CHAPTER 13 Nitrogen in the Atlantic Ocean Dennis A. Hansell and Michael J. Follows Contents 1. Introduction 597 2. Distribution of Nitrogen in the Atlantic 598 2.1. Nitrate 598 2.2. Dissolved organic nitrogen 602 2.3. Seasonal variations of surface nitrate 604 3. Sources of Nitrogen to the Euphotic Zone 609 3.1. Physical supply of nitrate 609 3.2. Geochemical investigations of nitrogen fixation 612 3.3. Atmospheric deposition and possible contribution to excess nitrate 617 4. Denitrification 619 5. The Atlantic as a Source of Nitrogen to the Atmosphere 619 6. Is the Atlantic a Source of Nitrogen to the Global Ocean? 620 7. Key Unresolved Issues 622 Acknowledgements 623 References 623 1. Introduction The location of the North Atlantic (between Europe and North America) has made it the focus of considerable study; some of the most important advances in marine nitrogen dynamics have been developed in the region (Lipschultz et al, 2002). The South Atlantic, in contrast, has undergone expeditionary surveys, but fewer process studies have been specific to that basin. The Atlantic is fed by the northward flow of upper ocean waters that are depleted in nitrate and phosphate, while North Atlantic deep waters, formed in the northern high latitudes, move southwards through the basin gathering remineralized nutrients. The proximity of the vigorous source of iron from Saharan dust suggests that productivity in the basin is limited by the availability of these macro-nutrients. The North Atlantic is unique among ocean basins with a nitrate:phosphate ratio (N:P) in thermocline waters in excess of the requirements of ‘‘average’’ Redfieldian organisms (Fanning, 1992), a situation commonly interpreted as a signature of N input by nitrogen fixation. Nitrogen in the Marine Environment # 2008 Elsevier Inc. DOI: 10.1016/B978-0-12-372522-6.00013-X All rights reserved. 597