Résumé
Diazotrophic activity and primary production (PP) were investigated along two transects (Belgica BG2014/14 and GEOVIDE cruises) off the western Iberian Margin and the Bay of Biscay in May 2014. Substantial N2 fixation activity was observed at 8 of the 10 stations sampled, ranging overall from 81 to 384 μmolNm-2 d-1 (0.7 to 8.2 nmolNL-1 d-1, with two sites close to the Iberian Margin situated between 38.8 and 40.7° N yielding rates reaching up to 1355 and 1533 μmolNm-2 d-1. Primary production was relatively lower along the Iberian Margin, with rates ranging from 33 to 59 mmol Cm-2 d-1, while it increased towards the northwest away from the peninsula, reaching as high as 135 mmol Cm-2 d-1. In agreement with the area-averaged Chl a satellite data contemporaneous with our study period, our results revealed that post-bloom conditions prevailed at most sites, while at the northwesternmost station the bloom was still ongoing. When converted to carbon uptake using Redfield stoichiometry, N2 fixation could support 1% to 3% of daily PP in the euphotic layer at most sites, except at the two most active sites where this contribution to daily PP could reach up to 25 %. At the two sites where N2 fixation activity was the highest, the prymnesiophyte.symbiont Candidatus Atelocyanobacterium thalassa (UCYN-A) dominated the nifH sequence pool, while the remaining recovered sequences belonged to non-cyanobacterial phylotypes. At all the other sites, however, the recovered nifH sequences were exclusively assigned phylogenetically to non-cyanobacterial phylotypes. The intense N2 fixation activities recorded at the time of our study were likely promoted by the availability of phytoplanktonderived organic matter produced during the spring bloom, as evidenced by the significant surface particulate organic carbon concentrations. Also, the presence of excess phosphorus signature in surface waters seemed to contribute to sustaining N2 fixation, particularly at the sites with extreme activities. These results provide a mechanistic understanding of the unexpectedly high N2 fixation in productive waters of the temperate North Atlantic and highlight the importance of N2 fixation for future assessment of the global N inventory.
| Langue d'origine | English |
|---|---|
| Pages (de-à) | 999-1017 |
| Nombre de pages | 19 |
| Journal | Biogeosciences |
| Volume | 16 |
| Numéro de publication | 5 |
| DOI | |
| Statut de publication | Published - mars 13 2019 |
Note bibliographique
Funding Information:Acknowledgements. We thank the Captains and the crews of R/V Belgica and R/V Pourquoi pas? for their skilful logistic support. A very special thank goes to the chief scientists Géraldine Sarthou and Pascale Lherminier of the GEOVIDE expedition for the great work experience and wonderful support on-board. We would like to give special thanks to Pierre Branellec, Michel Hamon, Catherine Kermabon, Philippe Le Bot, Stéphane Leizour, Olivier Ménage (Laboratoire d’Océanographie Physique et Spatiale), Fabien Pérault and Emmanuel de Saint Léger (Division Technique de l’INSU, Plouzané, France) for their technical expertise during clean CTD deployments. We thank Arnout Roukaerts and David Verstraeten for their assistance with laboratory analyses at the Vrije Universiteit Brussel. We acknowledge Ryan Barkhouse for the collection of the DNA samples during the GEOVIDE cruise, Jennifer Tolman and Jenni-Marie Ratten for the nifH amplification and tag sequencing. David Lherminer, Paul Tréguer, Emilie Grossteffan and Manon Le Goff are gratefully acknowledged for providing us with the shipboard physicochemical data including CTD and nitrate plus nitrite data from the GEOVIDE expedition. Ship time for the Belgica BG2014/14 cruise was granted by Operational Directorate “Natural Environment” (OD Nature) of the Royal Institute of Natural Sciences, Belgium. OD Nature (Ostend) is also acknowledged for their assistance in CTD operations and data acquisition on-board the R/V Belgica. This work was financed by the Flanders Research Foundation (FWO contract G0715.12N) and Vrije Universiteit Brussel, R&D, Strategic Research Plan “Tracers of Past & Present Global Changes”, and is a Belgian contribution to SOLAS. Additional funding was provided by the Fund for Scientific Research – FNRS (F.R.S.-FNRS) of the Wallonia-Brussels Federation (convention no. J.0150.15). Xuefeng Li was a FNRS doctorate aspirant fellow (mandate no. FC99216). This study was also supported, through the GEOVIDE expedition, by the French National Research Agency (ANR-13-B506-0014), the Institut National des Sciences de L’Univers (INSU) of the Centre National de la Recherche Scientifique (CNRS) and the French Institute for Marine Science (Ifremer). This work was logistically supported by DT-INSU and GENAVIR. This publication is also a contribution to the Labex OT-Med (ANR-11-LABEX-0061; http://www.otmed.fr/, last access: 15 April 2018) funded by the “Investissements d’Avenir”, French Government project of the French National Research Agency (ANR; http://www.agence-nationale-recherche.fr/, last access: 15 April 2018) through the A*Midex project (ANR-11-IDEX-0001-02), funding Virginie Riou during the preparation of the manuscript. Finally, this work was also supported by an NSERC Discovery grant and Ocean Frontier Institute (OFI) grant (Canada First Research Excellence Funds) to Julie LaRoche, and the OFI postdoctoral fellow Debany Fonseca-Batista.
Publisher Copyright:
© 2019 Author(s).
ASJC Scopus Subject Areas
- Ecology, Evolution, Behavior and Systematics
- Earth-Surface Processes