The aims were to investigate how the species succession and toxin production of a Baltic summer phytoplankton communities were affected by phosphorus, nitrogen and silicon deficiency versus sufficent conditions. A land-based enclosure experiment was performed in July 1998 with the Baltic Sea open waters. The phytoplankton communities were exposed to two levels of nitrogen, phosphorus and silicon deficiency and a non-deficient treatment. In deficient treatments, the daily added nutrients were added in order to give the same level of deficiency for either N or P concerning the Redfield ratio. For example the daily added N was 1.6 µM in the most N-deficient and P was 0.1 µM in the most P-deficient, i.e. N:P ratio of 16:1. The most N-deficient treatment was the one that forced the summer phytoplankton communities to the lowest levels of produced biomass. Although the daily added P was kept at the same N level of deficiency, the biomass in the P-deficient was higher than in the N-deficient treatments. Silicon limitation in the open waters of the Baltic Sea during the summer does not seem to exist, and this was proved by the initial concentrations of silicon (6.3 µM). Moreover, in the Si-deficient treatments the phytoplankton biomass were as high as in the N,P,Si sufficient treatment. In the most N-deficient treatment, the intracellular N-content was higher than the sum of all added nitrate suggesting that either nitrogen from organic source might have been used or N2-fixation was occurring. The N2-fixing toxic cyanobacteria, Nodularia spumigena, did grow well in all treatments, but as the other phytoplankton species, had the lowest biomass in the N-deficient treatments. The harmful dinoflagellate Heterocapsa triquetra was in higher numbers in the N and Si deficient treatments showing the ability of this algae of mixotrophy. In all three deficient treatments, small flagellates increased in cell numbers. The diatom Skeletonema costatum was in very high numbers in all treatments where N was added in excess to the needs of the phytoplankton cells, including the most Si-deficient treatment Toxin (nodularin) content was higher in the N. spumigena cells which were grown in the most P-deficient conditions. Since phosphorus has a very high turnover rate in natural marine systems and our experiment shows that a shortage of nitrogen will produce less phytoplankton biomass, we hypothesize that it is unlikely that a reduction of P input to the Baltic will lead to lower biomass or toxicity/toxin production.