Biogeosciences
www.biogeosciences.net
1726-4170
1726-4189
5
2
2008
10.5194/bg-5-311-2008
http://www.biogeosciences.net/5/311/2008/
http://www.biogeosciences.net/5/311/2008/bg-5-311-2008.html
http://www.biogeosciences.net/5/311/2008/bg-5-311-2008.pdf
311
321
2008-03-04
Distribution of micro-organisms along a transect in the South-East Pacific Ocean (BIOSOPE cruise) using epifluorescence microscopy
S. Masquelier
D. Vaulot
vaulot@sb-roscoff.fr
Station Biologique de Roscoff, UMR 7144, CNRS et Université Pierre et Marie Curie, Place G. Tessier, 29682 Roscoff, France
The distribution of selected groups of micro-organisms was analyzed along a
South-East Pacific Ocean transect sampled during the BIOSOPE cruise in 2004.
The transect could be divided into four regions of contrasted trophic
status: a High Nutrient Low Chlorophyll (HNLC) region (mesotrophic) near the
equator, the South-East Pacific Ocean gyre (hyper-oligotrophic), a
transition region between the gyre and the coast of South America
(moderately oligotrophic), and the Chile upwelling (eutrophic). The
abundance of phycoerythrin containing picocyanobacteria (PE picocyanobacteria), autotrophic and heterotrophic eukaryotes (classified
into different size ranges), dinoflagellates, and ciliates was determined by
epifluorescence microscopy after DAPI staining. Despite some apparent loss
of cells due to sample storage, distribution patterns were broadly similar
to those obtained by flow cytometry for PE picocyanobacteria and
picoeukaryotes. All populations reached a maximum in the Chile upwelling and
a minimum near the centre of the gyre. The maximum abundance of PE
picocyanobacteria was 70 10<sup>3</sup> cell mL<sup>−1</sup>. Abundance of autotrophic
eukaryotes and dinoflagellates reached 24.5 10<sup>3</sup> and 20 cell mL<sup>−1</sup>,
respectively. We observed a shift in the size distribution of autotrophic
eukaryotes from 2–5 μm in eutrophic and mesotrophic regions to less
than 2 μm in the central region. The contribution of autotrophic
eukaryotes to total eukaryotes was the lowest in the central gyre. Maximum
concentration of ciliates (18 cell mL<sup>−1</sup>) also occurred in the Chile
upwelling, but, in contrast to the other groups, their abundance was very
low in the HNLC zone and near the Marquesas Islands. Two key findings of
this work that could not have been observed with other techniques are the
high percentage of PE picocyanobacteria forming colonies in the HLNC region
and the observation of numerous dinoflagellates with bright green
autofluorescence.
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