An assessment of the vertical diffusive flux of iron and other nutrients to the surface waters of the subpolar North Atlantic Ocean

S C Painter,N Rogan,S A Henson,J Klar,M C Stinchcombe,S Steigenberger,A R Baker,E P Achterberg,A Forryan,C M Moore

doi:10.5194/bg-11-2113-2014

Abstract

Abstract. In this study we report diapycnal diffusive fluxes of dissolved iron (dFe), dissolved aluminium (dAl) and the major macronutrients to the surface waters of the North Atlantic subpolar gyre. Turbulent diffusivities at the base of the summer mixed layer ranged from 0.01 to 0.5 (median 0.07) cm2 s−1 and daily macronutrient fluxes into the surface mixed layer typically represented < 0.5% of integrated mixed layer inventories, although fluxes were highly variable. Elevated nutrient fluxes of up to 4% of mixed layer inventories were identified on the Greenland Shelf, where integrated nutrient pools were lowest due to localised shoaling of the mixed layer. Diffusive fluxes of dFe and dAl were typically <0.1% of mixed layer inventories but were also highly variable between stations. Approximations of daily phytoplankton nutrient and Fe uptake indicate that the diffusive flux may at best represent <10% of phytoplankton macronutrient uptake, and only 1% of daily phytoplankton Fe uptake. The daily turbulent diffusive flux of dFe was comparable in magnitude to coincident estimates of aeolian Fe supply but despite shallower than normal convective mixing in winter 2010 the diffusive supply was 22 and 59 times smaller than the annual convective supply of Fe to the Irminger and Iceland basins respectively. The general picture obtained from this study is one of small magnitude diffusive nutrient and Fe fluxes to the subpolar North Atlantic during the period of annual nutrient minima and indicates that the diffusive supply mechanism is unlikely to alleviate the recently identified presence of seasonal iron limitation within the North Atlantic subpolar gyre; a condition exacerbated by low dFe:NO3− ratios in subsurface source waters.

Highlights

In the subpolar North Atlantic, winter convective mixing represents the dominant nutrient supply process (Louanchi and Najjar, 2001)
The general picture obtained from this study is one of small magnitude diffusive nutrient and Fe fluxes to the subpolar North Atlantic during the period of annual nutrient minima and indicates that the diffusive supply mechanism is unlikely to alleviate the recently identified presence of seasonal iron limitation within the North Atlantic subpolar gyre; a condition exacerbated by low dFe : NO−3 ratios in subsurface source waters
We have demonstrated that the vertical daily diffusive flux of macronutrients into the mixed layer is a small source term in the subpolar North Atlantic Ocean typically representing < 0.5 % of mixed layer standing stocks

Summary

Introduction

In the subpolar North Atlantic, winter convective mixing represents the dominant nutrient supply process (Louanchi and Najjar, 2001). This annual event, which can reach 700–800 m depths or more (Bacon et al, 2003; Pickart et al, 2003; Yashayaev et al, 2007), recharges surface ocean nutrient inventories and subsequently fuels the spring phytoplankton bloom. During the remainder of the year nutrients are resupplied continuously or episodically through additional processes. Such processes include mesoscale and sub-mesoscale physical processes (e.g. eddies and fronts), vertical turbulent diffusion, lateral advection and atmospheric deposition. Surface NO−3 concentrations are not usually drawn down to exhaustion during the summer and in the Irminger and Iceland sub-basins have been shown to remain at concentrations

Methods

Results

Conclusion