Iron cycling during the decline of a south Georgia diatom bloom

Joanna Ainsworth, Alex J. Poulton, Maeve C. Lohan, Mark C. Stinchcombe, Alastair J. M. Lough, C. Mark Moore

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)
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Abstract

The Southern Ocean is the largest high nutrient low chlorophyll (HNLC) oceanic region, where iron limits phytoplankton growth and productivity and ultimately influences the Biological Carbon Pump (BCP). Natural exceptions to the HNLC regime occur where island wakes cause iron to be mixed into surface waters from sediments, enabling large, prolonged phytoplankton blooms and increased carbon drawdown. Interactions between iron and phytoplankton are reciprocal in blooms: with plankton regulating the (re)cycling of iron through cellular uptake and remineralisation. The depth of iron remineralisation then influences either re-supply to the surface mixed layer biota or sequestration into deeper waters. Water column trace metal observations and shipboard experiments, using bioassays and radioisotope (55Fe, 32Si, 14C) cycling, were undertaken to investigate surface mixed layer phytoplankton iron limitation, iron uptake, and mesopelagic iron remineralisation relative to carbon and silica within the November 2017 bloom downstream of South Georgia. Surface phytoplankton residing in the iron depleted mixed layer were iron limited throughout the four-week sampling period. Experiments designed to investigate particulate water column (re)cycling revealed limited iron remineralisation from freshly produced upper ocean particles. The main pathway of iron transfer from particulates into the dissolved phase was through rapid (<2 d) release of extra-cellular adsorbed iron, which, if occurring in situ, could contribute to observed higher sub-surface dissolved Fe concentrations. This was accompanied by a small loss of cellular carbon, likely through respiration of the fixed 14C, and limited dissolution of particulate 32Si to dissolved 32Si. Decoupling of the remineralisation length scales for Fe, C and Si, with Fe having the fastest turnover, is thus likely in the upper mesopelagic zone beneath the bloom.

Original languageEnglish
Article number105269
JournalDeep-Sea Research Part II: Topical Studies in Oceanography
Volume208
Early online date20 Jan 2023
DOIs
Publication statusPublished - Apr 2023

Keywords

  • Iron
  • Mesopelagic
  • Phytoplankton
  • Remineralisation
  • South Georgia bloom

ASJC Scopus subject areas

  • Oceanography

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