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Short-term variability of ultraplankton in the northwestern Mediterranean in late summer 2004. Evidence for pulsed mineralisation in the water column

DENIS Michel

The relationship between pelagic processes and exported carbon flux is highly complex. Interactions between physical and biological controls may occur on top of possible bathymetry forcing and so at different temporal and spatial time scales. In particular, the role of the diverse heterotrophic organisms in carbon mineralisation raises questions that remain to be clarified. Studies aiming at carbon export prediction usually are not considering the diversity of heterotrophs, their community structure, the succession of dominant species and their function. This is a major issue to understanding the impact of heterotrophic processes on matter and energy fluxes. In a previous study? Denis et al. (2003), suggested the existence of pulsed mineralisation triggered by diel vertical migration of zooplanktonic organisms. In the frame of a national program (PECHE) investigating the production and exportation of carbon and more specifically the control by heterotrophs at short-time scales, we studied by flow cytometry the short-term variability of ultraplankton abundance and tested the above hypothesis in the 0-1200 m water column. This study took place during the DYNAPROC 2 cruise (12 September - 17 October 2004) in the northwestern Mediterranean, away from the Ligurian current to minimise advection. Several time series were conducted in the upper 200 m with a frequency of 6 h during 5 days and 3 h over 36 h and also down to 1200 m with a frequency of 3 h during 36 h. Results will be presented to illustrate the short-term variability of ultraplankton in the upper layer and the heterogeneity of the bacterial community and its structural changes with depth monitored by staining their nucleic acids. The bacterial viability was also investigated for each sample by using the nucleic acid double staining (NADS) of Grégori et al. (2001). Results support the existence of pulsed mineralisation in the water column, shedding a new light on heterotrophic activity in the aphotic layer.