Temporal and Spatial High-Frequency Monitoring of Phytoplankton by Automated Flow Cytometry and Pulse-Shape Analysis

Phytoplankton was investigated with automated high frequency flow cytometry to address their patchiness and short-term variability. To document the latter, we deployed a submersible flow cytometer (CytoSub, www.cytobuoy.com) in the Bay of Marseille (North Mediterranean), at 1.5 m depth. This instrument involves pulse shape analysis and can analyse cells (1-1000 µm) and even chains at a flow rate of 8 µl.s-1. Phytoplankton was monitored in situ every 30 min during summer 2005. The seven clusters resolved in the size range 1-50 µm, behaved as independent entities, suggesting that they could be considered as functional groups. The spatial heterogeneity of oceanic phytoplankton distribution was addressed by running the CytoSub on board a 33 m schooner (Fetia Ura, www.seanergies.com) between Azores and French Britany in April 2007. The flow cytometric analysis was triggered every 15 min (spatial resolution of 2.8 km). Five clusters were resolved in the pumped surface water and specific relationships were determined between their distributions within the different water masses sampled during the cruise. The evidenced variabilities are critical to explain the impact of intrinsic and extrinsic factors on phytoplankton spatial and temporal distributions. Automated in situ flow cytometry appears as a powerful tool to investigate phytoplankton assemblages at high frequency and at the single cell level. Undergoing technological developments are extending this capacity to the whole microbial ecosystem