scholarly journals Aphotic N<sub>2</sub> fixation along an oligotrophic to ultraoligotrophic transect in the Western Tropical South Pacific Ocean

2018 ◽  
Author(s):  
Mar Benavides ◽  
Katyanne M. Shoemaker ◽  
Pia H. Moisander ◽  
Jutta Niggemann ◽  
Thorsten Dittmar ◽  
...  
Keyword(s):  
N2 Fixation ◽  
New Caledonia ◽  
Nifh Gene ◽  
South Pacific ◽  
French Polynesia ◽  
Mode Water ◽  
South Pacific Ocean ◽  
Fixation Activity ◽  
Nitrogen Inputs ◽  
Cyclotron Mass

Abstract. The western tropical South Pacific (WTSP) Ocean has been recognized as a global hotspot of dinitrogen (N2) fixation. Here, as in other marine environments across the oceans, N2 fixation studies have focused in the sunlit layer. However, studies have confirmed the importance of aphotic N2 fixation activity, although until now only one had been performed in the WTSP. In order to increase our knowledge of aphotic N2 fixation in the WTSP, here we measure N2 fixation rates and identify diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. Because non-cyanobacterial diazotrophs presumably need external dissolved organic matter (DOM) sources for their nutrition, we also identified DOM compounds using Fourier Transform Ion Cyclotron Mass Spectrometry (FTICRMS). N2 fixation rates were low (average 0.63 ± 0.07 nmol N L−1 d−1), but consistently detected across all depths and stations, representing ~ 6–88 % of photic N2 fixation. N2 fixation rates were not significantly correlated to DOM compounds. The analysis of nifH gene amplicons revealed a wide diversity of non-cyanobacterial diazotrophs, majorly matching clusters 1 and 3. Interestingly, a distinct phylotype from the major nifH subcluster 1G dominated at 650 dbar, coinciding with the oxygenated Sub-Antarctic Mode Water (SAMW). This consistent pattern suggests that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure. While the data available is still too scarce to elucidate the distribution and controls of mesopelagic non-cyanobacterial diazotrophs in the WTSP, their prevalence in the mesopelagic layer and the consistent detection of active N2 fixation activity at all depths sampled during our study suggest that aphotic N2 fixation may contribute significantly to fixed nitrogen inputs in this area.

scholarly journals Aphotic N<sub>2</sub> fixation along an oligotrophic to ultraoligotrophic transect in the western tropical South Pacific Ocean

2018 ◽  
Vol 15 (10) ◽  
pp. 3107-3119 ◽  
Author(s):  
Mar Benavides ◽  
Katyanne M. Shoemaker ◽  
Pia H. Moisander ◽  
Jutta Niggemann ◽  
Thorsten Dittmar ◽  
...  
Keyword(s):  
N2 Fixation ◽  
Water Mass ◽  
New Caledonia ◽  
Hot Spot ◽  
Nifh Gene ◽  
South Pacific ◽  
French Polynesia ◽  
Mode Water ◽  
Fixation Activity ◽  
Nitrogen Inputs

Abstract. The western tropical South Pacific (WTSP) Ocean has been recognized as a global hot spot of dinitrogen (N2) fixation. Here, as in other marine environments across the oceans, N2 fixation studies have focused on the sunlit layer. However, studies have confirmed the importance of aphotic N2 fixation activity, although until now only one had been performed in the WTSP. In order to increase our knowledge of aphotic N2 fixation in the WTSP, we measured N2 fixation rates and identified diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. Because non-cyanobacterial diazotrophs presumably need external dissolved organic matter (DOM) sources for their nutrition, we also identified DOM compounds using Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) with the aim of searching for relationships between the composition of DOM and non-cyanobacterial N2 fixation in the aphotic ocean. N2 fixation rates were low (average 0.63 ± 0.07 nmol N L−1 d−1) but consistently detected across all depths and stations, representing ∼ 6–88 % of photic N2 fixation. N2 fixation rates were not significantly correlated with DOM compounds. The analysis of nifH gene amplicons revealed a wide diversity of non-cyanobacterial diazotrophs, mostly matching clusters 1 and 3. Interestingly, a distinct phylotype from the major nifH subcluster 1G dominated at 650 dbar, coinciding with the oxygenated Subantarctic Mode Water (SAMW). This consistent pattern suggests that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure. While the data available are still too scarce to elucidate the distribution and controls of mesopelagic non-cyanobacterial diazotrophs in the WTSP, their prevalence in the mesopelagic layer and the consistent detection of active N2 fixation activity at all depths sampled during our study suggest that aphotic N2 fixation may contribute significantly to fixed nitrogen inputs in this area and/or areas downstream of water mass circulation.

scholarly journals Preface: The Oligotrophy to the UlTra-oligotrophy PACific Experiment (OUTPACE cruise, 18 February to 3 April 2015)

2017 ◽  
Vol 14 (13) ◽  
pp. 3207-3220 ◽  
Author(s):  
Thierry Moutin ◽  
Andrea Michelangelo Doglioli ◽  
Alain de Verneil ◽  
Sophie Bonnet
Keyword(s):  
Biological Diversity ◽  
New Caledonia ◽  
Austral Summer ◽  
South Pacific ◽  
Adaptive Strategy ◽  
French Polynesia ◽  
Biogenic Elements ◽  
General Context ◽  
Western Boundary ◽  
South Pacific Ocean

Abstract. The overall goal of OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) was to obtain a successful representation of the interactions between planktonic organisms and the cycle of biogenic elements in the western tropical South Pacific Ocean across trophic and N2 fixation gradients. Within the context of climate change, it is necessary to better quantify the ability of the oligotrophic ocean to sequester carbon through biological processes. OUTPACE was organized around three main objectives, which were (1) to perform a zonal characterization of the biogeochemistry and biological diversity of the western tropical South Pacific during austral summer conditions, (2) to study the production and fate of organic matter (including carbon export) in three contrasting trophic regimes (increasing oligotrophy) with a particular emphasis on the role of dinitrogen fixation, and (3) to obtain a representation of the main biogeochemical fluxes and dynamics of the planktonic trophic network. The international OUTPACE cruise took place between 18 February and 3 April 2015 aboard the RV L'Atalante and involved 60 scientists (30 onboard). The west–east transect covered  ∼  4000 km from the western part of the Melanesian archipelago (New Caledonia) to the western boundary of the South Pacific gyre (French Polynesia). Following an adaptive strategy, the transect initially designed along the 19° S parallel was adapted along-route to incorporate information coming from satellite measurements of sea surface temperature, chlorophyll a concentration, currents, and diazotroph quantification. After providing a general context and describing previous work done in this area, this introductory paper elucidates the objectives of OUTPACE, the implementation plan of the cruise and water mass and climatological characteristics and concludes with a general overview of the other papers that will be published in this special issue.

scholarly journals N<sub>2</sub> fixation in eddies of the eastern tropical South Pacific Ocean

2016 ◽  
Vol 13 (10) ◽  
pp. 2889-2899 ◽  
Author(s):  
Carolin R. Löscher ◽  
Annie Bourbonnais ◽  
Julien Dekaezemacker ◽  
Chawalit N. Charoenpong ◽  
Mark A. Altabet ◽  
...  
Keyword(s):  
Pacific Ocean ◽  
N2 Fixation ◽  
South Pacific ◽  
Open Ocean ◽  
Cyclonic Eddy ◽  
Geochemical Data ◽  
Intermediate Water ◽  
N Loss ◽  
Mode Water ◽  
South Pacific Ocean

Abstract. Mesoscale eddies play a major role in controlling ocean biogeochemistry. By impacting nutrient availability and water column ventilation, they are of critical importance for oceanic primary production. In the eastern tropical South Pacific Ocean off Peru, where a large and persistent oxygen-deficient zone is present, mesoscale processes have been reported to occur frequently. However, investigations into their biological activity are mostly based on model simulations, and direct measurements of carbon and dinitrogen (N2) fixation are scarce.We examined an open-ocean cyclonic eddy and two anticyclonic mode water eddies: a coastal one and an open-ocean one in the waters off Peru along a section at 16° S in austral summer 2012. Molecular data and bioassay incubations point towards a difference between the active diazotrophic communities present in the cyclonic eddy and the anticyclonic mode water eddies.In the cyclonic eddy, highest rates of N2 fixation were measured in surface waters but no N2 fixation signal was detected at intermediate water depths. In contrast, both anticyclonic mode water eddies showed pronounced maxima in N2 fixation below the euphotic zone as evidenced by rate measurements and geochemical data. N2 fixation and carbon (C) fixation were higher in the young coastal mode water eddy compared to the older offshore mode water eddy. A co-occurrence between N2 fixation and biogenic N2, an indicator for N loss, indicated a link between N loss and N2 fixation in the mode water eddies, which was not observed for the cyclonic eddy. The comparison of two consecutive surveys of the coastal mode water eddy in November 2012 and December 2012 also revealed a reduction in N2 and C fixation at intermediate depths along with a reduction in chlorophyll by half, mirroring an aging effect in this eddy. Our data indicate an important role for anticyclonic mode water eddies in stimulating N2 fixation and thus supplying N offshore.

scholarly journals Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N<sub>2</sub> fixation activity and diversity in the western tropical South Pacific Ocean

2018 ◽  
Vol 15 (12) ◽  
pp. 3795-3810 ◽  
Author(s):  
Mathieu Caffin ◽  
Hugo Berthelot ◽  
Véronique Cornet-Barthaux ◽  
Aude Barani ◽  
Sophie Bonnet
Keyword(s):  
Food Web ◽  
N2 Fixation ◽  
Secondary Ion Mass Spectrometry ◽  
Heterotrophic Bacteria ◽  
South Pacific ◽  
Open Ocean ◽  
Trophic Levels ◽  
Planktonic Food ◽  
South Pacific Ocean ◽  
Fixation Activity

Abstract. Biological dinitrogen (N2) fixation provides the major source of new nitrogen (N) to the open ocean, contributing more than atmospheric deposition and riverine inputs to the N supply. Yet the fate of the diazotroph-derived N (DDN) in the planktonic food web is poorly understood. The main goals of this study were (i) to quantify how much of DDN is released to the dissolved pool during N2 fixation and how much is transferred to bacteria, phytoplankton and zooplankton, and (ii) to compare the DDN release and transfer efficiencies under contrasting N2 fixation activity and diversity in the oligotrophic waters of the western tropical South Pacific (WTSP) Ocean. We used nanometre-scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labelling and flow cytometry cell sorting to track the DDN transfer to plankton, in regions where the diazotroph community was dominated by either Trichodesmium or by UCYN-B. After 48 h, ∼ 20–40 % of the N2 fixed during the experiment was released to the dissolved pool when Trichodesmium dominated, while the DDN release was not quantifiable when UCYN-B dominated; ∼ 7–15 % of the total fixed N (net N2 fixation + release) was transferred to non-diazotrophic plankton within 48 h, with higher transfer efficiencies (15 ± 3 %) when UCYN-B dominated as compared to when Trichodesmium dominated (9 ± 3 %). The pico-cyanobacteria Synechococcus and Prochlorococcus were the primary beneficiaries of the DDN transferred (∼ 65–70 %), followed by heterotrophic bacteria (∼ 23–34 %). The DDN transfer in bacteria was higher (34 ± 7 %) in the UCYN-B-dominating experiment compared to the Trichodesmium-dominating experiments (24 ± 5 %). Regarding higher trophic levels, the DDN transfer to the dominant zooplankton species was less efficient when the diazotroph community was dominated by Trichodesmium (∼ 5–9 % of the DDN transfer) than when it was dominated by UCYN-B (∼ 28 ± 13 % of the DDN transfer). To our knowledge, this study provides the first quantification of DDN release and transfer to phytoplankton, bacteria and zooplankton communities in open ocean waters. It reveals that despite UCYN-B fix N2 at lower rates compared to Trichodesmium in the WTSP, the DDN from UCYN-B is much more available and efficiently transferred to the planktonic food web than the DDN originating from Trichodesmium.

scholarly journals N<sub>2</sub> fixation in eddies of the eastern tropical South Pacific Ocean

2015 ◽  
Vol 12 (22) ◽  
pp. 18945-18972 ◽  
Author(s):  
C. R. Löscher ◽  
A. Bourbonnais ◽  
J. Dekaezemacker ◽  
C. N. Charoenpong ◽  
M. A. Altabet ◽  
...  
Keyword(s):  
Pacific Ocean ◽  
N2 Fixation ◽  
South Pacific ◽  
Open Ocean ◽  
Cyclonic Eddy ◽  
Geochemical Data ◽  
Intermediate Water ◽  
N Loss ◽  
Mode Water ◽  
South Pacific Ocean

Abstract. Mesoscale eddies play a major role in controlling ocean biogeochemistry. By impacting nutrient availability and water column ventilation, they are of critical importance for oceanic primary production. In the eastern tropical South Pacific Ocean off Peru, where a large and persistent oxygen deficient zone is present, mesoscale processes have been reported to occur frequently. However, investigations on their biological activity are mostly based on model simulations, and direct measurements of carbon and dinitrogen (N2) fixation are scarce. We examined an open ocean cyclonic eddy and two anticyclonic mode water eddies: a coastal one and an open ocean one in the waters off Peru along a section at 16° S in austral summer 2012. Molecular data and bioassay incubations point towards a difference between the active diazotrophic communities present in the cyclonic eddy and the anticyclonic mode water eddies. In the cyclonic eddy, highest rates of N2 fixation were measured in surface waters but no N2 fixation signal was detected at intermediate water depths. In contrast, both anticyclonic mode water eddies showed pronounced maxima in N2 fixation below the euphotic zone as evidenced by rate measurements and geochemical data. N2 fixation and carbon (C) fixation were higher in the young coastal mode water eddy compared to the older offshore mode water eddy. A co-occurrence between N2 fixation and biogenic N2, an indicator for N loss, indicated a link between N loss and N2 fixation in the mode water eddies, which was not observed for the cyclonic eddy. The comparison of two consecutive surveys of the coastal mode water eddy in November and December 2012 revealed also a reduction of N2 and C fixation at intermediate depths along with a reduction in chlorophyll by half, mirroring an aging effect in this eddy. Our data indicate an important role for anticyclonic mode water eddies in stimulating N2 fixation and thus supplying N offshore.

scholarly journals The Oligotrophy to the UlTra-oligotrophy PACific Experiment (OUTPACE cruise, Feb. 18 to Apr. 3, 2015)

2017 ◽  
Author(s):  
Thierry Moutin ◽  
Andrea Doglioli ◽  
Alain De Verneil ◽  
Sophie Bonnet
Keyword(s):  
Biological Diversity ◽  
New Caledonia ◽  
Austral Summer ◽  
South Pacific ◽  
Adaptive Strategy ◽  
French Polynesia ◽  
Biogenic Elements ◽  
General Context ◽  
Western Boundary ◽  
South Pacific Ocean

Abstract. The overall goal of OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) was to obtain a successful representation of the interactions between planktonic organisms and the cycle of biogenic elements in the western tropical South Pacific Ocean across trophic and N2 fixation gradients. Within the context of climate change, it is necessary to better quantify the ability of the oligotrophic ocean to sequester carbon through biological processes. OUTPACE was organized around three main objectives which were: (1) To perform a zonal characterization of the biogeochemistry and biological diversity of the western tropical South Pacific during austral summer conditions, (2) To study the production and fate of organic matter (including carbon export) on three contrasting trophic regimes (increasing oligotrophy) with a particular emphasis on the role of dinitrogen fixation, and (3) to obtain a representation of the main biogeochemical fluxes and dynamics of the planktonic trophic network. The international OUTPACE cruise took place between 18 February and 3 April 2015 aboard the RV L'Atalante and involved 60 scientists (30 onboard). The west-east transect covered ~ 4 000 km from the western part of the Melanesian Archipelago (New Caledonia) to the western boundary of the South Pacific gyre (French Polynesia). Following an adaptive strategy, the transect initially designed along the 19° S parallel was adapted along-route to incorporate information coming from satellite measurements of sea surface temperature, chlorophyll a concentration, currents, and diazotrophs quantification. After providing a general context and describing previous work done in this area, this introductory paper elucidates the objectives of OUTPACE, the implementation plan of the cruise, water mass and climatological characteristics, and concludes with a general overview of the other papers that will be published in this special issue.

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

2008 ◽  
Vol 5 (2) ◽  
pp. 323-338 ◽  
Author(s):  
P. Raimbault ◽  
N. Garcia
Keyword(s):  
Pacific Ocean ◽  
Nitrogen Uptake ◽  
N2 Fixation ◽  
Inorganic Nitrogen ◽  
South Pacific ◽  
Dinitrogen Fixation ◽  
The South ◽  
New Production ◽  
Marquesas Islands ◽  
South Pacific Ocean

Abstract. One of the major objectives of the BIOSOPE cruise, carried out on the R/V Atalante from October-November 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 13C/15N isotope technique was used to estimate the carbon fixation rates, inorganic nitrogen uptake (including dinitrogen fixation), ammonium (NH4) and nitrate (NO3) 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 N2-fixation of the South Pacific is estimated to 21×1012g, of which 1.34×1012g is for the SPG only. Even if our "snapshot" estimates of N2-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 N2-fixers which are not yet identified.

scholarly journals Mesozooplankton structure and functioning in the western tropical South Pacific along the 20° parallel south during the OUTPACE survey (February–April 2015)

2018 ◽  
Author(s):  
François Carlotti ◽  
Marc Pagano ◽  
Loïc Guilloux ◽  
Katty Donoso ◽  
Valentina Valdés ◽  
...  
Keyword(s):  
Primary Production ◽  
New Caledonia ◽  
Size Structure ◽  
South Pacific ◽  
French Polynesia ◽  
Grazing Impact ◽  
Coral Sea ◽  
Carbon Demand ◽  
Excretion Rates ◽  
Density And Biomass

Abstract. This paper presents results on the spatial and temporal distribution patterns of mesozooplankton in the western tropical South Pacific along the 20 °S south visited during austral summer (February–April 2015). By contributing to the interdisciplinary OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) project (Moutin et al., 2017), the specific aims of this study dedicated to mesozooplankton observations were (1) to document the responses of zooplankton in terms of species diversity, density and biomass along the transect, and (2) to characterize the trophic pathways from primary production to large mesozooplanktonic organisms. Along a West-East transect of 4000 km from New Caledonia to the French Polynesia, 15 short-duration stations (SD-1 to SD-15, 8 hours each) dedicated to a large-scale description, and three long-duration stations (LD-A to LD-C, 5days each), respectively positioned (1) in offshore northern waters of New Caledonia, (2) near Niue Island, and (3) in the subtropical Pacific gyre near the Cook Islands, were sampled with a Bongo Net with 120 μm mesh size net for quantifying mesozooplankton abundance, biomass, community taxonomy and size structure, and size fractionated content of δ15N. Subsequently, the contribution of Diazotroph Derived Nitrogen (DDN (%) to zooplankton δ15N (ZDDN) values at each station was calculated, as well as an estimation of zooplankton carbon demand and grazing impact and of zooplankton excretion rates. The mesozooplankton community showed a general decreasing trend in abundance and biomass from West to East, with a clear drop in the ultra-oligotrophic waters of the subtropical Pacific gyre (LD-C, SD-14 and SD-15). Higher abundance and biomass corresponded to higher primary production of more or less ephemeral blooms linked to complex mesoscale circulation in the Coral Sea and between the longitudes 170–180 °W. Copepods were the most abundant group (68 to 86 % of total abundance), slightly increasing in contribution from west to east while, in parallel, gelatinous plankton decreased (dominated by appendicularians) and other holoplankton. Detritus in the net tow samples represented 20–50 % of the biomass, the lowest and the highest values being obtained in the subtropical Pacific gyre and in the Coral Sea, respectively, linked to the local primary production and the biomass and growth rates of zooplanktonic populations. Taxonomic compositions showed a high degree of similarity across the whole region, however, with a moderate difference in subtropical Pacific gyre. Several copepod taxa, known to have trophic links with Trichodesmium, presented positive relationships with Trichodesmium abundance, such as the Harpacticoids Macrosetella, Microsetella and Miracia, and the Poecilostomatoids Corycaeus and Oncaea. At the LD stations, the populations initially responded to local spring blooms with a large production of larval forms, reflected in increasing abundances but with limited (station LD-A) or no (station LD-A) biomass changes. Diazotrophs contributed up to 67 and 75 % to zooplankton biomass in the western and central Melanesian Archipelago regions respectively, but strongly decreased to an average of 22 % in the subtropical Pacific gyre (GY) and down to 7 % occurring in the most eastern station (SD-15). Using allometric relationships, specific zooplankton ingestion rates were estimated between 0.55 and 0.64 d−1 with the highest mean value at the bloom station (LD-B) and the lowest in GY, whereas estimated weight specific excretion rates ranged between 0.1 and 0.15 d−1 for NH4 and between 0.09 and 9.12 d−1 for PO4. Daily grazing pressure on phytoplankton stocks and daily regeneration by zooplankton were as well estimated for the different regions showing contrasted impacts between MA and GY regions. For the 3 LD stations, it was not possible to find any relationship between the abundance and biomass in the water column and swimmers found in sediment traps. Diel vertical migration of zooplankton, which obviously occurs from observed differences in day and night samples, might strongly influence the community of swimmers in traps.

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