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

Abstract

Abstract. Dinitrogen fixation is now recognized as one of the major sources of bio-available nitrogen in the ocean. Thus, N2 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 N2 fixation. This new development leads us to develop and test an explicit N2 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 N2 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 biogeochemical models) implicit parameterization of N2 fixation showed that the latter leads to an underestimation of N2 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.

Diazotrophy as the main driver of the oligotrophy gradient in the western tropical South Pacific Ocean: results from a one-dimensional biogeochemical–physical coupled model

“…A similar shift between nitracline and phosphacline depths was observed 10 • further south than our studied area, with a nitracline about 20 m deeper than the phosphacline (Law et al, 2011). In those N-depleted regions, diazotrophs may outcompete non-diazotroph organisms, using the unlimited atmospheric N 2 (Agawin et al, 2007;Dutkiewicz et al, 2014). However, their development also requires sufficient light intensity and other nutrients such as P and Fe, and the debate on their expected limitation or co-limitation is of great interest to the ocean biogeochemical community (Falkowski, 1997;Wu et al, 2000;Sañudo-Wilhelmy et al, 2001;Mills et al, 2004;Moutin et al, 2005;Monteiro et al, 2011).…”

Remote sensing of <i>Trichodesmium</i> spp. mats in the western tropical South Pacific

“…Finally, Dutheil et al (2018) explore the regional and seasonal budget of the N 2 fixation due to Trichodesmium in a numerical model based on physical and biogeochemical properties that does not take into consideration the spatial charac-teristic of Trichodesmium colonies to aggregate in mats. One interesting aspect will be to find a way to integrate our results into such a model to better estimate the regional effects of that species.…”

“…This lack of inorganic nitrogen favors the growth of dinitrogen (N 2 )-fixing organisms (or diazotrophs), which have the ability to use the inexhaustible pool of N 2 dissolved in seawater and convert it into bioavailable ammonia. Several studies have reported high N 2 fixation rates in the WTSP Bonnet et al, 2009;Bonnet et al, 2015;Garcia et al, 2007), which has recently been identified as a hot spot of N 2 fixation . During austral summer conditions, N 2 fixation supports nearly all new primary production and organic matter export Knapp et al, 2018) as nitrate diffusion across the thermocline and atmospheric sources of N are < 10 % of new N inputs.…”

Role of Iron in the Marquesas Island Mass Effect

“…Further details on the PISCES model functioning are available in Aumont et al (2015). PISCES has been widely used in a variety of configurations from global scale simulations (e.g., Aumont & Bopp, 2006;Dufresne et al, 2013) to basin or regional scale studies, of which some were noticeably focused on the Pacific region (Dutheil et al, 2018;Gorgues et al, 2005;Gorgues et al, 2010).…”