Sedimentation rates were determined for various diatom species, and both average and maximum sinking velocities of sedimenting diatoms were calculated during a spring bloom investigation in the central Baltic Sea in 1986. Up to 25 and 50% of the Chaetoceros spp. and Thalassiosira levanderi populations, respectively, sedimented daily. Daily sedimentation rates of other diatoms, dinoflagellates and Mesodinium rubrum, however, were less than 1% of their respective standing stocks. The T. levanderi population was divided into two subpopulations: while one was sinking, the second was actively dividing (recognizable by paired-cell stages) with a specific growth rate of > 0.2 to 0.3 d-1. These paired cells were never found in sediment trap samples. The average sinking velocity of Chaetoceros spp. was 15 to 30 m d-1; that of T. levanderi was higher. The maximum sinking velocity of cells was at least 70 m d-1. According to these observations, the formation of aggregates (which enhances sinking velocity), and their sedimentation, represent a highly selective process. This indicates that diatom aggregates do not act as roving filters, sweeping the water clear while sinking.
