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| Titel |
Distribution of ultraphytoplankton in the western part of the North Pacific subtropical gyre during a strong La Niña condition: relationship with the hydrological conditions |
| VerfasserIn |
M. Girault, H. Arakawa, A. Barani, H. J. Ceccaldi, F. Hashihama, S. Kinouchi, G. Grégori |
| 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 ; 10, no. 9 ; Nr. 10, no. 9 (2013-09-11), S.5947-5965 |
| Datensatznummer |
250085328
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| Publikation (Nr.) |
 copernicus.org/bg-10-5947-2013.pdf |
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| Zusammenfassung |
| The distribution of ultraphytoplankton was investigated in the western North
Pacific Subtropical Gyre (NPSG) during La Niña, a cold phase of El
Niño Southern Oscillation (ENSO). Observations were conducted in a
north-south transect (33.6–13.25° N) along the 141.5° E
meridian in order to study the ultraplankton assemblages in various
oligotrophic conditions. Analyses were performed at the single cell level by
analytical flow cytometry. Five ultraphytoplankton groups
(Prochlorococcus, Synechococcus, picoeukaryotes,
nanoeukaryotes and nanocyanobacteria-like) defined by their optical
properties were enumerated in three different areas visited during the
cruise: the Kuroshio region, the subtropical Pacific gyre and a transition
zone between the subtropical Pacific gyre and the Warm pool.
Prochlorococcus outnumbered the other photoautotrophs in all the
investigated areas. However, in terms of carbon biomass, an increase in the
relative contribution of Synechococcus, picoeukaryotes and
nanoeukaryotes was observed from the centre of the subtropical gyre to the
Kuroshio area. In the Kuroshio region, a peak of abundance of nanoeukaryotes
observed at the surface suggested an increase in nutrients likely due to the
vicinity of a cold cyclonic eddy. In contrast, in the salinity front along
the isohaline 35 and anticyclonic eddy located around 22.83° N, the
mainly constant distribution of Prochlorococcus from the surface
down to 150 m characterised the dominance by these microorganisms in high
salinity and temperature zone. Results suggested that the distribution of
nanocyanobacteria-like is also closely linked to the salinity front rather
than low phosphate concentration. The maximum abundance of ultraphytoplankton
was located above the SubTropical Counter Current (STCC) at depths
> 100 m where higher nutrient concentrations were measured. Finally,
comparison of the ultraphytoplankton concentrations during El Niño (from
the literature) and La Niña (this study) conditions seems to demonstrate
that La Niña conditions lead to higher concentrations of
Synechococcus in the Subtropical gyre and a lower abundance of
Synechococcus in the Kuroshio region. Our results suggest that the
west part of NPSG is a complex area, where different water masses, salinity
fronts and eddies lead to a heterogeneous distribution of ultraphytoplankton
assemblages in the upper layer of the water column. |
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