Distribution of micro-organisms along a transect in the South-East Pacific Ocean (BIOSOPE cruise) from epifluorescence microscopy
Résumé
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), the transition region between the gyre and the coast of South America (moderately oligotrophic), and the Chile upwelling (eutrophic). The abundance of phycoerythrin containing picocyanobacteria, autotrophic and heterotrophic eukaryotes in different size ranges, dinoflagellates, and ciliates was determined by epifluorescence microscopy after DAPI staining. All populations reached a maximum in the Chile upwelling and a minimum near the centre of the gyre. Picocyanobacteria reached a maximum abundance of 70×10³ cell mL-1. In the HNLC zone, up to 50% of picocyanobacteria formed colonies. Autotrophic eukaryote and dinoflagellate abundance reached 24.5×10³ and 200 cell mL-1, 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-1) 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.
Origine : Accord explicite pour ce dépôt
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