Iron cycling during the decline of a South Georgia diatom bloom

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.

Iron, Mesopelagic, Phytoplankton, Remineralisation, South Georgia bloom

10.1016/j.dsr2.2023.105269

0967-0645

Ainsworth, Joanna Jane

f39170cb-c746-424b-a251-1cebafbc6981

Poulton, A.

ddf1ce85-0567-4d18-bb10-f5a1893196a8

Lohan, Maeve

6ca10597-2d0f-40e8-8e4f-7619dfac5088

Stinchcombe, Mark C.

433dd398-15f7-4730-9f1e-992d65bec70b

Lough, Alastair J.M.

64be5e9d-9ca2-438c-b566-cfee88ff1a37

Moore, C. Mark

7ec80b7b-bedc-4dd5-8924-0f5d01927b12

April 2023

Ainsworth, Joanna Jane

f39170cb-c746-424b-a251-1cebafbc6981

Poulton, A.

ddf1ce85-0567-4d18-bb10-f5a1893196a8

Lohan, Maeve

6ca10597-2d0f-40e8-8e4f-7619dfac5088

Stinchcombe, Mark C.

433dd398-15f7-4730-9f1e-992d65bec70b

Lough, Alastair J.M.

64be5e9d-9ca2-438c-b566-cfee88ff1a37

Moore, C. Mark

7ec80b7b-bedc-4dd5-8924-0f5d01927b12

Ainsworth, Joanna Jane, Poulton, A., Lohan, Maeve, Stinchcombe, Mark C., Lough, Alastair J.M. and Moore, C. Mark (2023) Iron cycling during the decline of a South Georgia diatom bloom. Deep Sea Research Part II: Topical Studies in Oceanography, 208, [105269]. (doi:10.1016/j.dsr2.2023.105269).

Record type: Article

Abstract

Text

Ainsworth et al. 2023 preprint - Accepted Manuscript

Available under License Creative Commons Attribution.

Download (8MB)

Text

1-s2.0-S096706452300019X-main - Version of Record

Available under License Creative Commons Attribution.

Download (7MB)

More information

Accepted/In Press date: 19 January 2023

e-pub ahead of print date: 20 January 2023

Published date: April 2023

Additional Information: Funding Information: This work was supported by the Natural Environment Research Council grant for the COMICS project. Grant codes: NE/M020835/1, NE/M020835/2, NE/M02072X/1, Studentship (1942696).The authors would like to thank the crew and technicians on the RSS Discovery for a very productive cruise, the COMICS project team for providing a very comprehensive dataset, funded by NERC (Natural Environment Research Council) and Hannah East for analysing the POC. The in situ chlorophyll time series was collected, calibrated and processed by Nathan Briggs and Filipa Carvalho, funded through a European Research Council Consolidator grant (GOCART, agreement 724416, PI: Stephanie Henson). We would also like to thank Sea Technology Services (STS) for technical assistance with glider deployments, this work was supported by South Africa's Department of Science and Innovation (DST/CON 0182/2017) and the National Research Foundation (SANAP: SNA170522231782). Associated CTD data is available online through the British Oceanographic Data Centre (BODC). Funding Information: This work was supported by the Natural Environment Research Council grant for the COMICS project. Grant codes: NE/M020835/1 , NE/M020835/2 , NE/M02072X/1 , Studentship (1942696). Funding Information: The authors would like to thank the crew and technicians on the RSS Discovery for a very productive cruise, the COMICS project team for providing a very comprehensive dataset, funded by NERC ( Natural Environment Research Council ) and Hannah East for analysing the POC. The in situ chlorophyll time series was collected, calibrated and processed by Nathan Briggs and Filipa Carvalho, funded through a European Research Council Consolidator grant ( GOCART , agreement 724416 , PI: Stephanie Henson). We would also like to thank Sea Technology Services (STS) for technical assistance with glider deployments, this work was supported by South Africa's Department of Science and Innovation (DST/CON 0182/2017) and the National Research Foundation ( SANAP: SNA170522231782 ). Associated CTD data is available online through the British Oceanographic Data Centre (BODC). Publisher Copyright: © 2023 The Authors

Keywords: Iron, Mesopelagic, Phytoplankton, Remineralisation, South Georgia bloom

Learn more about Institute for Life Sciences research Learn more about School of Ocean and Earth Science research