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| Titel |
The significance of nitrogen fixation to new production during early summer in the Baltic Sea |
| VerfasserIn |
U. Ohlendieck, K. Gundersen, M. Meyerhöfer, P. Fritsche, K. Nachtigall, B. Bergmann |
| 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 ; 4, no. 1 ; Nr. 4, no. 1 (2007-01-18), S.63-73 |
| Datensatznummer |
250001499
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| Publikation (Nr.) |
 copernicus.org/bg-4-63-2007.pdf |
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| Zusammenfassung |
| Rates of dinitrogen (N2) fixation and primary production were measured
during two 9 day transect cruises in the Baltic proper in June–July of 1998
and 1999. Assuming that the early phase of the bloom of cyanobacteria lasted
a month, total rates of N2 fixation contributed 15 mmol N m−2
(1998) and 33 mmol N m−2 (1999) to new production (sensu Dugdale and
Goering, 1967). This constitutes 12–26% more new N than other annual
estimates (mid July–mid October) from the same region. The between-station
variability observed in both total N2 fixation and primary productivity
greatly emphasizes the need for multiple stations and seasonal sampling
strategies in biogeochemical studies of the Baltic Sea. The majority of new N
from N2 fixation was contributed by filamentous cyanobacteria. On
average, cyanobacterial cells >20 µm were able to supply a major part
of their N requirements for growth by N2 fixation in both 1998 (73%)
and 1999 (81%). The between-station variability was high however,
and ranged from 28–150% of N needed to meet the rate of C incorporation
by primary production. The molar C:N rate incorporation ratio
(C:NRATE) in filamentous cyanobacterial cells was variable (range 7–28) and the average almost twice as high
as the Redfield ratio (6.6) in both years. Since
the molar C:N mass ratio (C:NMASS) in filamentous cyanobacterial cells
was generally lower than C:NRATE at a number of stations, we suggest that
the diazotrophs incorporated excess C on a short term basis (carbohydrate ballasting
and buoyancy regulation), released nitrogen or utilized other
regenerated sources of N nutrients. Measured rates of total N2 fixation
contributed only a minor fraction of 13% (range 4–24) in 1998 and 18%
(range 2–45) in 1999 to the amount of N needed for the community
primary production. An average of 9 and 15% of total N2 fixation
was found in cells <5 µm. Since cells <5 µm did not show any
detectable rates of N2 fixation, the 15N-enrichment could be
attributed to regenerated incorporation of dissolved organic N (DON) and
ammonium generated from larger diazotroph cyanobacteria. Therefore, N
excretion from filamentous cyanobacteria may significantly contribute to the
pool of regenerated nutrients used by the non-diazotroph community in
summer. Higher average concentrations of regenerated N (ammonium) coincided
with higher rates of N2 fixation found during the 1999 transect and a
higher level of 15N-enrichment in cells <5 µm. A variable but
significant fraction of total N2 fixation (1–10%) could be
attributed to diazotrophy in cells between 5–20 µm. |
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