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

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.