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High-resolution on-line measurements of chlorophyll a, particles, salinity and temperature supplemented with stationary vertical casts of CTD, nutrients and phytoplankton, from August 1990 were used to investigate the spatio-temporal patterns of summer cyanobacterial blooms along the axis of the Gulf of Finland, the Baltic Sea. These measurements were supplemented with information about phytoplankton species composition and the vertical distributions of chlorophyll and nutrients while the ship was stationary. The results showed that the Gulf of Finland is divided into several sub-basins which are separated from each other by quasi-stationary fronts related to the bottom topography and circulation patterns of the Gulf. The basins and fronts had their own characteristic bloom dynamics — wind-induced external energy inputs controlled bloom formation within the sub-basins, whereas the vertical, front-related transport processes had an additional impact on the bloom in frontal areas. Nutrient reserves and inorganic N:P ratios below the seasonal thermocline differed between sub-areas.