Eddy iron fluxes control primary production in the open Southern Ocean

Takaya Uchida, Dhruv Balwada, Ryan Abernathey, Galen McKinley, Shafer Smith & Marina Lévy

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Aug 08, 2019
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Received Date: 30th July 19

Primary productivity of the Southern Ocean ecosystem, and the 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.Existing estimates of vertical iron transport focus on one-dimensional processes such as mixed-layer entrainment; however, evidence from the North Atlantic and from Lagrangian simulations suggests that eddy transport may be a highly effective pathway for nutrient supply.In this study, high-resolution physical/biogeochemical simulations of an open-Southern-Ocean ecosystem forced with a realistic seasonal cycle reveal that iron transport across the mixed layer base is primarily due to (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 and parametrized runs show that this transport is predominantly associated with (sub)mesoscale isopycnal stirring, rather than mixed-layer instability.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 biogeochemical pump on climate timescales.

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This is an abstract of a preprint hosted on an independent third party site. It has not been peer reviewed but is currently under consideration at Nature Communications.

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