Modeling how surface nitrogen fixation influences subsurface nutrient patterns in the North Atlantic

Abstract

We represented mechanistically the process of nitrogen (N) fixation and associated N* anomalies in the Atlantic Ocean using a three-dimensional coupled physical/biogeochemical model. Available direct measurements of N fixation rates in the Atlantic Ocean are compiled, and these, along with observed N* anomalies, constrain the model. The model N fixation rate for the whole Atlantic domain is 2.1 × 1012 molN yr-1. The model-generated N* anomaly shows the observed feature of a subsurface maximum. When plotted on isopycnal surfaces, the model-generated N* anomaly bears little relation to the pattern of N fixation at the surface. However, the highest N fixation rates should be spatially related to N* distribution if particulate export is remineralized at depths in the same region where the N fixation occurred. We performed case studies varying remineralization and advection to clarify the genesis of the N* anomaly and to determine the reasons underlying differences between N* anomalies and N fixation rate patterns. These studies indicated that the difference between these two patterns was created by both horizontal advection of excess N compared to phosphorus (P) and preferential remineralization of P compared to N. N fixation and preferential P remineralization create high N* anomalies both at the surface and in subsurface waters in the tropical Atlantic, which are transported into the northwestern North Atlantic by western boundary currents and subsequently subducted. As a result, the highest N* anomalies are located not in the tropics but in the northwestern North Atlantic. ©2013. American Geophysical Union. All Rights Reserved.