Dissolved organic nitrogen in the global surface ocean: Distribution and fate Robert T. Letscher, 1,2 Dennis A. Hansell, 1 Craig A. Carlson, 3 Rick Lumpkin, 4 and Angela N. Knapp 1 Received 16 July 2012; revised 13 November 2012; accepted 27 November 2012; published 26 February 2013. [1] The allochthonous supply of dissolved organic nitrogen (DON) from gyre margins into the interior of the ocean’s oligotrophic subtropical gyres potentially provides an important source of new N to gyre surface waters, thus sustaining export production. This process requires that a fraction of the transported DON be available to euphotic zone photoautotroph communities via mineralization. In this study, we investigated the biological and physical controls on the distribution and fate of DON within global ocean surface waters. Inputs of nitrate to the euphotic zone at upwelling zones fuel net accumulation of a DON pool that appears to resist rapid microbial remineralization, allowing subsequent advective transport into the subtropical gyres. Zonal gradients in DON concentrations across these gyres imply a DON sink in the surface layer. Assessment of the physical dynamics of gyre circulation and winter mixing revealed a pathway for DON removal from the mixed layer via vertical transport to the deep euphotic zone, which establishes the observed zonal gradients. Incubation experiments from the Florida Straits indicated surface-accumulated DON was largely resistant (over a few months) to utilization by the extant surface bacterioplankton community. In contrast, this same material was remineralized three times more rapidly when exposed to upper mesopelagic bacterioplankton. These results suggest the primary fate of surface DON to be removal via vertical mixing and subsequent mineralization below the mixed layer, implying a limited role for direct DON support of gyre export production from the surface layer. DON may contribute to export production at the eastern edges of the subtropical gyres, but only after its mineralization within the deep euphotic zone. Citation: Letscher, R. T., D. A. Hansell, C. A. Carlson, R. Lumpkin, and A. N. Knapp (2013), Dissolved organic nitrogen in the global surface ocean: Distribution and fate, Global Biogeochem. Cycles, 27, 141–153, doi:10.1029/2012GB004449. 1. Introduction [2] Over much of the surface global ocean (upper 200 m), most of the standing stock of fixed nitrogen (N) is in the form of dissolved organic nitrogen (DON) [Bronk, 2002; Aluwihare and Meador, 2008]. Accumulation of N within this pool results from a decoupling of DON production and consumption processes primarily carried out by autotrophic plankton and heterotrophic bacterioplankton, respectively. This biologically recalcitrant material can accumulate via direct production [McCarthy et al., 2004] or diagenetic alteration of the molecular structure [Amon and Benner, 1996]. However, if a portion of the euphotic zone (<100 m) bulk DON pool eventually becomes bioavailable to the resident microbial community, then the remineralized N can represent a potential source of new N to support primary and export production in oligotrophic systems. [3] Prior efforts to quantify the sustenance of upper ocean productivity by DON have focused on the Atlantic Ocean where the greatest density of DON observations exists and a clear east-to-west gradient in DON concentration has been observed (>5 mmol kg -1 in the east to ~4.5 mmol kg -1 in the west) [Mahaffey et al., 2004; Roussenov et al., 2006; Charria et al., 2008; Torres-Valdés et al., 2009]. These studies have considered the allochthonous input of semila- bile DON (lifetime of months to years), generated at the productive gyre margins, to the subtropical gyre interior and its potential role as an organic nutrient for enhancing export production there. In order for allochthonous DON to be a quantitatively important source of new N, a substantial fraction of the advected DON must become bioavailable to the photoautotrophic community within the euphotic zone. Mechanisms that make DON bioavailable to photoautotrophs include direct assimilation [Bronk et al., 2007], extracellular 1 Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA. 2 Now at Earth System Science, University of California, Irvine, California, USA. 3 Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California, USA. 4 Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, Florida, USA. Corresponding author: Robert T. Letscher, Earth System Science, University of California, Irvine, California, USA. (robert.letscher@uci.edu) ©2012. American Geophysical Union. All Rights Reserved. 0886-6236/13/2012GB004449 141 GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 27, 141–153, doi:10.1029/2012GB004449, 2013