Two High-Nutrient Low-Chlorophyll phytoplankton assemblages: the tropical central Pacific and the offshore Perú-Chile Current
1Laboratoire d'Océanographie et de Biogéochimie, CNRS UMR 6535, Centre d'Océanologie de Marseille, Université de la Méditerranée, 163 avenue de Luminy, 13288 Marseille, France
2UPMC Univ. Paris 06, UMR 7093, Laboratoire d'Océanographie de Villefranche, 06230 Villefranche-sur-Mer, France
3CNRS, UMR 7093, LOV, 06230 Villefranche-sur-Mer, France
4Station Marine de Wimereux, Université des Sciences et Technologies de Lille-Lille1, FRE 2816 ELICO CNRS, 28 avenue Foch, BP 80, 62930 Wimereux, France
Abstract. 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 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 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 Rhizosolenia bergonii 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.
Gómez, F., Claustre, H., Raimbault, P., and Souissi, S.: Two High-Nutrient Low-Chlorophyll phytoplankton assemblages: the tropical central Pacific and the offshore Perú-Chile Current, Biogeosciences, 4, 1101-1113, doi:10.5194/bg-4-1101-2007, 2007.