Distribution of micro-organisms along a transect in the South-East Pacific Ocean (BIOSOPE cruise) using epifluorescence microscopy

Abstract. 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 70 10³ cell mL⁻¹. Abundance of autotrophic eukaryotes and dinoflagellates reached 24.5 10³ and 20 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. 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.