Data from a mesocosm experiment where the effects of nitrate and phosphate fertilization and copepod grazing pressure on Baltic sea plankton community were followed. Oversupply of nitrogen and phosphorus compounds fertilise aquatic systems but also change the stoichiometry of available nutrients. Here we manipulated N and P concentrations to create a range of Si:N ratios. Plankton and dissolved nutrient samples were collected three times per week for the duration (20 days) of the experiment. Unicelular plankton was fixed with lugol solution after sampling and enumerated following Utermohl method. Picoplankton was enumerated using flow cytometry. Initially, plankton biomass was the highest in high nutrient low Si:N treatments but after the bloom period, this trend switched and phytoplankton biomass was sustained longer in low nutrient high Si:N treatments. Copepods had minor effects on total phytoplankton biomass but did affect the biomass of selected species.