Modelling N 2 fixation related to Trichodesmium sp.: driving processes and impacts on primary production in the tropical Pacific Ocean

Dutheil, Cyril; Aumont, Olivier; Gorguès, Thomas; Lorrain, Anne; Bonnet, Sophie; Rodier, Martine; Dupouy, Cécile; Shiozaki, Takuhei; Menkes, Christophe

Dinitrogen fixation is now recognized as one of the major sources of bio-available nitrogen in the ocean. Thus, inline-formulaN2 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 inline-formulaN2 fixation. This new development leads us to develop and test an explicit inline-formulaN2 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 inline-formulaN2 fixation) in the tropical Pacific. This simulation displayed a Trichodesmium regional distribution that extends from 150inline-formula E to 120inline-formula W in the south tropical Pacific, and from 120inline-formula E to 140inline-formula 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 biogeochemical models) implicit parameterization of inline-formulaN2 fixation showed that the latter leads to an underestimation of inline-formulaN2 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.

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Dutheil, Cyril / Aumont, Olivier / Gorguès, Thomas / et al: Modelling N2 fixation related to Trichodesmium sp.: driving processes and impacts on primary production in the tropical Pacific Ocean. 2018. Copernicus Publications.

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