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Phytoplankton dynamics were investigated at the mesoscale in the northern part of the Strait of Sicily in July-August 1997 on fractionated samples (< 3 and> 3 µm) using HPLC pigment analysis and flow cytometry. Distribution, diversity and photoacclimation varied within the different water masses and features present at the time of sampling, including a surface filament of deep, cold water. Picophytoplankton (< 3 µm) accounted for 80% of total chlorophyll on average, and was numerically dominated by cyanobacteria of the genus Prochlorococcus, with an average concentration of 5.2× 10 4 cells ml–1. The biomass and pigment diversity of picophytoplankton was higher in the deep chlorophyll maximum (DCM) and was related to hydrological and biological features, whereas larger phytoplankton (> 3 µm) appeared to respond to different cues. Chlorophyll pigment content per cell of Synechococcus spp …