Seasonal effects of photophysiology and chlorophyll a abundance on phytoplankton group-specific primary production in the Kuroshio region as revealed by SeaStar/SeaWiFS

Abstract. To evaluate the group-specific primary production of diatoms, haptophytes, and cyanobacteria in the Kuroshio region, a novel satellite observation methodology using the SeaStar/SeaWiFS satellite instrument was developed. The method used bio-optical relationships between the group-specific production and bio-optical properties such as the photosynthetic quantum yield and chlorophyll a specific optical absorption coefficient of phytoplankton, the last two of which were also estimated together with the group-specific production rather than assumed a priori. A global property of the absorption coefficient of phytoplankton, that the coefficient value at the wavelength of 510 nm was close to their spectral average, was highlighted in the method for a use of multi-spectral ocean color satellite data. Our results showed that the derived quantum yield index was higher for diatoms than haptophytes and cyanobacteria. Furthermore, intraspecific variation in the index, emerged as a latitudinal gradient: the values for cyanobacteria increased towards the higher latitudes. The group-specific primary production in the Kuroshio region showed that the climatological average of 134, 72 and 40 mg C m⁻² day⁻¹ for diatoms, haptophytes, and cyanobacteria, respectively. A comparison among variability of the group-specific primary production, the quantum yield index, and the absorption coefficient suggested that, in the Kuroshio region, the primary production due to diatoms was driven by their abundance through the year, whereas that due to cyanobacteria by photophysiology. The production due to haptophytes was seasonally regulated by both abundance and photophysiology.