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

During 2010-2012 in the northwestern region of the North Pacific subtropical gyre, we examined seasonal variability of the phytoplankton community with respect to structure and photo-physiological status using the chemotaxonomy program CHEMTAX and fast-repetition-rate fluorometry. Total chlorophyll a standing stock in the upper 150 m (E integral TChl a) varied from 25.5 to 89.0 mg m(-2) during periods of relatively deep vertical mixing (40-207 m), but was fairly constant (18.6-27.2 mg m(-2)) during stratification periods. Prymnesiophytes, chlorophytes, chrysophytes, prasinophytes, and/or diatoms comprised major portions of the E integral TChl a during mixing periods. Diatoms became the most abundant group (29-43 %) in February 2011, when large phytoplankton blooms were observed, whereas Prochlorococcus was the dominant component (15-46 %) during stratification periods. During mixing periods, nitrate and nitrite (NO3 + NO2) concentrations occasionally exceeded 0.5 mu mol kg(-1) in surface waters, and F (v)/F (m) (photochemical efficiency of photosystem II) ranged from approximately 0.40-0.50 within the euphotic zone. During stratification periods, however, NO3 + NO2 concentrations were very low (< 0.1 mu mol 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. The seasonal variability of phytoplankton community structure in this region was related to the nutrient supply from deep waters. Additionally, we compared our data to published values at the time-series station ALOHA, located in the eastern region of the NPSG. At Sta. ALOHA in 2011, E integral TChl a was almost constant (19.5-26.4 mg m(-2)) throughout the year and was consistently dominated by Prochlorococcus (61-81 %). These results suggest that the phytoplankton community in the northwestern region of the NPSG is seasonally variable and composed of diverse groups compared with the eastern region of the NPSG.