Spring diatom blooms in temperate waters are often terminated by aggregation of the cells into large flocs and subsequent mass sedimentation of the phytoplankton to the sea floor. The rate of aggregate formation by physical coagulation depends on the concentration of suspended particles, on the turbulent shear that makes particles collide, and on their stickiness (= probability of adhesion upon collision). During a mixed diatom bloom in a shallow Danish fjord, for 3 weeks we monitored the concentration and stickiness of suspended particles and the species composition of the phytoplankton at 2–3 d intervals and we estimated the turbulent shear rate from observations of wind velocity. By means of coagulation theory these observations were combined into a predictor of aggregation rate. We also quantified the sedimentation of phytoplankton, other suspended particles and of aggregates by means of moored sediment traps. The sinking velocity of suspended particles and the sedimentation of aggregates varied in concert during the 3-week period and were closely mimicked by the coagulation-based predictor of aggregate formation. The population dynamics of the five quantitatively significant diatom species were all similar. During the first week of observation all increased approximately exponentially in abundance; thereupon the populations fluctuated around species-specific concentrations (100 – 104 cells ml−1), even though nutrients were not limiting their growth. This pattern is consistent with a simple coagulation model of Jackson (1990), according to which growth and coagulation (and subsequent sedimentation) will balance at a certain equilibrium concentration. The equilibrium concentrations observed were accurately predicted by the model, and the stickiness coefficients estimated by the model were in accordance with those reported in the literature. Thus, aggregate formation by coagulation appears to account for the vertical flux of particles and for the dynamics of the diatom bloom in the fjord studied.