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The Alboran Sea (western Mediterranean) is characterized by a well-defined hydrological structure, the Almeria (Spain)-Oran (Algeria) geostrophic front. During the Almofront-2 cruise (November 22, 1997 to January 18, 1998), high frequency sampling (At = 0.5 h) of autotrophic picoplankton in surface waters was performed for 17 days (December 24 to January 9) using an automatic sampler. Cell abundance and several cellular parameters were measured by flow cytometry (FCM): forward and right-angle light scatters (FALS, PALS), phycoerythrin (PE) and chlorophyll (Chl) fluorescence. Analysis of abundance of Prochlorococcus, Synechococcus and picoeukaryotes allowed the distinction of two major types of systems: mesotrophic conditions (with waters characterized by detectable nutrients) dominated by picoeukaryotes and more oligotrophic areas (with low to undetectable nutrient levels) dominated by Prochlorococcus and, to a lower extent, Synechococcus. Most of the cellular parameters exhibited a clear diel periodicity, suggesting that cell growth and division processes were tightly coupled to the daily light cycle despite strong gradients of temperature and salinity. Cell growth for both Synechococcus and picoeukaryotes began at dawn and stopped at dusk. In contrast, the increase of Prochlorococcus light scattering only began in late afternoon, a very unusual behaviour that was apparently associated with strong quenching of Prochlorococcus Chl fluorescence during the day. Our data also suggested Chl quenching in Synechococcus but not in picoeukaryotes. Fourier analysis established unambiguously the 24 h diet periodicity for all cellular parameters as well as for cryptophyte abundance, but not for the abundance of other phytoplankters. 'The burst of division for picophytoplanktonic cells, inferred from the timing of the decrease of light scatter, occurred at dusk for both Synechococcus and picoeukaryotes, and later at night for Prochlorococcus. The ratio between maximum and minimum scatter (RALS max/min ) was useful in assessing the physiological state of the different picoplankten: high values suggested that populations probably had high division rates, especially picoeukaryotes, suggesting a quasi-optimal physiological activity of the cells in all water types encountered during this cruise despite strong hydrological gradients. This study reveals that the growth rate of the populations may be very little in these ecosystems.

Short-timescale variability of picophytoplankton abundance and cellular parameters in surface waters of the Alboran Sea (western Mediterranean)