Evidence for efficient regenerated production and dinitrogen fixation in nitrogen-deficient waters of the South Pacific Ocean: impact on new and export production estimates

Abstract

Abstract.One of the major objectives of the BIOSOPE cruise, carried out on the R/V Atalante from OctoberNovember 2004 in the South Pacific Ocean, was to establish productivity rates along a zonal section traversing the oligotrophic South Pacific Gyre (SPG). These results were then compared to measurements obtained from the nutrient -replete waters in the Chilean upwelling and around the Marquesas Islands. A dual 13 C/ 15 N isotope technique was used to estimate the carbon fixation rates, inorganic nitrogen uptake (including dinitrogen fixation), ammonium (NH 4 ) and nitrate (NO 3 ) regeneration and release of dissolved organic nitrogen (DON). The SPG exhibited the lowest primary production rates (0.15 g C m −2 d −1 ), while rates were 7 to 20 times higher around the Marquesas Islands and in the Chilean upwelling, respectively. In the very low productive area of the SPG, most of the primary production was sustained by active regeneration processes that fuelled up to 95% of the biological nitrogen demand. Nitrification was active in the surface layer and often balanced the biological demand for nitrate, especially in the SPG. The percentage of nitrogen released as DON represented a large proportion of the inorganic nitrogen uptake (13-15% in average), reaching 26-41% in the SPG, where DON production played a major role in nitrogen cycling. Dinitrogen fixation was detectable over the whole study area; even in the Chilean upwelling, where rates as high as 3 nmoles l −1 d −1 were measured. In these nutrient-replete waters new production was very high (0.69±0.49 g C m −2 d −1 ) and essentially sustained by nitrate levels. In the SPG, dinitrogen fixation, although occurring at much lower daily rates (≈1-2 nmoles l −1 d −1 ), sustained up to 100% of the new production (0.008±0.007 g C m −2 d −1 ) which was two orders of magnitude lower than that measured in the upwelling. The annual N 2 -fixation of the South PacificCorrespondence to: P. Raimbault (patrick.raimbault@univmed.fr) is estimated to 21×10 12 g, of which 1.34×10 12 g is for the SPG only. Even if our "snapshot" estimates of N 2 -fixation rates were lower than that expected from a recent ocean circulation model, these data confirm that the N-deficiency South Pacific Ocean would provide an ideal ecological niche for the proliferation of N 2 -fixers which are not yet identified.

Heterotrophic organisms dominate nitrogen fixation in the South Pacific Gyre

“…Depthintegrated rates (Table 1) were generally lower for the center of the gyre (12-190 mmol N m À2 day À1 , mean 93.8 ± 61.3 mmol N m À2 day À1 ) than for the southern transect (103-160 mmol N m À2 day À1 , mean 134.3 ± 29 mmol N m À2 day À1 ). These rates were comparable to rates previously reported for the central gyre (48-135 mmol N m À2 day À1 ) (Moutin et al, 2008), and were not significantly different from the rates (30-60 mmol N m À2 day À1 ) estimated by Raimbault and Garcia (2008), or those modeled for the South Pacific (55 mmol N m À2 day À1 ) (Deutsch et al, 2007). Similar rate estimates have also been given for the North Pacific (Zehr et al, 2001;Montoya et al, 2004).…”

“…Raimbault and Garcia (2008) found the highest N 2 -fixation rates (1.8 nM N day À1 ) in the upper 50 m of the water column in the eastern and central SPG, but these rates declined rapidly below 50 m. In contrast, our measured nitrogenfixation rates were similar throughout the surface mixed layer in the center of the gyre during December 2006/January 2007 (Figure 3, Supplementary Figure S3). The SPG usually reaches its maximum expansion and thus the minimum chl a concentrations in September, although the magnitude of seasonal variation is not as large as that in other subtropical gyres (McClain et al, 2004;Morel et al, 2007).…”

“…Primary productivity was B10-fold higher at sites along the southern rim of the gyre (stations 8-11) than along the northern transect (Figure 2a). Although B3-fold higher rates (B6.9 nM C h À1 ) were measured for the north-eastern part of the SPG in another study (Raimbault and Garcia, 2008), all rates reported thus far are very low relative to more mesotrophic oceanic regions like the equatorial west Pacific (120 nM C h À1 ) (Raimbault and Garcia, 2008). In comparison, incubations amended with 15 NH 4 þ and 15 N-leucine significantly enhanced 13 C-bicarbonate uptake in the center of the gyre (up to 10.2 nM C h À1 ; t-test, Po0.05), and such enhancement was generally light dependent (Figure 2, Supplementary Figure S4), indicating that primary production in the center of the SPG was nitrogen limited.…”

“…In contrast to the subtropical gyres of the northern hemisphere, investigations of N-cycling in the southern hemisphere are scarce. Very few nitrogen fixation rates have been reported so far for the central SPG (Moutin et al, 2008;Raimbault and Garcia, 2008). Even less is known about the organisms responsible for N 2 -fixation therein.…”

High levels of heterogeneity in diazotroph diversity and activity within a putative hotspot for marine nitrogen fixation

“…However, comparisons made in the North Pacific and Atlantic Oceans indicate that this method could lead to N 2 fixation underestimates of 50% (Wilson et al, 2012; or greater, depending on the composition of the diazotroph community (Großkopf et al, 2012). Dekaezemacker et al, 2013) and the tropical South Pacific Gyre (⩽3 nmol l − 1 day − 1 ; Raimbault and Garcia 2008), the maximum conservative rates of N 2 fixation reported here during the Austral winter (91 nmol l − 1 day − 1 ) are relatively high. Indeed, our observations, along with those previously reported (Montoya et al, 2004;Raes et al, 2014), support the proposition that the tropical waters of northern Australia are a 'hotspot' of diazotroph activity within the Southern Hemisphere.…”