Seasonal variability of phytoplankton community structure in the subtropical western North Pacific

During 2010–2012 in the northwest region of the North Pacific subtropical gyre, we examined the seasonal variability of the phytoplankton community with respect to structure and photo-physiological status using the chemotaxonomy program CHEMTAX , microscopy, and fast-repetition-rate fluorometry. Total chlorophyll (Chl) a standing stock in the upper 150 m (ʃTChl a) varied from 25.5 to 89.0 mg m^–2 during periods of relatively deep vertical mixing (40–207 m), but was almost constant (18.6 to 27.2 mg m^–2) during stratification periods. Prymnesiophytes, chlorophytes, chrysophytes and/or diatoms comprised major portions of the ʃTChl a during mixing periods. Diatoms became the most abundant group (29–43 %) in February 2011, when large phytoplankton blooms were observed. In contrast, the genus Prochlorococcus was the dominant component (15–46 %) during stratification periods. During mixing periods, concentrations of nitrate and nitrite (NO₃+NO₂) occasionally exceeded 0.5 μmol kg^–1 in the surface waters, and F_v/F_m (photochemical efficiency of photosystem II) ranged from approximately 0.40 to 0.50 within the euphotic zone. During stratification periods, however, NO₃+NO₂ concentrations were very low (<0.1 μmol kg^–1) from the surface to 50–95 m, and F_v/F_m was in the range of 0.25–0.40 in the upper mixed layer. These results suggest that the seasonal variability of phytoplankton community structure in this region is related to nutrient concentrations in the water column.