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Iron depletion in the deep chlorophyll maximum: mesoscale eddies as natural iron fertilization experiments
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  • Nicholas J. Hawco,
  • Benedetto Barone,
  • Matthew J Church,
  • Lydia Babcock-Adams,
  • Daniel J Repeta,
  • Emma Wear,
  • Rhea K Foreman,
  • Karin M. Björkman,
  • Shavonna Bent,
  • Benjamin A.S. Van Mooy,
  • Uri Sheyn,
  • Edward F. DeLong,
  • Marianne Acker,
  • Rachel L. Kelly,
  • Alex Nelson,
  • John Ranieri,
  • Tara Clemente,
  • David M Karl,
  • Seth G John
Nicholas J. Hawco
University of Hawaiʻi at Mānoa

Corresponding Author:[email protected]

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Benedetto Barone
University of Hawaii at Manoa
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Matthew J Church
University of Montana
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Lydia Babcock-Adams
Woods Hole Oceanographic Institution
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Daniel J Repeta
Woods Hole Oceanographic Institution
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Emma Wear
University of Montana
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Rhea K Foreman
University of Hawaii - Manoa
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Karin M. Björkman
University of Hawaii at Manoa
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Shavonna Bent
Woods Hole Oceanographic Institution
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Benjamin A.S. Van Mooy
Woods Hole Oceanographic Institution
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Uri Sheyn
University of Hawaii at Manoa
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Edward F. DeLong
University of Hawaii
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Marianne Acker
Woods Hole Oceanographic Institution
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Rachel L. Kelly
University of Southern California
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Alex Nelson
University of Hawaii Manoa
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John Ranieri
University of Montana
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Tara Clemente
University of Hawaii at Manoa
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David M Karl
Department of Oceanography, University of Hawaii, Honolulu, HI 96822, USA
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Seth G John
University of Southern California
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Abstract

In stratified oligotrophic waters, phytoplankton communities forming the deep chlorophyll maximum (DCM) are isolated from atmospheric iron sources above and remineralized iron below. Reduced supply leads to a minimum in dissolved iron (dFe) near 100 m, but it is unclear if iron limits growth at the DCM. Here, we propose that natural iron addition events occur regularly with the passage of mesoscale eddies, which alter the supply of dFe and other nutrients relative to the supply of light, and can be used to test for iron limitation at the DCM. This framework is applied to two eddies sampled in the North Pacific Subtropical Gyre. Observations in an anticyclonic eddy center indicated downwelling of iron-rich surface waters, leading to increased dFe at the DCM but no increase in productivity. In contrast, uplift of isopycnals within a cyclonic eddy center increased supply of both nitrate and dFe to the DCM, and led to dominance of picoeukaryotic phytoplankton. Iron addition experiments did not increase productivity in either eddy, but did enhance leucine incorporation at ambient light in the cyclonic eddy, a potential indicator of iron stress among Prochlorococcus. Rapid cycling of siderophores and low dFe:nitrate uptake ratios also indicate that a portion of the microbial community was stressed by low iron. However, near-complete nitrate drawdown in this eddy, which represents an extreme case in nutrient supply compared to nearby Hawaii Ocean Time-series observations, suggests that recycling of dFe in oligotrophic ecosystems is sufficient to avoid iron limitation in the DCM under typical conditions.
Dec 2021Published in Global Biogeochemical Cycles volume 35 issue 12. 10.1029/2021GB007112