| Summary: | Sustaining phytoplankton primary production requires the physical supply of nutrients to the thermocline and the mixed layer. In the extensive downwelling regions of the subtropical gyres, the pathways of this nutrient supply remain unclear. Global estimates of mesoscale eddy mixing and internal-wave driven diapycnal mixing and climatological nitrate, salinity, and temperature data are combined to explore the relative importance of microscale turbulence and eddy stirring in supplying nutrients to phytoplankton in the North Atlantic subtropical gyre. The nutrient fluxes by eddy stirring and microscale turbulence are estimated in terms of their advective and diffusive transfers associated with the evolution of a nutrient. Mesoscale eddy stirring and microscale turbulence together supply an average of 0.10 mol N m -2 yr -1 to the mixed layer over the gyre. There is substantial spatial variation in the relative importance of the eddy- and turbulence-driven supplies across the gyre, where eddy stirring dominates over microscale turbulence in the northern and south-western gyre and vice versa in the south-eastern gyre. Given the combined nitrate supply by eddy stirring and microscale turbulence to the mixed layer falls short of nitrate export over 90% of the gyre, we suggest that mesoscale eddy stirring and microscale turbulence does not sustain phytoplankton primary production in the subtropical gyres alone. Other mixing mechanisms such as time-varying eddy circulations and double diffusion by salt fingers are expected to be important.
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