Biogeosciences
www.biogeosciences.net
1726-4170
1726-4189
4
6
2007
10.5194/bg-4-1101-2007
http://www.biogeosciences.net/4/1101/2007/
http://www.biogeosciences.net/4/1101/2007/bg-4-1101-2007.html
http://www.biogeosciences.net/4/1101/2007/bg-4-1101-2007.pdf
1101
1113
2007-12-14
Two High-Nutrient Low-Chlorophyll phytoplankton assemblages: the tropical central Pacific and the offshore Perú-Chile Current
F. Gómez
fernando.gomez@fitoplancton.com
H. Claustre
P. Raimbault
S. Souissi
Laboratoire d'Océanographie et de Biogéochimie, CNRS UMR 6535, Centre d'Océanologie de Marseille, Université de la Méditerranée, 163 avenue de Luminy, 13288 Marseille, France
UPMC Univ. Paris 06, UMR 7093, Laboratoire d'Océanographie de Villefranche, 06230 Villefranche-sur-Mer, France
CNRS, UMR 7093, LOV, 06230 Villefranche-sur-Mer, France
Station Marine de Wimereux, Université des Sciences et Technologies de Lille-Lille1, FRE 2816 ELICO CNRS, 28 avenue Foch, BP 80, 62930 Wimereux, France
The phytoplankton (>15 μm) composition and abundance was investigated along
a ~8000 km transect between the Marquesas Islands Archipelago and the
Chilean coasts off Concepción. In the southern limit of the central
Equatorial Pacific (at 8° S, 141° W), in High-Nutrient
Low-Chlorophyll (HNLC) warm waters, the microphytoplankton assemblage was
dominated by the lightly silicified diatoms <i>Pseudo-nitzschia delicatissima</i>
and <i>Rhizosolenia bergonii</i>. The morphology of these
species, a small pennate diatom that exhibited a tendency to form "ball of
needles" clusters and large centric diatom (>500 μm long), are interpreted as
two anti-grazing strategies in an environment dominated by small
micrograzers. Surprisingly, this a priori typical HNLC phytoplankton
assemblage was also found in the temperate offshore waters of the
Perú-Chile Current between 2000 and 600 km off Chile. This observation
suggests that a common set of environmental factors (obviously other than
temperature and salinity) are responsible for the establishment and
maintaining of this distinctive phytoplankton in these geographically and
hydrologically distant regions. Both regions are characterized by a surface
nitrate-silicic acid ratio ranging from 1–3. Occasionally <i>Rhizosolenia bergonii</i>
showed frustules anomalously fragmented, likely the result of extreme weakly silicified
phytoplankton. We suggest that silicon deficiency may be responsible of the
occurrence of HNLC phytoplankton assemblage in the tropical central Pacific
as well as the offshore Perú-Chile Current during the austral summer.
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