How widespread and important is N2 fixation in the North Atlantic Ocean?

The spatial extent of N fixation in the Atlantic Ocean is examined by determining the isotopic composition of N in suspended particulate organic nitrogen (?N PON). The samples were collected from zonal and meridional transects of the Atlantic Ocean during a 3-year period. There is a consistent depleted ?N PON signal extending over the center of the northern subtropical gyre, which partly coincides with a region where the tracer N* increases westward following the gyre circulation. This nonconservative behavior of N* implies that N fixation is responsible for the depleted ? N PON . A mixing model suggests that N fixation over parts of the northern gyre provides up to 74% of the N utilized by phytoplankton. However, since the PON represents only a small fraction of the total N pool, N fixation probably only plays a minor role in supplying new N to the euphotic zone in the surface waters of the northern subtropical gyre. Copyright 2007 by the American Geophysical Union.