Sources of Nitrogen to the Euphotic Zone

The availability of nitrogen in the euphotic zone is an important, potentially limiting factor for productivity and the biological sequestration of carbon in the ocean. There are three principal routes by which new nitrogen makes its way into the euphotic zone of the Atlantic Ocean Physical transport of nitrate, nitrogen fixation by diazotrophic organisms, and aeolian transport and deposition. 

In the subpolar gyre and in the northern sector of the subtropical gyre, the nutrient stream encounters the deep winter mixed-layers and outcrops (Fig. 13.9A), thus sustaining the annual convective supply of nitrate to the surface (WUhams and 

Follows, 2003). There, seasonal variations of surface ocean mixing drive a wintertime supply of nitrate to the euphotic zone (Glover and Brewer, 1988), fueUingmost of the annual export production. 

While the North Atlantic subtropical gyre shows the classical signatures of ohgotrophy, with low surface nutrient concentrations and htde surface chlorophyll, 

While some of the nutrient stream nitrate originates in the southern hemisphere with SAMW, a fraction has recirculated within the subtropical gyre (Fig. 13.9B). Jenkins and Doney (2003) hypothesize a nutrient spiral in which enhanced diapycnal mixing associated with the eddy-rich, western boundary current also leads to a significant vertical transfer of nutrients (and other properties including helium) into the mixed-layer of the subtropical gyre. 

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