Modelling N₂ fixation related to Trichodesmium sp.: driving processes and impacts on primary production in the tropical Pacific Ocean

Cyril Dutheil (1, 2) , Olivier Aumont (3) , Thomas Gorgues (4) , Anne Lorrain (5, 6) , Sophie Bonnet (7) , Martine Rodier (8) , Cecile Dupouy (1, 7) , Takuhei Shiozaki (9) , Christophe E. Menkès (10, 1)

1 IRD [Nouvelle-Calédonie] - Institut de Recherche pour le Développement
2 LOCEAN - Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques
3 NEMO R&D - Nucleus for European Modeling of the Ocean
4 LOPS - Laboratoire d'Océanographie Physique et Spatiale
5 LEMAR - Laboratoire des Sciences de l'Environnement Marin (LEMAR)
6 IRD - Institut de Recherche pour le Développement
7 MIO - Institut méditerranéen d'océanologie
8 EIO - Ecosystèmes Insulaires Océaniens (UMR 241)
9 JAMSTEC - Japan Agency for Marine-Earth Science and Technology
10 VARCLIM - Océan et variabilité du climat

Dinitrogen fixation is now recognized as one of the major sources of bio-available nitrogen in the ocean. Thus, N 2 fixation sustains a significant part of the global primary production by supplying the most common limiting nutrient for phytoplankton growth. The "Oligotrophy to UlTra-oligotrophy PACific Experiment" (OUTPACE) improved the data coverage of the western tropical South Pacific , an area recently recognized as a hotspot of N 2 fixation. This new development leads us to develop and test an explicit N 2 fixation formulation based on the Trichodesmium physiology (the most studied nitrogen fixer) within a 3-D coupled dynamical-biogeochemical model (ROMS-PISCES). We performed a climatological numerical simulation that is able to reproduce the main physical (e.g. sea surface temperature) and biogeochemical patterns (nutrient and chlorophyll concentrations, as well as N 2 fixation) in the tropical Pacific. This simulation displayed a Trichodesmium regional distribution that extends from 150 • E to 120 • W in the south tropical Pacific, and from 120 • E to 140 • W in the north tropical Pacific. The local simulated maximuma were found around islands (Hawaii, Fiji, Samoa, New Caledonia, Vanuatu). We assessed that 15 % of the total primary production may be due to Trichodesmium in the low-nutrient low-chlorophyll regions (LNLC) of the tropical Pacific. Comparison between our explicit and the often used (in biogeochem-ical models) implicit parameterization of N 2 fixation showed that the latter leads to an underestimation of N 2 fixation rates by about 25 % in LNLC regions. Finally, we established that iron fluxes from island sediments control the spatial distribution of Trichodesmium biomasses in the western tropical South Pacific. Note, this last result does not take into account the iron supply from rivers and hydrothermal sources, which may well be of importance in a region known for its strong precipitation rates and volcanic activity.

Mots clés: DINITROGEN-FIXATION; PHOSPHATE AVAILABILITY; ENVIRONMENTAL CONTROLS; DIAZOTROPH-DERIVED NITROGEN; CYANOBACTERIUM-TRICHODESMIUM; MARINE CYANOBACTERIUM; BUOYANCY REGULATION; EQUATORIAL PACIFIC; IRON LIMITATION; CARBON; ACL

Domaines: Géophysique [physics.geo-ph]; Biodiversité et Ecologie; Océan, Atmosphère

Cyril Dutheil, Olivier Aumont, Thomas Gorgues, Anne Lorrain, Sophie Bonnet, et al.. Modelling N₂ fixation related to Trichodesmium sp.: driving processes and impacts on primary production in the tropical Pacific Ocean. Biogeosciences, 2018, 15 (14), pp.4333 - 4352. ⟨10.5194/bg-15-4333-2018⟩. ⟨hal-01860661⟩