Vertical eddy iron fluxes support primary production in the open Southern Ocean

Takaya Uchida,Dhruv Balwada,Marina Lévy,Galen A Mckinley,Ryan P Abernathey,Shafer K Smith

doi:10.1038/s41467-020-14955-0

Abstract

The primary productivity of the Southern Ocean ecosystem is limited by iron availability. Away from benthic and aeolian sources, iron reaches phytoplankton primarily when iron-rich subsurface waters enter the euphotic zone. Here, eddy-resolving physical/biogeochemical simulations of a seasonally-forced, open-Southern-Ocean ecosystem reveal that mesoscale and submesoscale isopycnal stirring effects a cross-mixed-layer-base transport of iron that sustains primary productivity. The eddy-driven iron supply and consequently productivity increase with model resolution. We show the eddy flux can be represented by specific well-tuned eddy parametrizations. Since eddy mixing rates are sensitive to wind forcing and large-scale hydrographic changes, these findings suggest a new mechanism for modulating the Southern Ocean biological pump on climate timescales.

Highlights

The primary productivity of the Southern Ocean ecosystem is limited by iron availability
The model configuration is identical to a companion paper[26] where we quantify the relative contribution of submesoscale and mesoscale dynamics on the total vertical iron transport
We focus on the biogeochemical effect of eddy iron transport on primary production and whether eddy parametrizations in non-eddying runs can replicate this unresolved flux

Summary

Introduction

The primary productivity of the Southern Ocean ecosystem is limited by iron availability. Eddy-resolving physical/biogeochemical simulations of a seasonally-forced, open-Southern-Ocean ecosystem reveal that mesoscale and submesoscale isopycnal stirring effects a cross-mixed-layer-base transport of iron that sustains primary productivity. Calculating iron concentration in the reference frame of Lagrangian particles, Rosso et al.[19,20] argued that submesoscale iron fluxes could enhance primary productivity by a factor of two While suggestive, their simulation technique did not implement a full ecosystem model, account for the strong seasonal cycle in both turbulence and biology, nor include fluxes from vertical mixing or mixed-layer entrainment. By varying the model resolution, we resolve, suppress, or parametrize the eddies and show that eddy iron transport modulates primary production in the open Southern Ocean

Methods

Results

Conclusion