Sampling along the water column was performed with Niskin bottles associated to a CTD profiler. At each station, several discrete depths were sampled, according to station depth and hydrological vertical profiles. Study Extent 5 oceanographic surveys were carried out in the North Adriatic Sea, in front of the Po River delta. Method step description: Samples were fixed with Ca(HCO3)2-buffered formaldehyde (0.8% final concentration) and cell abundance and species composition of phytoplankton were estimated according to Utermöhl’s method (1958), using an inverted microscope equipped with phase contrast, at 200-320-400x final magnifications.