Distinct iron cycling in a Southern Ocean eddy

Michael J Ellwood,Philip W Boyd,Thomas W Trull,Peter G Strutton,Marion Fourquez,Robert F Strzepek

doi:10.1038/s41467-020-14464-0

Abstract

Mesoscale eddies are ubiquitous in the iron-limited Southern Ocean, controlling ocean-atmosphere exchange processes, however their influence on phytoplankton productivity remains unknown. Here we probed the biogeochemical cycling of iron (Fe) in a cold-core eddy. In-eddy surface dissolved Fe (dFe) concentrations and phytoplankton productivity were exceedingly low relative to external waters. In-eddy phytoplankton Fe-to-carbon uptake ratios were elevated 2–6 fold, indicating upregulated intracellular Fe acquisition resulting in a dFe residence time of ~1 day. Heavy dFe isotope values were measured for in-eddy surface waters highlighting extensive trafficking of dFe by cells. Below the euphotic zone, dFe isotope values were lighter and coincident with peaks in recycled nutrients and cell abundance, indicating enhanced microbially-mediated Fe recycling. Our measurements show that the isolated nature of Southern Ocean eddies can produce distinctly different Fe biogeochemistry compared to surrounding waters with cells upregulating iron uptake and using recycling processes to sustain themselves.

Highlights

DFe concentrations in the upper 200 m were 35–68 and 57–210 pmol kg−1 for the SAZ and Southern Ocean Time Series (SOTS) stations, respectively (Fig. 1), consistent with measurements for this region and the Southern Ocean generally (Table 1)[15]
Background dissolved Fe concentration beforeFe infusion. #Fe:C and Fe uptake rates are for days 1–3 after Fe infusion.Fe uptake rate25 ± 5 12 ± 2 28 ± 6 3.07# 290–360 not reported 60 55 77 ± 10Dissolved Fe 440 ± 70
Our study focused on one such eddy to understand the biogeochemical cycling of Fe in this isolated structure

Summary

Introduction

DFe concentrations in the upper 200 m were 35–68 and 57–210 pmol kg−1 for the SAZ and SOTS stations, respectively (Fig. 1), consistent with measurements for this region and the Southern Ocean generally (Table 1)[15]. Like dFe, in-eddy particulate Fe (pFe) concentrations were extremely low with values between 13 and 29 pmol kg−1 for the upper 200 m (Fig. 3; Supplementary Table 3).

Results

Conclusion