We investigated the seasonal variability of the phytoplankton community in the western subarctic gyre (WSG) of the northwestern North Pacific with respect to structure (abundance, size, and taxonomic composition) and photophysiological state from 2006 to 2012 by using the chemotaxonomy program CHEMTAX, microscopy, and fast-repetition-rate fluorometry. Chlorophyll a standing stock (#Chl a) varied seasonally from 20 to 52 mg m 22 and increased frequently to . 40 mg m 22 in June and July. Diatoms (20-35%) and prymnesiophytes (13-23%) comprised major portions of the #Chl a during the bloom period. Diatoms decreased to , 23% during the postbloom period, and prymnesiophytes became the most abundant group (24-35%). Mean Fv : Fm ratios (potential photochemical efficiency of photosystem II) in the mixed layer were relatively high (0.41-0.47) in winter and early spring, decreased rapidly to 0.32-0.39 concomitant with bloom development, and remained at low levels in the summer and autumn, although macronutrients [NO
were not fully consumed during the remainder of the year, the result being that the WSG was a highnutrient, low-chlorophyll (HNLC) region. The deep ML reduced primary production by reducing light availability in winter, whereas primary production was enhanced by strong light availability in the shallower ML as summer progressed. However, primary production was often attenuated by a reduction of light availability attributable to dense sea fog in summer. We found a significant relationship between primary production and light availability in this HNLC region. However, chlorophyll a was less variable seasonally than primary production. The highest depth-integrated chlorophyll a was observed in summer (54.6 6 13.4 mg m 22 ), but chlorophyll a remained high in winter (45.3 6 7.7 mg m 22 ). Reduced light availability depressed primary production, but a reduction of the chlorophyll a concentration was prevented by a relaxation of grazing in the deep ML during winter. We found that light availability exerted an important control on the seasonal variability of primary production and phytoplankton biomass in the WSG.
An empirical method is presented for the estimation of basin‐scale distribution of partial pressure of carbon dioxide (pCO2) in the North Pacific using satellite‐derived sea surface temperature (SST), chlorophyll‐a concentrations (chl a), and climatological sea surface salinity (SSS). In this approach, multiple regression equations were developed to compute mixed layer dissolved inorganic carbon (DIC) based on SST, SSS and Chl a, whereas mixed layer total alkalinity (TA) was linearly regressed with SSS. The DIC‐SST relation exhibited three different slopes at SST < 20°, 20° < SST < 27.5° and SST>27.5°C. Therefore data have been grouped with reference to SST. Regression equations were developed for two seasons (spring and summer). The regression errors for DIC and TA were 10.5 and 5 μmol kg−1, respectively. The pCO2 was computed from the estimated DIC and TA using dissociation constants given by Mehrbach et al. (1973), refit by Dickson and Millero (1987). The derived pCO2 agreed with the shipboard pCO2 observations within an error of 17–23 μatm. The sensitivity test on the regression equations for DIC estimation indicated that SSS is the most influencing parameter, followed by SST and Chl a. Using the monthly average SST and Chl a fields derived from the Advanced Very High Resolution Radiometer (AVHRR) and SeaWiFS (Sea‐viewing Wide Field of view Sensor), respectively, and climatological SSS, monthly basin‐scale pCO2 fields were computed. The statistical model derived pCO2 results are in agreement with underway pCO2 in the North Pacific. This study strongly suggests that satellite‐based techniques are promising tools for estimation of pCO2 fields on a basin scale but the associated error bars are larger than required to study anthropogenic carbon uptake by the oceans. Incorporation of more in situ shipboard data may help in refining the estimating equations and reducing the errors further.
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