Phytoplankton was studied in the Kandalaksha Bay, White Sea, and exposed in situ for 18 days with the addition of 180 micromol of nitrogen in the form of nitrate, urea, ammonium, and glycine. Species composition, abundance, and biomass of algae were estimated every three days. After the enrichment, the abundance of phytoplankton increased, and its structure changed. On the 18th day of the experiment, the biomass of phytoplankton communities assimilating different nitrogen substrates was subequal. Because all the environmental factors except nitrogen substrates were identical and the level of grazing was insignificant, the similarity of the dynamics of phytoplankton assimilating organic and nitrogen demonstrates that the algae compete for organic nitrogen. Competitive parameters of algae were related to the form of nitrogen source. For instance, the competitive ability of Cylindrotheca closterium was the highest in communities that assimilated organic nitrogen, while the competitive ability of Licmophora oedipus was the highest in communities that assimilated nitrate. The competitive ability of species also depended on the type of organic substrate. For example, in L. oedipus the ability to compete for urea was higher than the ability to compete for glycine. On the contrary, in small pennate diatoms and in Nitzschia sp. the ability to compete for glycine was higher.