This database contains the abundance data of four phytoplankton size classes from the Long Term Ecological Research Station MareChiara (st. LTER-MC, 40°48.5’ N, 14°15’ E) over the years 1984-2015. Samples were collected with a Niskin bottle in surface waters, fixed with neutralized formaldehyde (0.8-1.6% final concentration), and counted at the inverted microscope. Linear measurements were taken on a number of phytoplankton specimens for each species and biovolumes were calculated based on species-specific shapes. The species were grouped in size classes (< 5 µm, 5-15 µm, 15-30 µm, > 30 µm) based on their equivalent spherical diameter (ESD).