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Preprints
https://doi.org/10.5194/bg-2020-300
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-300
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  18 Aug 2020

18 Aug 2020

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A revised version of this preprint is currently under review for the journal BG.

Cross-basin differences in the nutrient assimilation characteristics of induced phytoplankton blooms in the subtropical Pacific waters

Fuminori Hashihama1,2, Hiroaki Saito3, Taketoshi Kodama4,5, Saori Yasui-Tamura1, Jota Kanda1, Iwao Tanita4,6, Hiroshi Ogawa3, E. Malcolm S. Woodward7, Philip W. Boyd2, and Ken Furuya4,8 Fuminori Hashihama et al.
  • 1Department of Ocean Sciences, Tokyo University of Marine Science and Technology, Tokyo 108-8477, Japan
  • 2Institute for Marine and Antarctic Studies, University of Tasmania, Hobart TAS 7004, Australia
  • 3Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba 277-8564, Japan
  • 4Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan
  • 5Fisheries Resources Institute, Japan Fisheries Research and Education Agency, Kanagawa 236-8648, Japan
  • 6Fisheries Technology Institute, Japan Fisheries Research and Education Agency, Okinawa 907-0451, Japan
  • 7Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK
  • 8Graduate School of Science and Engineering, Soka University, Tokyo 192-8577, Japan

Abstract. To better understand the nutrient assimilation characteristics of subtropical phytoplankton, deep water addition incubation experiments were carried out on surface waters collected at seven stations across the subtropical North and South Pacific Ocean. These deep water additions induced phytoplankton blooms with nutrient drawdown at all stations. The drawdown ratios of dissolved inorganic nitrogen (DIN) to phosphate (PO4) varied from 14.1 to 29.6 at the PO4-replete stations in the central North Pacific (CNP) and eastern South Pacific (ESP). These ratios were similar to the range represented by the canonical Redfield ratio (16) through to typical particulate N:P ratios in the surface subtropical ocean (28). In contrast, lower DIN:PO4 drawdown ratios (8.0–12.9) were observed in induced blooms at the PO4-depleted stations in the western North Pacific (WNP). The DIN:PO4 drawdown ratios in the PO4-replete ESP were associated with eukaryote-dominated blooms, while those in PO4-depleted WNP were associated with eukaryotic and cyanobacterial blooms. The surplus PO4 assimilation, relative to DIN, by phytoplankton in the WNP was not expected based on their typical cellular N:P ratio, and was likely due to the high PO4 uptake capability as induced by low PO4-adapted phytoplankton. The low and high P* (=PO4-DIN/16) regimes geographically corresponded to the low and high DIN:PO4 drawdown ratios in the WNP and the CNP or ESP, respectively. The basin-wide P* distribution in the oligotrophic Pacific surface waters showed a clear regional trend from low in the WNP (<50 nM) to high in the ESP (>100 nM). These results suggest that the subtropical phytoplankton blooms as observed in our experiments could be an important factor controlling P* as well as the commonly recognized dinitrogen fixation and denitrification characteristics.

Fuminori Hashihama et al.

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Fuminori Hashihama et al.

Fuminori Hashihama et al.

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Short summary
We investigated the nutrient assimilation characteristics of deep-water induced phytoplankton blooms across the subtropical North and South Pacific Ocean. Nutrient drawdown ratios of dissolved inorganic nitrogen to phosphate were anomalously low in the western North Pacific, likely due to the high phosphate uptake capability of low phosphate-adapted phytoplankton. The anomalous phosphate uptake might influence the maintenance of chronic phosphate depletion in the western North Pacific.
We investigated the nutrient assimilation characteristics of deep-water induced phytoplankton...
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