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Title: Longitudinal variability of size-fractionated N2 fixation and DON release rates along 24.5°N in the subtropical North Atlantic
Authors: Benavides, Mar
Bronk, Deborah A.
Agawin, Nona S.R.
Perez Hernandez, Maria Dolores 
Hernández-Guerra, Alonso 
Arístegui, Javier 
UNESCO Clasification: 2510 Oceanografía
Keywords: Diazotrophy
DON release
Dissolved 15N2
Unicellular diazotrophs
Saharan dust
Issue Date: 2013
Project: Expedición de Circunnavegación Malaspina 2010: Cambio Global y Exploración Del Océano Global 
Intercambio Plataforma-Oceano en El Ecosistema Marino de Las Islas Canarias-Peninsula Iberica (Caibex):Afloramiento de Cabo Guir 
Zonas de Mezcla y Frentes en El Océano Oscuro Como ¿Hot-Spots? de Biodiversidad y Flujos Biogeoquímicos A Través Del Mar Mediterráneo y Atlántico Nordeste - I. 
Journal: Journal of geophysical research. Oceans 
Abstract: Dinitrogen (N2) fixation and dissolved organic nitrogen (DON) release rates were measured on fractionated samples (>10 µm and <10 µm) along 24.5∘N in the subtropical North Atlantic. Net N2 fixation rates (N2 assimilation into biomass) ranged from 0.01 to 0.4 nmol N L−1 h−1, and DON release rates ranged from 0.001 to 0.09 nmol N L−1 h−1. DON release represented ∼14% and ∼23% of >10 µm and <10 µm gross N2 fixation (assimilation into biomass plus DON release), respectively. This implies that by overlooking DON release, N2 fixation rates are underestimated. Net N2 fixation rates were higher in the east and decreased significantly toward the west (rs = −0.487, p = 0.002, and rs = −0.496, p = 0.001, for the >10 µm and <10 µm fractions, respectively). The sum of both fractions correlated with aerosol optical depth at 550 nm (AOD 550 nm) (rs = 0.382, p = 0.017) and phosphate (PO43−) concentrations (rs = 0.453, p = 0.018), suggesting an enhancement of diazotrophy as a response to aerosol inputs and phosphorus availability. In contrast, DON release was constant among size fractions and did not correlate with any of these variables. We also compared N2 fixation rates obtained using the 15N2 dissolved and bubble methods. The first gave average rates 50% (49% ± 39) higher than the latter, which supports the finding that previously published N2 fixation rates are likely underestimated. We suggest that by combining N2 fixation and DON release measurements using dissolved 15N2, global N2 fixation rates could increase enough to balance oceanic fixed nitrogen budget disequilibria.
ISSN: 2169-9275
DOI: 10.1002/jgrc.20253
Source: Journal of geophysical research. Oceans [ISSN 2169-9275], v. 118 (7), p. 3406-3415
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