A two year survey of ultraphytoplankton in the bay of Marseilles

The Bay of Marseilles is under the influence of a large urban concentration and its maritime activities. All of them discharge compounds (hydrocarbons, excess nutrients, heavy metals, chemicals, etc.) that can alter the marine ecosystem. To investigate whether ultraphytoplankton (<10 microm) could be used as biosensors for their own ecosystem, a 2-year survey was conducted in the Bay of Marseilles. Seven stations monitored water mass and potential anthropic effects in the bay. Seawater samples were collected monthly or bimonthly at three depths, prefiltered, fixed, and kept in liquid nitrogen until flow cytometric analysis. Five categories were created: Prochlorococcus, picoeukaryotes (<2 microm), nanoeukaryotes I (2--6 microm), nanoeukaryotes II (6--10 microm), and Synechococcus (<1.5 microm). Artificial neural network analysis (Kohonen self-organizing maps) produced the same number of clusters as cluster analysis with Winlist software (Verity Software House). In addition to the wide variabilities in abundance and biomass, there were a strong seasonal signal and sporadic events. Lessons are derived from this study for future monitoring of marine microorganisms.