Cyanobacteria are a diverse group of unicellular and multicellular photosynthetic prokaryotes (Castenholz & Waterbury, 1989; Rippka, 1988); they are often referred to as blue-green algae, even though it is now known that they are not related to any of the other algal groups. Seasonal patterns of picoplankton abundance have been observed in many studies, revealing a strong relation with water temperature. A study on picophytoplankton populations conducted by Alonso and colleagues (2007) in north-west Mediterranean coastal waters showed a peak during the winter for picoeukaryotes, and peaks in spring and summer for Synechococcus. Meanwhile, Prochlorococcus was more abundant from September to January
