During the spring of 1994, we determined the factors responsible for the decline of the seasonal diatom bloom in the Gullmar fjord, on the west coast of Sweden. Four species constituted >75% of the biomass—Detonula confervacea, Chaetoceros diadema, Skeletonema costatum and Thalassiosira nordenskioeldii—reaching concentrations of 4900, 350, 8200 and 270 cells ml−1, respectively. Growth of phytoplankton was exponential (growth rate = 0.12 day−1) from 3 to 21 March, after which a gale with winds >15 m s−1 caused massive aggregation. A maximum of 130 p.p.m. (v/v) of marine snow aggregates was observed by in situ video at the peak of the bloom. Critical concentrations (Jackson, Deep-Sea Res., 37, 1197–1211, 1990) were similar to observed showing that coagulation theory could explain the sudden decline of the bloom. The heterotrophic dinoflagellate Gyrodinium cf. spirale increased exponentially after the peak of the bloom with maximum (temperature-adjusted) growth rates. After the rapid aggregation and sedimentation of the bloom, they were able to control any further growth of diatoms. Nitrate and silicate were never depleted, but phosphate may have been limiting by the end of the study period. We conclude that mass aggregation during a gale marked the end of the bloom, and that intense grazing by heterotrophic dinoflagellates prevented any subsequent increase of diatoms.