| Summary: | Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 28 (2014): 103-114, doi:10.1002/2013GB004652. A new observational synthesis of diazotrophic biomass and nitrogen fixation provides the opportunity for systematic quantitative evaluation of these aspects in biogeochemical models. One such model of the Atlantic Ocean is scrutinized, and the simulated biomass is found to be an order of magnitude too low. Initial attempts to increase biomass levels through decreasing grazing and other loss terms caused an unrealistic buildup of nitrate in the upper ocean. Two key changes to the model structure facilitated a closer match to the observed biomass and nitrogen fixation rates: addition of a pathway for export of diazotrophically fixed organic material and uptake of inorganic nitrogen by the diazotroph population. These changes, along with a few other revisions to existing model parameterizations, facilitate more accurate simulation of basin-scale distributions of diazotrophic biomass, as well as mesoscale variations contained therein. The resulting solutions suggest that the Trichodesmium spp. populations of the North Atlantic export the vast majority of the nitrogen they fix, a finding that awaits assessment through direct observation. Support of this research by the National Science Foundation and the National Aeronautics and Space Administration is gratefully acknowledged. 2014-08-20
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