Mathieu Caffin

Hot spot of N-2 fixation in the western tropical South Pacific pleads for a spatial decoupling between N-2 fixation and denitrification

Nitrogen (N) is the building block of life. Quantifying the sources and sinks of N to the ocean is essential for predicting its productivity and potential carbon sequestration. In his paper, Gruber (1) seeks for “elusive marine nitrogen fixation” following results from Knapp et al. (2), who measured unexpectedly low N input through N2 fixation in the eastern tropical South Pacific (ETSP), seriously bringing into question the proposed close spatial coupling between N input (through N2 fixation) and loss (through denitrification) (3). Here, we compile data from recently published and unpublished studies revealing a hot spot of N2 fixation in the western tropical South Pacific (WTSP) arguing for a spatial decoupling between N sources and sinks in the South Pacific. Based on four cruises performed between 2012 and 2015 during austral winter and summer conditions, with a total of more than 600 15N2 incubations-based measurements, and particularly a 4,000-km zonal transect at ∼20°S in … [↵][1]1To whom correspondence should be addressed. Email: sophie.bonnet{at}univ-amu.fr. [1]: #xref-corresp-1-1

Longitudinal contrast in Turbulence along a ∼ 19S section in the Pacific and its consequences on biogeochemical fluxes

Microstructure measurements were performed along the OUTPACE longitudinal transect in the tropical Pacific (Moutin and Bonnet, 2015). Small-scale dynamics and turbulence in the first 800m surface layer were characterized based on hydrographic and current measurements at fine vertical scale and turbulence measurements at cm scale using a vertical microstructure profiler. The possible impact of turbulence on biogeochemical budgets in the surface layer was also addressed in this region of increasing oligotrophy to the East. The dissipation rate of turbulent kinetic energy, , showed an interesting 5 contrast along the longitudinal transect with stronger turbulence in the West, i.e. the Melanesian Archipelago, compared to the East, within the South Pacific Subtropical Gyre, with a variation of by a factor of 3 within [100m−500m]. The layer with enhanced turbulence decreased in vertical extent traveling eastward. This spatial pattern was correlated with the energy level of the internal wave field, higher in the West compared to the East. The difference in wave energy mostly resulted from enhanced wind power input into inertial motions in the West. Moreover, three long duration stations were sampled along the cruise 10 transect, each over three inertial periods. The analysis from the western long duration station gave evidence of an energetic baroclinic near-inertial wave that was responsible for the enhanced , observed within a 50m-250m layer, with a value of 810−9Wkg−1, about 8 times larger than at the eastern long duration stations. Averaged nitrate turbulent diffusive fluxes in a 100-m layer below the top of the nitracline were about twice larger west of 170W due to the higher vertical diffusion coefficient. In the photic layer, the depth-averaged nitrate turbulent diffusive flux strongly decreased eastward with an 15 averaged value of 11μmolm−2d−1 West of 170W to be compared with the 3μmolm−2d−1 averaged value East of 170W. Contrastingly phosphate turbulent diffusive fluxes were significantly larger in the photic layer. This input may have an important role in sustaining the development of N2-fixing organisms that were shown to be the main primary contributors to the biological pump in the area. The time-space intermittency of mixing events, intrinsic to turbulence, was underlined but its consequences on micro-organisms would deserve a dedicated study. 20 Copyright statement.