Evidence of high N2 fixation rates in productive waters of the temperate Northeast Atlantic

Abstract. Diazotrophic activity and primary production (PP) were investigated along two transects (Belgica BG2014/14 and GEOVIDE cruises) off the western Iberian Margin and the Bay of Biscay (38.8–46.5° N; 8.0–19.7° W) in May 2014 close to the end of the spring bloom. We report substantial N2 fixation activities, reaching up to 65 nmol N L−1 d−1 and 1533 µmol N m−2 d−1 close to the Iberian Margin between 38.8° N and 40.7° N. Similar figures in the basin have only been reported in the temperate and tropical western North Atlantic waters with coastal, shelf or mesohaline characteristics, as opposed to the mostly open ocean conditions studied here. In agreement with previous studies, the qualitative assessment of nifH gene diversity (encoding the nitrogenase enzyme that fixes N2) suggested a predominance of heterotrophic diazotrophs, and the absence of filamentous cyanobacteria. At the sites where N2 fixation activity was highest sequences affiliated to UCYN-A1, obligate symbiont of eukaryotic prymnesiophyte algae, were recovered. The remaining phylotypes were non-cyanobacterial diazotrophs, known to live in association with suspended particles and zooplankton (i.e., Bacteroidetes, Firmicutes and Proteobacteria). Outside the area of exceptional activity, N2 fixation in the open ocean and at shelf-influenced sites was also relatively high, ranging from 81 to 384 µmol N m−2 d−1, but was undetectable in the central Bay of Biscay. We propose that the unexpectedly high heterotrophic N2 fixation activity recorded at the time of our study was sustained by the availability of phytoplankton derived organic matter (dissolved and/or particulate) resulting from the ongoing to post spring bloom. We pose that this organic material not only sustained bacterial production, but also provided sufficient nutrients essential for the nitrogenase activity (e.g., phosphorus). Dissolved Fe was supplied through atmospheric dust deposition during the month preceding our study and through advection of surface waters from the subtropical region and the shelf area. Our findings stress the need for a more detailed monitoring of the spatial and temporal distribution of oceanic N2 fixation in productive waters of the temperate North Atlantic to better constrain the basin-scale nitrogen input to the ocean inventory.

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Evidence of high N2 fixation rates in the temperate northeast Atlantic

Biogeosciences ◽

10.5194/bg-16-999-2019 ◽

2019 ◽

Vol 16 (5) ◽

pp. 999-1017 ◽

Cited By ~ 5

Author(s):

Debany Fonseca-Batista ◽

Xuefeng Li ◽

Virginie Riou ◽

Valérie Michotey ◽

Florian Deman ◽

...

Keyword(s):

Primary Production ◽

North Atlantic ◽

Surface Waters ◽

N2 Fixation ◽

Active Sites ◽

Chl A ◽

Future Assessment ◽

Fixation Activity ◽

Carbon Concentrations ◽

Iberian Margin

Abstract. Diazotrophic activity and primary production (PP) were investigated along two transects (Belgica BG2014/14 and GEOVIDE cruises) off the western Iberian Margin and the Bay of Biscay in May 2014. Substantial N2 fixation activity was observed at 8 of the 10 stations sampled, ranging overall from 81 to 384 µmol N m−2 d−1 (0.7 to 8.2 nmol N L−1 d−1), with two sites close to the Iberian Margin situated between 38.8 and 40.7∘ N yielding rates reaching up to 1355 and 1533 µmol N m−2 d−1. Primary production was relatively lower along the Iberian Margin, with rates ranging from 33 to 59 mmol C m−2 d−1, while it increased towards the northwest away from the peninsula, reaching as high as 135 mmol C m−2 d−1. In agreement with the area-averaged Chl a satellite data contemporaneous with our study period, our results revealed that post-bloom conditions prevailed at most sites, while at the northwesternmost station the bloom was still ongoing. When converted to carbon uptake using Redfield stoichiometry, N2 fixation could support 1 % to 3 % of daily PP in the euphotic layer at most sites, except at the two most active sites where this contribution to daily PP could reach up to 25 %. At the two sites where N2 fixation activity was the highest, the prymnesiophyte–symbiont Candidatus Atelocyanobacterium thalassa (UCYN-A) dominated the nifH sequence pool, while the remaining recovered sequences belonged to non-cyanobacterial phylotypes. At all the other sites, however, the recovered nifH sequences were exclusively assigned phylogenetically to non-cyanobacterial phylotypes. The intense N2 fixation activities recorded at the time of our study were likely promoted by the availability of phytoplankton-derived organic matter produced during the spring bloom, as evidenced by the significant surface particulate organic carbon concentrations. Also, the presence of excess phosphorus signature in surface waters seemed to contribute to sustaining N2 fixation, particularly at the sites with extreme activities. These results provide a mechanistic understanding of the unexpectedly high N2 fixation in productive waters of the temperate North Atlantic and highlight the importance of N2 fixation for future assessment of the global N inventory.

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Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N2 fixation activity and diversity in the western tropical South Pacific Ocean

Biogeosciences ◽

10.5194/bg-15-3795-2018 ◽

2018 ◽

Vol 15 (12) ◽

pp. 3795-3810 ◽

Cited By ~ 12

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.

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Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N2 fixation activity and diversity in the Western Tropical South Pacific

10.5194/bg-2017-572 ◽

2018 ◽

Cited By ~ 2

Author(s):

Mathieu Caffin ◽

Hugo Berthelot ◽

Véronique Cornet-Barthaux ◽

Sophie Bonnet

Keyword(s):

Food Web ◽

N2 Fixation ◽

Secondary Ion Mass Spectrometry ◽

Heterotrophic Bacteria ◽

South Pacific ◽

Open Ocean ◽

Planktonic Food ◽

Fixation Activity ◽

Zooplankton Communities ◽

Secondary Ion

Abstract. Biological dinitrogen (N2) fixation provides the major source of new nitrogen (N) to the open ocean, contributing more than atmospheric and riverine inputs to the N supply. Yet the fate of the diazotroph-derived N (DDN) in the planktonic food web is poorly understood due to technical limitations. The main goals of this study were to (i) quantify how much of DDN is released to the dissolved pool during N2 fixation and how much is transferred to bacteria, phytoplankton and zooplankton, (ii) to compare the DDN release and transfer efficiencies under contrasting N2 fixation activity and diversity the oligotrophic waters of the Western Tropical South Pacific (WTSP) Ocean. We used nanometer 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 were the diazotroph community was either dominated by 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 %). Most of the DDN (> 90 %) was transferred to picoplankton (Synechococcus, Prochlorococcus and bacteria) in all experiments. The cyanobacteria Synechococcus and Prochlorococcus were the primary beneficiaries (~ 65–70 % of the DDN transfer), followed by heterotrophic bacteria (~ 23–34 % of the DDN transfer). The DDN transfer in bacteria was the highest (34 ± 7 %) when UCYN-B were dominating the diazotroph community. Regarding higher trophic level, the DDN transfer to the dominant zooplankton species was more efficient when the diazotroph community was dominated by Trichodesmium (~ 5–9 % of the DDN transfer) than when it is 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 coming from Trichodesmium.

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Observed seasonal regimes of North-South Atlantic Ocean interbasin exchange

10.31223/osf.io/ykm29 ◽

2019 ◽

Author(s):

Hamed D. Ibrahim

Keyword(s):

North Atlantic ◽

Climate Models ◽

South Atlantic ◽

Meridional Overturning Circulation ◽

Volume Flux ◽

The South ◽

The North ◽

Basin Scale ◽

Interbasin Exchange ◽

North And South

North and South Atlantic lateral volume exchange is a key component of the Atlantic Meridional Overturning Circulation (AMOC) embedded in Earth’s climate. Northward AMOC heat transport within this exchange mitigates the large heat loss to the atmosphere in the northern North Atlantic. Because of inadequate climate data, observational basin-scale studies of net interbasin exchange between the North and South Atlantic have been limited. Here ten independent climate datasets, five satellite-derived and five analyses, are synthesized to show that North and South Atlantic climatological net lateral volume exchange is partitioned into two seasonal regimes. From late-May to late-November, net lateral volume flux is from the North to the South Atlantic; whereas from late-November to late-May, net lateral volume flux is from the South to the North Atlantic. This climatological characterization offers a framework for assessing seasonal variations in these basins and provides a constraint for climate models that simulate AMOC dynamics.

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The Marine Environment

10.1093/oso/9780199233267.003.0001 ◽

2017 ◽

Author(s):

N. Penny Holliday ◽

Stephanie Henson

Keyword(s):

Primary Production ◽

North Atlantic ◽

Physical Environment ◽

Seasonal Cycle ◽

Physical Processes ◽

The North ◽

Basin Scale ◽

The North Atlantic ◽

Physical Features ◽

Growth Distribution

The growth, distribution, and variability of phytoplankton populations in the North Atlantic are primarily controlled by the physical environment. This chapter provides an overview of the regional circulation of the North Atlantic, and an introduction to the key physical features and processes that affect ecosystems, and especially plankton, via the availability of light and nutrients. There is a natural seasonal cycle in primary production driven by physical processes that determine the light and nutrient levels, but the pattern has strong regional variations. The variations are determined by persistent features on the basin scale (e.g. the main currents and mixed layer regimes of the subtropical and subpolar gyres), as well as transient mesoscale features such as eddies and meanders of fronts.

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Evidence for strong relations between the upper Tagus loess formation (central Iberia) and the marine atmosphere off the Iberian margin during the last glacial period

Quaternary Research ◽

10.1017/qua.2020.119 ◽

2021 ◽

pp. 1-30

Author(s):

Daniel Wolf ◽

Thomas Kolb ◽

Karolin Ryborz ◽

Susann Heinrich ◽

Imke Schäfer ◽

...

Keyword(s):

North Atlantic ◽

Moisture Transfer ◽

Sea Temperature ◽

Last Glacial ◽

Atmospheric Circulation Patterns ◽

The North ◽

Terrestrial Environments ◽

The North Atlantic ◽

Iberian Margin ◽

The Last Glacial

Abstract During glacial times, the North Atlantic region was affected by serious climate changes corresponding to Dansgaard-Oeschger cycles that were linked to dramatic shifts in sea temperature and moisture transfer to the continents. However, considerable efforts are still needed to understand the effects of these shifts on terrestrial environments. In this context, the Iberian Peninsula is particularly interesting because of its close proximity to the North Atlantic, although the Iberian interior lacks paleoenvironmental information so far because suitable archives are rare. Here we provide an accurate impression of the last glacial environmental developments in central Iberia based on comprehensive investigations using the upper Tagus loess record. A multi-proxy approach revealed that phases of loess formation during Marine Isotope Stage (MIS) 2 (and upper MIS 3) were linked to utmost aridity, coldness, and highest wind strengths in line with the most intense Greenland stadials also including Heinrich Events 3–1. Lack of loess deposition during the global last glacial maximum (LGM) suggests milder conditions, which agrees with less-cold sea surface temperatures (SST) off the Iberian margin. Our results demonstrate that geomorphological system behavior in central Iberia is highly sensitive to North Atlantic SST fluctuations, thus enabling us to reconstruct a detailed hydrological model in relation to marine–atmospheric circulation patterns.

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