Two High-Nutrient Low-Chlorophyll phytoplankton assemblages: the tropical central Pacific and the offshore Perú-Chile Current
Résumé
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 micro-phytoplankton assemblage was dominated by the lightly silicified diatoms Pseudo-nitzschia delicatissima and Rhizosolenia bergonii. 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 micro-grazers. 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-silicate ratio ranging from 1–3. Occasionally Rhizosolenia bergonii showed frustules anomalously fragmented, likely the result of extreme weakly silicified phytoplankton. We suggest that silicate-deficiency may be responsible of the occurrence of HNLC phytoplankton assemblage in the tropical central Pacific as well as offshore Perú-Chile Current during the austral summer.
Origine : Accord explicite pour ce dépôt
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