Importance of N2-Fixation on the Productivity at the North-Western Azores Current/Front System, and the Abundance of Diazotrophic Unicellular Cyanobacteria.

• Published in: PLOS ONE
• Main Authors: RIOU, Virginie, Fonseca Batista, D., Roukaerts, A., Biegala, Isabelle C., Prakya, S. r., Loureiro, C. m., Santos, M., Muniz-piniella, A. e., Schmiing, M., Elskens, M., Brion, N., Martins, M.A., Dehairs, F.
• Other Authors: Institut méditerranéen d'océanologie (MIO), Institut de Recherche pour le Développement (IRD)-Aix Marseille Université (AMU)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Toulon (UTLN)-Centre National de la Recherche Scientifique (CNRS), 3, Institut de Recherche pour le Développement (IRD)-Aix Marseille Université (AMU)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Toulon (UTLN)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Aix Marseille Université (AMU)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Toulon (UTLN)-Centre National de la Recherche Scientifique (CNRS), ANCH-VUB, IMAR-UAç, CIBIO-UAç, IPMA-UAç, IMAR-MARE-UAç
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2016
• Subjects: North Atlantic; Azores Current Front System; N2-fixation; Primary production; Microcosm; Nitrogen compounds; Nitrate Isotopes; UCYN; Eddies; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; [SDV.EE.ECO]Life Sciences [q-bio]/Ecology; environment/Ecosystems
• Online Access: https://hal.archives-ouvertes.fr/hal-01455155; https://hal.archives-ouvertes.fr/hal-01455155/document; https://hal.archives-ouvertes.fr/hal-01455155/file/PONE-D-15-34460_R2.pdf; https://doi.org/10.1371/journal.pone.0150827

International audience 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.