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Sinking and sedimentation rates of a natural phytoplankton community were simultaneously measured during the course of a diatom winter/spring bloom in a 13m3 experimental mesocosm. Sinking rate was determined directly in settling columns and was calculated from sediment trap catches. The 2 methods yielded significantly different results. Whole-community as well as species- specific sinking rates varied over time. These variations were related to changes of the environmental conditions. Over a 26d study period, a total of 7.5g cm-2 was collected in the sediment traps. Viable phytoplankton cells were the primary component of the sedimented matter while zooplankton fecal pellets contributed on average less than 10 %. Assuming the Redfield atomic ratio for the collected material, the amount of carbon which sedimented during the winter/spring bloom could be predicted from pre-bloom nutrient concentrations. The daily sedimentation rate varied considerably over time and displayed a characteristic pattern. This pattern is evidently a function of both suspended phytoplankton biomass and the temporal variation in whole-community sinking rate.