Time series of phytoplankton abundance collected with a monthly frequency at 0.5, 5, 10 and 15 meters in the Gulf of Trieste. The time series station C1, located in the Gulf of Trieste in the northernmost part of the Adriatic Sea, is part of the Adriatic Long Term Ecological Research (LTER) site. It is a shallow coastal station (17 m depth) located at 0.2 Km from the coast, at the outer border of the Natural Marine Reserve of Miramare, Trieste. More information about activities carried out and about datasets from other periods can be found at: http://nettuno.ogs.trieste.it/ilter/GoTTs/en_index.html. Samples were fixed with Ca(HCO3)2-buffered formaldehyde (0.8% final concentration) and cell abundance was estimated according to Utermöhl’s method (1958), using an inverted microscope equipped with phase contrast, at 200-320-400x final magnifications. Abundant cells were counted in defined random fields, rarer species were counted on the whole chamber (Fonda Umani et al., 2007).