East‐West Variabilities of N2 Fixation Activity in the Subtropical North Pacific Ocean in Summer: Potential Field Evidence of the Phosphorus and Iron Co‐Limitation in the Western Area

Author:

Horii S.12ORCID,Takahashi K.2,Shiozaki T.3,Takeda S.4,Sato M.24ORCID,Yamaguchi T.25,Takino S.2,Hashihama F.6ORCID,Kondo Y.4ORCID,Takemura T.7ORCID,Furuya K.8

Affiliation:

1. Fisheries Resources Institute Japan Fisheries Research and Education Agency Nagasaki Japan

2. Department of Aquatic Bioscience Graduate School of Agricultural and Life Sciences The University of Tokyo Bunkyo Japan

3. Atmosphere and Ocean Research Institute The University of Tokyo Kashiwa Japan

4. Graduate School of Fisheries and Environmental Sciences Nagasaki University Nagasaki Japan

5. Fisheries Resources Institute Japan Fisheries Research and Education Agency Yokohama Japan

6. Department of Ocean Sciences Tokyo University of Marine Science and Technology Minato Japan

7. Research Institute for Applied Mechanics Kyushu University Kasuga Japan

8. Institute of Plankton Eco‐engineering Soka University Hachiohji Japan

Abstract

AbstractIn the subtropical North Pacific, the east‐west gradient of iron deposition as atmospheric Asian dust strongly affects the zonal distribution of biological N2 fixation activity in numerical models, but the in‐situ relationship at a basin‐scale is not well examined. We examined the trans‐Pacific zonal variation in N2 fixation activity on 23°N in summer along with environmental parameters that potentially influence diazotrophy. Dissolved inorganic iron (DFe) was consistently low (<0.4 nM) throughout the transect. The atmospheric dust iron (dust‐Fe) flux increased westward, whereas phosphate and labile phosphoric monoesters in the surface decreased westward. N2 fixation varied between 34.6 and 298 μmol‐N m−2 d−1 and was high (>200 μmol‐N m−2 d−1) in the central area (150–180°W). N2 fixation rates significantly increased with dust‐Fe input only in the western area (137–180°E), whereas the contribution of DFe diffused from below the euphotic zone was often larger in the eastern area (120–170°W). N2 fixation was considerably low in the phosphate‐depleted western area despite the excess amount of iron relative to phosphate, and it increased with the labile phosphoric monoesters stock. These indicated that N2 fixation was primarily limited by phosphorus in the western area, though this activity also increased with iron supply from dust, likely due to phosphorus‐iron co‐limitation. In contrast, in the phosphorus‐repleted eastern area, iron supplied from dust and below the euphotic zone appeared to limit N2 fixation. Overall, N2 fixation in the subtropical North Pacific was likely limited by zonally different factors relating to iron and phosphorus availability.

Funder

Japan Society for the Promotion of Science

National Institute for Physiological Sciences

Publisher

American Geophysical Union (AGU)

Subject

Earth and Planetary Sciences (miscellaneous),Space and Planetary Science,Geochemistry and Petrology,Geophysics,Oceanography

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