Department of Earth System Science

Cell size and ascent rate were measured in individual short chains (2-8 cells) and single cells of positively buoyant marine diatoms from natural populations and cultures. Maximum ascent rates for Rhizosolenia formosa, Rhizosolenia acuminata, Rhizosolenia castracanei, Rhizosolenia debyana, and Ethmodiscus rex suggest an upper limit to diatom ascent rates on the order of 7-8 m h- 1. Ascent rates were weakly correlated with cell diameter, volume, and surface: volume ratio across the pooled data for all species (n = 105). Among Rhizosolenia spp., ascent rate was strongly correlated with size-related measures. The Rhizosolenia spp. average diameter was highly correlated with average ascent rate (r2 = 0.99, n = 4). Calculated cell density was inversely related to cell diameter in Rhizosolenia spp. Cell diameter is a better predictor of ascent rate than volume in long, cylindrical cells owing to covariations of density and form resistance. The observed ascent rates are consistent with vertical migration by these large diatoms in oligotrophic seas.