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Short-term variation of the phytoplankton assemblage in the Bay of Marseille (France) monitored by in situ flow cytometry

Thyssen Melilotus

High frequency single cell analysis is of great importance in monitoring short-term ecosystem changes or perturbations due to human activity or natural events. The phytoplankton community encloses a wide sum of biological information (high biodiversity, fast growth rate) and thus may present fast responses when the external environment changes, and this on several counts such as composition, abundance, physiological state and metabolic activity. Due to limitations linked to sampling and the time consuming observation by optical microscopy, little is known about these short-term variations of phytoplankton communities. To address this question, we used a submersible flow cytometer (CytoSub, CytoBuoy b.v., Bodegraven, The Netherlands) and investigated the short-term variability of phytoplankton in an harbour on the coast of Marseille (North Mediterranean). This instrument can analyse phytoplankton in the size range 1-600 µm with a time interval as short as 10 min and a high flow speed, enabling detection of single cells, chains and "rare" events. Phytoplankton was monitored in situ at a fixed site, 1.5 m depth, during summer 2005. Seawater was analysed every 30 min. The data treatment was conducted on the basis of pulse-shape analysis of the signals that enables collection of much more information than usual softwares based on peak-intensity or peak-area analysis. Eight clusters were resolved. Daily sampling of nutrients (NO3-, NO2-, PO43-) and continuous information on salinity, temperature, wind speed and global light intensity allowed to distinguish the impact of environmental factors from that of nychtemeral cell cycle on phytoplankton abundance variations. Results lead to consider the flow cytometric clusters as functional groups.