Contribution of N2 fixation to new production and excess nitrogen in the subtropical northeast Atlantic margin

• Published in: Deep Sea Research Part I: Oceanographic Research Papers
• Main Authors: Benavides, M., Arístegui, J., Agawin, N.S.R. (Nona S.R.), Álvarez-Salgado, X.A. (Xosé Antón), Álvarez, M. (Marta), Troupin, C. (Charles)
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Centro Oceanográfico de A Coruña 2014
• Subjects: North Atlantic; Northeast Atlantic
• Online Access: http://hdl.handle.net/10508/8571; http://hdl.handle.net/10261/316585; https://doi.org/10.1016/j.dsr.2013.07.004

We used 15N-labeled substrates to measure dinitrogen (N2) fixation, nitrate and ammonium uptake, regeneration and associated dissolved organic nitrogen (DON) release in a coastal upwelling system (Cape Ghir, ∼31°N) and an open ocean grid (bounded between 25°–42°N and 20°W) in the Canary Current region during the summer of 2009. New production (Pnew= uptake+N2 fixation+DON released from uptake− regeneration) was higher in the upwelling than in the open ocean zone (0.126 and 0.014 µmol N L−1 h−1, respectively), while regenerated production (Preg= uptake+DON released from uptake+ regeneration) was similar in both zones (0.157 and 0.133 µmol N L−1 h−1, respectively). The resulting f-ratio (Pnew/Pnew+Preg) for the open ocean and upwelling zones was 0.08 and 0.48, respectively. The availability of nitrogen in excess of that expected from Redfield stoichiometry is generally attributed to N2 fixation. A previous study indicated that our open ocean grid zone had an excess nitrogen production rate of 40±22×1010 mol N yr−1. We revisited this budget including new dissolved organic matter and fluxes through the Strait of Gibraltar and estimated a revised nitrogen excess rate of 22±19×1010 mol N yr−1. The average volumetric rate of N2 fixation for this zone was only 1.3×10−3 nmol N L−1 d−1, indicating that its influence in Pnew and nitrogen excess production in this part of the Atlantic is negligible. 2