N<sub>2</sub> fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

Abstract

Abstract. We performed nitrogen (N) budgets in the photic layer of three contrasting stations representing different trophic conditions in the western tropical South Pacific (WTSP) Ocean during austral summer conditions (FebruaryMarch 2015). Using a Lagrangian strategy, we sampled the same water mass for the entire duration of each long-duration (5 days) station, allowing us to consider only vertical exchanges for the budgets. We quantified all major vertical N fluxes both entering (N 2 fixation, nitrate turbulent diffusion, atmospheric deposition) and leaving the photic layer (particulate N export). The three stations were characterized by a strong nitracline and contrasted deep chlorophyll maximum depths, which were lower in the oligotrophic Melanesian archipelago (MA, stations LD A and LD B) than in the ultra-oligotrophic waters of the South Pacific Gyre (SPG, station LD C). N 2 fixation rates were extremely high at both LD A (593 ± 51 µmol N m −2 d −1 ) and LD B (706 ± 302 µmol N m −2 d −1 ), and the diazotroph community was dominated by Trichodesmium. N 2 fixation rates were lower (59 ± 16 µmol N m −2 d −1 ) at LD C, and the diazotroph community was dominated by unicellular N 2 -fixing cyanobacteria (UCYN). At all stations, N 2 fixation was the major source of new N (> 90 %) before atmospheric deposition and upward nitrate fluxes induced by turbulence. N 2 fixation contributed circa 13-18 % of primary production in the MA region and 3 % in the SPG water and sustained nearly all new primary production at all stations. The e ratio (e ratio = particulate carbon export / primary production) was maximum at LD A (9.7 %) and was higher than the e ratio in most studied oligotrophic regions (< 5 %), indicating a high efficiency of the WTSP to export carbon relative to primary production. The direct export of diazotrophs assessed by qPCR of the nifH gene in sediment traps represented up to 30.6 % of the PC export at LD A, while their contribution was 5 and < 0.1 % at LD B and LD C, respectively. At the three studied stations, the sum of all N input to the photic layer exceeded the N output through organic matter export. This disequilibrium leading to N accumulation in the upper Published by Copernicus Publications on behalf of the European Geosciences Union.

Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N&lt;sub&gt;2&lt;/sub&gt; fixation activity and diversity in the western tropical South Pacific Ocean

“…The DDN was transferred to pico-plankton, which dominated the WTSP, suggesting that N 2 fixation fuelled the growth of biomass in the N-depleted environment. This is consistent with Caffin et al (2018), who revealed that N 2 fixation provides > 90 % of the new N to the photic layer of the WTSP. On a larger-scale view, the simulation performed by Dutheil et al (2018) predicts that diazotrophs will support a large part of primary production (∼ 15 %) in low nutrient low chlorophyll regions of the Pacific Ocean, comprising the WTSP.…”

“…This suggests a possible coupling between Synechococcus and Trichodesmium as ever mentioned by Campbell et al (2005), who report higher Synechococcus abundances inside Trichodesmium blooms compared to surrounding waters, while it is not the case for other plankton groups. This difference may also be linked with the communities present at the time of the experiments: Prochlorococcus accounted for ∼ 65 % of pico-phytoplankton in E1 and E2, while it accounted for ∼ 80 % in GY in E3.…”

“…In these N-depleted areas of the tropical and subtropical ocean, biological dinitrogen (N 2 ) fixation (the reduction of atmospheric N 2 into bioavailable ammonia) sustains the major part of new production and organic matter export (Bonnet et al, 2009;Caffin et al, 2018;Capone et al, 2005;Karl et al, 2012). At the global scale, N 2 fixation is the major source of new N to the ocean, before atmospheric deposition and riverine inputs (100-150 Tg N yr −1 : Duce et al, 2008;Gruber, 2008).…”

Major Impact of Dust Deposition on the Productivity of the Arabian Sea

“…During a campaign in May along a transect along 65°E through the central basin of the AS, Capone et al (1998) found higher rates in the upper 0.5 m (130 μmol N·m −2 ·day −1 max rate) in a Trichodesmium bloom at their station located at 65°E -10°N but a maximum rate of 36 μmol N·m −2 ·day −1 in the waters between 0.5 and 100 m at the same station. Our modeled rates when including atmospheric deposited iron show that overall N 2 fixation rates over the AS (Figure 3) are moderate compared to other oceanic regions, such as the western tropical South Pacific, where the N 2 fixation process has been shown to be a major path for new nitrogen (Caffin et al, 2018). In the AS, maximum rates are on the order of 30 μmol N·m −2 ·day −1 in April/May (Table 1 and Figure S12), in good agreement with the available in situ measurements for the same season in the same area (70°E -10°N; Capone et al, 1998).…”

Has Nitrogen Supply to Coral Reefs in the South Pacific Ocean Changed Over the Past 50 Thousand Years?

“…As well as subsurface N supply, the low N:P ratio of surface water nutrients in the SPSG is known to promote N 2 fixation (Raimbault et al, 2008). Indeed, many studies suggest that this is a major contributor to the surface N pool of the SPSG (Caffin et al, 2018;Dekaezemacker et al, 2013;Moutin et al, 2007). However, using isotopic evidence, Knapp et al (2018) found that available N in the SPSG was ultimately sourced from subsurface NO 3 − rather than N 2 fixation, at least along its eastern margin and away from any island effects.…”