Seasonal variability of phytoplankton community st

Authors: Tetsuichi Fujiki Kosei Sasaoka Kazuhiko Matsumoto Masahide Wakita Yoshihisa Mino

Publish Date: 2016/01/12

Volume: 72, Issue: 3, Pages: 343-358

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Abstract

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 photophysiological status using the chemotaxonomy program CHEMTAX and fastrepetitionrate fluorometry Total chlorophyll a standing stock in the upper 150 m ʃTChl a varied from 255 to 890 mg m−2 during periods of relatively deep vertical mixing 40–207 m but was fairly constant 186–272 mg m−2 during stratification periods Prymnesiophytes chlorophytes chrysophytes prasinophytes 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 whereas Prochlorococcus was the dominant component 15–46  during stratification periods During mixing periods nitrate and nitrite NO3 + NO2 concentrations occasionally exceeded 05 μmol kg−1 in surface waters and F v/F m photochemical efficiency of photosystem II ranged from approximately 040–050 within the euphotic zone During stratification periods however NO3 + NO2 concentrations were very low 01 μmol kg−1 from the surface to 50–95 m and F v/F m was in the range of 025–040 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 timeseries station ALOHA located in the eastern region of the NPSG At Sta ALOHA in 2011 ʃTChl a was almost constant 195–264 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 NPSGWe thank the captain and crew of the R/V Mirai for their support during the cruise and the staff of Marine Works Japan and Global Ocean Development for their onboard analysis and deck work We also thank M C Honda and two anonymous reviewers for comments and suggestions on this manuscript This work was carried out as part of the K2S1 project funded by the Environmental Biogeochemical Cycle Research Program of the Japan Agency for MarineEarth Science and Technology

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