Vertical Eddy Iron Fluxes Support Primary Production in the Open Southern Ocean

Abstract

The primary productivity of the Southern Ocean ecosystem, and associated biological carbon pump, is limited by the availability of the micronutrient iron. Riverine sediments and atmospheric dust supply iron at the ocean margins, but in the vast open ocean, iron reaches phytoplankton primarily when iron-rich sub-surface waters enter the euphotic zone, linking vertical transport processes to ecosystem productivity. In addition to mixed-layer entrainment, recent studies in the North Atlantic and Southern Ocean suggest that eddy transport may be a highly effective pathway for nutrient transport. Here, high-resolution physical/biogeochemical simulations of an open-Southern-Ocean ecosystem forced with a realistic seasonal cycle reveal that primary production is sustained via iron supply across the mixed layer base primarily due to mesoscale and submesoscale turbulence (hereafter ``(sub)mesoscale eddies''). As model resolution is increased from 20 km to 5 km to 2 km, vertical eddy iron flux and phytoplankton biomass increase strongly, despite shoaling of the mixed layer. Diagnostics from eddy resolving runs show that the increase in primary production is supported by iron supply due to (sub)mesoscale isopycnal stirring. We also highlight that properly tuned eddy parametrizations in non-eddying runs can replicate this isopycnal flux and consequently the amount of biomass. One important consequence is that iron recycling is second-order importance in explaining sustained summertime productivity, as eddies continue to supply iron to the mixed layer throughout the year. Since eddy mixing rates are sensitive to wind forcing and large-scale hydrographic changes, these findings open a new mechanism for modulating the Southern Ocean biological pump on climate timescales.