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East-west variabilities of N2 fixation activity in the subtropical North Pacific Ocean in summer: the field evidence of iron and phosphorus co-limitation in the western area
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  • Sachiko Horii,
  • Kazutaka Takahashi,
  • Takuhei Shiozaki,
  • Shigenobu Takeda,
  • Mitsuhide Sato,
  • Tamaha Yamaguchi,
  • Shota Takino,
  • Fuminori Hashihama,
  • Yoshiko Kondo,
  • Toshihiko Takemura,
  • Ken Furuya
Sachiko Horii
Fisheries Resources Institute, Japan Fisheries Research and Education Agency

Corresponding Author:shorii@affrc.go.jp

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Kazutaka Takahashi
The Unioversity of Tokyo
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Takuhei Shiozaki
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Shigenobu Takeda
Nagasaki University
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Mitsuhide Sato
Nagasaki University
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Tamaha Yamaguchi
National Research Institute of Fisheries Science
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Shota Takino
Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo
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Fuminori Hashihama
Tokyo University of Marine Science and Technology
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Yoshiko Kondo
Nagasaki University
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Toshihiko Takemura
Kyushu University
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Ken Furuya
Institute of Plankton Eco-engineering, Soka University
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Biological dinitrogen (N2) fixation is an important new nitrogen source in oligotrophic subtropical oceans. In numerical model studies, the east-west gradient of iron deposition as atmospheric Asian dust strongly affects the zonal distribution of N2 fixation activity in the North Pacific, but the in-situ relationship at a basin-scale is not well examined. We examined the trans-Pacific longitudinal variation in N2 fixation activity from 120°W to 137°E at 23°N in summer with environmental parameters that potentially influence diazotrophy. The dissolved inorganic iron concentration in surface water was consistently low (<0.4 nM) throughout the study area. The modelled deposition flux of iron as atmospheric dust (dust-Fe) largely increased westward, whereas labile phosphorus (phosphate and labile phosphoric monoesters) in the surface water decreased westward. N2 fixation varied between 34.6–298 µmol N m-2 day-1 and was high (>200 µmol m-2 day-1) in the central area (150–180°W), where both dust-Fe input and the phosphorus stock were in intermediate ranges. The rates of N2 fixation showed an increasing trend with dust-Fe input in the eastern and western parts of 180°, indicating that increasing dust input enhanced N2 fixation activity. However, compared with that of the eastern region, the effect of enhancement on activity was smaller in the western region, where phosphate concentration in the euphotic zone was low (<50 nM), presumably due to the higher iron requirement to utilize organic phosphorus. Our data show that phosphorus availability substantially controls the longitudinal distribution of N2 fixation through co-limitation with iron in the subtropical North Pacific.