the physical and chemical factors that control the community structures of phytoplankton.