Biogeosciences, 2018,
15 (14), p. 4333-4352 ISSN 1726-4170
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 degrees E to 120 degrees W in the south tropical Pacific, and from 120 degrees E to 140 degrees 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 produc-tion may be due to Trichodesmium in the low-nutrient lowchlorophyll regions (LNLC) of the tropical Pacific. Comparison between our explicit and the often used (in biogeochemical 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.
