title: Importance of N2-Fixation on the Productivity at the North-Western Azores Current/Front System, and the Abundance of Diazotrophic Unicellular Cyanobacteria.
abstract: To understand the impact of the northwestern Azores Current Front (NW-AzC/AzF) system on HCO3--and N2-fixation activities and unicellular diazotrophic cyanobacteria (UCYN) distribution, we combined geochemical and biological approaches from the oligotrophic surface to upper mesopelagic waters. N2-fixation was observed to sustain 45-85% of the HCO3--fixation in the picoplanktonic fraction performing 47% of the total C-fixation at the deep chlorophyll maximum north and south of the AzF. N2-fixation rates as high as 10.9 μmol N m-3 d-1 and surface nitrate δ15N as low as 2.7‰ were found in the warm (18-24°C), most saline (36.5-37.0) and least productive waters south of the AzF, where UCYN were the least abundant. However, picoplanktonic UCYN abundances up to 55 cells mL-1 were found at 45-200m depths in the coolest nutrient-rich waters north of the AzF. In this area, N2-fixation rates up to 4.5 μmol N m-3 d-1 were detected, associated with depth-integrated H13CO3--fixation rates at least 50% higher than observed south of the AzF. The numerous eddies generated at the NW-AzC/AzF seem to enhance exchanges of plankton between water masses, as well as vertical and horizontal diapycnal diffusion of nutrients, whose increase probably enhances the growth of diazotrophs and the productivity of C-fixers.
authors.affiliation: Aix-Marseille Université, Mediterranean Institute of Oceanography (MIO), UM 110 CNRS/INSU, IRD, 13288 Marseille, Université du Sud Toulon-Var, 83957, La Garde, France.|IMAR-Institute of Marine Research, Centre of IMAR at the University of the Azores, Horta, Portugal.
authors.forename: Virginie
authors.initials: V
authors.lastname: Riou
authors.affiliation: Aix-Marseille Université, Mediterranean Institute of Oceanography (MIO), UM 110 CNRS/INSU, IRD, 13288 Marseille, Université du Sud Toulon-Var, 83957, La Garde, France.|Analytical, Environmental and Geo-Chemistry & Earth System Sciences, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Debany
authors.initials: D
authors.lastname: Fonseca-Batista
authors.affiliation: Analytical, Environmental and Geo-Chemistry & Earth System Sciences, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Arnout
authors.initials: A
authors.lastname: Roukaerts
authors.affiliation: Aix-Marseille Université, Mediterranean Institute of Oceanography (MIO), UM 110 CNRS/INSU, IRD, 13288 Marseille, Université du Sud Toulon-Var, 83957, La Garde, France.
authors.forename: Isabelle C
authors.initials: IC
authors.lastname: Biegala
authors.affiliation: IMAR-Institute of Marine Research, Centre of IMAR at the University of the Azores, Horta, Portugal.
authors.forename: Shree Ram
authors.initials: SR
authors.lastname: Prakya
authors.affiliation: CIBIO, Research Center in Biodiversity and Genetic Resources, InBIO Associated Laboratory, Department of Oceanography and Fisheries, Horta, Portugal.|DOP/UAz - Department of Oceanography and Fisheries, University of the Azores, Azores, Portugal.
authors.forename: Clara
authors.initials: C
authors.lastname: Magalhães Loureiro
authors.affiliation: IPMA, I.P.-Portuguese Institute of Ocean and Atmosphere, Lisbon, Portugal.|MARE-Marine and Environmental Sciences Centre, Lisbon, Portugal.
authors.forename: Mariana
authors.initials: M
authors.lastname: Santos
authors.affiliation: Analytical, Environmental and Geo-Chemistry & Earth System Sciences, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Angel E
authors.initials: AE
authors.lastname: Muniz-Piniella
authors.affiliation: IMAR-Institute of Marine Research, Centre of IMAR at the University of the Azores, Horta, Portugal.|MARE-Marine and Environmental Sciences Centre, Lisbon, Portugal.|DOP/UAz - Department of Oceanography and Fisheries, University of the Azores, Azores, Portugal.
authors.forename: Mara
authors.initials: M
authors.lastname: Schmiing
authors.affiliation: Analytical, Environmental and Geo-Chemistry & Earth System Sciences, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Marc
authors.initials: M
authors.lastname: Elskens
authors.affiliation: Analytical, Environmental and Geo-Chemistry & Earth System Sciences, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Natacha
authors.initials: N
authors.lastname: Brion
authors.affiliation: CIBIO, Research Center in Biodiversity and Genetic Resources, InBIO Associated Laboratory, Department of Oceanography and Fisheries, Horta, Portugal.|DOP/UAz - Department of Oceanography and Fisheries, University of the Azores, Azores, Portugal.
authors.forename: M Ana
authors.initials: MA
authors.lastname: Martins
authors.affiliation: Analytical, Environmental and Geo-Chemistry & Earth System Sciences, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Frank
authors.initials: F
authors.lastname: Dehairs
mesh_terms: D001394:Azores; D002244:Carbon / Q000032:analysis; D000458:Cyanobacteria / Q000378:metabolism*; D005843:Geography; D009584:Nitrogen / Q000032:analysis; D009586:Nitrogen Fixation; D009587:Nitrogen Isotopes; D010316:Particle Size; D052638:Particulate Matter / Q000032:analysis; D010758:Phosphorus / Q000032:analysis; D012623:Seawater / Q000382:microbiology; D014872:Water Movements
chemical_list: D009587:Nitrogen Isotopes; D052638:Particulate Matter; D010758:Phosphorus; D002244:Carbon; D009584:Nitrogen