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| Titel |
Dinitrogen fixation and dissolved organic nitrogen fueled primary production and particulate export during the VAHINE mesocosm experiment (New Caledonia lagoon) |
| VerfasserIn |
H. Berthelot, T. Moutin, S. L'Helguen, K. Leblanc, S. Hélias, O. Grosso, N. Leblond, B. Charrière, S. Bonnet |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 13 ; Nr. 12, no. 13 (2015-07-07), S.4099-4112 |
| Datensatznummer |
250118015
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| Publikation (Nr.) |
 copernicus.org/bg-12-4099-2015.pdf |
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| Zusammenfassung |
| In the oligotrophic ocean characterized by nitrate (NO3−)
depletion in surface waters, dinitrogen (N2) fixation and
dissolved organic nitrogen (DON) can represent significant nitrogen
(N) sources for the ecosystem. In this study, we deployed large in situ mesocosms in New Caledonia in order to investigate (1) the contribution
of N2 fixation and DON use to primary production (PP) and
particle export and (2) the fate of the freshly produced particulate
organic N (PON), i.e., whether it is preferentially accumulated and
recycled in the water column or exported out of the system. The
mesocosms were fertilized with phosphate (PO43-) in order to prevent
phosphorus (P) limitation and promote N2 fixation. The diazotrophic
community was dominated by diatom–diazotroph associations (DDAs)
during the first part of the experiment for 10 days (P1) followed by
the unicellular N2-fixing cyanobacteria UCYN-C for the last 9
days (P2) of the experiment. N2 fixation rates averaged 9.8
± 4.0 and 27.7 ± 8.6 nmol L−1 d−1 during P1 and P2,
respectively. NO3− concentrations (< 0.04 μmol L−1) in the
mesocosms were a negligible source of N, indicating that N2
fixation was the main driver of new production throughout the
experiment. The contribution of N2 fixation to PP was not
significantly different (p > 0.05) during P1 (9.0 ± 3.3 %)
and P2 (12.6 ± 6.1 %). However, the e ratio that
quantifies the efficiency of a system to export particulate organic
carbon (POCexport) compared to PP (e ratio = POCexport/PP) was significantly higher (p <
0.05) during P2 (39.7 ± 24.9 %) than during P1 (23.9 ±
20.2 %), indicating that the production sustained by UCYN-C was
more efficient at promoting C export than the production sustained
by DDAs. During P1, PON was stable and the total amount of N
provided by N2 fixation (0.10 ± 0.02 μmol L−1) was
not significantly different (p > 0.05) from the total amount of PON
exported (0.10 ± 0.04 μmol L−1), suggesting a rapid and
probably direct export of the recently fixed N2 by the
DDAs. During P2, both PON concentrations and PON export increased in
the mesocosms by a factor 1.5–2. Unlike in P1, this PON production
was not totally explained by the new N provided by N2
fixation. The use of DON, whose concentrations decreased
significantly (p < 0.05) from 5.3 ± 0.5 μmol L−1 to 4.4
± 0.5 μmol L−1, appeared to be the missing N source. The
DON consumption (~ 0.9 μmol L−1) during P2 is higher
than the total amount of new N brought by N2 fixation
(~ 0.25 μmol L−1) during the same period. These results
suggest
that while DDAs mainly rely on N2 fixation for their N
requirements, both N2 fixation and DON can be significant
N sources for primary production and particulate export following
UCYN-C blooms in the New Caledonia lagoon and by extension in the
N-limited oceans where similar events are likely to occur. |
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