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

摘要

The distribution of selected groups of micro-organisms was analyzed along aSouth-East Pacific Ocean transect sampled during the BIOSOPE cruise in 2004.The transect could be divided into four regions of contrasted trophicstatus: a High Nutrient Low Chlorophyll (HNLC) region (mesotrophic) near theequator, the South-East Pacific Ocean gyre (hyper-oligotrophic), atransition region between the gyre and the coast of South America(moderately oligotrophic), and the Chile upwelling (eutrophic). Theabundance of phycoerythrin containing picocyanobacteria (PE picocyanobacteria), autotrophic and heterotrophic eukaryotes (classifiedinto different size ranges), dinoflagellates, and ciliates was determined byepifluorescence microscopy after DAPI staining. Despite some apparent lossof cells due to sample storage, distribution patterns were broadly similarto those obtained by flow cytometry for PE picocyanobacteria andpicoeukaryotes. All populations reached a maximum in the Chile upwelling anda minimum near the centre of the gyre. The maximum abundance of PEpicocyanobacteria was 70 103 cell mL−1. Abundance of autotrophiceukaryotes and dinoflagellates reached 24.5 103 and 20 cell mL−1,respectively. We observed a shift in the size distribution of autotrophiceukaryotes from 2–5 μm in eutrophic and mesotrophic regions to lessthan 2 μm in the central region. The contribution of autotrophiceukaryotes to total eukaryotes was the lowest in the central gyre. Maximumconcentration of ciliates (18 cell mL−1) also occurred in the Chileupwelling, but, in contrast to the other groups, their abundance was verylow in the HNLC zone and near the Marquesas Islands. Two key findings ofthis work that could not have been observed with other techniques are thehigh percentage of PE picocyanobacteria forming colonies in the HLNC regionand the observation of numerous dinoflagellates with bright greenautofluorescence.