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.
Highlights
Nitrogen is known to be the most common limiting nutrient for phytoplankton growth in the modern world ocean (Moore et al, 2013), especially in the low-nutrient low-chlorophyll (LNLC) ecosystems (Arrigo, 2005; Gruber, 2005)
The iron flux from the sediments around the islands affects the spatial structure of Trichodesmium chlorophyll (Fig. 9e, f), most noticeably in the South Pacific, with maxima shifted from the South Pacific islands region (e.g. Fiji, New Caledonia, Vanuatu) in the TRI simulation to the coastal regions near Australia and Papua New Guinea in the TRI_NoFeSed simulation
This sensitivity test demonstrates that Trichodesmium are highly sensitive to the iron distribution in our model and that the spatial patterns of Trichodesmium chlorophyll in the southwest Pacific are tightly controlled by the release of iron from the coastal sediments of the Pacific islands
Summary
Nitrogen is known to be the most common limiting nutrient for phytoplankton growth in the modern world ocean (Moore et al, 2013), especially in the low-nutrient low-chlorophyll (LNLC) ecosystems (Arrigo, 2005; Gruber, 2005). They provide the major external source of reactive nitrogen to the ocean (Gruber, 2008), and support up to 50 % of new production in tropical and subtropical (LNLC) regions (Bonnet et al, 2009; Capone, 1997; Deutsch et al, 2007; Karl et al, 1997; Moutin et al, 2008; Raimbault and Garcia, 2008) These organisms are physiologically and taxonomically diverse including cyanobacteria, bacteria, and archaea (Zehr and Bombar, 2015; Delmont et al, 2018)
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