The specific physiological mechanisms governing the process of diatom sedimentation were not well understood. This study investigated the possibility of a short term energy requirement for buoyancy maintenance in marine diatoms which varied predictably with physiological state. The maximum sinking rate (MSR) of Ditylum brightwellii was measured under progressive energy limitation which allowed the determination of the energydependent potential for a cell to reduce its MSR. There was systematic

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... xperimental evidence that this diatom's ability to gain energy from storage products via respiration is the principal determinant of its sinking rate. This study provides the first quantification of the negative exponential relationship between respiration rate and sinking rates. Preliminary studies with two other species indicate that a spectrum of energy-dependence may exist. Diatom blooms are the annual periods of highest "new" production and carbon sedimentation. Results from a 5-year study in Auke Bay, Alaska showed that termination of the spring bloom consistently occurred at limiting nitrate concentrations. The sinking response of diatoms to ambient nutrients influenced both species succession during the spring bloom and the subsequent sedimentation of new production. Threshold nitrate concentrations approximating Ks values of the species present, were found to signal initiation of increased sedimentation. Results suggested genus-specific differences in sinking-rate sensitivity to nitrate exhaustion. Overall, sinking rates of the three principal genera ranked (high to low) Thalassiosira spp.--> S. costatum --> Chaetoceros spp., while the nitrate sensitivities of the sinking rates of the genera ranked (high to low) Thalassiosira spp. --> Chaetoceros spp. --> S. costatum. For Thalassiosira aestivalis, sinking rates over all five years did not vary significantly with physical water properties such as ambient temperature, salinity or sigma-t. However daily irradiance showed a significant [...]